When Nikon introduced their flagship D1 DSLR in 1999 they ushered in a new era of photography - but with it came a new era of lens technology and marketing to deal with certain parts of the D1’s design, along with the design of all its successors (save the D3 and D700, of course).
This design feature was the 16x24mm CCD sensor, which is a little less than half the size of a frame of 35mm film, which is 24x36mm. Nikon dubbed this smaller sensor the “DX format”, and its larger brother received the name “FX” with the release of the D3. This diagram shows an actual size comparison:
This usage of a smaller format is both a good and a bad thing – it's easer and cheaper to manufacture, takes up less space in the camera, and uses less battery power - but on the other hand, it's more prone to noise and sharpness-robbing diffraction above a certain pixel concentration.
The main effect, however, is the “crop factor”, or the effect it has on a lens’ field of view. This is the reason you hear people say “it becomes an x millimetre when I put it on the D40”, or “I can’t wait to use a proper 14mm on the D3”. This is also the reason Nikon offers you lenses marked “DX”, but we’ll come back to them.
This sounds very complicated, but is in fact very simple. What it means is that the smaller sensor simply crops the centre out of the full image circle produced by the lens – hence the name. Consider this example:
Let’s assume that the original was taken with a 75mm lens on a D3, with its FX sensor. The inset shot was taken with the exact same 75mm lens on a DX-format camera, like the D40. It’s the same lens and the same scene – but because the size of the sensor is different, the field of view is different. This is the same effect as opening Photoshop or Picasa and cropping the middle out of the picture – that’s it.
This forms the explanation for that phrase “it becomes an xx millimetre when I put it on the D40”; the reason being that because the people who first had to deal with crop factor were migrating from film, they naturally instituted a system of comparing equivalent focal length. These people knew what 50mm looked like on a film camera, what 100mm looked like on a film camera, and so on. Take one of these lenses and put it on a digital, however, and it looks different. So they compare equivalent lengths – a 50mm on digital looks like a 75mm on film, a 300mm on digital looks like a 450mm on film. At no point does the lens actually change its focal length – it simply appears to.
The DX sensor has a 'crop factor' of 1.5x (i.e. an FX sensor is 1.5 times larger), so to find the focal length with an equivalent field of view, take the lens focal length and multiply it by 1.5. (It might be easier to half the focal length, then add that half - for example, a 200mm lens looks like a 300mm on digital because half of 200 is 100, which added makes 300).
Having got that down-pat we can now return to the DX lens, and the accompanying question of “image circle”.
When the D1 came out with its DX sensor, many purchasers were disappointed that their previously ultra-wide 14- and 16mm lenses took on the appearance of comparatively narrow-angle 21- and 24mm lenses, with horrific distortion to boot. It was obvious to Nikon that wider lenses would be needed to fill this gap - but wider lenses are big and expensive. This is partly because of the requirement to fill the 35mm frame with an image, circular fisheye lenses not being in vogue for most professional photographers.
To do this, a large 'image circle' is needed. The image circle - that is, the circle of light projected onto the film/sensor by the lens - by necessity must be larger than the piece of film or sensor that its projecting onto. Nikon noted that this could work either way - ultra-wide lenses were difficult to develop because they had to have a a suitable image circle, but a DX sensor didn't need such a large image circle. “If we’re going to go for smaller sensors now,” they hypothetically rationalised, “then all our lenses can have smaller image circles”. And so the DX lens was born – the smaller sensor allowing for a smaller image circle, which then meant for a smaller amount of glass in a smaller lens, offering savings for Nikon and lower weight, size, and price for you.
The difference in image circle sizes is now shown by superimposing them on the earlier sensor size diagram.
As you can see, the blue image circle of a DX lens needs only to be big enough to cover the blue DX sensor. The red 35mm image circle is adequate for both.
Accordingly Nikon introduced the AF-S DX Zoom-Nikkor 12-24mm 1:4G lens, which was followed by the AF DX Fisheye-Nikkor 10.5mm 1:2.8G – respectively recreating the perspectives of an 18-36mm and 16mm lens. The reduction in required size for the image circle meant that Nikon could reduce the size of the lens significantly.
Meanwhile this was of little concern to fans of telephoto shots – who simply found that the camera was saving them the effort of cropping, and Nikon was saving them the money of buying full-frame lenses.
The side effect of this is that you cannot use a DX lens on film or FX cameras because there isn't enough picture to cover the film - like this:
… which is what you get if you use the AF-S DX 12-24/4 at 12mm on a D700.
(image by Flickr user nateOne, used under CC BY 2.0 license).
One source of confusion with DX lenses, though, is whether or not the focal length displayed on them is corrected for the sensor’s cropping of the image. This post, from the Digital Photography School forums, is one of many:
“I have the Nikon 18-55mm DX lens. On my D40 am I seeing 18mm or am I seeing 18mm x 1.5 based on the crop factor of the sensor for an actual view of 27mm? This being a DX lens I'm not sure…”
The answer here is that the poster is seeing the equivalent of 27mm on FX, achieved by calculating 18*1.5, or half of 18 again, because DX lens are not adjusted for crop factors. He/she would see exactly the same thing if he/she looked through an 18mm full-frame lens, which would also look like a 27mm lens. The only difference is the smaller image circle.
Therefore, all in all, we can compress this down to these statements:
- Smaller sensor size offers many benefits but is misleading when comparing focal length measurements because the…
- Smaller sensor crops centre out of image circle produced by lens. This creates the impression that lens focal length has changed.
- It hasn’t – field of view has. Calculate the field of view equivalent to a lens on a 35mm film camera by taking the focal length and multiplying it by the crop factor of 1.5 or by halving the focal length and adding that half again. I.e. a 50mm lens on digital looks like a 75mm lens on film.
- DX lenses are different because they produce a smaller image circle – this makes them smaller and cheaper.
- Due to this, you will waste film if you use a DX lens on a film camera because the image circle will not cover the film.
- Furthermore, DX lenses are not adjusted for the smaller format – you still need to apply the crop factor.
- And finally, for those aspiring to a D3 or D700 – you can use DX lenses on the D3, but it will automatically crop down to DX size for you and throw away 7 of its 12 megapixels. If you plan on moving up to a pro-end Nikon DSLR (as they’re all likely to go FX in the next few years), it’s probably best to start picking non-DX lenses now where you can.