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06-23-2008, 07:02 PM   #31
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QuoteOriginally posted by Wheatfield Quote
I'm not so sure that this isn't jibberish. Depth of field is tied to image magnification (on the sensor/film) and aperture. Focal length plays no part in it, other that deciding camera to subject difference.
A 58mm lens on the DSLR gives pretty much exactly the same field of view as an 85mm lens does on the 35mm film format, and gives very similar DOF at equivalent apertures, if the subject magnification is maintained.
This is why lazy thinkers can get away with their mind bending equivalencies where they say that a 50mm lens is a 75mm lens on the DSLR, when the fact is, it is still a 50mm lens.
What will change, and will change very dynamically, is subject to background relationships if you change focal length (but not format) and maintain the same magnification of the subject. Even then, depth of field will not alter, presuming identical apertures are used.
What you are saying is interesting. There have been a number of threads on this site with discussions of magnification vs. crop factor in regards to depth of field and perspective. Perhaps some of you with a good background in optical theory could put together a short tutorial on a new thread explaining the relationship of focal length and working distance to depth of field and perspective and then include a few paragraphs explaining what crop factor is and what it is not! (A smaller sensor does not make a normal lens into a short tele, though for many purposes, it might as well be.)

Steve

06-23-2008, 07:03 PM   #32
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QuoteOriginally posted by Wheatfield Quote
I'm not so sure that this isn't jibberish. Depth of field is tied to image magnification (on the sensor/film) and aperture. Focal length plays no part in it, other that deciding camera to subject difference.
A 58mm lens on the DSLR gives pretty much exactly the same field of view as an 85mm lens does on the 35mm film format, and gives very similar DOF at equivalent apertures, if the subject magnification is maintained.
This is why lazy thinkers can get away with their mind bending equivalencies where they say that a 50mm lens is a 75mm lens on the DSLR, when the fact is, it is still a 50mm lens.
What will change, and will change very dynamically, is subject to background relationships if you change focal length (but not format) and maintain the same magnification of the subject. Even then, depth of field will not alter, presuming identical apertures are used.
All you have to do is mount a 50mm lens on a crop body and view the scene with both eyes open to see a 50 is a 50, even if after cropping, your Field of View is similar to that of a 75mm lens.
06-23-2008, 07:07 PM   #33
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And having said all that, i regret having sold my Jupiter-9 85/2 lens (M42), not knowing how much more i get out of every lens on the K20D compared to the 30D. should i want a 85 down the road, i'll pick up another Jupiter-9 maybe. If i use it only with natural light or strobes, i won't have to fret the P-TTL flaw of full power flash on manual lenses
06-23-2008, 07:39 PM   #34
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QuoteOriginally posted by jms698 Quote
Looking over the Pentax lens lineup and comparing it with Nikon and Canon lenses, I wish there was a sub-$400 85mm f/1.8 prime lens. Both Nikon and Canon have an excellent lens like this: affordable, fast, lightweight with great image quality. Just the kind of thing I would buy in an instant.

The Pentax 77mm lens is great, of course, but I just can't justify spending $700 for it. The 70mm pancake is also nice, but I don't agree with spending $500 for a lens that is one stop slower than the competitions' lens in that category (I don't care too much about the small size).

So why can't Hoya/Pentax bless us with a 85mm f/1.8 lens?
Overall I think Pentax lens line up is pretty good as long as it is at or shorter than 200mm. I agree that only 85mm is the exception. But it is covered by both 70mm & 77mm Ltd. Lenses at F1.8 grade/range is not expected to be very affordable .

I do not have it either. But if push comes to shove, I can shoot film (FF) with my FA135mm. I should have used more of my film camera. Still have a roll of Reala there for a month now.

