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Understanding Focal length and Ratios


New Member
If I have a lens of Focal length 4.12mm (mobile phone) with a F2.2 lens (I assume this is the largest size) does this mean the largest aperture size in mm is:

4.12 / 2.2 = 1.8728mm

If so then if I look at the Exif data for a picture and it displays the following values:

Focal length : 4.3
F number : 2.1

Does this mean the aperture size is now:

4.3 / 2.1 = 2.04mm

Which is a figure larger than 1.87828mm then...

How do I explain this! Is there something wrong in my figures or understanding?

Roger S

Crazy Canuck
Hello and welcome to the forum.

Photographically, you are trying to split hairs with the calculations. Due to the constrictions of the camera software and general lens specs, those numbers are being rounded out in your EXIF data. The slight differences you are talking about in your post aren't worth worrying about, or the software designers would have included more decimal places into their algorithms.


Always on
I agree with Roger but can add one extra point that can account for differences. The aperture your calculating is as it appears at the lenses entrance pupil, not at the iris itself. If the lens elements are moving relative to the iris this can change how the aperture looks at the entrance pupil...
I woun't expect this to have much effect on a camera phone, but it can on big zooms.