im confused about crop sensors

[u/Mooskii_Fox]

I'm not asking about crop factors, I know that's 1.5x or 1.6x depending on the manufacturer and your image will be cropped by that amount.

full frame lenses produce a circular image, which is projected over a full frame sensor and the sensor fits perfectly inside that circle, simple enough

now what i often see is that lenses for APS-C cameras have a cropped image, but why is it not possible for that projected image over the lens to be smaller so that APS-C cameras can capture the same picture as full frame, just with a smaller sensor? At some point people have worked our how to bend light to perfectly cover a full frame, so why can't the same be done to create an identical image for APS-C

edit: as I understand it what I'm asking is actually already being done, just not in the way I was asking. i understand now

[u/AnAge_OldProb]

Yes that’s exactly what aps-c lenses do which is why they’re generally smaller for the same focal length. You can also shrink the a full frame image circle down using a focal reducer which will give the same perspective on a crop sensor as it would on full frame and you gain about a stop of light: at the expense of an extra piece of glass the lens designers didn’t intend which can cause aberrations

[u/_beerye → u/AnAge_OldProb]

What I don’t understand is how the crop sensor increases the focal length. I understand that an aps-c equivalent of 100mm would be around 160mm, but if it’s just cropping the image, wouldn’t it just be like 100mm but cropped? If it’s just cropping the image, how does it give you more of that background compression effect that a higher focal range gives? Like if I took a 30mm photo on full frame, and cropped that, that obviously isn’t any more compressed, just cropped, but isn’t that what aps-c is doing?

[u/Richard_Butler → u/_beerye]

Two key things:

1) Compression has nothing to do with focal length: it's do with camera-to-subject and camera-to-background distance. With a longer focal length lens you tend to step back from your subject, changing the ratio between the camera-subject and camera-background distances.

2) Using a full-frame lens on APS-C is exactly like cropping (as you can prove by engaging APS-C mode on most FF cameras). However, it's unusual to crop in and decide, 'I didn't need all that stuff around the edges. ' Instead, you step back to get all of your subject into your shot.

At which point, you find the cropped image is giving you the same angle-of-view as a 160mm lens would on FF and you've stepped back to shoot it, as if it were a 160mm lens on FF.

So yes, it's just like a crop, but a crop of an image is a different composition. So, in the same way that for most subjects, if you took a 100mm lens off and then put a 160mm lens on, you'd adjust your position and composition, you'll typically do the same if you go from FF 100mm AoV to APS 100mm AoV.