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Lens ‘mm’ and Zoom

My Rebel XT has a 1.6 crop factor, which basically means that - in 35mm terms - a 135mm lens is the equivalent to a 216mm lens on a full-frame camera. This much I understand - simple math.

What I'm not so sure about is "zoom" factors. I realize that "zoom factors" are not the way people like to think about still cameras - it's more a P&S term or a digital video camera term - but I have one simple question: does a standard 100mm lens show 4x as much as a 200mm lens? (half, but in two directions, height and width, resulting in one-quarter the viewing size)? In other words, is a 200mm lens "zoomed" in twice as far as a 100mm lens (regardless of the crop factor)?

I ask because I'm trying to think about lenses, and what I may want to add. I could have taken some nice pictures recently if I had more reach. I own two lenses currently - the wide kit 18-55mm, and a 28-135mm IS that does the job nicely and is, for all intents and purposes, a good "walking around" lens. But I couldn't even get close enough to some geese without scaring them…

The 70-200 F4L comes highly recommended and checks in at around $600, but I don't know if the overlap with the 28-135 is worth it.

6 Responses to "Lens ‘mm’ and Zoom"

  1. I dunno what the relationship is, but frankly I have a simple point of view: get as much optical as I can and pretend that digital zoom even exists. Seems no matter how much optical I have I want more. And every time I try to use digital zoom, it looks like Sepia tried to hump the Mosaic filter and left me with an ugly image as the offspring. Bleh!

  2. How does one determine these factors? Does it have to do with the size of the CMOS element vs the size of a 35mm negative?

  3. Yes, the CCD/CMOS sensor on a digital camera is usually 1.5–1.6 times smaller (in both width and height) than a 35 mm negative. The result is that the camera takes a smaller part of the image formed in the image plane.

    The benefits are that the image is bigger and suffers less from vignetting (darker corners, or sometimes even edges). The drawbacks is that when you view/print the picture, you magnify it more than a 35 mm negative. It is of course not a problem with the sensor's resolution (the pixels are tiny compared to Kodachrome 200) but if the optics are of lower quality, you magnify the resolution defects more as well. The other drawback is that you lose some field on wide angle lenses.

    There are lenses made specially for digital cameras that are optimised for the smaller sensor. They would fit on a 35 mm camera as well, but I bet the vignetting would be terrible.

  4. As long as you don't do macro (i.e. provided the distance to the object is much larger than the focal length) yes, you can assume a 200 mm lens magnifies twice as much as a 100 mm, resulting in half the field in both height and width. For example at 10 m, the magnification ratio between a 200 mm and a 100 mm is 2.02. At 2 m it's 2.11, but you don't often use a 200 mm so close anyway.

    I have always been puzzled by the 'n times zoom' advertised on compact digital cameras and camcorders. I belive it's about the ratio between the long and short focal lengths of the zoom, which is not very useful because then a 28–140 and a 80–400 are both 5x zooms, although they're definitely not comparable.

  5. When a point and shoot camera is referred to as having a 3x zoom, it means that the focal length has a 3x range (i.e. 25-75mm or 10-30mm). The relationship between the field of view of different focal length lenses is trigonometric. This article explains it well, including in the context of "crop factors" for digital SLRs. It also includes information about fisheye lenses which may not be applicable.

  6. A possible way to get around the number crunching from differences in different optical systems, from various cameras and other optics. is to start thinking of your lens collection in terms of the angle of view instead; literally use a rule of thumb and go about like you see movie directors apparently do, making little frames with their fingers-- know that your hand at arms length will cover a certain number of degrees, as will that particular lens you might be using with that camera, or a certain number of fingers held up, side by side or spanned open, will do likewise. Or know that a wide angle will cover a certain number of people a certain distance away. I used to keep in mind if I wanted to get a person head to toe lengthwise on 35mm film with my 17mm, I had to be 2 or three feet away from them. You'll just have to develop that feel again for digital, if you had it before for another format.