Lenses all have one focal length, they never change.
Crop sensors crop
images.
Take a photo with a 18mm on a FF, now crop it way down, that is the cropped photo, no change in focal length or aperture
So look out a window and see outside, now hold a toilet paper tube up to one eye, you are looking at a cropped full frame image.
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You might have come upon a tip somewhere about "calculating crop factor" or how to calculate what focal len your lens will be on a cropped sensor camera.
Those are technically false information and the real answer is more complex.
You do know that images appear differently on different focal lengths? The wide angle view bends lines that are straight and long lenses flatten everything out? Moved from one size sensor to another, this is still true. So if you were told a 50mm looks like 70mm in a crop sensor, try taking a test shot of someone's face with a 70mm on a FF and a 50 on a cropped camera and compare. The one shot with a 50mm, the person's nose will look bigger, the lines going into the distance will converge sooner, the lens is different, you will not see the same image in both, even though you will see about the same items at the same distance.
Last edited by jack002; 12-13-2019 at 10:17 AM.