Good exemples of the DoF difference between FF and crop.

This is the reason I actually prefer to use my 1.6 crop body in most situations. I am usually trying to get as much DOF as possible, as the long focal lengths I usually use already limit DOF quite a bit.

I don't like close-up portraits of animals such as deer when the eye is sharp and in focus, but the nose and ears are very soft and OOF. I really want the whole face and head to be sharp, and often the only way I can accomplish that, at a given distance, is by using my crop body 50D instead of my full frame 5D.

Crop bodies are indeed great for wildlife and other situation when a larger DoF is needed. I also love my crop body for day street photography. It's light and unobstrusive and permit me to reach more far at 85mm.

TweakMDS wrote in post #14938457
I'm expecting SkipD to come in here and link his sticky any moment, but in his absence:

What you posted doesn't really hold up in comparing DoF differences between crop and full frame.

For any comparison to be "fair", the subject distance should be a constant. As soon as you move in closer, your foreground subject might be framed the the same size, but your background will become wider, so you will have a different composition.
If you move further away with a long lens, the DoF will appear to become more narrow, but that's an effect of the smaller relative amount of your background in the frame.

Everyone in doubt of the effects of this should really try to take some shots of a subject with a clear background at various distances and focal lengths and try to frame the foreground the same size.

Oh, I know all that. What I'm saying is that if the camera position remains the same, the focal length remains the same, the aperture remains the same and the only change is from cropper to FF, then why would the DoF change? How does the circle of confusion know what size sensor it is shining onto?

Tiberius47 wrote in post #14944965
Oh, I know all that. What I'm saying is that if the camera position remains the same, the focal length remains the same, the aperture remains the same and the only change is from cropper to FF, then why would the DoF change? How does the circle of confusion know what size sensor it is shining onto?

The DOF is dependent upon the degree of enlargement of the sensor image to make the final print. The larger area surrounding the subject in the FF frame means about 8x enlargement for 8x10 print, while the APS-C image needs about 13x enlargement. In such a case, the FF actually has more DOF than APS-C image, but the subject itself is 1.6x smaller in the 8x10 FF print, too.

Using 100mm f/8 lens at 100' on both bodies,

• FF captures area of 24'x36' in the frame, and 8x10 has DOF zone of 193'
  
• APS-C captures area of 15'x22.4', and 8x10 has DOF zone of 89'
  

Tiberius47 wrote in post #14944965
Oh, I know all that. What I'm saying is that if the camera position remains the same, the focal length remains the same, the aperture remains the same and the only change is from cropper to FF, then why would the DoF change? How does the circle of confusion know what size sensor it is shining onto?

Enlargement factor to get back to same display size.

Tiberius47 wrote in post #14944965
Oh, I know all that. What I'm saying is that if the camera position remains the same, the focal length remains the same, the aperture remains the same and the only change is from cropper to FF, then why would the DoF change? How does the circle of confusion know what size sensor it is shining onto?

One of the things I really like about these DOF discussions is that, under the conditions you state above (same distance to subject, same focal length, same aperture) the crop camera will have LESS DOF than the full frame camera (Wilt's calculations above demonstrate this). This one's a good betting proposition when discussing DOF trivia - most people will say that the full frame has shallower DOF under all conditions compared to a crop.

With regard to "circle of confusion," that's merely an assumed parameter in doing depth of field calculations, related (as the posts above imply) to the amount of enlargement to a "standard" sized display (usually assumed to be 8X10 inches), which is how sensor size is taken into account in calculating DOF. There is no "circle of confusion" as such in the actual optics and image. The circle of confusion is an assumption about how out-of-focus a point in the image can be and still appear to be in acceptable focus when viewing an 8X10 display of the image at an assumed standard viewing distance with assumed standard visual acuity.

stsva wrote in post #14945748
With regard to "circle of confusion," that's merely an assumed parameter in doing depth of field calculations, related (as the posts above imply) to the amount of enlargement to a "standard" sized display (usually assumed to be 8X10 inches), which is how sensor size is taken into account in calculating DOF. There is no "circle of confusion" as such in the actual optics and image. The circle of confusion is an assumption about how out-of-focus a point in the image can be and still appear to be in acceptable focus when viewing an 8X10 display of the image at an assumed standard viewing distance with assumed standard visual acuity.

The true "circle of confusion" is seen when a group of folks, arranged in an approximate circle, start discussing depth of field concepts.

Joking, of course....

SkipD wrote in post #14945961
The true "circle of confusion" is seen when a group of folks, arranged in an approximate circle, start discussing depth of field concepts.

Joking, of course....

Quite true, joking or not.

I know the DOF difference of APS-C vs. FF (in this case, at f/2.8 (0.8' for APS-C vs. 0.45' for FF), but I didn't inherently know the blur in the far field differences. The near background object (Colloquial Spanish case)was 3' behind the subject vase, and the shooting distance was about 7', and the far field vase and blinds were about 20' back from the subject vase. The sequence was f/2.8, f/4, f/5.6 and f/8 for both cameras

I apologize for the exposure errors in the 40D images causing the b/g to be exposed differently, but at least I got it right when rushing thru the 5D images! I used 1/200 f/2.8 for the first shot, then 1/100 f/4 for the second shot, the foreground subject had ceiling bounce ETTL flash, and I can't explain why the b/g in the second shot was so underexposed. The third shot was 1/50 f/5.6, and the fourth shot was 1/25 f/8...the background should have been identical! I believe that ETTL screwed up with full power flashes (not communicating correctly thru the Canon off-camera cord!) on shots 2 and 3, leading to needing to reduce exposure in Lightroom.

40D