Doc Nickel wrote:I was reading a reference briefly earlier today, which suggested that the 350D's "kit" lens (18-55mm) was the "equivalent" of a 29-88mm lens.
Now, I understand the "crop factor", but I was under the impression that the EF-S lenses, being specifically made for the 1.6-crop bodies, would be designated by the actual focal length. IE, instead assuming the lens will 'act' like a 29mm lens, it will actually be what it says, an 18mm lens.
In other words, if I use an EF-S lens, it provides it's actual stated focal length. And that I only need to consider the "crop factor" multiplication if I'm using a normal EF series lens on the crop bodies.
.... Um, right?
Doc.
The "crop factor" thing is extremely misunderstood, probably by the majority of folks who buy APS-C cameras (like your 350D).
The focal length of a lens does not change based on what format the camera body that the lens is mounted to uses. Only the field of view changes when you move a lens (EF, for example) from a 35mm body to a 350D. The focal length is still the same.
The "crop factor" is referencing the field of view provided by a particular focal length lens mounted to a 35mm film camera (or a "full-frame" digital camera), and confuses the heck out of most folks who have just purchased a digital SLR as their first "real" camera and have never used a 35mm camera.
There is only one valid reason to consider the "crop factor". That is when someone takes a photo with a 100mm lens on a 35mm camera and you want to calculate what focal length to use to duplicate the shot (from the same position used by the 35mm shooter). Then you would divide the 100mm by 1.6 and choose a 62.5mm lens (if there were one available - or use a zoom) to duplicate the shot. Another example - 80mm lenses are the "standard" recommendation for portrait lenses for 35mm cameras. What focal length duplicates that for your 350D? Divide 80mm by 1.6 and you get 50mm.
What you need to understand about your 350D is that 28mm to 31mm is the approximate focal length of a "normal" lens. Any lens with a shorter focal length is considered wide-angle, and any lens with a longer focal length is considered a telephoto. To understand "how much" wide angle or telephoto, just use the 31mm figure as a reference. For example, a 100mm lens on your 350D is about a 3X telephoto and a 15mm lens on your 350D is about a 0.5X wide angle.
The "normal" focal length for a 35mm camera is 50mm, and you do calculations for how much wide-angle or telephoto based on the 50mm "normal" as a reference when using 35mm cameras (or "full-frame" digitals).
EF-S lenses provide the same ANGLE of view as lenses of the same focal lengths used on 35mm cameras. That is to say, that for a subject that would show up completely in both cameras, the angle defined by the subject's width with the camera as the reference point is the same. EF lenses provide a larger image circle than EF-S lenses. That is because they need to fill a 35mm frame with the image. EF-S lenses are designed to fill only an APS-C sensor with the image, but the ANGLE of view of a 20mm EF-S lens is identical to the angle of view of a 20mm EF lens. The total FIELD of view is different depending on which format body the EF lens is connected to (same would be true for the EF-S, but you can't connect them to a 35mm camera) because the 35mm camera can record more of the image from the lens. An EF-S lens, if you could mount it to a 35mm camera, would produce an image with a dark circle surrounding what image was on the 35mm film, but the size of the subject material that did show up (on the film) would be the same as the size of the image projected by the same lens on the sensor of your 350D.
. Doc's 
