What is Crop Factor?

When looking for a lens for a Digital SLR you may have come across a term called crop factor. The crop factor is a value that compares the size of a sensor on a Digital SLR to a sensor on a 35mm film camera. One of the reasons why it is important is because it has an impact on the focal length of a lens attached to a Digital SLR.

Do All Digital SLRs have a Crop Factor?
Some Digital SLRs have a sensor that is the same size as those you find on a 35mm film camera. These tend to be among the more expensive Digital SLRs. These cameras are known as "full frame" cameras and do not have a crop factor.

Is the Crop Factor Always the Same?
To further complicate matters the crop factor can differ between camera bodies. Even the same manufacturer can produce camera bodies with different crop factors. Cheaper Digital SLRs tend to have the smallest sensors and therefore the largest crop factors. If you are buying a starter Digital SLR the crop factor is likely to be in either 1.5x or 1.6x. Check the specification of the camera body you are considering buying to confirm the actual crop factor the camera has.

What Impact Does Crop Factor Have on a Lens?
You will notice that the focal length of a lens is described using numbers. A typical lens may have a focal length of 28 - 70mm. The focal length described here is when the lens is used with either a full frame Digital SLR or with a 35mm camera. If you were to use the same lens with a Digital SLR with a crop factor then the field of view of the lens changes.

For example for a lens with a focal length of 28 - 70mm and a camera with a crop factor of 1.5x the focal length of the lens becomes 42 - 105mm. This can be good news when zooming in on a subject as the telephoto effect is increased, but not so good if you are looking to take wide angle shots.

If you already own a number of lenses you previously used with a 35mm camera then the crop factor is likely to have a significant impact if you switch to a Digital SLR that is not full frame. The likelihood is that you will need to consider adding new wide angle lenses to your collection with the focal length starting around the 18mm mark. An 18mm lens used on a Digital SLR with a 1.5x crop factor will produce the same effect as a 27mm lens on a 35mm camera.