Understanding Crop Factors and Nikon DX Lenses
I wrote this last month for a Nikon user group on Flickr which quite often got question about these two topics, but I never actually posted it. I've decided this would be a perfect place for it though. Please feel free to comment on any oversights I may have made so I can correct them.
Understanding Crop Factors and Nikon DX Lenses.
When Nikon introduced their flagship D1 DSLR in 1999 they ushered in a new era of photography, but also a new era of lens technology and lens marketing to deal with certain parts of the D1’s design, along with the design of all its successors (save the D3, 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” last August. This diagram shows an actual size comparison:
This usage of a smaller format is both a good and a bad thing – it saves Nikon and you money and uses less power, but on the other hand, it is more prone to noise and sharpness-robbing diffraction above a certain pixel concentration. The main effect however, is the “crop factor”, that is to say, the effect it has on a lens’ field of view. This is the reason you hear people say “it becomes an xx 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 (35mm equivalent) sensor. The inset shot was taken with the exact same 75mm lens on a DX-format camera like the D70. It’s the same lens, 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 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, what 100mm looked like, and so on, when on a film camera. 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. This is calculated by taking the lens’ focal length, halving it, then adding it (i.e. 400/2=200 then 400+200=600) to get the equivalent.
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 the 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. Nikon filled this gap by introducing the AF-S DX Nikkor 12-24mm 1:4G lens, which was followed by the AF DX Nikkor 10.5mm 1:2.8G – respectively recreating the perspectives of an 18-36mm and 16mm lens.
This however was of little concern to fans of telephoto shots – who simply found that the camera was saving them the effort of cropping, and so we come to the real reason for the existence of DX lenses and the explanation for why, if you don’t plan on using a D3 anytime soon, you don’t really need to care.
One of the qualities exhibited by all lenses is the image circle, that is, the size of the circle of light projected onto the film/sensor by the lens. Naturally, this needs to be larger than the recording medium it’s projecting onto, a fact that Nikon noted could work either way. “If we’re going to go for smaller sensors now,” they hypothetically rationalised, “then 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 side effect of this is that you cannot use a DX lens on film or FX cameras because their image circles are too small, as shown by this illustration:
Ken Rockwell further proves this by mounting a DX 10.5mm lens on a film body and posting the image (which can be found by following the link and scrolling up)
This is where more confusion mounts (he he – mounts – like a lens, geddit!), because it’s a common misconception, grown from a lack of information, that DX lenses are corrected for the sensor’s crop in the form of an adjusted focal length scale. 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 (1.5 for Nikon) 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 – 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.
And there you have it, folks. Hopefully this article has gone a ways to helping you better understand crop factors and DX lenses. If you have any questions there are many very helpful people on the board who are more than willing to clear up what I may have missed.
Nikon D40: AI Nikkor 35mm 1:2 | AF-S DX Nikkor 18-55mm 1:3.5-5.6GII ED
Canon AL-1: Canon FD 50mm 1:1.8 | Tokina SD 70-210mm 1:4-5.6