Crop Factors

Started Sep 20, 2008 | Discussions thread
Sean Nelson
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Re: Crop Factors
In reply to lacix, Sep 20, 2008

You can’t get more DOF while have the same perspective! More
DOF=wider angle of view.

That's true FOR THE SAME CAMERA.

But if we're comparing cameras with different sensor sizes, and using lenses which give the same field of view (and therefore the same perspective), then at the same aperture the depth of field is shallower on the camera with the larger sensor.

This is because depth of field is determined by the absolute aperture of the lens (ie, a 100mm f/4 lens has an absolute aperture of 100/4 = 25mm). On a camera with a larger sensor, you have to scale the optics of the lens up in order to retain the same field of view, and therefore the absolute aperture is larger.

For example, the DOF calculator at http://www.dofmaster.com/dofjs.html shows the following for a picture taken with a field of view equivalent to a 35mm lens on a 35mm film camera at f/4 with a subject distance of 3 feet:

Canon A650 at 7.4mm, DOF = 3 ft to 14.9ft
Pentax K100Ds at 23.2mm, DOF = 4.09 to 6.44 ft

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