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12-04-2018, 08:22 AM   #106
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New soft focus lens

I just pulled the trigger on a "like new" copy of Pentax 67 120mm f 3.5 soft focus, the idea being to have soft focus capability on the 645D and Z. Photos coming when available.

12-05-2018, 09:02 AM   #107
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I'm very proud of the original Pentax PK adapter
It comes useful for three 6x7 lenses.
Both the 120mm SF and the 135mm Macro work well on smaller formats, then I have a Pentacon 5.6/500mm that was converted long ago to Pentax 67 mount.
All the other 6x7 optics I own don't make much sense.
A pity the 120mm is not very portable, like all the other 6x7/67 lenses.
For this reason I almost never use them (I haven't shot analog in a while).
Maybe one day I will follow your example and go for a bigger sensor...
12-05-2018, 10:53 AM   #108
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QuoteOriginally posted by cyberjunkie Quote
Like this?



I'm very proud of the original Pentax PK adapter
It comes useful for three 6x7 lenses.
Both the 120mm SF and the 135mm Macro work well on smaller formats, then I have a Pentacon 5.6/500mm that was converted long ago to Pentax 67 mount.
All the other 6x7 optics I own don't make much sense.
A pity the 120mm is not very portable, like all the other 6x7/67 lenses.
For this reason I almost never use them (I haven't shot analog in a while).
Maybe one day I will follow your example and go for a bigger sensor...

Haven't seen mine yet, but now I see what it looks like! I have seven of the 67 lenses, and I concur that a great deal of the line is redundant if you have similar focal lengths in K-mount or 645 mount. I do love the 55mm f 4.0 on both the K-1 and the 645D/Z as well as the super fast 105mm f 2.4. The 135mm macro is also nice. I think I have become as much a lens collector as a photographer, and lots of times I go back and shoot the same scenes with different lenses just to test and compare lenses with each other. There can be no redundancy if you can afford the stuff and it is just a hobby! The attached photo courtesy of Pentax K-1 and the Pentax 67 55mm f 4.0, one of Pentax's truly great lenses in my view.

---------- Post added 12-05-18 at 01:03 PM ----------

QuoteOriginally posted by cyberjunkie Quote
Like this?



I'm very proud of the original Pentax PK adapter
It comes useful for three 6x7 lenses.
Both the 120mm SF and the 135mm Macro work well on smaller formats, then I have a Pentacon 5.6/500mm that was converted long ago to Pentax 67 mount.
All the other 6x7 optics I own don't make much sense.
A pity the 120mm is not very portable, like all the other 6x7/67 lenses.
For this reason I almost never use them (I haven't shot analog in a while).
Maybe one day I will follow your example and go for a bigger sensor...
I have that 500mm lens also, but it is in M42 mount. I sure wish I could find a way to convert it to a Pentax mount, either K-mount or 645 mount. Any ideas?
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PENTAX K-1  Photo 

Last edited by ivanvernon; 12-05-2018 at 10:55 AM. Reason: Grammar police.
12-05-2018, 04:09 PM   #109
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QuoteOriginally posted by ivanvernon Quote
I have that 500mm lens also, but it is in M42 mount. I sure wish I could find a way to convert it to a Pentax mount, either K-mount or 645 mount. Any ideas?
No easy, practical suggestion. Sorry.
As far as I remember mine had the Pentacon Six to M42 adapter on, so if you have/find such adapter you can use it for M2/PK cameras.
The adaptation was done by my friend/repairman using a lathe. I remember I gave him a 6x7 extension ring to be used for the bayonet... but I got it back, he had masterfully done it using a slice of a high grade aluminum rod. All done manually, with a small lathe and a micrometric burr.
Actually he made two bayonets, not one.
The other was used to convert to 6x7 mount a nice octagonal bellows (originally in Pentacon Six mount). I had in mind to convert another P6 lens, the Zenitar 35mm fisheye, but it was a very complex job, got postponed, and in the end I gave up and sold it.

