Real focal length and online focal length calculators

Started Nov 5, 2012 | Questions thread
Andy Westlake
Senior MemberPosts: 2,928Gear list
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Couple of points
In reply to Detail Man, Nov 11, 2012

You're still using the wrong value for magnification, as you're using the nominal rather than effective focal length to calculate it. Although the most obvious thing to do, of course, is estimate it directly using (sensor width)/(measured object width), which gives a value of about 1/28.

Even if you use this number to calculate a pupillary magnification, it's only of academic interest, as it's only a constant for unit-focusing primes. With internal focus zooms like on the SX50HS, it varies continuously with both focal length and focus distance (as focusing changes the relationship between the lens groups).

I can't help but think you're making a linguistic error too. 'Focus breathing' is purely a descriptive term for the change in angle of view with focus distance. It makes no attempt to define the underlying optical causes.

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Andy Westlake
dpreview.com

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