FX, DX and foc al len gth

The size of the sen­sor in your cam­era makes a dif­fer­ence

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Nikon’s ‘full-frame’ FX sen­sor for­mat has the same di­men­sions as a frame of 35mm film and when fo­cal lengths are de­scribed as be­ing stan­dard, wide-an­gle or tele­photo, it’s this for­mat that ev­ery­one refers to. How­ever, the same fo­cal length on a smaller-sen­sor DX cam­era be­haves as if it were half as long again. So, a 28mm wide-an­gle lens on an FX cam­era would give the view of a 42mm fo­cal length lens on a DX cam­era – not re­ally wide-an­gle at all. Look at this the other way around and to get the equiv­a­lent of that 28mm wide-an­gle view, you’d need 18mm. Hence the term ‘equiv­a­lent fo­cal length’ that’s of­ten listed in a lens’s spec­i­fi­ca­tions.

A DX sen­sor has a ‘crop fac­tor’ of x1.5. Mul­ti­ply the fo­cal length of the lens by this amount to dis­cover the ‘equiv­a­lent fo­cal length’ – or the one re­quired to give the same view as with an FX sen­sor

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