Depth of field of extension tube vs dedicated macro
Re: Cropping and enlarging alters f stop...
1
Joseph S Wisniewski wrote:
JamesMorgan wrote:
However, my understanding of post-processing cropping is that whilst this does reduce DoF, it does so in a linear manner rather than according to a square rule if increasing lens focal length. So, for example, if I increase focal length from 150mm to 300mm, DoF reduces to a quarter, however, if I crop the 150mm image to achieve the same field of view DoF only reduces by around a half. This can be useful to extract the maximum possible DoF if you don't mind losing a few pixels.
What you're seeing is the crop factor affecting effective f stop along with effective focal length. You gain more DOF, but you burn resolution. Open up to get the resolution back, and you've lost the DOF advantage.
Agreed. It does depend on what you final use is for the image as to whether this will be an issue. If I crop a 20MP image to change FoV from 150mm to 300mm I will end up with a 5MP image. This would be fine for viewing on an HD monitor or printing up to 8-10 inches. If I want greater resolution than that, then cropping is not a viable option.
If should also be remembered that DoF is not fixed and depends on how the viewer views the final image. Most calculators assume viewing the image as a 10x8 photograph viewed from 10 inches. If I need to view a larger image than that, then the DoF will be smaller than indicated by the calculator. This is all goverened by the Circle of Confusion.
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