Lenses, sen­sors & im­age cir­cles

NPhoto - - Feature -

DX su­per-wide zoo ms: Smaller choice

If you want a su­per-wide-an­gle lens for a DX-for­mat Nikon like the D500, you’ve no choice – you have to get a DX-for­mat lens, typ­i­cally a 10-24mm zoom, and al­most all are vari­able-aper­ture ‘con­sumer’ lenses.

DX stand ard Zoo ms: More for am­a­teurs

Again, the only way to get a use­ful range of fo­cal lengths in a stan­dard zoom for a DX-for­mat Nikon is with a DX-only lens, typ­i­cally in the range 18-55mm or 18-105mm. There are some con­stant-aper­ture lenses, but the only Nikon one is the old and ex­pen­sive non-VR 17-55mm f/2.8.

Tele­pho­tos : DX has an ad­van­tage

With tele­pho­tos, though, an FX-for­mat lens makes sense, even on a DX cam­era. The ef­fec­tive fo­cal length in­crease is a ben­e­fit rather than a drawback, so Nikon’s 70-200mm f/2.8 be­comes a 105-300mm f/2.8 equiv­a­lent – and you can still use it if you switch to an FX-for­mat Nikon later. You can use FX lenses on DX cam­eras but the lens’s ef­fec­tive fo­cal length in­creases by a ‘crop fac­tor’ of 1.5. All lenses need to pro­duce an im­age cir­cle large enough to cover the sen­sor area. Nikon’s DX sen­sor is smaller than its FX sen­sor, so it’s pos­si­ble to make smaller, cheaper and lighter lenses specif­i­cally for this for­mat. The drawback is that these lenses are lit­tle use on FX-for­mat Nikons (ex­cept in Nikon’s lower-res­o­lu­tion ‘crop’ mode). Some­times, though, a DX-for­mat lens is the only op­tion for own­ers of DX cam­eras be­cause al­though you can use FX-for­mat lenses too, their longer fo­cal lengths usu­ally rule them out for every­day and wider-an­gle pho­tog­ra­phy.

The larger area of the FX-for­mat sen­sor in the D750 means that you need lenses with a larger im­age cir­cle to cover it – in other words, full-frame or FX-for­mat lenses. You can phys­i­cally mount smaller DX lenses on these cam­eras, but they must switch to a ‘crop’ mode that uses a smaller area of the sen­sor with half as many pix­els (ac­tu­ally, rather fewer than half), so this is an emer­gency mea­sure rather than some­thing you’d want to do ev­ery day. The larger im­age cir­cle eas­ily cov­ers DX sen­sors too, of course, so you can use FX lenses on DX cam­eras as well.

Dx sen­sor 24x16mm Sma llerDXlens imag e cir­cle

50mm lens on a ‘crop’ sen­sor

Ac­tu­ally looks like 75mm

fx sen­sor La rger FX lens imag e cir­cle 36x24mm

Ac­tu­ally looks like 50mm

50mm lens on FX cam­era

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