FX/DX lenses

Smart Photography - - Ask Uncle Ronnie -

1. FX lenses are made for FX bod­ies and DX lenses are made for DX bod­ies. So a 50mm lens be­haves as a 50 mm lens on an FX body and it be­haves as a 75 mm lens on a DX body. Why does a 50mm DX lens de­signed for a DX body be­have like a 75mm lens on a DX body? 2. Would the use of FX lenses in a DX body, com­pared to DX lenses in a DX body, yield much bet­ter re­sults? 3. Which one is a bet­ter up­grade for a Nikon D7000 user--- D7100 or D600 (for Por­trait and Travel)? Amar Saha, via email Great ques­tions! 1. When you use a FX lens on a DX body, the pe­riph­ery of the im­age is cut off be­cause the DX body has a smaller sen­sor. Hence the man­u­fac­turer saves on cost and weight by pro­duc­ing a smaller and lighter DX lens that just cov­ers the DX sen­sor. Mind you, the fo­cal length is not changed, only the size and weight is. So ob­vi­ously, when this DX lens is at­tached to the DX body, it too be­haves like a 75 mm lens. By that we mean that the 50 mm DX lens pro­vides an an­gle of cov­er­age that a 75 mm lens would have of­fered in the 35mm for­mat. Just for your knowl­edge, the size of the sen­sor also de­ter­mines the an­gle of view. 2. When you use a FX lens on a DX body, only the cen­tral por­tion of the lens is used to pass the light through. This re­sults in bet­ter edge sharp­ness on the DX sen­sor (sharp­ness at the pe­riph­ery of a lens is al­ways less com­pared to the mid­dle. Us­ing only the mid­dle por­tion of the lens thus avoids loss of sharp­ness at the edges). It also helps to re­duce cor­ner dark­en­ing. DX lenses (and mod­ern FX lenses) are de­signed dif­fer­ently. They are ‘tele­cen­tric’, mean­ing to say that the light com­ing out of the lens does not spread out. This per­forms a very im­por­tant task – it makes the light hit the photo-diodes (on the imag­ing sen­sor) per­pen­dic­u­larly. This is an im­por­tant re­quire­ment for the photo-diodes – it helps to avoid/re­duce cor­ner dark­en­ing and loss of sharp­ness at the edges. So to come back to your ques­tion – “Would the use of FX lenses in a DX body yield much bet­ter re­sults?” – the an­swer is ‘no’. But if you do use an older ver­sion of an FX lens (one that is not tele­cen­tric), then you stand to gain some ad­van­tage. 3. As a bet­ter up­grade for D7000 (for ‘por­trait and travel’), I would opt for the D600 over the D7100. Both are 24 megapixel, but the D600 is a full­frame ( FX) model. This means bet­ter/smoother tonal­i­ties, lesser dig­i­tal noise (es­pe­cially at higher ISOs), and bet­ter con­trol over the depth of field.

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