DIGITAL CORNER
Sensor Magnification
This month we'd like to address two of the things a lot of photographers fairly new to digital photography find perplexing. First, why do digital cameras give an apparent magnification and what are the tradeoffs? The apparent magnification of a digital camera starts with the relative size difference between 35 mm film and the electronic sensor used for digital image capture: "35 mm" film has an effective image size of 24 mm high and 36 mm wide for a "horizontal" image. When we photograph something through a lens, we record a certain size image of the subject on the film.
Digital cameras have sensors that vary in size, and except for a few very high end cameras, the sensor is smaller than the 35 mm image size. In the case of the Canon EOS 20D, the sensor is 15 mm high and 22.5 mm wide. If you do a quick calculation you'll see that the dimensions of the 35 mm image are 1.6 times bigger then the digital sensor.
If we assume the same conditions when we photograph the same subject, that is camera to subject distance and focal length of the lens, the image on the digital sensor will be the same size as the image on the film plane. That's just standard photographic optics. But remember the size of the sensor is smaller than the 35 mm film frame.
Now the real impact of digital! The software in the camera enlarges the image to give an equivalent 35 mm image size.
In doing so, this software magnifies the image on the sensor by the same amount that is needed to make the image sensor look like the 35 mm image, in our example of the Canon EOS 20D, this is 1.6x.
OK, we now have a l.6x magnification, what did it cost? If we had used a 1.6 teleconverter we'd have lost some of the image because the angle of view would have been decreased as the effective focal length of the lens increased, the same happens with the digital, but in this case, the information that the lens gathered was focused beyond the edges of the sensor so it was lost; the same effect as reduced angle of view. The second thing we lose with a teleconverter is light, namely the effective f stop of the lens is increased by about 1 stop. (f4 to f5.6 for example) In the case of a digital camera, this is not the case. The camera will still show the f stop as the same. BUT, the resolution of the sensor (number of pixels per unit of area) is fixed so a slight increase in what is equivalent to grain will be seen. Digital camera noise reduction software does a very good job of smoothing out this grain effect, so the apparent magnification gained is pretty close to free.
Now let's think about a few other things that may have slipped by in our discussion. First is the aspect ratio. That's a fancy mathematical term for the relative size of the horizontal and vertical dimensions. 24 x 36 or 15 x 22.5 have the same ratio, 2 to 3. This has a real impact in printed image size and can readily explain the popularity of printing image in 8 x 12 size instead of the venerable 8 by 10. 8 by 12 does not require cropping of one dimension. When digital scanning and printing became popular, the long held 8 by 10 dimension was challenged and quickly abandoned.
The second thing to think about is some of the new lenses being marketed. If you look at the magazine adds for some new products, such as Canon EF-S lenses, you'll see a note indicating these lenses are only for digital cameras like the 20D. This is because they focus the image not to a full 24 by 36 mm area but to the size of the image sensor. Remember we said earlier that the equivalent of reduced angle of view was due to the information falling off of the edge of the sensor? This doesn't happen with these new lenses. The effect if these lenses were used with a film camera body, assuming the computer in the camera would allow the photo to be taken, would be a smaller image on the film plane.