12-05-2018, 07:38 PM   #110
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Northeast Harbor Maine with skim ice.
Put the Fujinon 85mm f4 Soft lens on my K-5 today. One thing it does better than the Pentax lenses is shoot sharp as well as soft.
Order is mixed up but f11, on top then f4 then f8 then f5.6. I didn't try f16 but I will.
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12-05-2018, 11:40 PM - 1 Like   #111
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QuoteOriginally posted by kcjonez Quote
Put the Fujinon 85mm f4 Soft lens on my K-5 today. One thing it does better than the Pentax lenses is shoot sharp as well as soft.
I like Fuji soft focus optics.
The large format version has been a favourite of pro portrait photographers for a few decades.
I believe the 85mm for 35mm film cameras to be as good.
I have a 210mm in Copal shutter, one of the few large format soft focus objectives that had coated glasses.
Most of the legendary, hyper expensive SF lenses did not, cause they were made pre-war.
When I collected large format portrait and soft focus lenses I bought all those I could find at remotely affordable prices, including two rare birds: the Cooke Portrait Series IIE (the only coated Cooke Portrait) and a Marcucci Pictor 270mm.
I think of them with a mix of awe and guilt. It took a lot of effort to find the uncommon ones, and now I don't shoot analog anymore...

The soft focus lens that dominated the market for quite a long time, at least in Europe, was the Rodenstock Imagon.
Born as Kuhn portrait objective, and made by Staeble (if I remember correctly), was later sold to Rodenstock and produced in huge numbers, compared with other SF optics.
I still have a few, in shutter and in barrel, up to the focal of 420mm, but none of them is really usable on a modern DSLR, because of the long focals.
I yearned over a long time for one of the shorter focals, that were used (with a focusing mount) on medium format film cameras.
They are always quite expensive, especially those in Hasselblad mount.
Well... after countless lost auctions I have finally acquired an old 120mm in focusing mount. It will not come dirty cheap, but at least I can afford it!
Here it is:

Kochman 120mm Jmagon in Korelle mount


I will post more details and pictures on the "Your last acquisition" thread.

cheers

Paolo
12-06-2018, 03:26 AM   #112
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QuoteOriginally posted by kcjonez Quote
Northeast Harbor Maine with skim ice.
Put the Fujinon 85mm f4 Soft lens on my K-5 today. One thing it does better than the Pentax lenses is shoot sharp as well as soft.
Order is mixed up but f11, on top then f4 then f8 then f5.6. I didn't try f16 but I will.

Love these images! However, I do not quite understand your meaning when you state: "One thing it does better than the Pentax lenses is shoot sharp as well as soft." My SMC Pentax-F 85mm f 2.8 Soft shoots with decreasing qualities of softness moving from f 2.8 to f 8.0, but at f 8.0 and smaller apertures becomes a regular sharp-focussing lens that can be used for landscape or other applications. Are you saying that while both Fujinon and Pentax shoot sharp, Fujinon does it better? I am not trying to be quarrelsome, just trying to understand. Thank you.
12-06-2018, 05:35 PM   #113
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QuoteOriginally posted by ivanvernon Quote
Love these images! However, I do not quite understand your meaning when you state: "One thing it does better than the Pentax lenses is shoot sharp as well as soft." My SMC Pentax-F 85mm f 2.8 Soft shoots with decreasing qualities of softness moving from f 2.8 to f 8.0, but at f 8.0 and smaller apertures becomes a regular sharp-focussing lens that can be used for landscape or other applications. Are you saying that while both Fujinon and Pentax shoot sharp, Fujinon does it better? I am not trying to be quarrelsome, just trying to understand. Thank you.
I will have to find one then. Thanks.

12-06-2018, 10:52 PM   #114
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Soft focus shots on snowy December day

Shots taken on a snowy December day.

