In a post on choosing between investing in lenses vs bodies, I recommended that even if you are currently shooting on a camera with a APS-C (crop factor) camera, it's wise to start building up an arsenal of full frame lenses now. There are a number of reasons for operating this way, including improved image quality, better build quality and durability, and the fact that these lenses often hold their value a lot better over the years. One thing that you definitely need to consider when mixing full frame and crop sensor systems, is the effect of your cameras crop factor on what you will see through the lens.
In the days of film photography as the primary medium of expression, 35mm film was the reference point due to it's mass adoption. Any photographer knew, at least approximately, what the field of view would look like when using a lens of a given focal length.
Today however, while most lenses made by the same manufacturer are interchangeable between camera bodies, different bodies have varying “crop factors” which will affect the field of view of images taken with the same lens.
Full frame cameras offer the equivalent field of view to a 35mm film SLR camera, thus becoming the new baseline for lens focal length. Besides full frame DSLRs however, there are cameras made with many varying crop factor ratios on the market. It's not important to know all of them, but you should know what - if any - crop factor is present in your camera.
Different Crop Factors
The most common crop factor is identified as APS-C which is used by Nikon (their DX line), Pentax, Sony, and Canon. This is where it can get a bit confusing however, as the APS-C crop ratio differs between some manufacturers.
Nikon, Pentax and Sony produce APS-C cameras with a 1.5x crop factor. Canon on the other hand manufactures their APS-C cameras with a 1.62x crop factor. Besides APS-C, Canon also makes some popular cameras with a 1.3x crop factor.
So how does the crop factor affect my photos?
Your cameras crop factor will affect the field of view a given lens will produce on your camera when compared to a standard reference 35mm or full frame camera. For example, the image above shows how one lens - let's say 100mm - would affect the field of view on 4 different cameras.
The YELLOW box is the field of view produced by our 100mm lens on either a 35mm film camera or a full frame DSLR.
The GREEN box is the result of the 100mm lens on a Canon 1.3X crop sensor camera.
The BLUE box is the result of the 100mm lens on a 1.5X APS-C sensor found in cameras by Nikon, Pentax and Sony.
The PINK box is the result of the 100mm lens on a 1.6X Canon APS-C sensor.
It's worth noting that while it may seem like you are able to get “more zoom” by using a crop factor camera, that is not in fact the case. Essentially it is the same as cropping the photo on your computer, chopping off and discarding the unused pixels that fall outside of the given box, ultimately reducing image resolution.
Equivalent Focal Lengths
Let's say that you've been shooting on an APS-C or other crop factor camera for a while, and have found that the majority of the photos you shoot are at a focal length of 50mm. It's useful to know that if/when you decide to upgrade to a full frame, to achieve the same the same field of view, you will require a different length of lens.
To find out the equivalent focal length on a 35mm/full frame system, divide the focal length by your cameras crop factor. The table below illustrates how some common focal lengths compare.
Effect on Aperture and Depth of Field
People often ask whether using a given lens on different systems has any effect on either the minimum/maximum aperture, or on the depth of field.
The answer to both of these is no, it does not. The focal length remains the same, and the lens reproduces the same image on any system, but the sensor captures only the portions of that image that it can.
I hope this post has clarified any questions you had about how crop factors relate to the field of view. If it is still unclear or you have further questions, let me know in the comments!