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Crop Sensors

In the digital world, cameras have different sensor sizes. This is an economic consideration as it is simply more expensive to create larger sensors. This has created a lot of confusion in respect to focal lengths of lenses, as well as performance of the sensors. This discussion centers mostly on DSLRs as they have to deal with lens issues more than compact cameras having fixed lenses.

In a 35mm film camera, the film size was 36mm x 24mm. Today, few DSLRs have a sensor this size, and those that do are generally known as "full-frame" cameras, or in Nikon's case "Fx". Cameras having smaller sensors are known as "cropped" cameras. Nikon uses the term "Dx" to describe these cameras, and other brands have their own notation. The figure below shows the relative size difference of the common sensor sizes.

The following table shows the relative size of the sensors between select common cameras. It should be no surprise that the more expensive full-frame cameras have larger sensors. Generally the larger the sensor, the better the performance - especially in low light. A larger sensor can simply gather more light. And for a given MegaPixel count, the individual pixels can be larger on a larger sensor - again improving the sensor's light gathering capability.

From this table you should see that stuffing 14MegaPixel sensors into a typical Compact Camera vs. a larger sensor camera would obviously result in smaller pixel "real estate", and will not provide the same low-light results. In some sense, marketing forces have gone amok at the camera manufacturers, as the uneducated buying public doesn't realize higher MegaPixel sensors often carry a performance penalty. While new technologies such as back-light sensors can improve a small sensor, there is no reason this technology cannot also be applied to larger sensors for an even better improvement.

 

Camera Model
Sensor
Crop Factor
CoC MM
Size MM
SQ MM
Nikon D3x, D3s, D700, Canon 1Ds Mk III, 5D Mk II
Full Frame
None (1.0x)
0.030
36x24
864
Canon 1D Mk IV, 1D Mk III
APS-H
1.29x
0.023
28x19
519
Nikon DX DSLR
APS
1.5x
0.020
24x16
370
Canon 7D, 60D, 50D, Rebel T3i, T2i, T1i, XSi, T3, XS
APS-C
1.6x
0.019
22x15
329
Olympus Pen PL1, P2, Panasonic Lumix GF1, GF2
4/3
2.0x
0.015
17x13
225
Nikon J1, V1
Nikon CX
2.7x
0.011
13x9
116
Fujifilm X10
2/3"
3.9x
0.008
9x7
58
Nikon P7000, Canon G12
1/1.7"
4.5x
0.007
8x6
43
Most Compact Cameras
1/2.3 & 1/2.5""
5.6~6x
0.005
6x4
25~28
Most Cell Phone Cameras
1/6"
14x
0.002
2.5x2
4

 

 

Crop Factor

There is a misconception that using a lens on a cropped camera changes it's focal length. It does not. Focal length is determined by measuring the distance from the focal plane in the camera (the plane the sensor is at) to the front of the lens. Using a smaller sensor does not change the lens length, regardless of the crop factor. If it did, the lens would not focus properly. For example, all Nikon cameras, whether they be full-frame, DX, or 35mm film cameras all have the same 46.5mm distance from the sensor to the front of the lens mount.

What does change is the field-of-view, or the width and height of the photo. This results in the appearance of increasing the focal length of the lens, but in reality, the image magnification in the camera has increased. In a sense, this is somewhat like a "digital" zoom effect.

The "cropping" comes from the smaller sensor, and part of the image will fall outside of the sensor's edges.

Multiplying the lens length by the crop factor is a convenient way to provide an "equivalent" focal length in terms of the 35mm sensor size. However, the lens characteristics stay the same. For instance, a 35mm wide angle lens, when used on a 1.5x crop sensor camera will appear to be the same size as a 52mm lens on a full frame camera (35mm x 1.5 = 52.5m), however the perspective distortion characteristics of the 35mm lens will not change. So while the lens focal length appears to change in a cropped camera, the inherent focal length characteristics do not.

What about DX lenses, or those lenses designed to work in cropped cameras, do they change their apparent focal length too? Yes. In the DSLR world, there are two types of lenses; FX (full-frame) and DX (cropped lens). Either type lens can be used on a crop factor camera, and the same reduction in field-of-view and apparent focal length change will occur.

Since a crop camera doesn't use the full diameter of the lens, a DX lens is nothing more than a reduced diameter lens. This is commonly done as it is less expensive to build a smaller diameter lens. However, while a cropped camera can be used on either full-frame or cropped lens, a full-frame camera cannot use a cropped lens without Vignetting (darkened corners). However, some Nikon full-frame cameras have a DX mode, and some can automatically go into that mode when it senses a DX lens is attached. Canon full-frame cameras cannot be used on their EF-S (cropped) cropped lenses. If that were done, the mirror in the body would smack against the lens optics. For that reason, Canon cropped lenses have a pin preventing their mounting on full-frame bodies.

So which lenses are the best? If you never plan on a full-frame camera, you can safely purchase cropped lenses. However, if a full-frame camera is in your future, buy full frame lenses now. Also, there is sometimes an advantage of full frame lenses on a cropped camera. No lens can be made perfectly, and most aberrations and defects occur at the lens edge. By using a full frame lens on a cropped camera, you are using just the center-most part of the glass - sometimes called the "sweet spot". This often results in sharper photos than even a full-frame camera can produce.

Should you never buy cropped lenses - not necessarily. Consider the apparent up-shift in focal length. This makes telephoto lenses longer, which is desirable for most part, but it also makes wide-angle lenses longer - which is undesirable. For full frame lenses, 17mm is about the low end of the wide angle range, but when used on a cropped camera, this lens would have the same field-of-view as a 25mm lens (depending on your crop factor).

The only way to obtain the same wide angle coverage is to buy a cropped super-wide angle lens, which often are as short as 8mm to 11mm. Manufacturers can make these lenses for cropped cameras as the lens diameter doesn't have to be as large. Making this size lens for full-frame cameras be result in a very large diameter lens, and the quality and cost of such a lens would not be practical.

Other reasons to buy a cropped lens is cost, as they tend to be less expensive. Even if you plan on going to a full frame camera, a lens such as a cropped fisheye lens might be an attractive lower cost option. Of course, this doesn't apply to Canon lenses due to the mirror-smack problem.