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03-07-2010, 02:24 PM   #1
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35mm Film lenses on DSLRs?

Can someone tell me the conversion in terms of size? Like I heard that a 135mm becomes like a 200mm on a DSLR. Is there some sort of formula or chart? Please and thank you.

03-07-2010, 02:53 PM   #2
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Try this:

https://www.pentaxforums.com/forums/pentax-lens-articles/90477-crop-factor-fo...ield-view.html
03-07-2010, 03:11 PM   #3
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Thank you Ole, I just read it and it makes much more sense now. So the formula I drew from that article you wrote would be... something like...

(Length of film lens/1.5) = Length of DSLR lens needed to get the same FOV as Film.
03-08-2010, 12:13 PM   #4
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Not just film lens: *any* lens. All 50mm lenses (for example) have the same focal length, whether they were designed for film or digital. A given lens will have different field of view between film and digital *cameras*, but if you find it useful to use the "crop factor" to compare the field of view between your cameras, you need to apply it to *all* lenses, not just film lenses.

03-08-2010, 01:56 PM   #5
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QuoteOriginally posted by Marc Sabatella Quote
Not just film lens: *any* lens. All 50mm lenses (for example) have the same focal length, whether they were designed for film or digital. A given lens will have different field of view between film and digital *cameras*, but if you find it useful to use the "crop factor" to compare the field of view between your cameras, you need to apply it to *all* lenses, not just film lenses.
Oh! Okay thanks there. So it's all just the camera's design? not so much the design of the lens. Okay I see, the film is larger than the receiver chip thing inside a usual DSLR?
03-08-2010, 02:43 PM   #6
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QuoteOriginally posted by NecroticSoldier Quote
Oh! Okay thanks there. So it's all just the camera's design? not so much the design of the lens. Okay I see, the film is larger than the receiver chip thing inside a usual DSLR?
It's all about the sensor (or film) frame size. See this great post: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/92150-field-vi...n-graphic.html

A lens forms an image on the frame. A smaller frame, with the same lens, gets a smaller slice of that image. So a 35mm full-frame (35/FF) camera or sensor has a frame that's 24x36mm, with a diagonal of 43.3mm. The APS-C sensor in your dSLR has a frame that's about 17x25mm, with a diagonal around 29mm. The APS-C picture is about 1/2 the area of the 35/FF frame.

The crop factor is calculated from the diagonals, not the areas. That's because not all frames have the same 2:3 aspect; some are square (1:1), many P&S's are 3:4, some pano.cams are 1:2 or 1:3, etc. The ratio of diagonals 43.3:29 is about 1.5, which is the number we use. This just means that a 100mm lens on your dSLR sees the same angle (or 'slice') of a view as a 150mm lens would on a 35/FF camera.

That doesn't mean that the PICTURE looks the same with 100mm on APS-C as 150mm on 35/FF. Those different lenses will have different perspective and depth-of-field (DOF). Perspective is the relationship of objects in the picture. DOF is the thickness of the view that looks sharp.

Look again at that picture in the link above. Each slice has the same lens (focal length), same aperture (f-stop), same distance from camera to subject. Relationships of stuff in the slices don't change; sharpness doesn't change. All that changes is how much of the view is in each slice.

That picture illustrates 3 different frame sizes. If you used different lenses and distances to get the same FOV, to fill the frame, the perspectives and DOFs would change. Back up with a longer lens focused on the couple; their features flattten, and the background shifts and gets blurry. Close in with a wider lens; they bulge more, and the background looks further away.

See, crop factor isn't everything when you're thinking about how different lenses behave. Focal length, aperture, focus distance, light, all shape the picture.
03-08-2010, 03:41 PM   #7
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Thanks for explaining, Yeah those things matter too! Thanks for the link too, I'll be sure to read in on it.
03-09-2010, 04:32 AM   #8
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QuoteOriginally posted by RioRico Quote
It's all about the sensor (or film) frame size. See this great post: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/92150-field-vi...n-graphic.html

A lens forms an image on the frame. A smaller frame, with the same lens, gets a smaller slice of that image. So a 35mm full-frame (35/FF) camera or sensor has a frame that's 24x36mm, with a diagonal of 43.3mm. The APS-C sensor in your dSLR has a frame that's about 17x25mm, with a diagonal around 29mm. The APS-C picture is about 1/2 the area of the 35/FF frame.

The crop factor is calculated from the diagonals, not the areas. That's because not all frames have the same 2:3 aspect; some are square (1:1), many P&S's are 3:4, some pano.cams are 1:2 or 1:3, etc. The ratio of diagonals 43.3:29 is about 1.5, which is the number we use. This just means that a 100mm lens on your dSLR sees the same angle (or 'slice') of a view as a 150mm lens would on a 35/FF camera.

That doesn't mean that the PICTURE looks the same with 100mm on APS-C as 150mm on 35/FF. Those different lenses will have different perspective and depth-of-field (DOF). Perspective is the relationship of objects in the picture. DOF is the thickness of the view that looks sharp.

Look again at that picture in the link above. Each slice has the same lens (focal length), same aperture (f-stop), same distance from camera to subject. Relationships of stuff in the slices don't change; sharpness doesn't change. All that changes is how much of the view is in each slice.

That picture illustrates 3 different frame sizes. If you used different lenses and distances to get the same FOV, to fill the frame, the perspectives and DOFs would change. Back up with a longer lens focused on the couple; their features flattten, and the background shifts and gets blurry. Close in with a wider lens; they bulge more, and the background looks further away.

