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The Crop Factor is a term that can be heard quite often in the world of digital photography. What does it mean that a camera has a crop factor of 1.6x and how does it affect your focal length? We try to untangle this issue and describe it as clearly as possible.

The subject of crop factors and focal length multiplier can be a bit confusing and hard to understand at first — I will try to explain it as simple as possible but yet informative enough for you to get the entire picture.

The size of the sensor is what controls the crop factor, and it’s always compared to a 35mm film size. So when people talk about Full Frame they are talking about cameras that have a sensor the same size as a 35mm film (24x36mm).

As you can see in the illustration above, a body with a crop factor captures only the center part of the image. The image itself is round because that’s what the lens produces, and the sensor only picks up the light that reaches it. A full frame sensor will capture edge to edge of what the lens capture.

It’s important to know that the crop factor changes the field of view, it doesn’t actually change the focal length, since that is something that is decided by the design of the lens. The same results could be produced by taking a photograph with a full frame camera and crop it to only show the center — however it would require a camera with a good enough sensor that such a small crop would still produce a good quality image. These high-end sensors are currently only found in the top of the line cameras from Nikon and Canon.

What it also means is that the perspective doesn’t change with the crop factor, if you stand on the same spot and take a photograph with a full frame camera with a 50mm lens and then take another photograph with a 1.6x crop factor camera you will get the exact same perspective (the 1.6x camera will however not have the same view of field).

When people say that a 50mm lens is the natural focal length they talk about perspective in relations with the human eye, and as stated above, this is the same even on cropped bodies, a 50mm lens still produces a natural looking photograph. To test this, use a 50mm lens and look through the viewfinder with one eye and have your other eye open, you will notice that the perspective looks the same for both eyes, no matter if you’re using a cropped body or not.

Lenses designed for Crop Factor cameras

To counter this issue, most lens manufacturers have designed some lenses that are only meant to be used on cameras with a crop factor (i.e. not on full frame bodies). These lenses often produces a smaller image circle so if they were used on full frame bodies they would create a black edge, much like circular fish eye lenses do. Since the problem with crop factor is mainly negative with wide-angle lenses it’s almost entirely that focal range that has designated digital lenses.

Here is a list of the name camera manufacturers call their digital only lenses.

  • Canon — EF-S
  • Nikon — DX
  • Sony — DT
  • Pentax — DA
  • Sigma — DC
  • Tamron — Di-II

One thing to remember is that the crop factor is still in use even on these cameras and you must multiply the given focal length to get the “correct” focal length. For example, Canon has an ultra wide EF-S lens that has a focal length of 10-22mm, on a body with a 1.6x crop factor the focal length corresponds to a 16-35mm. So don’t believe that you do not need to multiply the focal length just because you have a lens designed from cropped bodies.

How it affects your photographs

There are both positive and negative affects with the crop factor, let’s talk about the positive aspects first.
To get the “correct” focal length you need to multiply the focal length with the crop factor. When I say correct I mean the focal length that the lens acts like, not what it truly is. My camera has a crop factor of 1.6x and that means that a 50mm lens is a 80mm lens (50×1.6=80). It also means that a 100-400mm lens is actually a 160-640mm lens. That’s a very good thing in most situations. You loose 60mm on the short end but gain 240mm on the long end — rather significant. With telephoto lenses the crop factor is often desirable, since it extends your “reach” quite a bit.

Another positive affect is that almost all lenses are much sharper in the middle and softer on the edges. How can this be a good thing? Well with a camera that has a crop factor you only get the center of what the lens sees (unless it’s a lens designed for crop factors as described above). So with a crop factor you only get the center and therefore the best performance from your lens.

Now over to the not so good aspects of the crop factor. Wide-angle lenses, this is a big minus on the bodies with smaller sensors. As the example above, the 10-22mm ultra wide turns into a 16-35mm lens, and for example a 17-40mm lens turns into a 27-64mm lens. You simply do not get the same wide angles with a crop factor, the edges are cut off and field of view is more limited.

Another negative affect is that the extra focal length you gain also means that you increase the risk for blur due to camera shake. The rule that you should have a shutter speed of 1/focal length is no longer true. For this rule to be true you first need to multiply your focal length with the crop factor of the body.

Full frame camera often have a larger viewfinder as well. When I’m comparing my old 35mm film camera with my 1.6x crop DSLR it actually makes me a little bit sad inside. The viewfinder on the 35mm camera is so large and bright where as the DSLR’s viewfinder is although bright still very very small.

There is one more thing a smaller sensor affects, but it would be hard to classify it as either good or bad. Different sizes of sensors change the depth of field. A smaller sensor will give you greater depth of field (more in focus) and a full frame sensor will give you a narrower depth of field (less in focus). If this is good or bad is dependent on what you desire, either you want as much as possible in focus or you want to isolate your subject as effective as possible.

I hope I didn’t confuse you too much. This is an important part of digital photography to understand, at least the very basics of it.