Fuji APS-C vs FF for portrait DOF

Started 9 months ago | Discussions thread
57even Forum Pro • Posts: 13,153
Re: Wouldn't the 90mm give a shallower DOF due to smaller minimum focus distance?

Dr_Jon wrote:

57even wrote:

Dr_Jon wrote:

rlx wrote:

Christof21 wrote:

myfujilife wrote:


But the aperture size on full frame would be on a larger sensor so would it still be 30%?

Yes, sensor size does not enter in the equation.

The size of the airy disk "projected" in the focus plane corresponds to the aperture size.

Imagine you see a disk in the background and you focus on somebody. Imagine the disk is as big as the eye on the picture, then you can guess the aperture size used without further information !!

Christof is right. Sensor size is irrelevant here. Try this simple experiment. Cut out a circular disk from a sheet of paper of the same diameter as the lens aperture (focal length / aperture number -- 56 mm /1.2 = 47 mm if you have the 56). Ask a friend to be your model for a portrait shot outdoor and tell her/him to hold the disk close to his eye. Take a few pictures at different distances with the focus on the eye of your friend.

You will find out that the background is blurred by the same amount as the disk diameter. Note that the background needs to be far away; many times the distance between the camera and the subject.

You should get the same results whatever the size of the sensor. I have a Panasonic ZS40 (A tiny sensor) with a lens extending to 129 mm and I get a similar blur on that tiny sensor as I get from the same focal length on my Fuji at identical f-numbers. (For the same composition - say a head and shoulder. Of course I will be quite far from my subject with the zs40 at 720 mm eq focal length).

The reason you get more DoF with smaller sensors is you need a wider lens to get the same FoV from a particular spot, and that lens will have more DoF. There is also the effect that you're enlarging the smaller sensor's image more (for the same FoV) so diffraction blur will be enlarged too, hence smaller sensors can't live with as small apertures as larger ones.

Not really. Diffraction blur is only causing issues when it approaches the size of a pixel, which is not an issue at wide apertures. The main issue is aberration, which is normally worse on FF because of the wider image circle.

I think you'll find two pixels width is the point at which diffraction blur starts to become an issue (due to debayering). Some people say three, but I'm happier with two.

Whatever limit you set, it doesn't change one simple fact. Whatever the effect on APSC, the same effect will occur on FF 1 stop higher (given the same pixel count). To equalise DOF, the FF would need to stop down by one stop so the effect cancels out.

Really not convinced by the aberation idea though as for the same pixel count APS lenses have to resolve more lp/mm.

A lens designed specifically for APSC will have a smaller IC so it should also have slightly better resolution when not diffraction limited.

Its a bit like moving a slide projector closer to the screen.

Your premise would be true if using an FF lens on an APSC camera where it is only cropping a section from the centre of the image circle, but in that case the edge resolution would be better on the APSC camera.

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