DoF - FF vs 1.6x APSC - Explanation

ghibbett

Active Member
Hi All, :thumbsup:

Whilst having a look through some of Canons lens literature, I have come across the following information regarding Depth of Field differences between full frame and 1.6x APSC crop factor. Thought it would be good to share*

Depth of field and perspective
The EF 85mm f/1.8 USM lens is effectively a 136mm f/1.8 lens in terms of angle of view when attached to the EOS 30D. Looking at these specs, it would seem possible to take a photograph with a shallower depth of field than if the EF 135mm f/2L USM were used with 35mm film, but this is not the case. Since the focal length does not actually change, the depth of field in terms of the sensors and the blurred image in the out-of-focus area remain the same for the EF 85mm f/1.8 USM. And if the image is enlarged to the size of an A3 print (approx. 11 x 14 inches), the amount of enlargement required by the EOS 30D is greater than that for 35mm film size, because its screen size is smaller.
Nevertheless the depth of field on the print will be shallower for the latter combination, creating greater blur in the background. This means that if you want to achieve more blur in the background using the EOS 30D, you will have to shoot at a larger aperture.
Furthermore, perspective is related to angle of view, so even if the focal length is different for each particular lens, if the resulting angle is the same due to the difference in image size, as in the photographs shown above, the perspective will remain unchanged, too.

* although it is a bit of a headache inducer !! :rolleyes: :lesson: :eek:

Extract taken from here
 

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