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12-20-2010, 10:36 PM   #16
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QuoteOriginally posted by rlj Quote
So one can roughly say that the 150/2.8 on the 645D has the DOF of a 150/2.0 on 35mm film.
I don't look at it that way, I prefer make an equivalency based on the FOV of the lens and the DOF respective of the format it is projecting the image on. A 150mm f/2.8 on 35mm film will be a 150mm f/2.8 nothing more, nothing less. But if the sensor is 1.7X larger things will be a bit different: The DOF is going to get smaller because for a given magnification you will have to get 1.7X closer to your subject than you would have to with the same lens on a 24X36mm sensor. And in comparison to a 24X36mm sensor the focal length is going to appear to be wider with a 33X44mm sensor. Which means describing the 150mm f/2.8 as a 35mm equivalent of a 117mm f/1.1 isn't as far fetched as it seems.

I work with lens equivalence all the time when I am working with large format, believe it or not on 8X10 format a 300mm stopped down to f/6.7 delivers essentially the same FOV and DOF as a 50mm f/1.2 on 35mm film at a focus distance of 10m. Of course the OOF transitions on both lenses will be dramatically different from each other on their respective formats.


Last edited by Digitalis; 12-20-2010 at 11:03 PM.
12-21-2010, 11:36 AM   #17
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QuoteOriginally posted by Digitalis Quote
I don't look at it that way, I prefer make an equivalency based on the FOV of the lens and the DOF respective of the format it is projecting the image on. A 150mm f/2.8 on 35mm film will be a 150mm f/2.8 nothing more, nothing less. But if the sensor is 1.7X larger things will be a bit different: The DOF is going to get smaller because for a given magnification you will have to get 1.7X closer to your subject than you would have to with the same lens on a 24X36mm sensor. And in comparison to a 24X36mm sensor the focal length is going to appear to be wider with a 33X44mm sensor. Which means describing the 150mm f/2.8 as a 35mm equivalent of a 117mm f/1.1 isn't as far fetched as it seems.

I work with lens equivalence all the time when I am working with large format, believe it or not on 8X10 format a 300mm stopped down to f/6.7 delivers essentially the same FOV and DOF as a 50mm f/1.2 on 35mm film at a focus distance of 10m. Of course the OOF transitions on both lenses will be dramatically different from each other on their respective formats.

Digitalis, think about how big the front element of an 117mm/f1.1 lens will be. It is very evident, even without maths that the hipothetic 117mm/f1.1 on a 35FF camera is not "equivalent" (DOF) at the 150mm/f2.8 lens on the 645 or the 645D.

If you want to make the "same" photo (equivalent FOV and DOF) with the 645D and the 150/2.8 and a 35mmFF camera (the Canon 1DsII for example) you need this lens: 150mm/ crop factor 645D-35mmFF (1.27)= 118mm (more or less!) and the equivalent DOF: 2.8/crop factor (1.27)= 2.2

Sorry for my very poor english!

Ramn
12-22-2010, 04:57 AM   #18
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QuoteOriginally posted by Ramn Quote
Digitalis, think about how big the front element of an 117mm/f1.1 lens will be. It is very evident, even without maths that the hipothetic 117mm/f1.1 on a 35FF camera is not "equivalent" (DOF) at the 150mm/f2.8 lens on the 645 or the 645D.

If you want to make the "same" photo (equivalent FOV and DOF) with the 645D and the 150/2.8 and a 35mmFF camera (the Canon 1DsII for example) you need this lens: 150mm/ crop factor 645D-35mmFF (1.27)= 118mm (more or less!) and the equivalent DOF: 2.8/crop factor (1.27)= 2.2

Sorry for my very poor english!

Ramn
Your math are shoddy at best, lets review a bit the problem:
  1. 1. 645 and 35mm image ratios are different. Calculating a "crop factor" is a matter of personal taste, comparing 645 to 35mm is like comparing 6x6 and 6x7. What is the crop ratio of 6x6 compared to 6x7?
  2. 2. Once you have chosen a crop ratio (roughtly 1.3) you can determine equivalent FOV (diagonal, in length or in height) and equivalent focal length.
  3. 3. But to determine the equivalent apertures to obtain the same DOF, you cannot take the aperture number and divide it by the crop ratio.

DOF is much more dependant on distance to focus, than on aperture so you have to set before the distance of focusing to make comparisons.

DOF is a very complex subject, and get you things right, I suggest you have a look at this series of articles from the The Online Photographer

DOF is a very complex subject where focal length, distance to focus, capturing technology (film, digital, size of grain/ nb of pixels), aperture and size of print all play an important part. For the larger prints, the "look" of MF is definitively different than 35mm, which is different than APSC.