KIT: Pentax K-1 with Pentax-F 85mm f2.8.
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12-07-2018, 05:23 AM   #115
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QuoteOriginally posted by ivanvernon Quote
I do not quite understand your meaning when you state: "One thing it does better than the Pentax lenses is shoot sharp as well as soft." My SMC Pentax-F 85mm f 2.8 Soft shoots with decreasing qualities of softness moving from f 2.8 to f 8.0, but at f 8.0 and smaller apertures becomes a regular sharp-focussing lens that can be used for landscape or other applications. Are you saying that while both Fujinon and Pentax shoot sharp, Fujinon does it better?
I don't know the exact meaning of that sentence, but I do know that soft focus lenses are very different one from another in the way they render the subject.
Under diffused light the difference is not always easily visible, but under the right lighting it becomes quite apparent, even to an untrained eye.
This is due to the simple fact that the various lens makers used completely different approaches.
The SMC Pentax 2.2/85mm is an achromatic doublet, in practice it's an Imagon that has been reversed (diaphragm is behind, not in front of the objective) and stripped of the "tea strainer".
The Fuji soft focus lenses have a more elaborated design, and the strainer disk has to be inserted inside the lens. Unlike the Imagon, the disk is not adjustable to two different values. All the Fuji SF optics have the same design, across formats.
The Pentax F and FA 85mm share the same optical design, but the FA has a smarter way of dealing with the difficult task of correctly focusing an optic that is by definition "soft focus". The optical layout is rather complex: 5 elements in 4 groups.
I also own a very strange bird, the Pentax-FA 2.8/28mm Soft Focus. It is made of 5 elements in 5 groups and has an optical design that reminds of some very early retrofocus objectives of the fifties, revised, more complex and faster.
The 120mm for the Pentax 6x7/67 is a slightly simpler design (3G/4E), but has a completely different optical approach from the Imagon and the various triplets. There is an interesting university degree thesis that centers around the design and the peculiar rendering of different soft focus projects. For the record, the author criticises Pentax's approach and concludes that the simple Imagon design is still the best. If anybody is interested, I can provide the link or at least the title. It's in english.
All the various Pentax SF objectives control the amount of halation through the aperture. The closer the diaphragm the sharper the result.
The Rodenstock and Fuji approach is more refined, there are both the strainer disk and the diaphragm, and both can be used to control spherical aberration. Some very old SF lenses used chromatic aberration instead, but the advent of panchro film made them obsolete. Photographers who shot with those lenses had to juggle a lot to get correctly focused images. I spare you the details....
Those wanting to experiment on the cheap would find that even a simple 2-elements diopter lens is an achromat... and can be used as a simple SF lens without getting too crazy. The larger the diameter of the diopter, the better it is.
Many legendary soft focus lenses of the past had a specific ring that moved one element inside the lens, affecting the degree of spherical aberration.
One example is the pre-war Meyer Trioplan 270mm, that has the same basic Cooke triplet design of modern Trioplan's, but the central element could be adjusted to dial in the right amount of softness. The same way, all the legendary Cooke Portrait lenses had a similar softness ring, which controlled either the central or back elements of the triplet.
The only modern time objective with a dedicated softness ring is the Tamron 70-150mm Soft Focus. It's rare, expensive, and has the greatest level of control of the level/nature of halo. The use of both the SF ring and the diaphragm allows for a very fine adjustment of the rendition.
I have two of them, cause yes, I like that optics as much!
The last one sold for $735 on eBay, and when I did the search I couldn't find a single one on sale.
It would make sense to sell one and dedicate the money to some other vintage lens I dream of... but I am hesitant because of course I have only one example with me, and for some reason I didn't use it at all recently, so at least before selling I want to use it for a while.
Soft focus lenses have never been cheap, but some were exceptionally expensive. Karl Struss, the same man who won the first Academy Award for movie photography, designed some years before the Struss Pictorial Lens. It was a hand-ground SF aspherical meniscus lens for portrait photographers.
Grinding by hand an aspherical profile from large glass blanks was a very complicated affair. Any lens was slightly different from the other, and many were rejected. It was so expensive to produce, that the quartz (yes, from a huge crystal of quartz!) version had almost the same price of the optical glass one.
With the same amount of money you could either buy a large Struss, or 3 (three!!) Ford Model T
Btw, the second wave of american pictorialism, and the birth of the Hollywood movie industry, called for more high quality, crazy soft focus lenses, so after the Struss, a new quartz lens was made, the Hanovia Kalosat.
These lenses, along with the slightly cheaper Pinkham & Smith, were my objects of desire when I was actively collecting large format soft focus lenses.
When I definitively turned to digital-only I realised it was a shame to buy lenses that I would never use, so I gave up. Btw, most of those I don't have are hugely expensive and I would never, ever have enough funds to buy them

