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07-24-2009, 01:11 PM   #1
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SLR > DSLR focal length conversion?

I'm new to the DSLR world (just got a K2000 about a month ago), but am pretty familiar with film SLRs and lens terminology. However, I'm a bit confused over the film-to-digital focal length conversion thing.

I mean, I understand that on a film camera (like my ancient ME SE), the focal length is accurate based on or in relation to a 35 mm film negative. Since the size of the ccd/sensor in a DSLR is not the same size as a 35 mm film negative, the "effective" focal length changes. So, if I take the 50 mm lens off of the old ME and put it on the K2000, the effective focal length increases, right? I can't seem to find a table or calculator to determine what the focal length would be, but I've seen some people suggest that that 50 mm film lens would equate to 75 mm on the K2000.

So, doing some fiddling around with the 50-200 mm lens that came with my new camera, I took a few shots at the 50 mm setting. I then swapped on the 50 mm/1.7 lens from my ME and took a few shots of the same subject, thinking that the subject would be slightly enlarged (since a photo taken at 75 mm would be "closer up" than one taken at 50 mm). To my surprise, the images were nearly identical in relative size. The 50/1.7 image was brighter, crisper, and seemed to have better contrast, but the size of the subject in the photo was almost exactly the same between the two lenses.

So, my test wasn't exactly scientific, but the results seem to suggest that the effective focal length doesn't change. Or, do the manufacturers take this conversion into account when designating the lenses? That is, would the 50-200 mm DSLR lens decrease in effective focal length if I put it on a film camera?

07-24-2009, 01:14 PM   #2
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Any lens marked 50 mm is going to give you the same field of view on your DSLR, whether it's one from the film era or a digital-only lens. The crop only refers to the fact that the imaging area is smaller. I think this article explains it pretty well: Crop factor - Wikipedia, the free encyclopedia.
07-24-2009, 01:31 PM   #3
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Welcome to the wacky world of digital abuse of science.

as stated already focal length is an optical property and does not change regardless of what camera you put the lens onto.

What actually changes is the field of view of the lens, because you are projecting the image onto a smaller sensor, or performing the equivelent of a crop of a 35mm frame. Nothing more nothing less.

the result is that when you print you are actually performing an ellargement of 50% more with digital to get to any comparable print size that you shot on film.

the real issues with the new format are depth of field, and the old 1/focal length rule for shutter speed.

the depth of field has not changed for the lens, but because you blow things up bigger, the apparent "depth of field" is less

The same holds true for shutter speeds to avoid camera shake, because you blow things up more you need higher speeds to have the enlargement look as sharp.

Unfortunately camera companies and even photo magazines have latched onto the idea of stating focal lengths in equivelent field of view focal lengths on 35mm, hence the "focal length factor" they found this to be simpler than trying to explain cropping images.
07-24-2009, 01:53 PM   #4
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Welcome to the forum wedge,
You're not the only one who has been confused by this 35mm equivalent term.
Marketing is much to blame for this deception, and as long as you understand this point as described above, you won't be misled or disappointed when you come to buy a lens for a specific purpose.

07-24-2009, 02:01 PM   #5
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Just to build on what the others already stated, here's a "visual" example.

Take a 4x6 print. We'll call that your "full frame" 36mm x 24mm image.
Now trim off the top and sides, keeping the center constant ( cropping ), until the print is 2-2/3" by 4". Ignoring the "reality" that it is physically smaller, that's your APS-C image.

Nothing else much changes noticeably (in most photos) but the field of view. Yes, it is true that the "circle of confusion" causes depth of field variation but the overwhelmingly largest factor is field of view. On either format with the FA31 Limited at f/8 and a distance to subject of 25ft you're past the hyperfocal distance and have an "infinite" depth of field.
07-24-2009, 02:22 PM   #6
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this helped me finally understand it...

You're not getting 50% closer your seeing 33.3% less of the image...
07-24-2009, 02:30 PM   #7
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QuoteOriginally posted by mtroute Quote
this helped me finally understand it...

