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01-24-2008, 05:21 AM   #1
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Lens "conversion" to dSLR?

When reading lens reviews, I frequently come across statements that say a given lens has a different range when mounted on a dSLR as opposed to a film SLR. For example, a particular 30mm lens acts like a 50mm (or something) lens when mounted onto a dSLR. Why is this & does this happen for lenses advertised as "specifically designed for dSLR cameras"?

01-24-2008, 05:29 AM   #2
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I think the best explanation is here: Crop factor - Wikipedia, the free encyclopedia
01-24-2008, 05:45 AM   #3
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QuoteOriginally posted by arthur pappas Quote
When reading lens reviews, I frequently come across statements that say a given lens has a different range when mounted on a dSLR as opposed to a film SLR. For example, a particular 30mm lens acts like a 50mm (or something) lens when mounted onto a dSLR. Why is this & does this happen for lenses advertised as "specifically designed for dSLR cameras"?
I'm sure someone will give you a much more detailed response (we're all in the K20D threads right now), BUT here's the quick answer!

The reason is because most (all Pentax) sensors on digital SLRs are smaller than a 35mm film negative -- around 66.67%. So the "conversion factor" is actually a "crop factor" -- since the image that the lens is showing the camera is larger than the sensor, the sensor is effectively cropping it and converting the lens from a (say) 30mm to a 45mm (Pentax dSLRs convert at 1.5).

This is great for those of us who want to shoot longer telephotos, but bad news for the wide-angle folks.

But to keep things consistent (someone please correct me if I'm wrong about this), and not have two standards of measurement, YES, it is still an "issue" with "specially designed dSLR lenses" (such as the DA series), in the sense that if it says "30mm" what you're really getting is 45.

To be fair, imagine how confusing it would be otherwise! Some "30mm" lenses would BE 30mm, but some would be 45mm -- this way, you just have to keep in mind your conversion factor and apply it to EVERY number you see.

I actually started to say that it WASN'T an issue because it isn't a technical issue -- the "digitally designed" lenses are built to take the sensor size into account and "send" a smaller image to the camera and would show vignetting and/or a circular crop if you managed to mount them on a film camera or "full frame" sensor* -- but thought it through and came to the conclusion above.

Which I'm pretty sure is correct. Fingers crossed!


-------------------------------------------------------------------
*(which, as you might guess, is a sensor which is the same physical size as a piece of 35mm film and thus has no conversion factor -- such as the C**** EOS 5D)...


[edit: Darn, RB, you beat me to it -- now I look like a know-it-all follow-up poster. Crud.]

Last edited by amateur6; 01-24-2008 at 05:47 AM. Reason: see above.
01-24-2008, 06:21 AM   #4
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QuoteOriginally posted by amateur6 Quote
[edit: Darn, RB, you beat me to it -- now I look like a know-it-all follow-up poster. Crud.]
LOL!

No, no. You get points because you took the time to actually type the explanation. I just felt lazy

01-24-2008, 06:47 AM   #5
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focal length and crop factor

QuoteOriginally posted by amateur6 Quote
I'm sure someone will give you a much more detailed response (we're all in the K20D threads right now), BUT here's the quick answer!

The reason is because most (all Pentax) sensors on digital SLRs are smaller than a 35mm film negative -- around 66.67%. So the "conversion factor" is actually a "crop factor" -- since the image that the lens is showing the camera is larger than the sensor, the sensor is effectively cropping it and converting the lens from a (say) 30mm to a 45mm (Pentax dSLRs convert at 1.5).

This is great for those of us who want to shoot longer telephotos, but bad news for the wide-angle folks.

But to keep things consistent (someone please correct me if I'm wrong about this), and not have two standards of measurement, YES, it is still an "issue" with "specially designed dSLR lenses" (such as the DA series), in the sense that if it says "30mm" what you're really getting is 45.
To clarify: focal length is focal length, nothing more. The crop factor ONLY affects your FOV (field of view). You just have to crop less around the subject, since you FOV is narrower. Yes, a 50mm has a FOV of a 75mm lens in the SLR equivalent world. HOWEVER, it is NOT a substitute for magnification/resolving power of a lens. It does not make my 300/2.8 become a 450/2.8 lens. A teleconverter can increase the magnification, but often at a compromise of IQ (image quality).