I hope my little digest sparked some curiosity. If you want to learn more about the fascinating world of SF lenses, Internet can provide much food for thoughts, and many gorgeous portraits of Hollywood starlets of the twenties, shots with the kind of objectives I just mentioned, and many many others.
The level of technical competence, ingenuity and finesse of the photographers who actually shot those portraits frankly puts to shame the vast majority of present time professionals.
I have been so amazed by those masters of black and white that I checked on Google Books the availability of photographic books of the time.
Well, photography required a good level of wizardry, and the iconographic level of the images was impressively high.
Hats off.

EDIT:
I often find on the Web the theory that at present time soft focus lenses have no reason to exist, because the original image can be tweaked in post production.
I don't agree, and I think that most of those who back this opinion have never seen a beautiful print of a picture shot with a good soft focus lens.
A nice soft focus image has nothing to do with a blurry or poorly focused one.
A proper SF lens provides a sharp picture superimposed with a certain amount of halo, that "bleeds" from highlight areas into mid-tone areas.
It would take plenty of digital wizardry to get the same results in PP.

Last edited by cyberjunkie; 12-07-2018 at 06:04 AM.
12-07-2018, 07:39 AM   #116
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Thanks to cyberjunkie

QuoteOriginally posted by cyberjunkie Quote
I don't know the exact meaning of that sentence, but I do know that soft focus lenses are very different one from another in the way they render the subject.
Under diffused light the difference is not always easily visible, but under the right lighting it becomes quite apparent, even to an untrained eye.
This is due to the simple fact that the various lens makers used completely different approaches.
The SMC Pentax 2.2/85mm is an achromatic doublet, in practice it's an Imagon that has been reversed (diaphragm is behind, not in front of the objective) and stripped of the "tea strainer".
The Fuji soft focus lenses have a more elaborated design, and the strainer disk has to be inserted inside the lens. Unlike the Imagon, the disk is not adjustable to two different values. All the Fuji SF optics have the same design, across formats.
The Pentax F and FA 85mm share the same optical design, but the FA has a smarter way of dealing with the difficult task of correctly focusing an optic that is by definition "soft focus". The optical layout is rather complex: 5 elements in 4 groups.
I also own a very strange bird, the Pentax-FA 2.8/28mm Soft Focus. It is made of 5 elements in 5 groups and has an optical design that reminds of some very early retrofocus objectives of the fifties, revised, more complex and faster.
The 120mm for the Pentax 6x7/67 is a slightly simpler design (3G/4E), but has a completely different optical approach from the Imagon and the various triplets. There is an interesting university degree thesis that centers around the design and the peculiar rendering of different soft focus projects. For the record, the author criticises Pentax's approach and concludes that the simple Imagon design is still the best. If anybody is interested, I can provide the link or at least the title. It's in english.
All the various Pentax SF objectives control the amount of halation through the aperture. The closer the diaphragm the sharper the result.
The Rodenstock and Fuji approach is more refined, there are both the strainer disk and the diaphragm, and both can be used to control spherical aberration. Some very old SF lenses used chromatic aberration instead, but the advent of panchro film made them obsolete. Photographers who shot with those lenses had to juggle a lot to get correctly focused images. I spare you the details....
Those wanting to experiment on the cheap would find that even a simple 2-elements diopter lens is an achromat... and can be used as a simple SF lens without getting too crazy. The larger the diameter of the diopter, the better it is.
Many legendary soft focus lenses of the past had a specific ring that moved one element inside the lens, affecting the degree of spherical aberration.
One example is the pre-war Meyer Trioplan 270mm, that has the same basic Cooke triplet design of modern Trioplan's, but the central element could be adjusted to dial in the right amount of softness. The same way, all the legendary Cooke Portrait lenses had a similar softness ring, which controlled either the central or back elements of the triplet.
The only modern time objective with a dedicated softness ring is the Tamron 70-150mm Soft Focus. It's rare, expensive, and has the greatest level of control of the level/nature of halo. The use of both the SF ring and the diaphragm allows for a very fine adjustment of the rendition.
I have two of them, cause yes, I like that optics as much!
The last one sold for $735 on eBay, and when I did the search I couldn't find a single one on sale.
It would make sense to sell one and dedicate the money to some other vintage lens I dream of... but I am hesitant because of course I have only one example with me, and for some reason I didn't use it at all recently, so at least before selling I want to use it for a while.
Soft focus lenses have never been cheap, but some were exceptionally expensive. Karl Struss, the same man who won the first Academy Award for movie photography, designed some years before the Struss Pictorial Lens. It was a hand-ground SF aspherical meniscus lens for portrait photographers.
Grinding by hand an aspherical profile from large glass blanks was a very complicated affair. Any lens was slightly different from the other, and many were rejected. It was so expensive to produce, that the quartz (yes, from a huge crystal of quartz!) version had almost the same price of the optical glass one.
With the same amount of money you could either buy a large Struss, or 3 (three!!) Ford Model T
Btw, the second wave of american pictorialism, and the birth of the Hollywood movie industry, called for more high quality, crazy soft focus lenses, so after the Struss, a new quartz lens was made, the Hanovia Kalosat.
These lenses, along with the slightly cheaper Pinkham & Smith, were my objects of desire when I was actively collecting large format soft focus lenses.
When I definitively turned to digital-only I realised it was a shame to buy lenses that I would never use, so I gave up. Btw, most of those I don't have are hugely expensive and I would never, ever have enough funds to buy them

