Sensors contain millions of “photosites,” essentially buckets which capture the light coming through the DSLR's lens. The light is converted to an electric signal which gets measured, optimized, and converted to a digital image by the DSLR's processor.
The size of the buckets (photosites) is important to the sensor's sensitivity and its ability to accurately gather light in a variety of lighting conditions. Bigger buckets are better than smaller ones, because more light can be stored in them, without getting over-filled. Bigger buckets have a lesser tendency to impart noise into the image.
Early models of DSLR (Digital Single Lens Reflex) cameras contained sensors which were approximately 2/3 the size of the 35mm SLR format. Nikon calls this its DX format, while the other DSLR camera manufacturers (Canon, Pentax, and Sony) call it the APS-C format.
Nikon, Canon and the other DSLR manufacturers started with this size sensor due to serious light falloff problems, as much as two f/stops, of larger sized sensors. That meant the outer edge of the image would be considerably darker than the central area. This can occur because sensor chips require light to hit it almost directly, not at an angle. The expense of overcoming the problem in the early years of DSLRs would have made their price prohibitive.
Over time, sensor manufacturers have been able to overcome the problem of light falloff, however, full size sensors (FX) still cost more to produce, and to date are limited upper end DSLRs.
With both sensor size based DSLRs in existence, one has a difficult choice of which to purchase; APS-C (DX) or Full Size (FX). Each has its own advantages and disadvantages which must be considered.
Advantages of APS-C (DX) sensor format
- Cost – Smaller sensor cameras have a lower cost of ownership. Both APS-C cameras and lenses are less expensive than their FX counterparts.
- Lens sharpness and vignetting – As APS-C sensors use the center of FX lenses, professional lenses perform well on APS-C cameras. Vignetting is typically less pronounced on APS-C cameras than FX cameras, with older lenses. The latest FX lenses have significantly mitigated or eliminated this problem. High quality lenses manufactured specifically for APS-C cameras are generally sharp and without vignetting.
- Low-cost lenses – Lenses manufactured solely for APS-C cameras are smaller and more compact than FX lenses and generally less expensive.
- Reach – This is controversial, and I think it's generally misinterpreted. The field of view of a lens on a camera using an APS-C sensor is narrowed. This is the crop effect. The magnification, of the subject, via the lens, on the sensor is the same for both formats. What's different is that with sensors having the same number of pixels, the APS-C sensor has more pixels recording the subject, than the FX sensor, which has more image for the same number of pixels. That makes the resolution of the APS-C subject higher.
- Size and weight – Cameras with APS-C sensors are generally smaller and lighter than FX cameras. Of course, some may consider that a disadvantage.
- Noise in high ISO levels – This is the biggest disadvantage of APS-C based cameras. It's due to the small size of the sensor's photosites. For night photography, to keep noise low, one must use lower ISO levels (lower light sensitivity) and either longer exposures and/or wider apertures.
- Smaller dynamic range – Compared to FX, APS-C cameras have a smaller dynamic range, generally due to photosite size and density.
- Difficulty achieving wide-angle images – Due to the crop factor of the APS-C sensor, a normal wide angle lens of 24mm has a 36mm angle of view. Today there are wide angle lenses built specifically for the APS-C format, but they are often optically inferior to the lenses they are meant to emulate on FX cameras.
- DX lens incompatibility with FX – If you decide to move to an FX camera in the future DX lenses can only be used on the FX camera in “crop” mode.
- Lens diffraction – APS-C sensors sometimes cause more lens diffraction at small apertures than FX sensors.
- Smaller viewfinder size – The viewfinder on APS-C is smaller and not as bright as FX viewfinders.
- Higher sensitivity and lower noise – As FX sensors have larger photosites than APS-C sensors they have higher sensitivity to light, permitting higher ISO values, and have lower noise at similar ISO values.
- Large dynamic range – Larger photosites results allow for a larger dynamic range.
- Lens compatibility – FX lenses are compatible with APS-C sensor based cameras.
- Lens diffraction – FX sensors produce lower lens diffraction at small apertures than APS-C sensors.
- Larger and brighter viewfinder – FX sensor based cameras have larger and brighter viewfinders resulting in easier focusing.
- Wide-angle lenses are wide – Since there is no crop factor with FX sensors there is no reduction in a lens' field of view.
- High cost – FX sensors are more expensive to manufacture than APS-C sensors, as are FX lenses.
- Lens sharpness and vignetting – Older lenses may have reduced corner sharpness and vignetting when use on FX DSLRs. Newer lenses built for FX sensor cameras mitigate this problem.
- Size and weight – The larger internal components of FX sensor cameras cause the cameras to be larger and heavier. Personally, I don't view this as a problem.
To me, the purchase choice between an FX versus an APS-C camera primarily comes down to FX sensors' high light sensitivity and low noise at high ISO, versus APS-C sensors' lower camera and lens cost.
If low light and night photos, or action photos in difficult lighting circumstances, are important to you then you probably should purchase an FX sensor camera. If you need to keep the cost of your camera, and lenses more affordable, then go with an APS-C camera.