Digital specific lenses often have:
  1. specific coatings to optimize their resolving power on DSLRs (this claim by the manufacturer is questionable, since I shoot the (film) FA* lenses with stunning results)
  2. a smaller image circle, so you'll get vignetting in the corners on ultra-wide angles. My MZ-S film body has no vignetting on my DA 12-24 once it's at ~ 16mm and above.
Hope that helps!

Cheers,
Marc
01-24-2008, 09:12 AM   #6
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QuoteOriginally posted by 35mmfilm_user Quote
Digital specific lenses often have: specific coatings to optimize their resolving power on DSLRs (this claim by the manufacturer is questionable, since I shoot the (film) FA* lenses with stunning results)
I always suspected this; good to know! I'm especially wary of the "Digital UV" aftermarket filters, but that's probably because I've felt the sting of paying $100 for a 77mm version (needed it RIGHT THEN; only one option)! Once bitten....

QuoteQuote:
To clarify: focal length is focal length, nothing more. The crop factor ONLY affects your FOV (field of view). You just have to crop less around the subject, since you FOV is narrower. Yes, a 50mm has a FOV of a 75mm lens in the SLR equivalent world. HOWEVER, it is NOT a substitute for magnification/resolving power of a lens. It does not make my 300/2.8 become a 450/2.8 lens. A teleconverter can increase the magnification, but often at a compromise of IQ (image quality).
Marc, I don't mean to be rude, but isn't this just semantics? If the FOV of a 300mm 2.8 is the same as that of a 450mm, and the film from the 450 was scanned at the equivalent DPI and similarly cropped, would there be a visible difference in IQ? Barring the inherent differences between film and digital, of course...
01-24-2008, 11:52 AM   #7
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Just don't fall into the trap thinking that when you hear that a 300mm lens on a DSLR is like a 450mm lens on 35mm film that you can suddenly SEE farther. This is an illusion caused by the tighter crop of the digital sensor. A 300mm lens on a DSLR magnifies exactly the same as it does on a 35mm camera... thus, you cannot SEE farther with a DSLR than a 35mm...

From the wikipedia article cited earlier...
QuoteQuote:
Magnification factor
The crop factor is sometimes referred to as "magnification factor." This usage commonly derives from the telephoto effect, in which lenses of a given focal length seem to produce greater magnification on crop-factor cameras than they do on full-frame cameras. It should be noted that the lens casts the same image no matter what camera it is attached to, and therefore produces the same magnification on all cameras. It is only because the image sensor is smaller in many DSLRs that a narrower FOV is achieved. The end result is that while the lens produces the same magnification it always did, the image produced on small-sensor DSLRs will be enlarged more to produce output (print or screen) that matches the output of a longer focal length lens on a full-frame camera. That is, the magnification as usually defined, from subject to focal plane, is unchanged, but the system magnification from subject to print is increased.
01-24-2008, 12:24 PM   #8
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QuoteOriginally posted by MRRiley Quote
Just don't fall into the trap thinking that when you hear that a 300mm lens on a DSLR is like a 450mm lens on 35mm film that you can suddenly SEE farther. This is an illusion caused by the tighter crop of the digital sensor. A 300mm lens on a DSLR magnifies exactly the same as it does on a 35mm camera... thus, you cannot SEE farther with a DSLR than a 35mm...
Okay, but we're getting into subjective territory here, as with my thread disputing Luminous Landscape's assertion that "wide angle lenses, contrary to popular belief, don't have a greater depth of field" (which died a solitary death, unknown and unloved).

If you grant that, for the purposes of a 30" x 20" print, a 10 MP sensor (or, heck, let's say 14.6 MP, to choose a random number) is essentially the same quality as film, then the end (subjective) result is that you ARE "seeing farther" -- just as much as you would be if you printed the film image cropped to the sensor size. As you say, the actual magnification of the lens hasn't changed... but I'll bet that the illusion is hard to tell from reality.

Anyone want to loan me a 450mm so I can test? Oh, and I guess I'll need a 300mm too. And a film camera.

