Landscape & F stops

[ap4a]

Originally Posted by nikonian

IT is only the act of enlarging the small size of the cropped sensor by your PC that makes the C of C bigger.

If you compose an image in an APS-C, and then recreate the same composition with an APS camera the second image will exhibit a shallower DOF than the former, irrespective of what you do on a computer, because with an APS camera you will need to be closer to the subject to get the same composition.

[chris-p]

Originally Posted by ap4a

If you compose an image in an APS-C, and then recreate the same composition with an APS camera the second image will exhibit a shallower DOF than the former, irrespective of what you do on a computer, because with an APS camera you will need to be closer to the subject to get the same composition.

Exactly. If you maintain a constant field of view by maintaining the same 35mm equivalent focal length then the DoF is different for different sensor/film sizes.

Technically the DoF produced by any given lens doesn't change when you mount them on cameras with different sized sensors but to get the same image from a FF or APS-C camera requires a different field of view and so the DoF changes.

The other way to look at it is to image the physical aperture sizes using the true focal length.
If you shoot at 50mm FF at f/2.8 your aperture is 17.86mm in diameter
If you shoot at a 35mm equivalent of 50mm on a 4/3 camera (real focal length 25mm) your aperture is 8.93mm in diameter.
To get the same FoV using a smaller sensor requires a shorter true focal length which means your aperture is physically smaller, thereby increasing depth of field.

[flake]

Unfortunatley you fell into the trap Chris, the size of the aperture is not relevant to the Dof as it's a ratio. Although there is a commonly held belief that longer focal lengths give shallower depth of field, I'm afraid it's not correct. All lenses regardless of focal length will give the same depth of field provided that everything is equal, that includes aperture and very importantly magnification. The real thing which is changing in your example is magnification, the pysical size of the aperture does not make a contribution as it's a ratio which if kept the same is a constant.

There's an article about this here

[chris-p]

Thats sort of what I said. The lens doesn't change the DoF when it's placed on different bodies.

The point I'm trying to get across is that the DoF DOES change if you re-create the same field of view on different size sensors. As you said Flake, it's about magnification. Longer focal lengths also don't change DoF but they do alter perspective which is what (I think) gives the impression of a smaller DoF.

I know the aperture is a ratio so it doesn't really affect it but I was trying to illustrate my point (which I seem to have failed to do! ) As I said in my last post it's the field of view that changes and that creates the apparent change in DoF.