600 F5.6 DO

Started 2 months ago | Discussions thread
AKRover Regular Member • Posts: 224
Re: 600 F5.6 DO

H2ODoctor wrote:

I know how to calculate the effective focal length when cropping but am unaware of how to calculate the effective F value. Please share that information with me.

If you calculated a 'focal length multiplier' when calculating equivalent focal length, then equivalent f-number is actual f-number times focal length multiplier.  Another way, the easy way with a 100mm aperture diameter, is that equivalent f-number is equivalent focal length divided by actual aperture diameter.

Noise gets visually multiplied, too, when cropping so I always calculate a noise equivalence in terms of full frame ISO.  For that calculation, you have to square the focal length multiplier and multiply by the actual ISO used.  I find calculating equivalence for my crops to be helpful in understanding the barely acceptable images.  Sometimes it isn't just one thing that went wrong, it is everything.

I was moving a bit too fast yesterday so my numbers were actually wrong on two of the images.  I shoot almost every single shot with the 1.4X and with the aperture wide open so I completely missed the fact that I must have bumped the aperture dial for that grebe as it was shot at f/6.3 which means the crop has diffraction equivalent to f/22, not f/20.  The hawk was shot without the 1.4X (I should have remembered the scramble to remove it) so it is actually only about 1200f/12 equivalent.

 AKRover's gear list:AKRover's gear list
Canon EOS 5D Mark III Canon EOS 1200D Canon EF 50mm F1.8 II Canon EF 100mm f/2.0 USM Canon EF 200mm f/2.8L II USM +8 more
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