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Posted by Peter Cox on Tue, Dec 22, 2009, at 12:32 am

Crop Factors and Sensor Sizes Explained

A source of frequent confusion among newcomers to digital photography is the concept of ‘crop factor’ in some cameras and the effect this has on the field of view one gets from a lens.

It’s most often discussed in relation to cameras with interchangeable lenses, because the same lenses can often be used on both film and digital SLRs. However, depending on the size of the sensor in the digial SLR, the field of view will be different.

Focal Lengths and Fields of View

All lenses have a focal length which is a fixed optical property. It is defined as:

The distance from the center of the lens to the point at which an object at infinity is in focus.

The focal length is used in photography to describe the field of view given by a lens. Shorter focal lengths equal wider fields of view, and vice versa.

Below are two images, both taken from the same position. The first one was taken with a 24mm lens, and the second with a 200mm lens. The red rectangle in the first image shows the field of view given by the 200mm lens.

24mm

200mm

Within the same format, photographers get used to the field of view given by certain focal lengths. If a photographer puts a 50mm lens on their camera, they have a good idea what sort of field of view they’ll get.

Sensor Size and Fields of View

Once you change the size of the sensor (or film), you change the field of view of a given lens. The size of a frame of 35mm film is a rectangle 36mm wide by 24mm tall. Most digital SLRs have smaller sensors that are closer in size to the APS-C format. Canon cameras, for example have sizes in and around 22mm x 15mm.

This has the effect of giving a narrower field of view, as the edges of the image projected by the lens are ‘cropped off’ due to the smaller sensor. The image below illustrates this point.

This image was made with a 24mm lens on a full-frame digital SLR (having dimensions equal to a piece of 35mm film). The white rectangle shows the field of view a camera with an APS-C sized sensor would have with the exact same lens. As you can see, it’s quite a bit narrower.

In Practice

How is this used in practice? Digital SLRs are either described as ‘full-frame’, meaning they have a sensor the same size as a 35mm film frame, and therefore have no field of view modification, or as ‘crop-factor’ cameras which do.

If the camera has a crop factor, which all the entry- and mid-level cameras do (as of late 2009), then it is given as a multiplier. For example, most Canon low-mid range cameras have a 1.6x crop factor. This means that if a photograph is taken using, say, a 100mm lens, the field of view given will be equal to that of a photograph taken with a 160mm lens on a full-frame camera.

Therefore, for a given focal length, a lens on a crop-factor camera will have a longer reach, behaving like a longer telephoto lens on a full frame camera.

This is of benefit to people who need the extra reach – wildlife and sports shooters, for example. It’s not as good for people who want wide angles, such as landscape photographers. A 24mm lens on a 1.6x crop factor sensor will be equal to an almost 39mm lens on a full-frame sensor – which isn’t nearly as wide.

‘Digital’ Lenses

This problem led to the creation of special ‘digital-only’ lenses. All manufacturers have a line of these lenses, and they come in shorter focal lengths than usual for the 35mm format. For example, the kit lens that comes with most entry-level digital SLRs is an 18-55mm zoom. This is roughly equivalent to a 28-90mm zoom on a full frame camera, which is pretty much the focal length range that film SLR kit lenses used to come in.

These lenses will only work on crop-factor cameras as the image circle produced by the lens is just big enough to contain that 22×15mm rectangle. If you were able to mount the lens on a full-frame camera you would see the edges of the image circle in the corners of the frame. As such, some of the manufacturers have devised a specific mount for digital lenses so they won’t physically fit on full-frame cameras.

It’s important to note, however, that regular lenses work fine on APS-C format cameras. All Canon EF lenses will work on all Canon cameras, regardless of sensor size. All Nikon lenses will work on all Nikon cameras likewise (although older lenses may not have autofocus).

Full-frame or ‘Crop-Factor’ Cameras?

I often get asked for advice on whether to bother with a full-frame camera. There are advantages – the major ones being a slight increase in picture quality and dynamic range due to the larger pixel sizes. If you have two cameras of the same vintage – one full frame and one crop-factor, but both having 10 megapixel resolution, the pixels in the full frame camera are physically larger which gives them greater light gathering capability and reduced noise.

The other advantage is in the viewfinder – full-frame viewfinders tend to be larger and brighter than their smaller cousins, which makes composing the scene much easier.

That being said, neither of these is going to translate to a massive increase in image quality and you should go with what your budget permits.

In Conclusion

When buying a camera or lenses, it’s worth being aware of this issue. If you have a camera with a smaller sensor, but plan on upgrading to a full-frame camera at some stage, then you don’t want to invest too heavily in digital-only lenses. Any ‘normal’ lenses you buy will work with both cameras, however.

I hope that you have found this tutorial useful. If you’d like to improve your camera or post-processing techniques, I run regular workshops at my studio in West Cork and in Dublin.

On top of that, I also run ten-day touring workshops. Contact me for details.

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