Sensor Size, Crop Factor and Magnification


There are lots of discussions on this topic all over the Internet, so I figured I may as well add my own. Obviously, I enjoy photography, but I'm not a professional. I'm a software architect and developer, though, and that provides for a different angle of view, so to speak, on the concept.

Let's get some things out of the way first...
  1. A 300mm lens is a 300mm lens, always.
  2. There are other factors that go into the overall quality of an image beyond the number of pixels and how large it is. This is not a quality discussion.
For the purposes of this article, I'm going to use the Pentax K10d, K20d, and the Canon 5d Mark II. The first two are APS-C (1.5 crop) and the Canon is full frame (equivalent to 35mm film size). The specifications of interest are:
  • Pentax K10d - 2.7 million pixels per cm2 on APS-C.
  • Pentax K20d - 4.0 million pixels per cm2 on APS-C.
  • Canon 5d Mark II - 2.4 million pixels per cm2 on FF
Doing some ratios here, the K10d has 1.125 more pixels per area than the Canon. That means, over the same physical region of sensor, the K10d captures at a higher resolution and thus gets more detail in that space. So, that ratio makes a 300mm lens resolve comparable to a 337.5mm lens with a field of view equivalent to a 450mm lens in comparison to the Canon. The Canon will capture a larger field of view, but will contain less detail in the same space as the K10d. Of course, there's an advantage the Canon has: if you put a lens around the 337.5mm mark on it, it will capture the same detail, but it will have a much larger field of view because it's FOV will also be for 337.5mm. More picture for the same detail.

The game changes with the K20d though... The pixel ratio between it and the Canon is 1.666 (repeated) and so the K20d will capture comparable to a 500mm lens with a 450mm field of view. So, to match the detail, the Canon would need a 500mm lens, but then it gets less of the picture because the field of view is for 500mm, not 450mm.

Understand, however, that these numbers are not absolute numbers, they are comparison numbers between two cameras. If the Canon has a density to match the K20d, then there is no way for the K20d to outresolve it and the comparative magnification would change. The numbers can only work when you make a relative comparison between two cameras or, more specifically, two sensors.

So, what does that mean? Well, the crop factor of the sensor is a crop, you're definitely getting less of the image circle of the lens. However, the resolving power of the sensor may, potentially, give magnification beyond the focal length, if the lens can provide the detail to the sensor to capture it, but the magnification is relative to another camera. If the "native" capability of 35mm film is, for the sake of this discussion, 4.0 megapixels per cm2, then relative to traditional film, the Pentax K20d simply crops. If it is less, then the K20d magnifies, if it is larger, the K20d shrinks it.

So, hence the answer to the question... Yes, maybe. Of course, when you get down to the brass tacks about your camera, be it digital or film, the real question to ask is: do you enjoy using it? Nothing else really matters.

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