See, crop factor isn't everything when you're thinking about how different lenses behave. Focal length, aperture, focus distance, light, all shape the picture.
You need to be a little careful about DOF and perspective in your explanation,

By simply cropping the middle out of a 35 mm frame using the reduced sensor size of a DSLR, changes nothing with respect to the DOF and persoective of the residual shot.

a cropped section out of a frame changes nothing, what changes perspective and depth of field is when you move, using a dslr to get the same primary image size as you had with film.

Same goes with DOF. it is because you move the camera to compensate for the change in crop of the sensor that causes these things to change.

there is also another thing which causes change in depth of field using digitals, specifically when you look at the images, you enlarge the image more from digital because it was smaller, to get to the same print size, this also impacts depth of field because of the magnification you apply at the print stage.(or on the viewing screen)

03-09-2010, 09:37 AM   #9
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QuoteOriginally posted by Lowell Goudge Quote
You need to be a little careful about DOF and perspective in your explanation,

By simply cropping the middle out of a 35 mm frame using the reduced sensor size of a DSLR, changes nothing with respect to the DOF and persoective of the residual shot.

a cropped section out of a frame changes nothing, what changes perspective and depth of field is when you move, using a dslr to get the same primary image size as you had with film.

Same goes with DOF. it is because you move the camera to compensate for the change in crop of the sensor that causes these things to change.
That's exactly what I said. Re-read that paragraph; I specified that DOF and perspective change when MOVING to compensate for different focal lengths. And the previous paragraph: looking at different slices of the same image doesn't change DOF etc.

QuoteQuote:
there is also another thing which causes change in depth of field using digitals, specifically when you look at the images, you enlarge the image more from digital because it was smaller, to get to the same print size, this also impacts depth of field because of the magnification you apply at the print stage.(or on the viewing screen)
That's in the presentation, not the picture-taking. Viewing the picture at different distances, enlargements, angles, in different illuminations, with different eyes, changes the DOF because of circle-of-confusion issues. That's not under the photographer's control when capturing the image.

The presentation has GREAT impact. As I mentioned elsewhere, I've cropped 1mpx digicam pics square and printed them at 6x6 cm. I put them adjacent to 6x6cm Verichrome Pan contact prints, all on glossy paper, all matted and behind glass and framed. I hang those on the wall at eye level. Unless you move to within 6 inches, they can't be distinguished, even though the resolutions are vastly different. Small enough and/or far enough away, almost ANYTHING is sharp.
03-10-2010, 04:31 AM   #10
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QuoteOriginally posted by RioRico Quote
That's exactly what I said. Re-read that paragraph; I specified that DOF and perspective change when MOVING to compensate for different focal lengths. And the previous paragraph: looking at different slices of the same image doesn't change DOF etc.
I think I had more trouble with the statement "This just means that a 100mm lens on your dSLR sees the same angle (or 'slice') of a view as a 150mm lens would on a 35/FF camera." than anything else, but you're right, you make it much clearer further on. Anyway, no harm done

QuoteQuote:


That's in the presentation, not the picture-taking. Viewing the picture at different distances, enlargements, angles, in different illuminations, with different eyes, changes the DOF because of circle-of-confusion issues. That's not under the photographer's control when capturing the image.

The presentation has GREAT impact. As I mentioned elsewhere, I've cropped 1mpx digicam pics square and printed them at 6x6 cm. I put them adjacent to 6x6cm Verichrome Pan contact prints, all on glossy paper, all matted and behind glass and framed. I hang those on the wall at eye level. Unless you move to within 6 inches, they can't be distinguished, even though the resolutions are vastly different. Small enough and/or far enough away, almost ANYTHING is sharp.
The problem is that while DOF is much more in the presentation than the actual picture taking, because of the enlargement ratio, DOF is all based upon the assumption of the viewing format.

In fact, DOF is actually calculated for an 8x10 print ( approx) from a full frame image.

cropping in, or blowing up beyond 8x10 while "in the presentation" as you say, does impact the DOF . YOu can't remove it from the discussion because the presentation format is the basis of what is considered acceptable focus,
03-10-2010, 09:39 AM   #11
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QuoteOriginally posted by Lowell Goudge Quote
YOu can't remove it from the discussion because the presentation format is the basis of what is considered acceptable focus,
But there is no standard presentation format, not unless you arbitrarily choose one... in which case, it's YOUR standard and not necessarily anyone else's. (As we used to say in engineering: The nice thing about standards is, there are so MANY of them!") No matter how presented, in what format or size, DOF is still about an acceptable range of sharpness of focus. And it can be specified relative to a fixed point.

Say you've shot down a line of objects and hyperfocused so the central object and those adjacent are sharp, while those nearer and farther are blurry. You print that at 4x5". You print it again at 8x10" and again at 16x20". Viewed from the same distance, the larger prints aren't as sharp as the smaller. But the details within each print retain their same sharpness RELATIVE TO THE CENTRAL FOCUS. The central stuff looks sharper than the near and far stuff.

And an audience won't feel comfortable, viewing those at the same distance. They'll move closer for the 4x5", further back for the 16x20". Project that same image on a 32x40 FOOT cine screen -- those in the front row of the theatre likely aren't watching the screen, eh? Those elsewhere in the theatre will see that same range of sharpness.

Your point is (I think) that if that scene were shot once at FF and again at HF/APS-C, with the same FOV to fill each frame, and both shots were printed the same size, the FF would be sharper. Yes indeed. But to get the same FOV, you'd have to use different lenses and/or shooting distances, and THAT changes the DOF equation.

And yes, by careful selection of FL, distance, and aperture, you could get equivalent DOF -- but now the perspective is different, so you no longer have the same picture. And even with the FF print being sharper, all the elements within each picture would STILL have the same relative sharpness -- crisp center, fuzzy near-and-far.

I don't think we can usefully proceed further. Bye now.
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