BG blur / subject isolation with different lenses/crop?

[jdavtz]

Quick question:

In same scenario, fixed subject, fixed background.

Which should give more subject 'isolation' (not sure of the best word - i mean a more blurred background):

135mm f/2 and crop to give same size subject as...
200mm f/2.8 and don't crop

Actual lenses not really so important -- I'm interested in how you work it out.

(I would test it myself but have lent the 200mm lens to my sister).

Thanks!
Jonathan.

 

[Edge100]

Here

On full-frame...

135 f/2, with subject at 30ft - DoF = 1.78 ft
200 f/2.8, with subject at 30ft - DoF = 1.14 ft

Cropping after the exposure wont affect DoF...using a crop sensor camera to get the same subject size WILL (by moving you further away from the subject).

 

[toxic]

cropping does affect DoF since it enlarges the image more.

by cropping the 135mm image to match the 200mm FoV, you're simulating a 1.48x sensor. going to a DoF calculator and comparing 135/2 on 1.5x to 200/2.8 on 35mm, the 200mm has 5mm less DoF at 5m and 17mm less at 10m...which is much less than 1/3 of a stop, which means the difference is negligible.

the 200mm image will likely look better, though, since it isn't enlarged as much.