What is the Crop Factor?

The 35mm Film Format

The 35mm film standard was introduced in 1889, but became increasingly popular for still cameras after the Leica was introduced in 1913. The strip of film is 35mm wide (including the sprocket holes), thus giving the format's name. However, each frame is positioned lengthwise between the sprockets to maximize the film area in each frame. This gives a final frame size of 24mm by 36mm.

Any camera which wants to use standard 35mm film needs to implement the same sprocket arrangements, and allow the shutter to expose the same 24mm by 36mm area of film to the incoming light. Beyond that, there is a stunning diversity in nearly all other aspects of 35mm still film camera design.

The Expense of Digital Image Sensors

Given the ubiquitous 35mm film format, it was a natural design choice to make early digital cameras work with the same assumptions. Put a sensor chip in place of the film, and the rest of the camera should work pretty much unchanged.

The problems immediately became apparent, however. Although it may seem fairly small to the uninitiated, a surface area of 24mm by 36mm is actually a very large, very vulnerable piece of silicon. The larger the chip, the more power it requires and more heat is generated. The larger the chip, and fewer of them will fit on a production wafer. The larger the chip, and the more likely it would contain a microscopic flaw in the silicon which ruins the circuitry.

Since the first digital 35mm film cameras were created, the industry has worked on two separate technology paths. One path used very small chips that were well-suited for low resolution video output, while another path worked hard to make the best possible image regardless of the cost. The video-oriented chips formed the "point and shoot" line of consumer digital cameras, and the still-oriented sensors formed a professional product line.

Even though the megapixel counts are now roughly the same between these two technology strains, there are still some fundamental architectural differences in the chips. The video types are able to provide a reduced-quality live preview, shuttered constantly through an electronic discharge cycle. The still-camera sensors don't include any such live video capability, but instead focus on high quality receiving and filtering capability.

One thing that has not advanced is the overall cost of the silicon. Improvements in silicon chip production are happening at a slower pace, so the prime factor in a chip's cost is still the overall surface area of the chip. Smaller chips are more economical, and larger chips are immensely more expensive.

There has been great pressure on camera-makers to develop a middle-ground between the cheap digicams and the full-frame behemoth cameras. These compromise cameras use high-end sensor design but simply work in a frame size that is less than the full 24mm by 36mm frame area. This allows existing quality lenses and camera designs to be used while making the sensors affordably.

Does it Change the Perspective?

Some folks warn that if you have a crop factor, that it will change the perspective of the shot. This can be misleading.

It is important to note that perspective does not change with a crop factor. Perspective does not depend on the camera. Perspective does not depend on the focal length of the lens. Perspective is a matter of your vantage point: perspective is the result of the distance and the direction from your subject to your camera. (For the purposes of this discussion, we'll leave specialty lenses like tilt/shift or fisheye out of the equation.)

You can stand wherever you like; the crop factor doesn't affect the lens' ability to focus, or the camera's ability to expose properly. The perspective of two shots by any two cameras from the same position with any normal lens will be identical. Cropped cameras and longer lenses will just not have as much of the subject in view.

However, you may need to move farther back, to maintain roughly the same composition that you'd see with a full-frame camera. If you could just barely fit the subject in the full frame at ten paces, you may have to step back a few more paces to fit the subject in the cropped frame. Once you back up, you can include your entire subject in your frame, but the perspective of the shot will be altered by the new distance. You won't be able to match the whole composition if you step back, since background features will shift in relation to the foreground subjects, but you may be able to get a similar composition.

The crop factor won't change your optical perspective, but it may influence your compositional choices.

Is It Like Having a Longer Lens?

A lot of people describe the effect of cropping as being like having a longer focal length. The description is appealing: if a 20mm lens is on a 1.5x cropped camera, then it's just like having a 30mm lens. The 20mm times 1.5x is 30mm. On another camera, you might see a 100mm lens cropped times 1.6x to be similar to a 160mm lens.

The truth is that a lens has a specific focal length, regardless of the size of the film or sensor. That 20mm lens is always going to be a 20mm lens, whether it's on a full-frame camera or a cropped sensor or being held manually over a piece of paper. The lens doesn't change. The sensor just collects less of the resulting image.

Yet there's really little harm in describing an "crop factor equivalent" for lens focal lengths, as long as everyone understands what is really going on. It makes the math easy in the head. If you ignore any non-ideal lens distortions, the viewer couldn't tell the difference.

Some Benefits of the Crop Factor

It might seem like quite a price to pay: you lose all that image area, you're more susceptible to noise when compared to other sensors of the same pixel dimensions, and it's harder and harder to get a good wide-angle shot. So what are the benefits for the photographer who uses a cropped sensor camera?

Lower Sensor Cost
The first and foremost reason to love the sensor crop factor is a reduced cost for the sensor chip, and thus an affordable camera. The higher yield of good sensors per silicon wafer in production means more cameras can be produced and sold to a wider market. Full-frame sensors exist, but they're relegated to the higher end of professional gear, typically costing several thousand dollars.
Less Lens Vignetting
If you put a small black dot in the middle of the face of your lens, you would not get a small black dot in the middle of your image. However, the light would be diminished very slightly in the middle of the image, in a large round area. The same goes for any light lost around the edges of the lens, such as due to improperly sized lens hoods or filter rings. This darkening effect at the edges of the image is called vignetting. However, with a smaller sensor, the worst of the vignetting "fat" is trimmed away, leaving you with the best "meat" in the middle area of the image.
New, Lighter Lens Designs
Now that some consumers are getting used to crop factors, camera manufacturers are starting to adapt their lens designs to take advantage of the smaller chip size. Less glass is required to design a lens that only needs to cover the chip, instead of the full frame of 35mm film. Reflex mirrors can be reduced to allow the lens to get closer to the sensor, too. A lens with less glass is lighter, and less expensive to produce. Unfortunately, it also will not be useful for full-frame cameras, so if you expect to upgrade to a full-frame sensor someday, or you want to share with your film cameras, beware of these digital-specialized lenses.

Contact Ed Halley by email at ed@halley.cc.
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