( Don't bother clicking on the image above - that's as large as it gets.)
This has been fun because after 50-some years of playing with cameras, binoculars, and telescopes I at last understand how a lens works! And besides, when it comes to telephotos, the digital camera offers a real bonus that should be especially exciting if you're interested in capturing nature, sports, or candid portraits. Put simply, a telephoto on a digital camera packs more power for the buck, weight, and focal length. A 300mm telephoto on my Canon Digital Rebel acts like a 462mm telephoto!
If all this is old news to you, skip to the images in the album, "Lens games." If not - or if you just want to understand why - study the images above and read on.
First, you can think of your film - or electronic sensor - residing near the back of your camera as a screen. Your lens projects an image onto that camera back - screen. How big that image is, depends on the focal length of the lens - and focal length is simply the distance from your lens to your camera's film or other light sensor. A 28 mm lens, then, is a lot closer to your light sensor than a 300 mm lens - well about 272mm closer. That's why the 28mm is much smaller than the 300mm.
So how does the 300 mm lens "magnify" an image? Understanding that was my little "aha" moment. Think about standing in a dark room with a flashlight. On the wall is a painting. Point the light beam at the painting and move close to the painting. It only illuminates a little circle. Move it farther away and the illuminated part of the painting - the circle of light - gets larger.
Bingo - the light went on, for me, so to speak.
That's all a lens is doing. It's painting a picture on the back of your camera. The farther away it is from the back of the camera, the bigger the picture. Duh!
BUT. . . and here's the rub. . . your film only intercepts part of that picture.
And that's the key to understanding why the same lens on a digital camera 
magnifies more than if that lens were used on a 35mm camera and at no sacrifice of light. You see the image isn't actually any larger in the digital camera. It's just that its sensor - light sensitive area - is smaller than the 35mm film. It's 22.7mm wide, to be exact, but follows the same 3:2 ratio of width to height as the traditional 35mm frame. (The blue rectangle in th eillustrations represents the light sensitive material in your camera.)
Why does this appear to magnify more? Because in the case of the Digital Rebel you take that little rectangle of image, capture it on 6.3 megapixels, and then spread those pixels out over your computer screen in an image that measures 3072 pixels across! Or think of it this way. Make an 8x10 print of the full 35 mm negative and an 8x10 print of the full digital image and the digital image will appear to be magnified because you've spread a smaller area of the actual image over the same 80-square inches of paper.
Am I belaboring the obvious? Forgive me, if I am. It's just that I have been playing with optical instruments for years and have thought I understood how they worked - and indeed, have had a pretty good practical knowledge of them. But suddenly I have a more intuitive grasp of what's going on and for me that's refreshing.
Besides greater magnification, the other practical bonus of this business of taking a smaller slice of the image is that there is no additional sacrifice of light associated with this greater magnification. That's real useful because usually when you magnify an image you also have to slow down the shutter speed, or use "faster" film, or open up the lens wider to get more light to the sensor, be it film or electronic. But here the amount of light doesn't change, just the apparent magnification.
Cool!
So does this happen with all digital cameras? Yes. but your experience will vary with other cameras. First, it is most relevant in the single lens reflex world where there are a host of lenses available. What's more, the relationship between your camera and a 35mm in this respect will vary depending on the exact size of your camera's sensor chip. With the Canon Rebel it is 22.7 vs 35. That's a difference of 1.54. If you are used to the 35 mm world, then you consider 28mm to be a typical wide angle lens, 50mm to be a typical "standard" lens that sees what your eye sees, and 300mm to be about as much telephoto as you want to hand hold.
To understand what those lenses mean in the world of the Digital Rebel, multiply their focal length by 1.54.
28mm = 43mm
50mm = 77mm
300mm = 462mm
The lens that came with my camera is an 18-55mm which I think of as 28-85mm. In my 35mm days I would see that as a combination of a nice wide angle, standard, and portrait lens.
The lens I just ordered is a 28-200mm which I think of as a 43-308mm - in other words, standard to really huge telephoto. For me it's a dream to be carrying around a 300mm telephoto in a 200mm (shorter, lighter) package.
In the meantime, a friend has loaned me his 75-300mm Canon lens ( 47-462mm) and I'm having great fun doing some nature photography with it. And someday maybe I'll get a 500mm which will act like a 770mm! Whew. Well, dream on. meanwhile I'm having great fun using the 300mm to get shots such as this one of a sanderling at Horseneck this morning.
So are there any drawbacks to the digital telephoto world?
I'm sure there are and I'm sure I have a lot to learn. My main concern right now is focus - especially on something like a flying bird. And depth of field.
But while we're on the subject of telephoto, I see just about any image I take with the 6.3 megapixel offering another "telephoto" bonus. If I take an image of a seal, for example, with the 55mm (85mm) standard lens, that image stretches out over 3072 pixels. But when I go to display it on the Web screen all I need is 500 -1000 pixels. Bigger than that and most people won't be able to see the whole thing on their screen. So instead of reducing the image to 1000 pixels, I crop it to the size I want while at it's full width. To see exactly what this does, take a look at the two seal pictures. You'll find them here:
http://www.giveyoujoy.net/dancing/archives/000972.html
Oh - and btw, if you're wondering how the illustrations at the beginning of this entry were made, I simply used a flashlight in a dark room with a panting on the wall and the Digital Rebel. I took the results into Photoshop Elements 2, the free software that came with my camera, and played a bit. Man, this is fun! I love it when new toys actually work and teach me something!
Posted by Greg Stone at February 28, 2004 07:13 AM