In digital language 50mm film lens and equals 80mm digital lens , So if I am using 35mm-80mm zoom what are the digital mm numbers???
I have a working Canon XTi and a 50mm 1.8 and a 35mm - 80mm lens for it.
Question- What would be the advantage of a 18- 55 mm lens, and do I really need one?
I have a 28-105mm Sigma lens that works well on the old D60 but the XTi doesn't like it. Works fine indoors but not outdoors. Oh Welllll
My understanding of optics (viewing angle as it relates to focal length, etc.) is minimal, but the basic idea is this: you're talking about a ratio, the proportional relationship between the size of a traditional 35mm frame (24x36mm) and the sensor for your camera (22.2x14.8mm). This is often referred to as the "crop factor" -- the multiplier you need to get from 35mm terms to those of your camera.
In the Canon Rebel XTi's case, it's "1.6". That is, you multiply the "real" focal length of the lens -- by your example, 50mm, by 1.6 to get 80mm. This is not a real change, it's still 50mm in focal length, but you get (roughly) the same field of view on your XTi as a 35mm (or "full frame") camera would get with an 80mm lens. This is, of course, because you're essentially "cropping" a 22x2x14.8mm rectangle out of the center of the image circle thrown by the lens.
So: just multiply the focal length(s) of the lens by 1.6 to get the 35mm-equivalent field of view, if, like most of us over the age of 30, you have that pretty well stuck in your head already. For a 35-80mm lens, it would be "56-128mm".
As for the use of an 18-55; that's a useful range for a walk around lens, hence it's popularity. Applying the crop factor, it's a somewhat wide-angle ("28.8mm") to short telephoto/portrait ("88mm") with normal and wide-normal in between. Whether it's necessary depends on whether you find it useful, that's all. I like something like that when traveling, if only to avoid carrying a bunch of other lenses. But most of the time, I have a "prime" (fixed focal length, term stolen from movie cameras as I understand it) lens on my camera for weight, image quality (sometimes only imagined), and simplicity purposes.
