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A lot of people seem to be quite confused about the idea of camera “reach.” This idea relates to how much variance in magnification you actually get between different camera bodies. I thought I’d take a few minutes to clear this up once and for all. I assure you the following information explains the issue correctly. Once you’ve read and understood this post, you’ll have a perfect understanding of reach.

The primary myth is that size of the sensor makes a difference. This is not true. The sensor for an APS-C body and the sensor for a FF (Full-Frame) body are different sizes, sure enough, but they are located at the same distance from the lens inside the camera, and so this alone cannot change the amount of magnification a lens provides – the lens will put precisely the same image into both cameras, it is just that the APS-C sensor will intercept 62.5% of the image and miss the rest, as the light from that portion of the image is falling outside the edges of the sensor.

Instead, the size of the sensor affects the field of view. An FF sensor will see more (60% more) of the subject matter at the edges of a scene as compared to an APS-C sensor. There is a practical side effect of this and that is for an EF lens, that is, one that will work on both APS-C and FF Canon cameras, the edges of the lens’s projected image miss the sensor on the APS-C sensors, and so lenses that might have distortions or soft focus at the edges or corners may produce better images on the APS-C sensor, as it is really just using the center of the projected image.

What actually makes a difference to effective reach or magnification is the pitch, or density, of the sensels on the sensor. The smaller the pitch (or spacing) is, the more actual reach the camera has; that is, the more pixels will be involved in portraying the same object at the same distance with the same lens.

This information is significant because the more magnification a sensel array provides, the more difficult it is to get a lens to produce a sharp image; that is, one with high contrast from one pixel to the next. This is simply because the smaller the sensel, the tighter the lens must focus light rays in order to create an equivalently sharp appearance at the pixel level.

In addition, as the sensels get smaller, the degree of diffraction that it takes to cause an otherwise perfectly focused source of light rays to widen enough to spill over the edges of a single sensel is less and less; since diffraction effects are tied directly to the ƒ-stop setting of a lens, the practical effect of this is that the highest ƒ-stop that will not blur images because of diffraction for any one lens will be wider (a smaller ƒ-number) as the sensels get smaller.

Below you’ll find a chart I’ve prepared that relates common Canon DSLRs to one another with regard to the difference in reach between camera bodies.

Please keep in mind that these numbers reflect the detail achieved with a lens that is focused such that it can put a light ray almost entirely inside the bounds of a single sensel; as I mentioned previously, this is more and more difficult to achieve as the sensels get smaller. As you move away from this ideal situation due to any combination of mis-focus, diffraction and/or inherent sharpness limits of the lens itself, details will blur into neighboring sensels, and while technically the magnification remains numerically the same, the available pixel-level detail in the resulting image will be reduced.

Here’s how to use the chart: Pick the camera that you’re interested in at the left, then follow the colored bar across to the right; the percentage numbers tell you the difference in reach as compared to the camera body named at the top of each column. You can click on the chart to get a full size image.

For instance, the 40D, with its 5.7 µm² sensels, has 124.56% the magnification of the 300D across any row or column, and 82.46% of the magnification of the 50D across any row or column.

Sensel Pitch related to Reach