Re: Depth of field difference between APS-C and full frame

Neil Harrington wrote:
Paul Furman wrote
Neil Harrington wrote:
Paul Furman wrote

So sticking with the 50mm f/1.8 lens, if you move it from 35mm to APS & step back to get the same FOV,

But you can't do that. A 50mm lens on an APS camera cannot possibly deliver the same FOV as it does on a 35, no matter how far you step back. On a DX camera, the 50mm will always have the FOV of a 75mm lens on a 35.

You can step back to get the same *image size* as the same lens would give you on a 35, but only at one particular subject distance, because stepping back changes the perspective. You haven't changed the FOV.

This isn't a technical question but a perception question. Take the case of a head portrait. The framing of the face doesn't change significantly... well the nose gets a little longer looking but disregarding that, objects in the background will look (I think?) just as blurred in each case... (or not?). I guess it really is an unanswerable question because as you say the field of view is different, so you'd have to ask which objects in the background, at what distance. Maybe assume a particular hedge in the background at a given distance.

OK. We're disregarding perspective changes in the portrait like "the nose gets a little longer" and considering only the amount of blur in the background at some particular distance behind the subject, correct?

Right.

I think it should be possible to diagram this. Think of an imaginary cone with its base filling the aperture and its apex at the subject distance. Extending the lines of (the sides of) the cone into the background, what was a point at the subject distance becomes a circle increasing in diameter the farther it goes into the background. The larger this circle becomes, the larger the blur circle and the greater the apparent background blur in the final image, all else remaining the same.

Of course with a DX camera the image will have to be enlarged 1.5x more than with a 35 to get the same final image size, and any blur circle will also be enlarged 1.5 times. So really it takes only 2/3 the blur circle (at the "film" plane) in the case of the DX to give the same amount of apparent blur.

It seems to me this exactly balances out. Suppose a subject a few feet in front of the camera and a background a few feet behind the subject, the camera being either a 35 with a 75mm lens at f/2 or a DX with a 50mm lens at f/2, both shooting in exactly the same setup. The 35 produces a blur circle of a certain size at the background distance; the DX camera, since its physical aperture size (the base of the imaginary cone) is 2/3 the diameter of the 35's, will produce an imaginary circle (corresponding to blur circle size) at the background that's also 2/3 of the diameter of the 35's -- exactly the size needed to give the same amount of apparent blur in the same-size final print.

Whaddaya think?

I like the visualization idea but I can't really follow it without a drawing. I would plug it into that calculator but I'm not sure how to figure the field of view at different focal lengths and focal distances. That calculator does include focal distance and there is a drawing but I don't know how the light bends to reproduce the drawing for these scenarios:
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

If someone can tell me what focal distances will fit a face at 50mm on full frame and 50mm on APS, we can run the numbers and have an answer.


--
Paul Furman Photography
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Bay Natives Nursery
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