# Calculating image magnification (not sensor crop!) for various focal lengths.

• 12-02-2011, 06:03 PM
gong123321
Calculating image magnification (not sensor crop!) for various focal lengths.
Planning a shoot. Want to shoot a telephoto shot at a specific location. Going to take test photos of location with an 80mm, and then I want to crop into the image in photoshop such that I would get the view of a 135mm, 200mm, 300mm, etc. so that I can determine exactly what lens I need to rent. Shooting primes, zooms not an option. Everytime I search I get spammed with information about the sensor crop factor but I'm looking for the formula that determines how focal length effects image magnification. Is it just a linear transformation? 200mm lens gives 2x magnification of 100mm?..

Thanks!
• 12-02-2011, 06:15 PM
Chan Tran
Yes it is a simple linear relationship. Say you take the test shot with a 80mm lens and assumed that this lens has a wide enough angle of view to include everything you want and more. And then when you have the shot everything in the shot will be bigger in length proportion to the longer focal length you are going to use.
• 12-02-2011, 06:16 PM
Diapositivo
Image magnification: yes, 200mm lens gives you 2x magnification on film compared to 100mm lens. Somebody will point out: when lenses are focused at infinite.

Depth of field is not replicable this way. IIRC a cropped 100mm has more DoF compared to a 200mm, contrary to common sense and popular belief. You'll find interesting threads here on APUG on this matter.
• 12-03-2011, 03:29 AM
gong123321
Great, thanks for clarifying guys! Good to know about the dof too.