Does sensor or format size affect depth of field, or only the image formed by the lens?

Question

I saw a video claiming that format size (full frame, Super 35, etc.) does not change depth of field because the lens forms the image before it reaches the sensor, so a bigger or smaller sensor is just like a bigger or smaller wall catching that image. Is that accurate?

My understanding is that apparent depth of field depends on the acceptable circle of confusion in the final viewed image, so sensor/format size matters because it changes how much the image must be enlarged for display. In other words, the image at the sensor plane may be fixed, but the perceived depth of field in the final photo/video can still change with format size and viewing conditions.

Is the video mixing up depth of focus at the image plane with depth of field in the final image?

Steven Kersting · Accepted Answer

Your understanding is correct; the author is confusing depth of focus with depth of field... it's a fairly common confusion as the two are interrelated, but only one is common terminology.

The depth of focus is the relative sharpness of details at the image plane, and depth of focus is a fixed characteristic of an image. It is the non-variable component of depth of field; and it is dictated only by the physical size of the aperture opening/entrance pupil (not the f/#).

Depth of field is not a fixed aspect of an image; so when people talk about depth of field as a fixed aspect, they are really talking about the depth of focus.

Depth of field is dictated only by magnification... it is how apparent the depth of focus is made to the viewer. Magnification includes all of the other variables... focal length, subject distance, sensor area/cropping/enlargement, viewing distance, and even the viewer's visual acuity.

If the final/total magnification causes the image to have the same composition and relative size to the viewer, then the depth of field will remain the same and be dependent only on the depth of focus recorded. That's why the standard for image sharpness (and the calculators), assumes a standard viewing distance approximately equal to the image diagonal; and to where the image occupies the human's ~ 45˚ primary field of view, of a person with standard visual acuity (the "standard" circle of confusion).

Edited to add for completeness, and because I already had the image:

Many think that the depth of field is fixed once the image is recorded due to subject distance and FL... that's what the author is stating. But it is not, only the depth of focus (relative sharpness) is fixed.

This shows a 6x crop/enlargement in post. Note that the depth of field as measured linearly within the scene decreases with the magnification factor; because depth of field is variable. However, the percentage of the image area that remains acceptably sharp remains the same; because the depth of focus remains constant.

The exact same reduction in the depth of field would occur if I had used a longer FL, shorter distance, or crop sensor instead; because they all cause the same increase in magnification/relative size.

Does sensor or format size affect depth of field, or only the image formed by the lens?

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