Full frame DSLR: do you really need one?

Owning a full-frame DSLR, such as the Canon EOS 5D Mark II or the Nikon D700, is the ambition of many photographers. 'After all,' they think, 'bigger must be better' - and these cameras, with their supersize sensors, are what all the top professionals seem to use.

Switch to a full-frame DSLR and your pictures will automatically be better - or so the hype goes. But this is only partly true… A full-frame sensor camera just takes different - not necessarily better - shots than Nikon or Canon DSLRs with the more standard APS-C-sized sensor.

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So, what exactly do we mean by 'full-frame'? A full-frame DSLR from Nikon or Canon or Sony has a sensor that's the same size as a frame of traditional 35mm film, measuring 36x24mm. The more popular APS-C sensor cameras have much smaller 22x15mm sensors. This means, a full-frame sensor has over 2.5 times the surface area of an APS-C sensor.

Sure, size has certain advantages, but there are distinct drawbacks, too, to making the switch up to a full-frame DSLR.

Camera selection
To upgrade to a full-frame DSLR, prepare to pay a premium. For instance, the street price for a Canon 5D Mark II body is around £1,700, while Canon's flagship EOS-1Ds Mk III body is £5,250! The added production cost of the bigger sensors means that full frame DSLRs aren't launched all that often. Today's Nikon D800 announcement, nearly four years after the release of its predecessor, the Nikon D700, is a shining example of this.

Because full frame DSLRs are less frequent, though, this means you don't get as much choice as with APS-C.

Image quality The biggest advantage of full-frame is image quality and image size. Both the Canon EOS 5D Mark II and 1Ds Mark III full-frame DSLRs, for instance, come equipped with 21.1-megapixel sensors, and crucially these sensors are over two-and-a-half times bigger - and have much larger individual pixels (or photosites) - when compared to APS-C cameras.

It is not the number of pixels that really counts - APS-C models such as Canon's EOS 7D and EOS 550D have almost as many photosites, thanks to their 18-million pixel count - but the size of the photosites is crucial to image quality.

Bigger individual light sensors capture more light - and this means that less electronic noise is created. You notice this most as you increase the ISO setting - with this noise creating a coloured mosaic pattern that is particularly noticeable in shadow areas.

Crop-factor effect
The size of the sensor also changes the amount of the scene captured by the camera. Although APS-C and full-frame cameras can share many of the same lenses, the visual effect they provide is different. It is the angle of view that actually changes - as the smaller sensor covers less of the image projected by the lens.

This is known as the crop factor - which compares the angle of view with that seen by a traditional full-frame 35mm film SLR. With full-frame DSLRs the crop factor is 1x - so effectively can be forgotten. A 24mm lens gives the same angle of view as a 24mm lens before the age of digital cameras.

[caption id="attachment_534900" align="aligncenter" width="300" caption="Top: Full frame DSLR at 24mm. Bottom: APS-C sensor DSLR at 24mm."]Full Frame DSLR do you really need one Full Frame DSLR do you really need one [/caption]

An APS-C sensor sees a smaller angle of view - with a crop factor of 1.6x. This means the same 24mm lens actually captures the angle of view of a traditional 38mm focal length setting (24x1.6=38). So if you want to capture sweeping wide-angle vistas, a full-frame camera allows you to take in more of the scene in front of you than an APS-C model with the same lens.