You're not getting 50% closer your seeing 33.3% less of the image...
No lens ever gets you closer. Just think in terms of field-of-view and it will all make sense. A 50mm lens on APS-C will give you the same *field-of-view* as a 75mm on a 35mm film camera. The rest is all just Fun With Numbers.
07-24-2009, 02:31 PM   #8
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Honestly, people make this WAY more freakin complicated than it is with their "field of view" and magnification. FFS it's not brain surgery, just multiple by 1.5.

So if you shot with a 50mm lens on film, buy a 35mm for your DLSR ... jesus it's pretty fundamental math until you get into the depth of field discussion.

Ironically I just can't get used to a 50mm lens on a cropped DSLR but it's 75mm and the 77mm is one of Pentax's all time favourites. Go figure.

07-24-2009, 02:44 PM   #9
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QuoteOriginally posted by GaryM Quote
No lens ever gets you closer. Just think in terms of field-of-view and it will all make sense. A 50mm lens on APS-C will give you the same *field-of-view* as a 75mm on a 35mm film camera. The rest is all just Fun With Numbers.
really, so when I throw on a 100mm lens, I am not getting 100% closer (magnifying) to the image than I would be with my 50mm? (in film terms for simplicity)

Your confusing Focal Length and Field of View

Last edited by mtroute; 07-24-2009 at 02:55 PM.
07-24-2009, 02:48 PM   #10
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QuoteOriginally posted by Alfisti Quote
Honestly, people make this WAY more freakin complicated than it is with their "field of view" and magnification. FFS it's not brain surgery, just multiple by 1.5.

So if you shot with a 50mm lens on film, buy a 35mm for your DLSR ... jesus it's pretty fundamental math until you get into the depth of field discussion.

Ironically I just can't get used to a 50mm lens on a cropped DSLR but it's 75mm and the 77mm is one of Pentax's all time favourites. Go figure.
Remember when you were a beginner at something, say walking, would you prefer someone to be patient and help you or criticize you for not knowing how to do something they have been doing for decades.

I call it expertosis: the condition MOST experts in any field develop in which they forgot what it was like when they weren't and expert.

This is the OP's FIRST post, give them a break.
07-24-2009, 05:34 PM   #11
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QuoteOriginally posted by mtroute Quote
really, so when I throw on a 100mm lens, I am not getting 100% closer (magnifying) to the image than I would be with my 50mm? (in film terms for simplicity)
Obviously. You're only getting closer if you physically move closer. A longer focal length lens will produce an image with a narrower field of view than a shorter focal length lens, and that's what makes the subject look bigger on the print. Projecting the image onto a cropped sensor and producing a print of the same size from that sensor has exactly the same effect.

Now, it's entirely possible the longer lens will do so with more resolution than the cropped sensor, but that depends entirely on the specific lenses invovled as well as the specific film you are shooting and the resolution of the specific sensor you are using. Without knowing those variables, then indeed, for all practical purposes, cropping *is* exactly the same as using a longer focal length.

Back to the original question: a 50mm lens is a 50mm lens. So no surprise that the 50-200 at 50mm produces the same image as the 50/1.7. Doesn't matter if the lens was created for film or for digital, 50mm is 50mm.

the relevant comparison is not different lenses on the same camera, but one lens on two different cameras. Put that 50mm lens on the film camera then on the digital, and see which appears to have the longer focal length. You'll find that for a shot where the 50 on film would have shown head and shoulders, the same 50 on digital will show face only, etc.
07-25-2009, 03:50 AM   #12
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OK, I get it now. At least I think so. It's not a matter of the focal length (or "effective" focal length) changing/shifting/etc. It's a matter of the the image being cropped to the appropriate "target size" (ie, the sensor/CCD for digital, a film cell for, well, film).

Thanks for all the replies and patience dealing with a newbie such as I. Truth be told, I suffer greatly from expertosis, just not in this area. My full-time gig is in "Instructional Technology Support" and I often forget that the 40-60 year old faculty members that I support didn't necessarily grow up with computers and often don't understand terms like "right-click", "log on" or "copy & paste".