01-24-2008, 02:30 PM   #9
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Again, thank you all for the great info! Sorry for the crappy thead title, the term 'magnification factor' was what I was trying to parlay
01-24-2008, 02:32 PM   #10
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It's not subjective territory. The actual magnification of the lens has not changed. Because of the smaller size of the digital sensor, some of the image projected by the lens is being cropped out. Yes, you will probably get a very similar image using a 450mm lens on a film camera to the one you would get with a 300mm lens on a dSLR. However, that's not because somehow a 300mm lens magnifies more on a dSLR and less on a film. Magnification stays the same no matter what camera you use it on. It's because the camera crops out some of the projected image. It's not semantics, it's fact.

By the way, one thing I don't understand is if the DA lenses are designed for dSLRs and project an image to only cover the digital sensor, then why are they still subject to the crop factor? If they vignette on film, then doesn't that mean that the actual beam of the lens is thinner meaning the entire image is being projected onto the sensors, so no cropping?

Last edited by tux08902; 01-24-2008 at 02:38 PM.
01-24-2008, 03:13 PM   #11
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Arthur, apologies for hijacking your thread. I'm glad we answered your question! Now, as respectfully as possible...

QuoteOriginally posted by tux08902 Quote
It's not subjective territory. The actual magnification of the lens has not changed. Because of the smaller size of the digital sensor, some of the image projected by the lens is being cropped out. Yes, you will probably get a very similar image using a 450mm lens on a film camera to the one you would get with a 300mm lens on a dSLR. However, that's not because somehow a 300mm lens magnifies more on a dSLR and less on a film. Magnification stays the same no matter what camera you use it on. It's because the camera crops out some of the projected image. It's not semantics, it's fact.
If "you will probably get a very similar image" then it IS subjective; some people will find it to me more similar, some less.

Please don't introduce spurious statements such as "somehow a 300mm lens magnifies..." and then dispute them; no one ever said any such thing.

My point is -- as far as most of us are concerned, the magic (or horror, if you're of the wide-angle persuasion) of the crop factor, when switching from film to digital, IS like suddenly having all of your lenses multiplied... Anyone who says otherwise without providing evidence to demonstrate that there is a VISIBLE DIFFERENCE between the two is simply muddying the waters by taking the discussion into a realm of technical fetishism, in an effort to appear more "correct".

Marc (correctly) says that
QuoteQuote:
A teleconverter can increase the magnification, but often at a compromise of IQ (image quality)
Do you think there's a discernible difference between a 20x30 print from a negative shot with a 300mm lens with a 1.5 teleconverter, and a 20x30 print from a 14.6MP k20d with the same 300mm lens? After all, one has "real" magnification and the other is "just crop factor".

Maybe there is -- I don't know; but there is no evidence that anyone else in this thread so far does (based on visible evidence), either.



Oh -- just saw your edit. Good question...

QuoteQuote:
By the way, one thing I don't understand is if the DA lenses are designed for dSLRs and project an image to only cover the digital sensor, then why are they still subject to the crop factor? If they vignette on film, then doesn't that mean that the actual beam of the lens is thinner meaning the entire image is being projected onto the sensors, so no cropping?
If you're just basing that on what I said, was making a supposition, so I (personally) can't say for sure that they DO vignette on film. I'd also be willing to consider the idea that a DA at 50mm (say) is actually made to be 75mm, with a smaller circle, and intentionally mislabled as 50, to avoid confusion.

BTW, thanks to everyone for making this boring day at work much more stimulating!
01-24-2008, 04:34 PM   #12
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The FOV of a lens changes on a dSLR because the sensor is smaller and accepts a smaller portion of the projected image. The magnification of the lens will never change. It's a technicality. That's what I was getting at. That's what Mike was getting at.

Now, I'm really curious. I think I'm going actually to do some tests with my A 28/2.8, M 50/1.7, and my DA 18-55/3.5-5.6. I really need to answer my question about DA lenses once and for all.
01-24-2008, 05:12 PM   #13
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QuoteOriginally posted by tux08902 Quote
By the way, one thing I don't understand is if the DA lenses are designed for dSLRs and project an image to only cover the digital sensor, then why are they still subject to the crop factor? If they vignette on film, then doesn't that mean that the actual beam of the lens is thinner meaning the entire image is being projected onto the sensors, so no cropping?
The size of the imaging circle does not matter, only the size of the surface it is projected on.
01-24-2008, 05:30 PM   #14
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See the below diagram. The tree is the same size on the sensor or the film (or FF sensor), ie the same magnification, there is simply less surround. In other words the crop is greater, thus, 1.5 crop factor.

