Hi Rhys,
Well, now that I've got most of today's other rat-killing done, let me expand on my comment on the word "crop" used in various ways in connection with the implications of a sensor that is smaller than some other sensor.
Sometimes, the term is use in the sense that we can think of the smaller sensor as being a "cut down" (or "cropped") version of the larger sensor. That's a pretty benign metaphor - but not one that really well fits the traditional photographic implications of "cropping".
More often, the metaphor is that the smaller sensor, by virtue of its limited dimensions, "crops" some larger image (in particular, in this "sphere", an image 24 x 36 mm in size) to form a smaller image (22.5 x 15 mm in the case of an EOS 40D) - it crops it to 1/1.6 times its original linear dimensions (and thus the camera is said to have a "crop factor" of 1.6).
But what is that image that is cropped ? Does the lens on the 40D generate a 24 x 36 mm image, of which the 22.5 x 15 mm sensor "crops" out a portion 22.5 x 15 mm in size?
No. If the lens is an EF-S type, it generates a circular image, one large enough to embrace a rectangle 22.5 x 15 mm in size (but not big enough to embrace a rectangle of 36 x 24 mm size). If it is an EF type, it generates a circular image, one large enough to embrace a rectangle 36 x 24 mm in size. The sensor crops its 22.5 x 125 mm image out of one of those circular images.
And on a "full-frame 35-mm camera", said to be "non-crop", its 36 x 24 mm sensor crops a 36 x 24 mm portion out of the large circular image generated by the EF lens!
So, in the metaphor, what is the 36 x 24 mm image that is cropped by the sensor of the 40D to 22.5 x 15 mm? Well, the image captured by the sensor of some full-frame 35-mm camera, some place, some time.
It is as if I said that my 36 x 50 inch coffee is cropped out of a 60 x 76 inch coffee table in Vladimir Putin's office.
So I discourage the use of terms and phrases involving the term "crop" and its derivatives in connection with sensor smaller than 36 x 24 mm.
So, what terms can we use (especially if we are anxious to continue the use of the familiar numerical constants, such as "1.6" for the Canon EOS 40D and its siblings)?
Well, note that, although some decry it, the concept of "full-frame 35-mm equivalent focal length" is perfectly valid. Of course, that is not a focal length of the lens (on any camera). The word "equivalent" warns us of that.
And what is the use we mostly make of the constant 1.6 in connection with, say, an EOS 40D? We, about the only thing we do with it is to multiply the focal length of a lens, one we plan to use on a 40D, by 1.6 to get the "full-frame 35-mm equivalent focal length" of the lens when used on a 40D.
My general, policy is, "when all else fails, call it what it is". Thus, perhaps the best name for the factor 1.6 (which characterizes sensors whose size is about 22.5 x 15 mm) is "full-frame 35-mm equivalent focal length factor. But if we are operating in the sphere where the full-frame 35-mm camera is the "reference" (the sphere inhabited by cameras that are parts of families originally mostly using the full-frame 35-mm format), we can safely condense that to "equivalent focal length factor". Don't shrink from it - it is the name that describes the most prominent use of that constant.
Now, while I have my red pencil out, let me talk about "full frame". A format size we often make refernce to is one properly described as "full-frame 35-mm". What does full frame mean there? It distinguishes this format from the "half-frame 35-mm" format (24 x 18 mm), used on some cameras using 35 mm (type 135) film.
Now, what's a good "shorter name" for the term "full-frame 35-mm". Well, very few 35-mm cameras used the "half frame" format, and it is by definition an "alternative" format (the "half" is the clue). So if we speak of just the "35-mm" format, it is reasonable that we mean 36 x 24 mm. So that would be a good short name.
But unfortunately, many have gotten it the habit of using the short form "full frame" for the 36 x 24 mm format. That's greatly ambiguous. For my 8x10 cameras,. I have backs that operate in both the "full frame" and "half frame" formats (and full frame is 8 x 10 inches!). And for my APS film camera, the "full frame" output is 30.2 x 16.7 mm (also called "APS-H").
Another problem with the term "full frame" is that it sounds like the ne plus ultra of digital formats: if we have that, there is nothing bigger we could have, or even want . If we suddenly ended up with a camera with a 40 x 30 mm format, it is larger than "full frame". Wow, isn't that special.
So I discourage the term "full frame" to mean "36 x 24 mm format". Instead, I would suggest:
"36 x 24 mm" (!)
"Full-frame 35-mm"
"35 mm"
I of course use "full frame" to mean "not cropped " (and I don't mean in the sense of "cropped" I decry above). That is, if I post an image that is the entire image captured by my camera, when giving the technical information for the shot, I will say it is "full frame".
Again, the best practice in nomenclature is "call it what it is".
Best regards,
Doug