IMAGE SENSOR SIZES - FX DX CX

DEFINITION

FX, DX, and CX are Nikon’s nomenclature describing the size of an image sensor. Each camera and lens is designed around a specific image sensor size. All of Nikon’s image sensors are designed with a 3:2 aspect ratio. FX is the largest sensor size Nikon produces. It is also known as full frame, because its size is similar to that of 35mm film. DX is about 57% smaller in area compared to FX. CX is about 68% smaller than DX and 86% smaller than FX. Camera phones and point and shoots are significantly smaller than CX.

SIZE MATTERS

Why does size matter? Different sensor sizes affect both noise and depth of field. Noise is related to the sensitivity of a camera sensor. Because larger sensors have more real estate, they can have larger size pixels, or a lower pixel density. Larger pixels allow more photons to be collected for each picture. More photons mean more information. In low light situations, when using higher ISO (high sensitivity), larger pixel sizes yield images with less noise. Although noise can be considered pleasing with certain effects, typically noise is not desirable, as it takes away from the image detail.

EXAMPLE

Consider two scenarios, for example, an image taken with a 12MP FX camera sensor at ISO 1600 (Nikon D700) and an image taken with a 12MP DX camera sensor at ISO 1600 (Nikon D90). The only difference is that the FX sensor has larger size pixels because it has the same number of pixels on a larger size sensor. Both pictures are taken at the same ISO sensitivity. Yet less noise will be present in the FX sensor than the DX. To read more on noise and how it affects an image, read more about sensor sensitivity.

DEPTH OF FIELD

Depth of field also varies with sensor size. Depth of field is the distance from the focal plane in which an image is to be considered within focus. It is a subjective matter and depends on how an image is observed by the human eye. The relationship of the size of the observed image or print, to the size of the image captured by the sensor, affects depth of field. Larger sensors have less depth of field, and smaller sensors have more depth of field.

This depth of field concept is not easy to grasp at first. Now let us consider depth of field from a different perspective by defining crop factor. Crop factor describes a lens’ field of view compared with different sensor sizes. Typically crop factor is a referenced to an FX full frame 35mm sensor. DX size sensors have a crop factor of 1.5 compared to an FX sensor. Because the sensor is cropped the apparent zoom is closer. Therefore a focal length of 24mm on a DX sensor is equivalent to a 36mm (1.5 x 24) on FX. So the overall image appears zoomed in or closer on DX with lenses with the same focal length. CX size sensors have a crop factor of 2.7 compared to FX. So a 10mm focal length on CX is equivalent to a 27mm focal length on FX format.

Depth of field is directly related to focal length and aperture. Depth of field decreases as focal length increases and aperture decreases. The relationship between sensor size and depth of field is derived from focal length. Because smaller sensors require smaller focal lengths for the same zoom or field of view, the depth of field is greater for the same field of view for smaller sensors.

Depth of field is why smaller cameras don’t require much focus accuracy and can be fired quickly without thinking much about focus. Contrary, larger sensor cameras have complex autofocus systems that depending on the lens require high focus precision. Also larger sensors ultimately give the most depth of field control. They can be opened up for the shallowest depth of field, or they can be stopped down for increased depth of field. Shallow depth of field can be beneficial when blurring out undesired backgrounds or emphasizing depth.

COMPATIBILITY

Compatibility between formats is much like blood type when giving or receiving blood. Type O blood can donate to anyone but can only receive from type O, while type AB can receive blood from anyone but only donate to type AB. This is much like camera and lenses designed for different sensor formats.

FX lenses render images for full frame so they are capable of being used on cameras with smaller sensors. However lenses designed for smaller sensors cannot be used with larger size sensor cameras. For example, DX lenses cannot be used on FX cameras because the lenses do not render an image for full 35mm sensor size. Therefore the outer edges will be darker or vignetted when using a DX lens on an FX camera. This allows DX lenses to be smaller and lighter, because they do not need all the glass to render a 35mm sensor.

CONCLUSION

Overall, it is best to use lenses that are designed for the same sensor size of the camera. Using lenses that are designed for smaller sensors on larger sensor cameras will typically cause vignetting, and using lenses designed for larger sensors on smaller sensor cameras is not ideal because you pay for the extra glass in both price and weight!

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Photography Basics | ISO (Sensitivity) | Depth of Field