DOF on a 70-200 2.8

[fotog]

I use my 70-200 2.8 a lot. I do a great deal of family portraits with it. On the beach with a few different backgrounds including a large Victorian hotel. Been shooting an eos 3 for quite awhile with the 70/200 2.8. Now with the 10D besides having to back up 60% more than usual I don't seem to get the nice out of focus background as I did with the eos 3 combo, shooting at 2.8 The closer you get to something the better you can blow out the background. Now if I'm right my backgrounds are now 60% sharper having to back up 60%. Do I need a 200 f2 to get the same kind of shots?
Lost again. What do ya think???
thanks Bill

[hmhm]

Assuming the same composition between the two cameras, you'd expect the 10D to give depth of field that's a little deeper than the film camera, very roughly about 1 stops worth of aperture (e.g. f/2.8 on the EOS 3 should be close to f/2 on the 10D).

Achieving the same composition with the 10D either requires backing up or decreasing the focal length, both of which increase depth of field (the 10D requires greater enlargement, which decreases depth of field, but not enough to offset the effects of distance or focal length).
-harry

[DonCoon]

fotog wrote:
I use my 70-200 2.8 a lot. I do a great deal of family portraits with it. On the beach with a few different backgrounds including a large Victorian hotel. Been shooting an eos 3 for quite awhile with the 70/200 2.8. Now with the 10D besides having to back up 60% more than usual I don't seem to get the nice out of focus background as I did with the eos 3 combo, shooting at 2.8 The closer you get to something the better you can blow out the background. Now if I'm right my backgrounds are now 60% sharper having to back up 60%. Do I need a 200 f2 to get the same kind of shots?
Lost again. What do ya think???
thanks Bill

If I read your question correctly, you're trying to get the same DOF and image size/frame as you did with your 2.8 using film. If I didn't... : )

First, 125mm on the D10 equals 200mm on the EOS3. Given those focal lengths your depth of field at f2.8 and other options on the 10D will be approximately:

Dist EOS3 10D 10D@f/2 10D@f1.4
25' .6' .5' .9 .6
50' 2.7' 2' 3.7 2.6
100' 10'8' 8.1' 14.9 10.5

Arrrrg..... There go my spaces :(

Hope this helps

DOF calculator (Use .019 CoC for the 10D)
http://www.nikonians.org/html/resources/guides/dof/hyperfocal2.html