• Welcome to Photography Forum. Our photography community!

    Photography-forum is dedicated to those who have passion, desire and love of photography and want to improve their photographic technique. It doesn't matter what you photograph, landscapes, weddings, portraits or your photographic experience, it's about learning and loving what we do. Photography!

    If you want learn and expand your photography skills then there is one place to do it Photography Forum !!!

    You are viewing photography-forum as a guest which gives you limited access to view most forums and enjoy other features. By joining our free community you will be able to post photographs for critique, join in the monthly photography competitions, respond to polls, upload content and enjoy many other special features. Registration is fast, simple and absolutely free so please join Photography Forum.

    If you have any problems please contact us.

    The Photography-Forum Team
  • PLEASE SEE RULES BEFORE POSTING LINKS
    Click here to see Forum Rules

Understanding Focal length and Ratios

Walter ZAMBOTTI

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
Administrator
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.
 

Petrochemist

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.
 
Top