You probably heard already that the most digital cameras and DSLRs have a form factor (Crop-factor), it have an effect like a window enlargement. This implies that the millimeter information on the lens of a digital camera distinguished from a traditional 35-mm-camera. If you for example, use a traditional 85 mm lens with a DSLR than you will not really use 85mm.
A Canon works with a form factor of 1.6 so as to an 85mm you have 135mm effective. Nikon works with a form factor of 1.5 so as to an 85mm lens you are using a 127mm lens. This caused by people with changed from traditional 35mm to digital some perplexings.
Now there are still full-frame cameras – at these cameras are 85mm still 85mm. They don’t have any form factor, there is no multiplier – the lens is still what it is. There is some hook, like always! If you are screw a lens which was conceived for an APS-C camera (most cases) on a full-frame camera you get the effect of zooming.
If you are buying a full-frame DSLR, you just can use the advantages of it, if you are using the right lenses for this camera type. There are also some high-priced lenses who work excellent with full-frame-DSLRs and don’t cut the picture