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

• From: "Neil Harrington" <not@xxxxxxxxxx>
• Date: Sat, 15 Sep 2007 19:16:54 -0400

"Paul Furman" <paul-@xxxxxxxxxxxxx> wrote in message
news:pHSGi.2877\$ZA5.1923@xxxxxxxxxxxxxxxxxxxxxxx
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.

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.

That part at least should be easy. Face height : film (or sensor) height =
subject distance : focal length.

But I've decided my notion was all wet anyway, as I mentioned in a later
post. Very small sensor cameras do not have nearly the same DOF (or
background blurring) that 35s or DSLRs do, at equivalent focal lengths.

Neil

.

## Relevant Pages

• Re: Depth of field difference between APS-C and full frame
... 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. ... 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. ... 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. ...
(rec.photo.digital.slr-systems)
• Re: Depth of field difference between APS-C and full frame
... 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. ... to ask which objects in the background, at what distance. ... with a 35 to get the same final image size, and any blur circle will also be ...
(rec.photo.digital.slr-systems)
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