I hope my little digest sparked some curiosity. If you want to learn more about the fascinating world of SF lenses, Internet can provide much food for thoughts, and many gorgeous portraits of Hollywood starlets of the twenties, shots with the kind of objectives I just mentioned, and many many others.
The level of technical competence, ingenuity and finesse of the photographers who actually shot those portraits frankly puts to shame the vast majority of present time professionals.
I have been so amazed by those masters of black and white that I checked on Google Books the availability of photographic books of the time.
Well, photography required a good level of wizardry, and the iconographic level of the images was impressively high.
Hats off.

EDIT:
I often find on the Web the theory that at present time soft focus lenses have no reason to exist, because the original image can be tweaked in post production.
I don't agree, and I think that most of those who back this opinion have never seen a beautiful print of a picture shot with a good soft focus lens.
A nice soft focus image has nothing to do with a blurry or poorly focused one.
A proper SF lens provides a sharp picture superimposed with a certain amount of halo, that "bleeds" from highlight areas into mid-tone areas.
It would take plenty of digital wizardry to get the same results in PP.

Paolo:

Thank you for all this information, practical, theoretical, and historical, regarding the entire soft focus lens context. Reading your comments here on the forum is a little like taking a short college course in optics.

You are a treasure and a fantastic asset to Pentax Forum. I greatly value all your comments, and sincerely appreciate your willingness to share your knowledge with the rest of us. Please never allow any kind of false modesty to prevent you from expressing your experience and knowledge so that the rest of us can continue learning from your wide-ranging knowledge base and experience.

Ivan
12-07-2018, 10:56 AM - 1 Like   #117
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QuoteOriginally posted by ivanvernon Quote
Paolo:

Thank you for all this information, practical, theoretical, and historical, regarding the entire soft focus lens context. Reading your comments here on the forum is a little like taking a short college course in optics.

Ivan
Hahaha, I'm embarrassed
There are far more knowledgeable folks around, I'm sure.
I just happen to have a good memory, and remember a few things I have either read online, in books, or (very few) found myself.

One more thing about soft focus lenses.
They are not a thing of the past. Leica released short time ago a new version of the famous (and hugely expensive) Thambar.
Canon filed for a optical patent concerning soft focus designs in 2018:
https://www.dpreview.com/news/7576945995/canon-patent-application-teases-ful...oft-focus-lens
It confirms that even in a time of fake computational bokeh, soft focus lenses still have a place.
12-07-2018, 12:04 PM   #118
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QuoteOriginally posted by cyberjunkie Quote
Hahaha, I'm embarrassed
There are far more knowledgeable folks around, I'm sure.
I just happen to have a good memory, and remember a few things I have either read online, in books, or (very few) found myself.

