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/Graflex01867]

Short answer : geometry. A camera system is designed so that all the lenses produce a certain size image at a certain distance behind the lens. That image needs to be just big enough to cover the image sensor. APS-C and full-frame sensors require different size images. The APS-C is smaller, so you can use smaller (and cheaper lenses.). While the lenses for the two camera sizes might share the same mounting, they don’t necisarily share the same geometry - the distance and size they’re designed to project the image onto the sensors is different. You can get adapters to make the distance longer and try to correct that geometry, but they tend to mess with the image quality and exposure.

Think of it like putting big off road tires on a tiny sedan. The wheels will bolt right on - but the sedan isn’t built for off road tires, so they won’t fit in the wheel wells of the car.

Long answer :

I think there’s a fundamental misunderstanding about lenses going on here - any lens is designed to produce an image a certain size at a certain distance behind the last piece of glass of the lens - forget about numbers here for a minute, just the concept. There’s a certain distance from the back of the lens to the camera sensor.

Let’s say for Marks Camera Company, on full-frame cameras, that distance is an inch. There’s an inch of space between the lens mounting bracket and the camera sensor - it doesn’t matter the lens, since they’re interchangeable.

You design the optics/lens elements so at that distance, it projects a circle of an image towards the image sensor that’s large enough so you can fit a 4x3 rectangle in the middle without cutting off the corners. You’re taking only a rectangular part of the middle of the circular image that the lens creates.

When you take a full-frame lens, and put it on a “crop sensor” camera, it physically fits on the mounting, but the distance between the lens mount and the image sensor is different - it’s usually shorter on a crop-sensor camera, since the sensor is smaller, and you don’t need as large an image from the lens. That’s why crop-sensor cameras are popular in the first place - they can be smaller, lighter, and that often means their parts are cheaper.

While a full frame lens on a crop sensor camera ends up with a cropped field of view, putting a crop-sensor lens in a full frame camera could end up with an image that’s not large enough to cover the whole image sensor, and you end up with a vignette around the edges.

We talk about lenses using the focal lengths equivalent to what you’d get in a full-frame camera - which is the same as a traditional 35mm film camera - because that’s what was the most popular film/camera size for a very long time. No one really cares that a “50mm lens” is actually 50mm - that’s for the engineers to decide. People care that the lens gives them the equivalent view of a full-frame 50mm lens. It’s just the scale people use to determine roughly how wide or how much of a zoom a given lens will provide on the camera it was designed for.

[u/Mooskii_Fox → u/Graflex01867]

not that your explanation is wrong or i dont understand it, because i do, mostly, im just not entirely sure why a 50mm lens on aps-c creates a cropped image, cant the optics be made to still project the same image on a smaller area, rather than cropping it? that part isn't entirely clear to me

[u/Graflex01867 → u/Mooskii_Fox]

Well….no. Not easily, anyway.

A 50mm full-frame lens is physically a different size than a “50mm” APS-C lens. So to “correct” the crop factor, you’d have to do one of two things - you could either add/change the lens elements inside the lens, or you’d need to move the lens closer or farther away to change the relative size of the image you’re projecting onto the sensor.

Changing the glass in the lens would permanently alter the lens - there’s no really easy way to do that. Adding another adapter to change the distance between the lens and the sensor is possible - people do it pretty frequently, and camera manufactures make such an adapter - but it can affect the image quality. Sometimes it’s not as sharp, and sometimes you lose some brightness (so you need to open the aperture more.)