Although this is briefly covered in the DSLR Buying Guide, the question comes back very frequently: What is a a full-frame camera?
A full-frame camera is obviously a camera with a full-frame sensor! A full-frame sensor has the same size as a 35mm film negative which is why it is called full-frame. Other recent DSLRs use APS-C sensors which are either 1.5X or 1.6X smaller linearly. A full-frame sensor is therefore physically larger. Canon uses the 1.6X crop, everyone else uses 1.5X. Other sizes are used on older models: 1.3X for Canon APS-H, 1.7X for Sigma and 2X for Four-Thirds cameras.
Full-frame sensors are known to have higher quality because they have bigger pixels. Bigger pixels mean less image noise and higher dynamic-range but – as with everything – there are variations. If you compare modern cameras, then full-frame models indeed show better image quality. However, if you compare older models, you will find the latest APS-C sensors to be better than previous generations full-frame cameras. Full-frame sensors also allow for a more shallow depth-of-field which is commonly used in classic portraits and abstract photography.
It is important to know that a camera with a bigger sensor requires bigger lenses. Therefore, expect to buy bigger more expensive lenses for you bigger more expensive camera, for same grade of lenses of course.
There are two things to consider when comparing image quality, one is the lenses you use and the other is print size:
- The smaller you print, the less image quality differences show. A high megapixel count DSLR can make very large tack-sharp prints but if you only print common sizes, then you won’t be taking advantage of it.
- Lenses are one of the limiting factors of the system and unless you use top-quality lenses which are expensive and relatively heavy you will not see the full image quality your camera is capable of. If you buy cheaper lenses after spending more on a camera body, you cripple your camera.
The crop-factor applies to the field-of-view of your lenses too. So a lens on an APS-C camera has a smaller angle-of-view, just like a longer zoom, than the same lens on a full-frame camera. This can be an advantage for shooting wildlife and other distant subjects by requiring smaller and lighter lenses.
When deciding between an APS-C camera and a full-frame one, it is important to consider the whole system and its use. There are clear advantages to both and I know shooters who carry one of each to use at the appropriate time. Same-brand DSLRs can share some lenses. In particular, full-frame lenses always work on cropped-sensor cameras but not vice-versa.