One more thing about soft focus lenses.
They are not a thing of the past. Leica released short time ago a new version of the famous (and hugely expensive) Thambar.
Canon filed for a optical patent concerning soft focus designs in 2018:
Canon patent application teases full-frame 'soft-focus' lens designs: Digital Photography Review
It confirms that even in a time of fake computational bokeh, soft focus lenses still have a place.
I mean it.

Here is a challenge and a question:

Challenge: Give me a little essay some time on your five best lenses--the five that you would not consider ever getting rid of.

Question: It is sometimes said that a medium format lens of, say, 2.8 aperture is functionally equivalent to a wider aperture 135 format lens. For example, I have read where someone stated that the Mamiya 6445 80mm f 1.9 is functionally equivalent to a 135 lens of F 0.9. First, do you believe those sort of statements are valid? And, then second, when a medium format lens is adapted to a smaller format, say putting the Mamiya 645 80mm f 1.9 on the Pentax K-1, what then is its functional aperture behavior? I mean, does it act like a regular 135 f 2.8 lens or like an f 0.9 lens?
12-08-2018, 01:47 AM   #119
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QuoteOriginally posted by ivanvernon Quote
Here is a challenge and a question:
The challenge requires some thinking. I will use some time to give it a good thought...

The question is rather simple, but the answer is a bit more complex than what it seems to be.
I am much more knowledgeable in the history of camera lenses than in optical theory. Which means that I lack any basic background in mathematic and physics. I am not so sure I will not incur in practical mistakes, and I'm sure any formally correct answer would require the kind of background I don't have.
Let's start with the basic. A lens, of any kind/focal/aperture, would behave the same way independently from the format of the film/sensor.
An 80mm f/1.9 keeps its intrinsic characteristics whatever camera you use. The only difference is the area of the circle of coverage that is actually used.
Depending of the format, the angle of view (and the perspective!) changes, so that the same 80mm would be called a "normal" of a 6x6 camera, and a short tele on a 35mm/FF one.
BUT... what I just said is true ONLY if we keep the camera in the same place. If we want to cover the same angle of view, and try to take the same picture, we would see that when the 80mm is fitted to an FF camera we have to back off, in order to capture the same scene.
It is the difference in distance from the main subject that causes what we call perspective compression. If the size of the subject has to be the same across formats, the distance has to vary. This is the one and only cause of geometric distortion in wide angles. If we fit a 28mm on an APS-C camera we can stay away from the subject. If we fit the same lens on a FF camera, we have to go closer, and that change in perspective would deform the facial features, so that the nose seems to have grown bigger.
If we kept the same distance from the subject, the same face would be either smaller or bigger in the picture, but the way it looks would be the same.
Same thing with depth of focus.
It is widely accepted to consider a 50mm the "normal" lens for 35mm/FF cameras. In reality it would be a little less, to match human vision, but this is another story...
The same way it is widely accepted to consider a 300mm lens the "normal" for 8x10" film.
If we shoot the same scene we would see that the two pictures look very similar. Though it's not totally true, the SIZE of the subject is almost the same, but the DOF is not. This is because a 300mm has the same depth of field, across formats. Some lenses give the IMPRESSION of more DOF, because of the way they are designed. The sharper they are the more evident the difference between in focus and out of focus, so our eye is tricked into believing that the DOF is smaller. DOF is calculated using three values, it's physics. The human eye can be fooled, which is not a bad thing. Actually I'm quite happy that different lenses give a different "impression" of reality.
Of course this has very much to do with the way bokeh is rendered, because the circle of confusion of out of focus highlights looks different depending on the distance from the plane of correct focus.
Well, circle of confusion. Again, the answer is not straightforward because there is a variable.
In analog times the formula of DOF involved three values: focal, aperture and conventional circle of confusion. The latter was taken as a variable because the accepted circle of confusion (which determines what is acceptably sharp to the average human eye), was made to vary according to the format.
The reason was simple, larger negatives had to be enlarged less, so a level of sharpness that would be unacceptable with a 35mm film would be more than enough with a large sheet film. Contact print on 8x10", from negatives taken with horribly unsharp lenses, look gorgeous.
Some Ansel Adams pictures were taken with the back half of a Turner & Reich double anastigmat. Such lens allowed him to use a "tele", but the optical quality was awful. Nevertheless he managed to print amazing black & whites.
He did that stopping down like crazy, and contact printing (or enlarging just very little).
This is where I am at a loss. Is the relevance of circle of confusion in the equation still relevant?
In the first place most pictures are not printed but used with computer screens or projectors.
For example the image can be resampled to 72dpi with no adverse effect on the perceived quality.
The printing technology is also quite different from what it used to be. Does it make a difference or not?
In general, medium format lenses had to meet less stringent sharpness levels because of the use of a more relaxed circle of confusion (less enlargement).
This is generally true, BUT... some medium format objectives are no worse than contemporary small format ones, and surprise surprise, some optics sold for the small format were in reality capable of a much larger coverage. Anybody who went through the hassle of Leitax'ing an old Leitz long focus has probably found that such simple achromatic doublets work amazingly better than expected (if you knew the design and not the brand! ). This is because the APS-C (or even FF) frame covers a small part of the real circle of coverage. The huge amount of peripheral aberrations is left out of the frame.

