Digital Image Sensor Size – Digital Photography Tip of the Week

Last week I talked about how to shoot a sunrise or sunset. This week, I will get into something a little more technical, a brief discussion about digital image sensor size.
Many people coming from a film background with an SLR are already familiar with the issue of sensor size. Most digital SLR’s have a sensor that is smaller than that of a standard piece of film and the results of that have both advantages and disadvantages for the photographer.

If we look at any given 35 mm lens, we can see that it produces an image circle large enough to fully cover a 35 mm image. This image circle also covers film sizes smaller than 35 mm, though that is where the confusion sets in.

The image below is a representation of the differences between the image circle for a lens on a 35mm camera or a full frame (24mm x 26mm) digital camera, and APS-C sized digital image sensor and for comparison, a 1/1.8 sized digital image sensor. The lens is made to cover the entire image area of the full frame image, and the resulting image is shown outlined red. If we were to use the same lens (and therefor the same image circle) on a camera with and APS-C sized sensor (22.7 mm x 15.1 mm), shot from the same location, we would see the image that is outlined in blue. The image outlined in green is what we would see if we used the same lens on a camera with a 1/1.8 (7.2 mm x 5.3 mm) sized sensor, commonly used in digital point and shoot cameras.

As you can see, the image is the same size in each instance, what changes is the amount of the sensor the image occupies in relation to the 35mm, full frame size, or our benchmark. This is referred to as the crop factor, and commonly mistakenly referred to as the focal length multiplier. Because the image takes up proportionately more of the film size, the image appears to have been magnified. For the APS-C sized image, the crop factor is between 1.5 or 1.6.

This illusion of magnification works great for photographers using telephoto lenses. They are now able to bring their subjects in closer without having to purchase new, heavier, more expensive lenses. For instance, a Canon 200mm f2.8 L series lens retails for $899.95 CDN. With a 1.5 crop, it provides the same field of view as a 300mm (Canon 300 f2.8 IS USM L) which retails for $5199 CDN.

What is great for photographers using long, telephoto lenses is a thorn for photographers who like wide angle lenses. With a full frame sensor, 28 mm was a wide angle lens, and 20mm was considered ultra-wide. With an APS-C sized sensor, 31mm is considered normal. 18mm is equivalent to a 28 mm full frame view and it is becoming more common to find lenses in the 10 mm and 12 mm range for ultra-wide angle images.

There is a perceived change in depth of field due to crop factor as well. Depth of field changes as the distance between the subject and camera change. For the image above, depth of field would stay essentially the same for each image because distance between the subject and the image is not changing. However, in order to get the entire butterfly in the image, the photographer would need to back away from the subject. In doing so, the subject to camera distance would grow, as would the depth of field. There is a lot of science involved in depth of field that I will not get into here, but if you are interested, you can search Google.

Next week I will continue my discussion about sensor size and it’s impact on image quality in both digital SLR‘s and digital point and shoot cameras.

The digital photography tip of the week is written by the PCIN Assistant Editor, Chris Empey. Chris is a long time photographer and is currently the vice-president of the Niagara Falls Camera Club. You can see more of his photography at his Photo of the Day website.

If you have a tip to send Chris, or a question about digital photography he can address in the newsletter, send it to chris@pcin.net.

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