kendon wrote in post #11645971
just don't let that stop you from buying the right tool for the job! and in general: is it possible to explain in easy words how the smaller or larger image circle is achieved in terms of lens construction?
I agree! I shoot with both FF and crop sensor (1.6X) cameras. While most of my lenses are EF or full frame compatible... I do have and use one "crop only" lens, a Tokina 12-24/4. There just aren't any EF lenses that will give you a particularly wide view affordably. And I needed a wide lens to use on crop cameras.
As a side note, because this is a third party lens it doesn't use Canon's EF-S bayonet (which prevents mounting a lens onto Canon FF cameras at all). It just uses the standard EF mount, so it will fit onto a FF camera and some experimentation I've done with it shows it will work reasonably well on FF to about 18 or 19mm, before it starts to vignette very noticeably.
Don't worry too much about what you might shoot with in the future. Worry about what you need to have in your camera bag to get the shots now. If you get a good EF-S lens, then later switch to full frame, you can always sell the lens and quite possibly recoup most of what you put into it. In the meantime, you'll have the lens you need to do the job.
In the simplest terms, yes, an EF-S lens is different in that it throws a smaller image circle... enough for a crop sensor lens, but smaller than a FF lens would produce. (In fact, some FF lenses are designed to produce a larger than usual image circle, such as the Tilt-Shift series.... They need a larger circle to allow for the movements of the lens.)
Mostly it's at the wide angle end of the range that a "crop only" lens is necessary and useful. That's just because FF lens designs didn't need to be so wide. The 14mm and 16mm lenses designed for FF just aren't all that wide on crop. And they aren't cheap, either!
So a lens designed for "crop only" is likely to be more affordable. It's also possible to design a smaller and lighter lens, since it doesn't need to produce as large an image circle.
And, digital is different than film. The individual photo sensitive sites on the sensor sit inside of tiny "wells", with a lens over them. Because of this you can get light falloff at the corners and periphery of the sensor, due to the more acute angle at which the light travels to reach the sensor. In a sense, it gets "clipped" by the edges of the sensor wells. Modern lenses are designed to help compensate for this, try to align the beam of light in a more sensor friendly way. A lens designed for film didn't need to take this into consideration. This problem is more likely to be evident in wider lenses (shorter focal lengths).
There are various schemes to correct for this, besides just redesigning the lenses. Many current Canon have a peripheral image correction feature, which simply amps up the exposure for specific lenses on the camera. You can also do similar corrections in many post production softwares.
Another consideration is that digital sensors are more reflective than film. Light reflected from the sensor can bounce back off the rear element of the lens and cause some loss of contrast or even ghosting in the image. So modern lenses designed to be used with digital sensors often have improved coatings on the rear element, to reduce reflections.
Keep in mind that 50mm is still 50mm. Focal lengths don't change. But when the sensor (or film) size changes, the way a focal lenght performs changes with it. A 50mm on a point and shoot with a tiny chip might act like a super telephoto. On 1.6X crop camera it's a short telephoto, a nice portrait lens. On so-called "Full Frame" (which is based upon the 24x36mm image size of 35mm film, which once upon a time was called "miniature" format"), that same 50mm performs as a "normal" lens, approx the angle of view of the human eye. If you put the same 50mm on a medium format digital or film camera, with a larger image area such as 45x60mm, it becomes a wide angle.
So a 50mm is a 50mm, no matter what. It would have to be designed differently to perform properly on different formats, but it's still the same focal length.
Likewise, an f-stop is an f-stop. f2.8 will be f2.8 whether it's on a FF or crop sensor camera. It allows the same amount of light into the camera. However, it does perform a little differently, giving somewhat different effective Depth of Field. The reason for this is that angle of view has narrowed and you tend to stand a different distance from the subject to take the same shot, not tbecause DOF actually changes (similar to focal length not actually changing, just performing differently). That's why I refer to it as a different effective DOF.I'd estimate the difference between a FF camera and a 1.6X cropper is approx. one stop. f2.8 on a cropper will give results about the same as f4 on a FF camera. This can work for ya and against ya, depending upon what you are trying to accomplish. If you want a lot of DOF, the smaller format works in your favor. If you are trying to blur down a busy background, you will want to use a larger f-stop with a cropper.
Again, this is nothing new. We saw the same effects with f-stops on different size film formats, too. We might use f2.8 on 35mm film and f5.6 to similar effect on large format film.
The viewfinders of the cameras are effected by the format, too. The crop sensor camera's viewfinder is smaller and can be a little more difficult to work with in some respects. For example, maybe it's just me, but I think cropper VFs tend to exaggerate DOF, not give you a true representation of it. I find when using long lenses at large apertures it's easy to get too shallow DOF. To me it seems a little easier to keep this from happening when shooting with FF.
Another way of looking at all this, you might say that you have more control over DOF with a FF camera than you do with a cropper. Of course, you can get too shallow DOF on the FF, too. I just find it easier to predict my results with a FF camera. But maybe in part that's because I shot film for several decades before starting to use digital.
You do need to be aware of diffraction, though. With crop cameras this sets in at larger apertures than on FF. With a 1.6X cropper you start to see diffraction occur at f8 and it will be more and more evident at smaller apertures (f11, f16, yada, yada). It varies with the density of the camera's sensor... A higher resolution camera will see diffraction start to kick in at larger apertures than a lower resolution camera will. But, I find in the real world if you stick with f8 or larger apertures you likely won't be able to see it.
Just be aware that on your cropper, going to stops smaller than f8 you will likely start to see some softening and loss of detail, increasing as the stops get smaller and smaller. On a FF DSLR, the optimum might be f11 or f16.