Attachment 7822

This is why wide angle fans HATE the APC-C sensor size. They lose a lot of the capabilities of their wide lenses since only the center is captured by the sensor.

Last edited by MRRiley; 07-01-2008 at 09:23 AM.
01-24-2008, 05:52 PM   #15
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how about I explain it this way?

QuoteOriginally posted by amateur6 Quote
Marc, I don't mean to be rude, but isn't this just semantics? If the FOV of a 300mm 2.8 is the same as that of a 450mm, and the film from the 450 was scanned at the equivalent DPI and similarly cropped, would there be a visible difference in IQ? Barring the inherent differences between film and digital, of course...
How wude!!!!

Yes there is a difference! Think of it this way: you are trying to resolve the same subject with less "information" - fewer pixels are capturing the subject, because it is further away. Therefore there is less information for the sensor to capture, and thus the detail level is compromised. That's why a 300mm does not become a 450mm lens on a DSLR. EVER!!! When I put my 300mm from my MZ-S to my K10D, the subject didn't suddenly move so it's closer, right? There's just less space around the entire image, so there is less to crop...

Think about it: you'll never resolve more subject information using the same lens on a digital SLR as opposed to a film SLR (due to the crop factor), assuming good film or sensor is in the camera. The subject is still the same distance away! It's just a FOV (field of view) thing. I hope by saying it this way, you might see what I've been trying to explain here...

QuoteOriginally posted by amateur6 Quote
Arthur, apologies for hijacking your thread. I'm glad we answered your question! Now, as respectfully as possible...

Marc (correctly) says that
A teleconverter can increase the magnification, but often at a compromise of IQ (image quality)

Do you think there's a discernible difference between a 20x30 print from a negative shot with a 300mm lens with a 1.5 teleconverter, and a 20x30 print from a 14.6MP k20d with the same 300mm lens? After all, one has "real" magnification and the other is "just crop factor".
See above for explanation...

That image with the TC will be softer. I know, I've printed Elk images at 8x10 from a 300/2.8 with 1.7x TC and it's great! At 100% though, I feel it is quite soft, and not within my standards of sharpness. Printer interpolation is a different matter though, but doesn't apply to much in this situation. That's why the 8x10 is quite nice, but I'd be quite leery of printing it beyond an 11x14, I think it would start being soft at 16x20.

QuoteQuote:
If you're just basing that on what I said, was making a supposition, so I (personally) can't say for sure that they DO vignette on film. I'd also be willing to consider the idea that a DA at 50mm (say) is actually made to be 75mm, with a smaller circle, and intentionally mislabled as 50, to avoid confusion.
My DA 12-24 vignettes at anything below 16mm on my MZ-S film body. Besides, I've put a 50/1.4 on both my K10D and my MZ-S. Again, the subject isn't any closer, there's just less space around the entire image, so there is less to crop...


QuoteQuote:
BTW, thanks to everyone for making this boring day at work much more stimulating!
I agree, it is a very constructive dialog!

Side note: You may have noticed I tend to avoid or post very little in the New and Rumors section. Obviously I'll never get into a discussion with a certain vocal critic of Pentax, with a blog. I fear I would show my dark side and actually challenge him on his accountability. This includes a marked avoidance of posting any images to speak of his ability as a photographer, yet he criticizes many things needlessly. I do not wish to indulge in negative attitude or behavior! Everyone has a right to speak their mind, and yet I believe a balanced attitude is a good one. This will be the only time I say this publicly, unless I am challenged about it...

This is much a better conversation!

Cheers,
Marc

EDIT: I see Mike R. provided a very compelling diagram. Thanks Mike! This will only help my explanation - sorry it was so long-winded, but I felt it's important to clarify everything...

Last edited by Marc Langille; 02-03-2008 at 06:54 AM. Reason: clarification
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