Math to determine cropping given an image and a target focal length

E

eco_bach

Well-known member
Hi
Not sure If I am making sense, but for an upcoming shoot I need to rent the equivalent of approximately 20mm focal length on my 7D.

I have a test image shot at 16mm. (using Tokina 11-16)
I can rent the CANON EF 14MM F2.8L II USM

which I believe will give me 22.4 mm equivalent.

So, how much do I crop my test image to see if this will work?

(fell asleep during most high school math classes;)
 
eco_bach said:
Hi
Not sure If I am making sense, but for an upcoming shoot I need to rent the equivalent of approximately 20mm focal length on my 7D.

I have a test image shot at 16mm. (using Tokina 11-16)
I can rent the CANON EF 14MM F2.8L II USM

which I believe will give me 22.4 mm equivalent.

So, how much do I crop my test image to see if this will work?

(fell asleep during most high school math classes;)
Click to expand...

Is there a way to post an excel spreadsheet on DVXuser? If so, I have a focal length vs horizontal angle of view vs format spreadsheet for most common formats that I am happy to share. Works both ways, AOV for given FL for each format and FL for given AOV for each format.
 
I've got a FoV/DoF calculator here: http://www.similaar.com/foto/dof/dof.php

but mind you: when a manufacturer says "11-16mm", the lens doesn't actually go from 11.00mm to 16.00mm, usually it will be something like 11.36mm to 15.62mm (and when they say 180mm, it can be something like 170-190mm depending on how far you focus)
I've tested lots of 50mm lenses, a few 20mm, a couple of 135mm and 180mm, and they're never the same; in some cases, the difference can be quite noticeable
 
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