Anyhoo, long story short, if I want to use the 50/1.7 (or any other "film" lenses) on a DSLR, I shouldn't expect the images to look different in terms of FOV or magnification than they would on film. But, because of the sensor size compared to a 35 mm film cell, the digital image may be less sharp (with all other factors - shutter speed, EV compensation, aperture, etc. - being equal).

I guess the only other question is what to do with the aperture ring. A few of the lenses I have do have the "A" setting that the K2000 manual mentions, but not the 50/1.7 or the 40/2.8 ("pancake").
07-27-2009, 12:24 PM   #13
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QuoteOriginally posted by Marc Sabatella Quote
Obviously. You're only getting closer if you physically move closer. A longer focal length lens will produce an image with a narrower field of view than a shorter focal length lens, and that's what makes the subject look bigger on the print. Projecting the image onto a cropped sensor and producing a print of the same size from that sensor has exactly the same effect.

Now, it's entirely possible the longer lens will do so with more resolution than the cropped sensor, but that depends entirely on the specific lenses invovled as well as the specific film you are shooting and the resolution of the specific sensor you are using. Without knowing those variables, then indeed, for all practical purposes, cropping *is* exactly the same as using a longer focal length.

Back to the original question: a 50mm lens is a 50mm lens. So no surprise that the 50-200 at 50mm produces the same image as the 50/1.7. Doesn't matter if the lens was created for film or for digital, 50mm is 50mm.

the relevant comparison is not different lenses on the same camera, but one lens on two different cameras. Put that 50mm lens on the film camera then on the digital, and see which appears to have the longer focal length. You'll find that for a shot where the 50 on film would have shown head and shoulders, the same 50 on digital will show face only, etc.
Thanks for clarifying Marc, but there has to be an element of magnification involved. My research told me that FOV is the area of view, for example the human eye has an almost 180 degree FOV, changing that FOV to say 60 degrees will not make our eyes resolve at a greater distance just restrict the amount of information our eyes see. If we changed the focal length or our eyes (moved the retina further back) we would resolve objects at greater distances. Hence why long lenses are, well long. Are FOV and Focal length hopelessly connected?


Are we just saying the same thing in differing terms? Or am I just way off\?

Last edited by mtroute; 07-27-2009 at 01:54 PM.
07-27-2009, 12:34 PM   #14
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Say you had two hypothetical cameras, a 10 megapixel DSLR camera with a 1.5x crop factor, and a 10 megapixel DSLR with a full frame (35mm) size sensor. Say they also hypothetically had the same per-pixel sharpness, color accuracy, contrast, etc.

Would a picture of the same subject, at the same distance, taken with a 50mm lens at f/4 on the 1.5x camera look exactly the same as a picture taken with a 75mm lens at f/4 on the full frame camera? Or would there be some difference in DoF, distortion, or something else?
07-27-2009, 01:46 PM   #15
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QuoteOriginally posted by lunelson Quote
Say you had two hypothetical cameras, a 10 megapixel DSLR camera with a 1.5x crop factor, and a 10 megapixel DSLR with a full frame (35mm) size sensor. Say they also hypothetically had the same per-pixel sharpness, color accuracy, contrast, etc.

Would a picture of the same subject, at the same distance, taken with a 50mm lens at f/4 on the 1.5x camera look exactly the same as a picture taken with a 75mm lens at f/4 on the full frame camera? Or would there be some difference in DoF, distortion, or something else?
The only difference is that you would only see 66% of the full frame image on a DSLR with a 1.5 crop factor.

http://en.wikipedia.org/wiki/File:Crop_Factor.JPG

another way to think about it is if you had a film projector projecting a movie on a full size screen (FF DSLR) you would see the entire image. Now take that same projector and movie and put it on a screen that was only 66% of the original screen (APC-C), that is what crop factor is. The image qualities haven't changed, just how much of the image shows on the screen.

Last edited by mtroute; 07-27-2009 at 01:52 PM.
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