I do not see the point in producing a prime lens for EF-S mount, not unless it served a special niche, like a macro lens. An EF mount prime can be used on all DSLRs. An EF-S mount prime cannot.
As for the EF-S 24mm lens, and EF-S mount lens is smaller, which makes is less expensive to produce. Quality wide angle lenses, with wide apertures, can be costly. Canon produced a wide angle lens for their consumers using entry level DSLRs.
Many full frame lenses have a better build quality than an APS-C lens. As a general rule, image quality tends to drop off away from the center of the image circle produced by a lens. High quality full frame lenses include lens elements to help correct this natural phenomenom. A crop sensor camera body, will only use the best part of the image produced by the lens.
If you plan to shoot wide open aperture much of the time, then I suggest that you consider an "L" Series lens. Otherwise, many lenses produce their best images between f/6.3 and f/11.
Once upon a time, prime lenses would have remarkable image quality, which was superior to a zoom. In recent years, the image quality of zoom lenses has increased, and challenge the image quality of primes under all but the most demanding of shooting scenarios, like shooting at f/1.2.
The above photo was shot with an EF 16-35mm f/2.8 II USM zoom lens of a 6D. The focal length was 35mm. The shot was handheld. The shutter speed was 1/800. The aperture was f/4. The lens has no IS.
Thank you for the response. In a nut shell you are saying I am better off buying an ef lens like the one you used with a dedicated aperture than a prime lense? Also you are saying that it really isn't that big of a deal about the 1.6 difference you have to take in when using ef lenses?
The point that's being circled here is that the advantage you get with an APS-C camera at the telephoto end, you give back at the wide end. A lens has to have a very short focal length to be a wide-angle on an APS-C camera, and rectilinear lenses of very short focal length are difficult to design and expensive to manufacture.
Settling the why there are no UWA primes, let's explore something I don't believe you understand. First and foremost, focal length is what it is. It makes no difference what camera it is on. What you are really asking is about AOV, angle of view.
This is what you should be looking for.
If you put an ef 50mm prime lens on your Rebel and there was also an ef-s 50mm lens, they would be exactly the same. You could not tell the difference.
Canon does make a couple WA prime lenses like the EF 14mm f/2.8L II USM. Why they don't make an UWA prime is something you have to ask Canon. Perhaps it is because there is little to no reason to have primes when the zooms are so terrific these days.
If Canon does not make what you seek, you may have to look else where. Maybe the Sigma 4mm F1.8 DG HSM is appealing. It is a fantastic lens. Rokinon has a full line up of WA to UWA prime lenses. I think they go clear down to a 7mm prime lens.
Personally I have and just love a EF 8-15mm f/4L Fisheye USM. Yeah it isn't a prime but it is a terrific and fun lens. Another lens I don't currently own but have used a bit is the EF 11-24mm f/4L USM. Very good. Of course the UWA of choice for any Rebel owner is the EF-S 10-18mm f/4.5-5.6 IS STM. You can't beat the price to performance value with it.
So there may not be much in the prime choice but there are certainly great zooms out there just waiting for you.
BTW if you want that 50mm prime, AOV, look on your Rebel look at the EF 28mm f/2.8 IS USM or EF 35mm f/2 IS USM. The 35mil is outstanding.
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