Daniel

06-23-2008, 08:00 PM   #35
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QuoteOriginally posted by Wheatfield Quote
I'm not so sure that this isn't jibberish. Depth of field is tied to image magnification (on the sensor/film) and aperture. Focal length plays no part in it, other that deciding camera to subject difference.
A 58mm lens on the DSLR gives pretty much exactly the same field of view as an 85mm lens does on the 35mm film format, and gives very similar DOF at equivalent apertures, if the subject magnification is maintained.
This is why lazy thinkers can get away with their mind bending equivalencies where they say that a 50mm lens is a 75mm lens on the DSLR, when the fact is, it is still a 50mm lens.
What will change, and will change very dynamically, is subject to background relationships if you change focal length (but not format) and maintain the same magnification of the subject. Even then, depth of field will not alter, presuming identical apertures are used.
You are correct. I misused depth of field for the 'subject to background relationships' I explained in parenthesis.

Its more than lazy thinking though. I resent how people are constantly sold these equivalencies by the marketing machines for APS-C who allow people to buy that idea they are gaining a tele advantage. They don't outright tell you that, they just float the term 'equivalent' and allow the end user to believe its anything more than just a cropped field of view.

Interesting how this conversation actually pertains to the 85, its sort of a forgotten focal length in Pentax all APS-C world, especially when the 77 is more of an APS-C portait sweet-spot (considering the loss of FoV on the cropped sensor). I suspect Pentax will eventually have a full line of modern portrait primes after more consumer friendly APS-C choices have been produced and as a 'pro' model body unlocks an 85s market potential.
06-23-2008, 08:14 PM   #36
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I just started another thread with a comparison shot. Same subject, same magnification (image size on the sensor), same aperture, different focal lengths (50mm vs 85mm).

Step up and tell me which one is which and why!

https://www.pentaxforums.com/forums/general-pentax-photography/30632-85mm-vs-...tml#post275556
06-23-2008, 08:23 PM   #37
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QuoteOriginally posted by morfic Quote
All you have to do is mount a 50mm lens on a crop body and view the scene with both eyes open to see a 50 is a 50, even if after cropping, your Field of View is similar to that of a 75mm lens.
Yes! This is the most straight-forward way to see it.

And regarding some comment on depth-of-field (DoF), please everyone remember that while the FoV changes in direct proportion to the crop factor (so the FoV of an 85mm on film is the same FoV as an 85/1.53 = 55.6mm on a Pentax DSLR), the DoF doesn't. In fact you lose a whole 2 stops of DoF going from APS-C to film.

In layman's terms: Fred and Arnie each have a film and APS-C camera, respectively. Fred mounts an 85mm lens on his camera, while Arnie mounts a 56mm. They both zoom with their feet until they fill their respective frames with their model's face, and they find they are standing shoulder to shoulder. They set their apertures to f/1.4 and snap a picture. After waiting 1hr for Fred's film to develop at Walgreen's, they compare the photos. Both photos show exactly the same field of view, but Fred's has a much shallower DoF. They mess around all afternoon trying to figure out what's happening, and eventually they discover that for both pictures to look the same (same FoV and same DoF) they have to shoot the model from the same distance, but Fred has to use an aperture that's 2 stops smaller than Arnie. So if Arnie shoots at f/1.4, Fred must shoot at f/2.8 to achieve the same DoF.

"But what about the perspective of the model's face?" people cry out. Well, as they say at the end of the Conan films: that is another story...
06-23-2008, 08:32 PM   #38
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QuoteOriginally posted by Miserere Quote
Yes! This is the most straight-forward way to see it.



"But what about the perspective of the model's face?" people cry out. Well, as they say at the end of the Conan films: that is another story...
Same distance...same perspective...

(I learned the hard way on another thread...but only after I proved myself wrong with a few test shots...)

Steve

06-23-2008, 08:46 PM   #39
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Somebodys is getting lenses at an affordable price and I want to just shoot myself.

I missed out on a sale on craigslist a couple months ago that the 85 f1.8 went for $100.

I missed a sale last week that for $250 of a 50mm f/ 1:1.2 , 100mm f/4 macro, 24mm f/2.8, 40mm f/2.8 and a Viv S1 70-210 f/2.5

I have stayed away from sharp object since.
Bad thing about it was it was probably a dealer that will only benefit financially.

Sorry about the thread drift but I have been carrying that around for too long and it was really messing me up. Scarred for life
06-23-2008, 09:03 PM   #40
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QuoteOriginally posted by OrenMc Quote
Somebodys is getting lenses at an affordable price and I want to just shoot myself.

