FYI a 12 m mount review from Ashwin Rao

GaryLH

GaryLH

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I think he's incorrect about the DOF issue -- a lens designed to project an image onto a 35mm neg will have the same DOF characteristics whether it is projecting onto 35mm film or a 1.5x crop sensor. Nothing is changing the physical properties of the lens, you're only cropping out a portion of the image circle.
 
kzphoto, crop any imge and the circles of confusion get bigger compared to the image dimensions. Or, the absolute size of a circle of confusion doesn't tell much about the DOF if you don't have the size of the image it's part of. DOF is a property of images, not lenses.
 
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Maybe I'm not understanding this?

Okay, so in theory, I have a 28/2.8 lens (designed to work on a 35mm camera). If I stop the aperture down to f/8 and set my focus to 1.5m most everything from 3.5ft ~ 9ft. should be in focus.

If I use this same lens on a 1.5x crop sensor, you're telling me I have more things in focus?

I was under the impresssion that an equivalent focal length lens for an APC-C sensor would have more DOF, however the same 28mm lens would exhibit the same characteristics regardless of format.

My 28mm lens should always draw a circle roughly large enough to cover a 24x36mm neg.
 
I'll try to explain: If something is at the limit of DOF, it's not "in focus", it's "still acceptably sharp." Points are rendered as small circles. "Acceptably sharp" is usually defined as a ratio between the size of these circles and the image diagonal. (Different makers define it differently but it should be in a ballpark of 1/1000th or 1/2000th.) Anyway, crop factor 1.5 shortens the diagonal 1.5 times, so the same circles will now appear 1.5 times bigger relative to the diagonal. The effect is almost the same as opening the aperture by one stop, only in that case the diagonal remains the same but the circles grow wider, by 1.4 times.

So on a crop sensor with the same lens you will have not more, but less things "in focus."
 
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thanks t6un. i think that really cleared up a lot of the misunderstanding people have with using lenses designed for one format on another with a "crop factor".
 
I can't explain the technical reasons, but I was always pretty sure that smaller sensor meant more DOF.

[That's another confusing point - what people mean by "more DOF". some people mean more things in focus, some people mean more control of DOF or more ability to blur the background. Here, I am using it in the former sense.]

So take this to the extreme. Compare the ability of a large sensor like 35mm or medium format to a point in shoot tiny sensor to "blur the background" with equivalent focal lengths and apertures. To me it seems that the larger sensors have shallower DOF with the other things being equal.
 
It doesn't happen in the camera. If you shoot a 28mm on an M8 and an M9 then reproduce the image in a print pixel for pixel with the M8 print being ~70% the size of the M9 print reflecting the 1.33 crop factor, then the Depth Of Field will be the same. The Circle Of Confusion changes according to reproduction. If you take that M8 photo and reproduce it the same size as the M9 photo THEN the COC changes and your DOF will change accordingly.

The lens and the recording medium don't know or care about the COC or the DOF. It's in reproduction that these change.

Phil Forrest
 
Phil_F_NM said:
It doesn't happen in the camera. If you shoot a 28mm on an M8 and an M9 then reproduce the image in a print pixel for pixel with the M8 print being ~70% the size of the M9 print reflecting the 1.33 crop factor, then the Depth Of Field will be the same. The Circle Of Confusion changes according to reproduction. If you take that M8 photo and reproduce it the same size as the M9 photo THEN the COC changes and your DOF will change accordingly.

The lens and the recording medium don't know or care about the COC or the DOF. It's in reproduction that these change.

Phil Forrest
Click to expand...


^^^ This is what I hold to be true, I guess I don't get the rest. I need to read more. eeegh.
 
Phil_F_NM said:
It doesn't happen in the camera. If you shoot a 28mm on an M8 and an M9 then reproduce the image in a print pixel for pixel with the M8 print being ~70% the size of the M9 print reflecting the 1.33 crop factor, then the Depth Of Field will be the same. The Circle Of Confusion changes according to reproduction. If you take that M8 photo and reproduce it the same size as the M9 photo THEN the COC changes and your DOF will change accordingly.

The lens and the recording medium don't know or care about the COC or the DOF. It's in reproduction that these change.

Phil Forrest
Click to expand...

So I think this is still consistent with my limited understanding as above. I didn't say, but was assuming, equal print/viewing scales. However, I don't think viewing size affects the amount of DOF either way. I suppose it could affect how it is perceived, but I think DOF is a characteristic of the image when it is captured.
 
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grahamule said:
Just some quick googling yielded the following. I have no connection to the source, nor any indication of their accuracy or correctness.

About halfway down, they discuss DOF:
http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm
Click to expand...

This page as well addresses the issue of DOF and how it is tied to "field of view." It seems to me that as was said above, that the DOF is the same regardless of sensor, but that for an equivalent field of view, one has to move the smaller sensor further from the subject, and for the same focal length/aperture, doing this increases the DOF.

http://www.have-camera-will-travel.com/field_reports/full_frame_vs_crop_sensor_-.html
 
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