Personnaly, I gave up on making knowledgable calculations of equivalence etc... What is important is knowing what you can get from your equipment and knowing what equipement your need for the result you want to achieve.
12-22-2010, 05:30 AM   #19
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thanks for piping in Ghelary, DOF is indeed a complex subject and therefore it is poorly understood by a disproportionate number of photographers.

When I speak of equivalence I'm taking the focus distance into account. E.G: To do a tight head shot with the 645D you will have to be 1.7 times closer to your subject in comparison to 35mm full frame using the same lens. For portraiture subject to camera distances are very important because there is the ticklish subject of how close the photographer wants to be to the model, and the more practical aspect of how much lighting equipment you can fit between the model and the camera.

12-22-2010, 06:50 AM   #20
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QuoteOriginally posted by Digitalis Quote
thanks for piping in Ghelary, DOF is indeed a complex subject and therefore it is poorly understood by a disproportionate number of photographers.

When I speak of equivalence I'm taking the focus distance into account. E.G: To do a tight head shot with the 645D you will have to be 1.7 times closer to your subject in comparison to 35mm full frame using the same lens. For portraiture subject to camera distances are very important because there is the ticklish subject of how close the photographer wants to be to the model, and the more practical aspect of how much lighting equipment you can fit between the model and the camera.
Well that's precisely the point ^_^

If you make all your comparison for studio pictures taken at 2m, then you start to make more rigourous comparisons. But then, if you take square pictures (6x6 style) you would be cropping both 645 and 35mm picture, but cropping on 645 is more favorable than on 35mm for a square aspect ratio, so equivalent FOV will be different than if you work on a more streched ratio...

At the end, many people also forget that DOF is highly dependent on the size of the print, the same picture may look sharp on a small print (say 10x15 cm) and not sharp/in-focus enough for a larger print (say 40 x 60 cm @ 300dpi) I have seen the problem with my 14Mpx of my K20D so I guess the pb is even more accute with a 40Mpx camera with no AA filter...
12-22-2010, 05:09 PM   #21
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QuoteOriginally posted by ghelary Quote
the same picture may look sharp on a small print (say 10x15 cm) and not sharp/in-focus enough for a larger print (say 40 x 60 cm @ 300dpi) I have seen the problem with my 14Mpx of my K20D so I guess the pb is even more accute with a 40Mpx camera with no AA filte
That is largely a technical issue. The 645D provides essentially the same image quality you can get from Velvia 50 on a 4X5 view camera, so your technique had better be up to the task. High resolution cameras are much harder on everything, they are harder on lenses, you need stronger tripods,you need bigger camera bags for them....etc..etc..
12-22-2010, 05:18 PM   #22
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QuoteOriginally posted by ghelary Quote
Your math are shoddy at best, lets review a bit the problem:
  1. 1. 645 and 35mm image ratios are different. Calculating a "crop factor" is a matter of personal taste, comparing 645 to 35mm is like comparing 6x6 and 6x7. What is the crop ratio of 6x6 compared to 6x7?
  2. 2. Once you have chosen a crop ratio (roughtly 1.3) you can determine equivalent FOV (diagonal, in length or in height) and equivalent focal length.
  3. 3. But to determine the equivalent apertures to obtain the same DOF, you cannot take the aperture number and divide it by the crop ratio.

DOF is much more dependant on distance to focus, than on aperture so you have to set before the distance of focusing to make comparisons.

DOF is a very complex subject, and get you things right, I suggest you have a look at this series of articles from the The Online Photographer

DOF is a very complex subject where focal length, distance to focus, capturing technology (film, digital, size of grain/ nb of pixels), aperture and size of print all play an important part. For the larger prints, the "look" of MF is definitively different than 35mm, which is different than APSC.

Personnaly, I gave up on making knowledgable calculations of equivalence etc... What is important is knowing what you can get from your equipment and knowing what equipement your need for the result you want to achieve.

1. and 2. Maybe you don't have seen my 645D "equivalent" lenses table. I have posted the table here (april 2010) and in this thread.

https://www.pentaxforums.com/forums/attachment.php?attachmentid=78820&d=1293059317

As you can see there is a 35mmFF "equivalent" focal length for the horizontal, vertical and diagonal ratio.

Note also that I say in my post "same" "equivalent" (not same and equivalent) and "MORE OR LESS".


"Personnaly, I gave up on making knowledgable calculations of equivalence etc... What is important is knowing what you can get from your equipment and knowing what equipement your need for the result you want to achieve."

