Crop factor

I was thinking that it’s just a matter of time before I move to a full-frame camera. Full frame? Yes, full frame.

Back when everyone used film, cameras recorded an image 36 mm wide and 24 mm high on the film you loaded into the camera. Your prints were certainly larger, but that was the size of the image on the negative or slide. It was simple. Whether you used a cheap point-and-shoot or a professional SLR, the image your camera recorded was the same size if you used 35 mm film. 36 mm by 24 mm.

Note that the simulated film frame above is not to scale. It’s quite a bit larger than an actual film frame.

In the early 1990s, things started to change. Kodak introduced a digital SLR. They took a Nikon film camera and replaced the film mechanism with electronics and a digital sensor similar to those in video cameras. Back then, digital sensors were ruinously expensive. There was no way they could produce a sensor anywhere near the size of a 35 mm film frame. So what did they do? They used a smaller sensor. Even still, Kodak’s DCS 100 cost $25,000. And for that kind of money you got a digital SLR that captured a 1.3 megapixel (1320 × 1035) image.

These days, sensors are far more capable and much less expensive, but they’re still not cheap. For this reason, most digital SLRs continue to use sensors smaller than the 36 mm by 24 mm film frame. One common size is the APS‑C sensor. Even within this size format, there is slight size variation between manufacturers but I’m going to use the Nikon size in this example. Their APS‑C sensor size is 23.7 mm × 15.7 mm which is just about 2/3 the height and width of the full 35 mm frame. The smaller sensor has an important effect. This effect is so large that my current selection of lenses will have to change when I switch to a full frame camera.

Imagine you take a photo with your film SLR. You send the film off and when the print arrives, this is what you get:

Lovely!

But you want to try out your fancy new APS‑C digital SLR as well. After you took the photo with your film camera, you removed the lens, put it on your digital SLR, and took the same photo. You sent that image away to get printed as well. This is the print you get:

Now wait a minute! The photos are different even though you took them with the same lens! If you take some measurements, you’d realize that 1/6 of the image is missing around each outside edge. Fully 1/3 of the photo is missing! Wait a minute. This is familiar. Earlier I said that the APS‑C sensor used in most digital SLRs is 2/3 the size of a full film frame! Is this why 1/3 is missing? If you think so, you understand what’s going on.

Looking at the prints, the effect appears to be a free zoom, but it’s not. It appears that way because both images are printed at the same size. In reality, the images are not the same size at all. The sensor is smaller so your photo shows you less of the image the lens is delivering. For this reason, we describe the size reduction as a ‘crop factor.’ In this case the crop factor is 1.5 because the APS‑C sensor would need to be 1.5 times larger in height and width to match the size of a 35 mm film frame. That’s why the crop factor is 1.5, but we can say it in a different way to describe the effect of the crop factor. Your lens on an APS‑C camera will provide the same field of view as a lens with a focal length 1.5 times larger on a full frame camera. So for example, if you were using a 50 mm lens on an APS‑C camera, it would give the same field of view as a 75 mm lens on a full frame camera.

I’m not saying that the crop factor makes the 50 mm lens into a 75 mm lens. It doesn’t. It’s still a 50 mm lens. Only the field of view changes.

Lets step back a moment, okay? Showing what happens in the camera may make it clearer still.

Lenses project circular images. Your prints are rectangular because the film/sensor doesn’t capture all of the light the lens projects. In fact, most lenses are designed to project an image circle as small as possible while still covering a film frame or a full frame sensor. Taking the film option, this is a simplified representation of what’s going on inside the camera:

The round image is what the lens is projecting into the camera. The clear rectangle in the centre represents the portion of the image captured by the film. In the image below, the clear portion represents the portion of the image captured by the smaller APS‑C sensor:

The size of the image projected by the lens has not changed because the lens has not changed. The only difference is the portion of the projected image that’s used. The APS‑C sensor is smaller than a 35 mm film frame so it captures less of the image the lens projects.

When I say I’d need to change my lens selection if I went to a full frame camera, you realize that the cause isn’t the lenses themselves. The lenses I have would all suddenly offer a wider view. My wide-angle lens would be wider, which I like! My telephoto lens wouldn’t be so telephoto, which I do not like as much. This wouldn’t be an issue except that one builds a lens collection based on what one likes to shoot. I’d need a new lens to cover the wide end of normal to short telephoto range I enjoy. The change from a crop factor of 1.5 to no crop factor (or a crop factor of zero) would leave me with no lenses in this range.

So why would I bother switching when it will wreak havoc with my lens coverage? That’s a story for another time.