Understanding Crop Factor of Digital Camera Sensors

In digital photography, a crop factor is paternal to the ratio of the dimensions of a camera's imaging area compared to a reference format; most oftentimes, this quantity is applied to digital cameras, relative to 35 mm film format as a reference, which is considered 'full frame' size. If you likeness the size of the film in a normal SLR (film is 35mm) to the image sensor in most DSLRs you'll encounter that the size of the DSLRs sensor is mostly smaller (unless you get what's titled a ‘full frame' DSLR). 

Black – Full Frame
Red – 1.3x Crop Factor
Yellow – 1.5x Crop Factor
Green – 1.6x Crop Factor

This ratio is also commonly referred to as a focal length number ("FLM") since multiplying a lens focal length by the crop factor or FLM gives the focal size of a lens that would grant the similar region of scene if utilized on the reference format. A 50 mm lens on an APS-C format (crop factor 1.6) images the same yield of view as an 80 mm lens on a 35 mm camera.

The terms crop factor and focal length multiplier  were coined in past years in an pioneer to help 35 mm film format SLR photographers understand how their existing ranges of lenses would perform on fresh introduced DSLR cameras which had sensors smaller than the 35 mm film format, but oft utilized existing 35 mm film format SLR lens mounts. Most DSLRs on the market have nominally APS-C-sized image sensors, smaller than the standard 36 × 24 mm (35 mm) film frame. 

For most DSLR cameras, this factor is 1.3-2.0×. For admonition, a 28 mm lens delivers a moderately wide-angle FOV on a 35 mm full-frame camera, but on a camera with a 1.6 crop factor, an image made with the same lens will have the same field of view that a full-frame camera would make with a ~45 mm lens (28 × 1.6 = 44.8). Ultra-wide lens designs become merely wide; wide-angle lenses become 'normal'. A 300 mm lens on a camera with a 1.6 crop factor delivers images with the same FOV that a 35 mm film format camera would require a 480 mm long focus lens to capture.

Estimating Sensor Performance

For a given exposure, for example for a fixed focal-plane illuminance and exposure time, larger image sensors capture more photons and hence produce images with less noise and greater dynamic range than smaller sensors. The larger sensor has the smaller crop factor and the higher signal-to-noise ratio (SNR).

 

Digital Lenses

Most SLR camera and lens manufacturers have addressed the concerns of wide-angle lens users by designing lenses with shorter focal lengths, optimized for the DSLR formats. Lenses designed for the smaller digital formats include Canon EF-S lenses, Nikon DX lenses, Olympus Four Thirds System lenses, Sigma DC lenses, Tamron Di-II lenses, Pentax DA lenses, and Sony Alpha (SAL) DT lenses. Such lenses usually project a smaller image circle than lenses that were designed for the full-frame 35 mm format. 

Crop Factor of Point-and-Shoot Cameras

Smaller, non-DSLR, consumer cameras, typically referred to as point-and-shoot cameras, can also be characterized as having a crop factor or FLM relative to 35 mm format, even though they do not use interchangeable lenses or lenses designed for a different format. For example, the so-called "1/1.8-inch" format with a 9 mm sensor diagonal has a crop factor of almost 5 relative to the 43.3 mm diagonal of 35 mm film. In most cases, manufacturers label their cameras and lenses with their actual focal lengths, but in some cases they have chosen to instead multiply by the crop factor (focal length multiplier) and label the 35 mm equivalent focal length. 

Magnification Factor

The crop factor is sometimes referred to as "magnification factor", "focal length factor" or "focal length multiplier". This usage reflects the observation that lenses of a given focal length seem to produce greater magnification on crop-factor cameras than they do on full-frame cameras. Field-of-view crop in cameras of different sensor size but the same lens focal length.

Depth of Field

When a lens designed for 35 mm format is used on a smaller-format DSLR, besides the obvious reduction in field of view, there may be secondary effects on depth of field, perspective, camera-motion blur, and other photographic parameters.

The depth of field may change, depending on what conditions are compared. Shooting from the same position, with the same lens and same f-number as a non-cropped (full-frame) 35 mm camera, but enlarging the image to a given reference size, will yield a reduced depth of field. On the other hand, using a different lens with the same field of view as the non-cropped camera (matching the 35 mm-equivalent focal length), at same f-number, the smaller camera's depth of field is greater.

This means digital SLRs will have more depth of field when shooting at the same angle (shorter real focal length) than 35mm cameras at the same aperture. This also explains why compact point and shoot cameras have almost unlimited depth of field. For them a 6mm lens is normal!

Digital Camera Sensors: Size Really Matters

For sensors, size means everything, (sensors) size does matter. 

The larger the sensor, the better the picture and the higher the resolution it can push while maintaining a good image quality. Both compact cameras and SLRs can easily reach 14 megapixels, but the difference in image quality, clarity, and noise at that resolution is remarkable. My iPhone 4s can produce 8MP image, but the quality of the image produced is far behind Nikon D70s. For sensors, the larger the better.

Sensor size can vary even among digital SLRs. These cameras generally use either APS-C or full frame sensors. Full frame is based on 35mm film. A full frame camera's sensor is 35mm wide, the same size as a frame of 35mm film. 
 

These sensor sizes are important in digital SLRs not only for picture quality, but for lens choice. Non-full-frame cameras employ a "crop factor" when using lenses intended for 35mm cameras. APS-C cameras use a crop factor of 1.5 to 1.7, meaning a lens that's 50mm on a 35mm camera works the same as a 75 to 85mm-equivalent lens on an APS-C camera. Full frame SLRs, like Nikon's D3x and  Canon's 5D don't have a crop factor; a 50mm lens acts like a 50mm lens.  

Micro Four Thirds cameras sit between compacts and SLRs, with sensors measuring approximately 18 x 13.5mm. The sensors are still much larger than compact cameras, but don't even reach APS-C size.. 

On the other end, some professional photographers use much larger and more expensive cameras than even full frame SLRs. Medium format cameras, like those made by Hasselblad, use sensors much larger than full frame.

Crop Factor

The crop factor is the sensor's diagonal size compared to a full-frame 35 mm sensor. It is called this because when using a 35 mm lens, such a sensor effectively crops out this much of the image at its exterior (due to its limited size). the focal length multiplier relates the focal length of a lens used on a smaller format to a 35 mm lens producing an equivalent angle of view, and is equal to the crop factor. This means that a 50 mm lens used on a sensor with a 1.6X crop factor would produce the same field of view as a 1.6 x 50 = 80 mm lens on a 35 mm full frame sensor.

The lens focal length does not change just because a lens is used on a different sized sensor — just its angle of view. A 50 mm lens is always a 50 mm lens, regardless of the sensor type. At the same time, "crop factor" may not be appropriate to describe very small sensors because the image is not necessarily cropped out (when using lenses designed for that sensor).

 Depth of Field

As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. 

A shallower depth of field may be desirable for portraits because it improves background blur, whereas a larger depth of field is desirable for landscape photography. This is why compact cameras struggle to produce significant background blur in portraits, while large format cameras struggle to produce adequate depth of field in landscapes.

Pixel Size, Noise Level, and Dynamic Range

Larger sensors generally also have larger pixels (although this is not always the case), which give them the potential to produce lower image noise and have a higher dynamic range. Dynamic range describes the range of tones which a sensor can capture below when a pixel becomes completely white, but yet above when texture is indiscernible from background noise (near black). 

Further, larger pixels receive a greater flux of photons during a given exposure time (at the same f-stop), so their light signal is much stronger. For a given amount of background noise, this produces a higher signal to noise ratio — and thus a smoother looking photo.