I missed out on a sale on craigslist a couple months ago that the 85 f1.8 went for $100.

I missed a sale last week that for $250 of a 50mm f/ 1:1.2 , 100mm f/4 macro, 24mm f/2.8, 40mm f/2.8 and a Viv S1 70-210 f/2.5

I have stayed away from sharp object since.
Bad thing about it was it was probably a dealer that will only benefit financially.

Sorry about the thread drift but I have been carrying that around for too long and it was really messing me up. Scarred for life
Yikes!! Those are great prices! I think I might have to move north a few miles! Craigslist here in the Portland area is almost painfully dull.

Well jms698, there was your affordable 85mm!

Steve
06-23-2008, 09:21 PM   #41
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QuoteOriginally posted by OrenMc Quote
Sorry about the thread drift but I have been carrying that around for too long and it was really messing me up. Scarred for life
Nothing to apologise for! I still remember the guy selling a 2 month-old 77 Ltd on Craigslist for about $525. I still wonder who got that...
06-23-2008, 09:47 PM   #42
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QuoteOriginally posted by Miserere Quote
Yes! This is the most straight-forward way to see it.

And regarding some comment on depth-of-field (DoF), please everyone remember that while the FoV changes in direct proportion to the crop factor (so the FoV of an 85mm on film is the same FoV as an 85/1.53 = 55.6mm on a Pentax DSLR), the DoF doesn't. In fact you lose a whole 2 stops of DoF going from APS-C to film.

In layman's terms: Fred and Arnie each have a film and APS-C camera, respectively. Fred mounts an 85mm lens on his camera, while Arnie mounts a 56mm. They both zoom with their feet until they fill their respective frames with their model's face, and they find they are standing shoulder to shoulder. They set their apertures to f/1.4 and snap a picture. After waiting 1hr for Fred's film to develop at Walgreen's, they compare the photos. Both photos show exactly the same field of view, but Fred's has a much shallower DoF. They mess around all afternoon trying to figure out what's happening, and eventually they discover that for both pictures to look the same (same FoV and same DoF) they have to shoot the model from the same distance, but Fred has to use an aperture that's 2 stops smaller than Arnie. So if Arnie shoots at f/1.4, Fred must shoot at f/2.8 to achieve the same DoF.

"But what about the perspective of the model's face?" people cry out. Well, as they say at the end of the Conan films: that is another story...
I get a different result when I do the back-of-the-envelope math:

Suppose Fred and Arnie want a depth of field extending from 5m to 6m. With Fred's 85 mm lens and 35mm film, this requires an aperture setting of f/4

Arnie has to use a 58mm-ish lens to take a picture with the same FOV from the same position. To obtain a circle of confusion of the same diameter on the sensor/film, this requires an aperture setting of f/2 (DOF doubles when focal length is divided by the square root of two)

However, if Fred and Arnie both make prints of the same size, Arnie must enlarge an image from a smaller sensor/negative, and will obtain more blur in the print (for the same circle of confusion). To compensate, Arnie needs a sensor image with a smaller circle of confusion (say, 20 microns for the cropped sensor instead of 30 microns for 35mm film), and must stop down to f/2.8 to get the same DOF in the print as Fred.

By this argument, you lose one stop of DOF, not two, by going from a cropped sensor to film.


Am I missing something?
06-24-2008, 08:15 AM   #43
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Troy, good point. But let's assume Fred has a digital full-frame camera, and that both Fred and Arnie's cameras are 10MP. What then? Or what if Fred is still using film, but Arnie is shooting a prototype APS-C sensor with 21MP (which I believe surpasses the resolution of film)?
06-24-2008, 01:44 PM   #44
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QuoteOriginally posted by Miserere Quote
Nothing to apologise for! I still remember the guy selling a 2 month-old 77 Ltd on Craigslist for about $525. I still wonder who got that...

Or a mint F* 300 2.8 for $450. That was a sad, sad day.

.
06-24-2008, 02:35 PM   #45
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85mm Pentax

I have FA* 85mm f1,4 and I'm just happy with it. It's my second best lens
(I just have five F*/FA* lenses and none of those today's Ltd or DA* lenses).
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