When you use a 35mm lens on an APS-C camera, the effective focal length is determined by the camera's crop factor. The crop factor for most APS-C cameras is typically around 1.5x (for Nikon, Sony, and others) or 1.6x (for Canon).
To calculate the effective focal length:
Effective focal length = focal length X crop factor.
For a 35mm lens on a 1.5x crop factor APS-C camera:
35mm x 1.5 = 52.5mm
For a 35mm lens on a 1.6x crop factor APS-C camera:
35mm x 1.6 = 56mm
So, the effective focal length will be approximately 52.5mm on a 1.5x crop factor camera and 56mm on a 1.6x crop factor camera.
The simple reason why I have written this short post is because I just did a quick search on the Internet for the 35mm equivalent on an APS-C camera (popular on mirrorless cameras) and the answers that I got were confusing as far as I am concerned 😊. They were back to front. It looked chaotic to me. Maybe its me! It is the way you present the information and I hope this way is good.
In short, the answer has to be that if you use a 35mm lens designed for a full frame camera the effective focal length on a cropped sensor camera has to be longer because you're taking out the middle of the frame. It's a bit like a digital zoom. You are cropping the picture digitally in effect.
This means that you are zooming-in which means that the focal length is longer from 35mm on full-frame to to a little over 50 mm on APS-C.
So a 35mm lens on a full frame camera which is a wide-angle lens, becomes a standard lens on a cropped sensor camera such as the APS-C format.
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