Crop Sensor (APS-C) Cameras and Lens Confusion

5:23 PM


Despite the fact that so called "crop sensor" digital SLRs have been with us for over 5 years, there's still a huge amount of confusion out there about exactly what a crop sensor camera is and what effect is of using a lens with a crop sensor camera rather than a full frame camera. The photography forums are full of confused newcomers asking about focal length, field of view etc.
First, what is a crop sensor camera? Well, it's simple. A full frame 35mm camera ( whether it uses film or a digital sensor) records an image that is approximately 36mm x 24mm in size. In the early days of digital sensors it was not possible to make digital sensors that big in any sort of quantity, and the ones you could make were so expensive that hardly anyone would have been able to buy a camera which used one. So camera makers decided to use a smaller sensor, around 15mm x 22.5mm. This just happens to be close to the image size which was used with the short-lived APS film format, specifically the APS-C image size of 25.1 × 16.7 mm (there was also APS-H and APS-Panoramic format).


The "crop" name comes from the fact that if you take a full frame image (24x36mm) and crop the center 15x22.5mm out of it, you get an image the size of "crop" sensor cameras.
So why does the format size matter and what effect does it have on focal length? Well the answer to the second part of the question is "none". The focal length of a lens is the focal length of the lens. Whether you mount that lens on a 35mm camera, a medium format camera of a large format camera doesn't change its focal length. All 35mm lenses and lenses designed for use on APS-C DSLRs are marked with their true, actual, focal length.
The problem is that most of us have been trained to think in terms of focal length rather than field of view when comparing lenses. We've been trained to think that a 50mm lens is "normal", a 35mm lens is "wide normal", a 28mm lens is "wide", a 24mm lens is "very wide", a 20mm lens is "super wide", a 16mm lens is "ultrawide" and so on. In fact this is true ONLY if that lens is making a 36mm x 24mm image. The field of view (which is what "wide" is all about) is actually determined just as much by format size as by focal length. The diagram below shows why.




s you can easily see from the diagram, the larger the format, the wider the angle of view for a lens of a given focal length (shown by the red lines for the larger format and the blue lines for the smaller format). That's why a 28mm lens on a full frame 36x24mm gives a wide view (red lines), but on a smaller format camera such as one using and APS-C crop sensor, it's not so wide (blue lines). In fact if you put that same 28mm lens on a Canon EOS crop sensor camera, the angle of view decreases as you can see from the figure above. The angle of view decreases to the extent that it's now the same as that of a 44.8mm lens mounted on a full frame camera. It means that if you look through the viewfinder of an APS-C crop sensor camera with a 28mm lens mounted on it, you'll see exactly the same angle of view as if you looked through the viewfinder of a full frame camera with a 44.8mm lens mounted on it.
To get the same field of view as a 28mm lens on the full frame camera, you'd need a shorter focal length lens when used with the APS-C crop sensor. That's illustrated by the green lines in the image above. In the case of EOS DSLRs, the focal length would need to be 17.5mm. The relationship of these numbers will be explained next.


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