I agree. Because that I have used on film these formats:

35mm (Canon, Olympus)
35mm panoramic (Noblex)
4.5x6cm (Fuji and Pentax)
6x6 (Rolleiflex)
6x7 (Mamiya)
6x9 (Cambo)
5x12 panoramic (Noblex)
9x12 and 4x5" (Linhof, Arca-Swiss)
13x18 (Arca-Swiss)
8x10" (Arca-Swiss)

Now, on digital, I use 3 formats:
4/3 Olympus
APS-C Canon
35FF Canon
And Maybe 33x44mm Pentax in the near future...


"and knowing what equipement your need for the result you want to achieve."

One example:

Linhof Technika V, Nikkor T-ED 500/11, Sinar panorama 6x12 back, Fuji Velvia:

Luna sobre Aneto-Maladeta :: Fotonatura.org


Ramn
12-25-2010, 09:26 PM   #23
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Photo from 150/2.8 on 645D

QuoteOriginally posted by Digitalis Quote
believe it or not on 8X10 format a 300mm stopped down to f/6.7 delivers essentially the same FOV and DOF as a 50mm f/1.2 on 35mm film at a focus distance of 10m. Of course the OOF transitions on both lenses will be dramatically different from each other on their respective formats.
I do believe, it is supported by the math I gave previously. I've shot all formats for decades, and am about as aware of DOF issues as anyone. There are many factors involved. (One book from the '70s (?) that covered this topic rather nicely was Roger Hicks' book about Medium Format.)

To return to the original point of this post (instead of going in endless circles about theoretic DOF...): attached is an image from the 150/2.8 at f3.2 on the 645D. The shot was about 1.5m from subject's face. Since the fellow is about 1 year old, the brick background is not too far from his head, yet very much OOF. I truly like the 150 on the 645D. (So far none of my lenses seem to need any focus adjustments.)

Attached Images
 
12-28-2010, 04:26 PM   #24
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I looked at all these samples.
hmmm, they don't look sharp to me at all.
I hope if I read thew post correctly "manager"
states theses are unsharpened jpegs direct from the 645D/
I was looking at:
P645D_0040_FA35.jpg - Pentax 645D "" - managger - - iXBT
Which was taken with the 35mm FA at F22 1/5 sec.
Bad ca in the corners and not sharp anywhere.
or am i missing something here.

the FA 120mm at 5.6 looked good
P645D_0093_FA120M.jpg - Pentax 645D "" - managger - - iXBT

so does thew FA 200mm

hmmm I was about ready to buy the fa 35mm, to try out, but from this sample not looking so hot.

Steven






QuoteOriginally posted by managger Quote
Here you can find a lot of samples of different lenses on 645D
Pentax 645D - - iXBT

FA 150/2.8 is a "bokeh king" - as for me.
FA 120/4.0 is not only "macro lens for closed apertures" - portraits are not bad too.
I use both of this lenses from film era and tested them on 645D.

Here you can find several samples
Pentax 645D "" - managger - - iXBT
No sharpness in the camera or in the Photoshop !


Of course not.
You must divide on 1.61 if you want to understand equivalent on 645/645N/645NII.
150/1.61 ~ 93mm in 36x24 terms. Pure portrait lens.
You must divide on 1.27 if you want to understand equivalent on 645D.
150/1.27 ~ 118mm in 36x24 terms. Little bit tele portrait lens.
12-28-2010, 05:13 PM   #25
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The exif does not show where the focus was, alas. But to all accounts, that 35mm lens is truly superb. It may not have been aligned well with the camera, perhaps, but I surely would not discount it on the basis of a single image on the web.
12-28-2010, 05:24 PM   #26
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Smolk, I understand what you are saying, but geez, I wish I could find some other samples out there before spending lots of $$$$$
12-28-2010, 05:53 PM   #27
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yea, understood—I took to the A version, and it's a stunner on film. What the lens does on digital, I'm less sure, although on the K-x it's still stunning. Perhaps a good compromise? Even the A-version is not cheap, but you get 3 of them for one FA version....
There is a sample with this lens on the 645D at GANREF | PENTAX 645D??????????????????????? | ???????????

The FA version has an even better reputation, but on digital and more particularly on the 645D, I don't know.
12-28-2010, 09:22 PM   #28
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ndevlin, did you ever get around to testing this lens out on your 645d?

I just got my 120mm macro in today and man thats a sharp lens.
12-29-2010, 09:00 AM   #29
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Shuttershane did you get the fa 120 or the a 120
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