lens - What is the difference between DX format and FX format lenses, and which to choose for what purpose?

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What is the difference between DX format and FX format lenses, and which to choose for what purpose?

What are the pros and cons of both?

Answer

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  A FX lens is deigned to properly project an image onto an FX sensor - say 36mm x 24 mm nominal

  
  


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  A DX lens is designed to properly project an image onto a DX sensor say 18mm x 24 mm or half the area of an FX sensor.

  
  

Actual sensor sizes vary with manufacturer and even amongst sensors from the same manufacturer but the above is close enough or discussion.

A DX lens mounted on an FX body will usually not fully illuminate an FX sensor (usually it will at least vignette in the corners. But even if it does apparently fully illuminate the larger sensor it will not have been designed to perform well over such a wide area and the results at the edges can be expected to be bad.

If you use an FX lens on a DX body the middle half of the AREA of the lens will be used. This tends to have better performance (superior MTF etc) so use of an FX lens with a DX sensor usually improves sharpness and contrast. However, there are sometimes reports of a given FX lens working worse with DX sensors.

FX lenses are usually spec for spec, heavier , larger and substantially more expensive than a same spec DX lens. If you have a DX sensor system you may gain very little by using an FX lens of the same spec but the weight and cost and size be larger.
If your aim is to impress wit your musculature, disposable income and carrying power the FX lens may be better but otherwise may not be worth the difference in cost.

If you have a DX sensor system and buy FX lenses they will wok on an FX sensor if you upgrade (assuming same manufacturer /lens mount.) BUT if you buy DX lenses and upgrade they will not be suited to the FX system. The partial exception is that some FX cameras will accept DX lens and will mask out the outer half area of the sensor so that the lens and sensor act as if they are a full DX system. An examples is the Nikon D700 which provides 12 MP with an FX lens and 6MP with an DX lens.

A DX camera will image a lesser target area than a FX camera with the same lens because the lens still projects the same image onto the sensor location in each case BUT the DX sensor only occupies half the area being projected onto. Thus 100% of the DX sensor image consists of less target area than does 100% of the FX sensor image. This leads to the typical APS crop factor - typically but not always = 1.5 for APSC cameras.

This leads to effective focal length being multiplied by the crop factor. So a 50mm lens becomes 75mm, an 18mm becomes 27mm and a 500mm mirror lens becomes 750mm. This can be good and bad. A 50mm lens is usually bought because it is the classic prime lens. An f1.4, 0r 1.7 or 1.8 or all 3 are available from every major lens manufacturer. BUT on an APSC camera the 50mm becomes 75mm and the photographic perspectives are quite different. To get 35mm effective on an APSC camera an 33.33 mm would notionally be used and a 35mm is the usual nearest offering. The 500mm mirror becomes 750mm and this is usually acceptable. The 188mm wideangle becomes a 27mm semi wideangle - an entirely different appearance. The crop factor affects FX and DX lenses equally when used on a DX camera BUT which is used may affect future decisions. eg a DX camera owner may desire a 50mm f1.x prime. 50mm pries are almost always FX lenses, even when sold for DX use. On reason for this is that they are often legacy lenses based on glass designs which may have been tweaked a little but is still something from the film days. This often makes then even better on a DX system that uses only the lens centre but leads to upgrade agonisings.

If they have no intention of upgrading to an FX system then a 35mm may well be purchased to get the 50mm equivalent effect. If a DX to FX upgrade is envisaged at some stage then the choice is 50mm which acts like a 75mm now and like an 50mm with FX, OR a 35mm now which acts like a 50mm now and a 35mm with FX senor. Give the choice of 35mm FX or 50mm FX the 50mm would usually be the clear first choice for FX users and the 35mm for DX users unless the later sensor upgrade option was of importance.