So yes, your 1.9/80mm will stay the same wherever it goes, be it the original 4.5x6 format camera, a smaller digital medium format one, an FF, or an APS-C.
If the camera stays put, the only thing the varies is the size of the captured part of the scene.
Any formula that says anything different is based on one very unrealistic assumption: the subject always keeps the same size.
In reality, if you have a lens that behaves like a tele, it's likely that you would use it from further away.
I repeat it. If the camera stays in the same place, there is no variation in perspective nor in reproduction ratio. A smaller format captures a smaller portion of the circle of coverage, that's all.
If you move the camera to get the same reproduction ratio, then the perspective, the DOF and of course the rendition of the bokeh would change.
Regarding sharpness, it is likely that a medium format lens was designed to be a tad less sharp than a small format one, because of a smaller expected
enlargement ratio (which called for a more relaxed circle of confusion, to get the same level of sharpness in the final print).
It is not a fixed rule valid for all medium format objectives, but I believe that to some extent the same concept is still valid today.
If you print a file from your Pentax 645 the required enlargement would be smaller, so the optic can be just a tad less sharp than one used on smaller formats.

I hope I didn't create further confusion. I hope I haven't been too repetitive, I just tried my best to be clear.
English is my second language, not my mother language.

Last edited by cyberjunkie; 12-08-2018 at 02:22 AM.
12-08-2018, 02:54 AM   #120
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QuoteOriginally posted by cyberjunkie Quote
The challenge requires some thinking. I will use some time to give it a good thought...

