Depth of field - does sensor size affect it beyond lens multiplier effect?

Started Nov 12, 2008 | Discussions thread
Andre Affleck
Senior MemberPosts: 2,070
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Re: Depth of field - does sensor size affect it beyond lens multiplier effect?
In reply to gava, Nov 14, 2008

I think the problem is that it is hard to judge the blur magnitude, without bias, by looking at the lines. One can easily be fooled by the scale factor which can skew one's observation. The line thickness and spacing is changing at the same time as blur magnitude, which I think is skewing my observation. If you look at the edge of the ruler only, instead of the scale, and look at the absolute thickness change due to the blur (not factoring in magnification between the two) you then can see that blur is almost identical for both images, meaning that the 30D blur at the 22 mark is the same as the 5D blur at the 23 mark, confirming what you have said.

I agree now. The 5D does have 1.6 more DOF given the same aperture, distance, and focal length. I think the problem is that one can be fooled by the scale change, giving the impression of an equal DOF when it is not.

gava wrote:

Well it's clear, same focal length, same distance to subject, same
aperture. (So part 2 is the same.)

But does the DOF really look identical to you? It doesn't to me.

The shot with the larger sensor (and therefore less magnification has
quite significantly MORE DOF). The way I see it the 30D shot has
about 1.25 inches DOF and the 5D has almost 2. That is about the
difference you would expect from the crop factor:

1.6*1.25=2

But notice here the sting in the tail I mentioned before: the crop
sensor has LESS DOF. Once you adjust for field of view (i.e. change
the focal length) however you end up going to the normal observation
that the larger sensor gives pictures with less DOF, this is because
the DOF varies with the SQUARE of the focal length, but is LINEAR (in
the opposite direction) with the crop factor.

In the link you posted the writer is seeing what he wants to see
rather than what is actually there in the picture IMO.

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