DOF and Cropping take 2

Started 6 months ago | Discussions thread
Ian Stuart Forsyth
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Re: DOF and Cropping take 2
In reply to awaldram, 6 months ago

This is a very good reading material if you what to know how surface areas works with regard to Fstop and total light

Copy and past from here

http://www.dpreview.com/forums/post/52496321

The two basic parts are really:

  1. f/# determines image plane exposure
  2. exposure is "amount of light energy per area unit"

-So you get the light energy amount that your system "gathered" by:

(scene average luminance) / (f-stop)^2 * (sensor area) * (exposure time)

And from that you can get that the system "light gathering ability" when keeping scene average luminance and exposure time constant is:

Light gathering ability = (sensor area) / (f-stop)^2

Since (sensor area) is the square function of crop ratio this can be further simplified by doing a square root on both, and then you get:

Light gathering ability = (crop ratio) / (f-stop)

In the end that means that to collect as many photons per second from a certain scene, you need to change f/# with the same factor as you change the crop ratio.

If you feel the need to really argue this why not take it up in the Photographic Science and Technology department as I am sure they would love to teach you some valuable insight to sensor size ,cropping and total light?

Just to highlight Light gathering ability =(crop ratio)/(fstop)

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