The question is rather simple, but the answer is a bit more complex than what it seems to be.
I am much more knowledgeable in the history of camera lenses than in optical theory. Which means that I lack any basic background in mathematic and physics. I am not so sure I will not incur in practical mistakes, and I'm sure any formally correct answer would require the kind of background I don't have.
Let's start with the basic. A lens, of any kind/focal/aperture, would behave the same way independently from the format of the film/sensor.
An 80mm f/1.9 keeps its intrinsic characteristics whatever camera you use. The only difference is the area of the circle of coverage that is actually used.
Depending of the format, the angle of view (and the perspective!) changes, so that the same 80mm would be called a "normal" of a 6x6 camera, and a short tele on a 35mm/FF one.
BUT... what I just said is true ONLY if we keep the camera in the same place. If we want to cover the same angle of view, and try to take the same picture, we would see that when the 80mm is fitted to an FF camera we have to back off, in order to capture the same scene.
It is the difference in distance from the main subject that causes what we call perspective compression. If the size of the subject has to be the same across formats, the distance has to vary. This is the one and only cause of geometric distortion in wide angles. If we fit a 28mm on an APS-C camera we can stay away from the subject. If we fit the same lens on a FF camera, we have to go closer, and that change in perspective would deform the facial features, so that the nose seems to have grown bigger.
If we kept the same distance from the subject, the same face would be either smaller or bigger in the picture, but the way it looks would be the same.
Same thing with depth of focus.
It is widely accepted to consider a 50mm the "normal" lens for 35mm/FF cameras. In reality it would be a little less, to match human vision, but this is another story...
The same way it is widely accepted to consider a 300mm lens the "normal" for 8x10" film.
If we shoot the same scene we would see that the two pictures look very similar. Though it's not totally true, the SIZE of the subject is almost the same, but the DOF is not. This is because a 300mm has the same depth of field, across formats. Some lenses give the IMPRESSION of more DOF, because of the way they are designed. The sharper they are the more evident the difference between in focus and out of focus, so our eye is tricked into believing that the DOF is smaller. DOF is calculated using three values, it's physics. The human eye can be fooled, which is not a bad thing. Actually I'm quite happy that different lenses give a different "impression" of reality.
Of course this has very much to do with the way bokeh is rendered, because the circle of confusion of out of focus highlights looks different depending on the distance from the plane of correct focus.
Well, circle of confusion. Again, the answer is not straightforward because there is a variable.
In analog times the formula of DOF involved three values: focal, aperture and conventional circle of confusion. The latter was taken as a variable because the accepted circle of confusion (which determines what is acceptably sharp to the average human eye), was made to vary according to the format.
The reason was simple, larger negatives had to be enlarged less, so a level of sharpness that would be unacceptable with a 35mm film would be more than enough with a large sheet film. Contact print on 8x10", from negatives taken with horribly unsharp lenses, look gorgeous.
Some Ansel Adams pictures were taken with the back half of a Turner & Reich double anastigmat. Such lens allowed him to use a "tele", but the optical quality was awful. Nevertheless he managed to print amazing black & whites.
He did that stopping down like crazy, and contact printing (or enlarging just very little).
This is where I am at a loss. Is the relevance of circle of confusion in the equation still relevant?
In the first place most pictures are not printed but used with computer screens or projectors.
For example the image can be resampled to 72dpi with no adverse effect on the perceived quality.
The printing technology is also quite different from what it used to be. Does it make a difference or not?
In general, medium format lenses had to meet less stringent sharpness levels because of the use of a more relaxed circle of confusion (less enlargement).
This is generally true, BUT... some medium format objectives are no worse than contemporary small format ones, and surprise surprise, some optics sold for the small format were in reality capable of a much larger coverage. Anybody who went through the hassle of Leitax'ing an old Leitz long focus has probably found that such simple achromatic doublets work amazingly better than expected (if you knew the design and not the brand! ). This is because the APS-C (or even FF) frame covers a small part of the real circle of coverage. The huge amount of peripheral aberrations is left out of the frame.

So yes, your 1.9/80mm will stay the same wherever it goes, be it the original 4.5x6 format camera, a smaller digital medium format one, an FF, or an APS-C.
If the camera stays put, the only thing the varies is the size of the captured part of the scene.
Any formula that says anything different is based on one very unrealistic assumption: the subject always keeps the same size.
In reality, if you have a lens that behaves like a tele, it's likely that you would use it from further away.
I repeat it. If the camera stays in the same place, there is no variation in perspective nor in reproduction ratio. A smaller format captures a smaller portion of the circle of coverage, that's all.
If you move the camera to get the same reproduction ratio, then the perspective, the DOF and of course the rendition of the bokeh would change.
Regarding sharpness, it is likely that a medium format lens was designed to be a tad less sharp than a small format one, because of a smaller expected
enlargement ratio (which called for a more relaxed circle of confusion, to get the same level of sharpness in the final print).
It is not a fixed rule valid for all medium format objectives, but I believe that to some extent the same concept is still valid today.
If you print a file from your Pentax 645 the required enlargement would be smaller, so the optic can be just a tad less sharp than one used on smaller formats.

I hope I didn't create further confusion. I hope I haven't been too repetitive, I just tried my best to be clear.
English is my second language, not my mother language.
Thanks for this detailed explanation. I will have to read it a few more times to know if I am more confused or less confused, but everything you state seems well reasoned. :>)
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