Quick question on DoF & APS-C v 135 framing

I've done lots of searching and read lots of forums on depth of field comparisons between APS-C & 135 sized sensors. Depending on the terminology used and abstracting viewing distance & circle of confusion (itself confusing to many), is the following correct (assuming a mythical Pentax 16MP 135 frame camera is available):

If I framed a shot with my M135/3.5 at 4m distance with the mythical 16MP 135 frame camera, and then my K-5 (same aperture) from 6m (to get the same framing), I'd end up with the same 'apparent' depth of field?

Finally, if the 135 sized sensor is 24MP instead of 16MP, does this make a difference?

John.

It's best not to get too complex and confused with this, but basically whatever is behind a lens (film, sensor, any size) that does not change the characteristics of the lens. A 50mm lens will always be a 50mm lens and have the DOF of a 50mm lens.

However, on 135 format 50mm is a standard lens. On APS-C it is a telephoto lens. To get the same field of view without moving the camera, we need a 35mm lens (approx) for the APS-C. The smaller the format, the greater the DOF for a given aperture. At the extreme, a Minox uses a 15mm lens as its standard and has the immense DOF that you would expect, even wide open. 10" x 8" plate cameras would have miniscule DOF.

Now if we move the camera things change. The perswpective changes for one thing. But let us suppose we fill the frame with a 50mm macro on our K-5. Now we put on a 100mm macro, but we have to move twice as far away to get all the subject in. However, the magnification remains the same and the DOF with the two lenses will be identical.

Hope that helps rather than confusing it further!

No
As John Points out your 135mm lens is still a 135mm lens and will give the same DoF on either format, however you have to move nearer to the subject on a larger format body to get the same field of view and it's the moving closer that reduces the DoF. Likewise moving further away to get the same framing on a smaller format increases DoF because your further from the subject.

If you took the shots from the same distance and then cropped the larger format image to the same as the smaller format then the DOF would be identical because nothing has changed other than the size of the sensor.