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!