why would DOF for a lens changes depending on sensor size ?

Helen B · May 9, 2013

First, it is only a suggestion. If you wish you can use the pixel size or the printer ppi projected to the magnification at the sensor. It's your choice, not theirs.

The suggested format-specific MACC value is based on the typical viewing distance of a print in relation to its diagonal, and average human acuity. It's the approach taken with lenses that have DoF markings on the barrel (which happen to depend on the format the lens is meant for).

There always have been departures from that typical suggested approach and resulting MACC value - whether from a desire to maximize the system resolution or to tailor the sharpness criterion up or down to match the print quality (eg printer ppi or dpi) and likely viewing distance, for example.

amolitor · May 9, 2013

Just to elaborate on what Helen is saying, the standard basis for DoF calculations comes from the notion that people tend to view bigger prints from farther away. We "usually" view prints in such as a way as they take up pretty much the same portion of the field of view. The print's "visual size" is pretty much the same, regardless of the physical size of the print.

If your prints are going to be viewed in an unusual way, perhaps you're making large prints to hang in a narrow hallway, or quite small prints to be hung quite high, you may want to use a different CoC for your DoF calculations.

I'm doing a moderate amount of contact printing from 4x5 negatives these days, which presents one of these unusual cases. Normally, this is a very small print. Viewed from normal distances, the entire thing appears to be in focus, pretty much no matter what I do, creating a sensation of "shallow DoF" with a 4x5 print hung in a normal way at a normal viewing distance is pretty difficult, regardless of format. On closer examination, which the small print does invite, the experience of the print becomes quite different.

gregtallica · May 10, 2013

One confusing question I've had for the longest time with this is what about "APS-C equivelant" focal lenghts - would those have the same DOF as a FF? Example:

35mm on crop sensor is "about the same" as view of 50mm on full frame. Let's say you are shooting same settings, and same distance to your subject. Would the be similar? Not exact, but close?

ecphoto · May 10, 2013

Because the focusing distances are based on mm of the lens. When the sensor size changes so do the distances based on the crop factor e.g. x1.3 or x1.6. The change is usually proportional though.

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amolitor · May 10, 2013

Same subject, same distance, "equivalent" focal length lens, same aperture -- resulting in the same "picture" on the sensor?

At the same aperture, the hole in the lens will be bigger for bigger sensors, and smaller for smaller sensors. This means that the cone of light between the lens and the subject will be fatter for bigger sensors, and skinnier for little sensors. This, in turn, means that things the same distance off the plane of focus will be rendered more and less out of focus, respectively, as measured at the plane of the sensor.

For the small sensor camera, you can imagine that you're simply miniaturizing the entire assembly -- except that the subject is now "further away" in a sense. If you took the large sensor camera and simply shrank the whole thing by a factor of 1.5 or whatever, you'd have the smaller sensor, shorter lens, *same* aperture. The subject would be the same distance as before the shrinkage, though.

The effect is exactly the same as leaving everything alone at the camera side of things, and moving the subject further away while simultaneously making it bigger.

The varying CoC as used for one sensor over another will have the opposite effect, canceling some of this effect, but not entirely. You still get more calculated DoF with the smaller sensor.