With the announcement of the new Nikon D7100 there is a lot of commentary on the Net regarding the lack of a full lineup of DX lenses made by Nikon. The argument goes that, in order to properly serve the DX user base, Nikon should fill all (or at least most of) the gaps in their lineup of DX lenses irrespective of the availability of FX lenses of the focal length in question.
I cannot say I entirely disagree with this point of view. At the same time, however, I feel it should be (and perhaps is, in fact) a much smaller deal than it is made to be.
What makes a lens good? Every application has its own order of priority, so the list below can be shuffled in a number of ways:
- sharpness and resolution in the center
- sharpness and resolution corner to corner
- color reproduction
- vignetting (lack of thereof)
- chromatic aberration (lack of thereof)
- transmission
- distortion
- maximum aperture (considered in relationship with the maximum focal length)
- near focus distance
- VR
- AF
- Handling
- Weight
- Size
- Weather sealing
- Flare resistance
As you move up the ladder for each of those characteristics, price and sometimes size and weight increase, often in a non linear fashion.
Popular wisdom says that DX lenses have the advantage of being lighter, smaller and cheaper than their FX counterparts. That is correct, everything else being equal. Everything else is, for example, vignetting, softness in the corners, CA, maximum aperture.
However, everything is NOT equal when mounting an FX lens on a DX body.
Anyone who uses full frame cameras (film or digital) knows that lens quality becomes a problem as the distance from the center of the frame increases. Vignetting, softness in corners, CA are all too common for most full frame lenses. The center 1/3rd of the frame is generally fine. We do have the option of getting FX lenses that are near perfect corner to corner. As soon as we mortgage the house. And sell the car. And hire a Sherpa to carry them.
DX lenses aim to replicate the response of an average FX lens for the DX image circle. As a result, we have to deal with the same flaws as the FX shooters. How much more expensive would a DX lens be if it provided the same corner to corner performance as the top FX lenses? We don’t really know, because very few are made. The ones that are made are expensive, so here goes one advantage of DX.
On the other hand, if you take a decent FX lens and mount it on a DX body, you eliminate the flaws that make them average on an FX body and retain the best. The DX sensor only covers the center part of the FX circle produced by the FX lenses. We can only speculate how much more would a DX lens cost if we wanted to make it as good as the center of good FX lenses. My guess is that the price differential between DX and FX lenses would be much smaller.
The one advantage of DX lenses that remains is size. Even building up DX lenses to provide exceptional quality would leave them smaller and lighter than their FX counterparts.
On the flip side, buying DX lenses will lock you into the DX format or DX crop mode on FX bodies. FX lenses work fine on either format body. There are many photographers who use both FX and DX bodies. Using one set of lenses for both bodies generally makes sense.
As always, my advice is to understand what you want to do and get the best lens for your shooting style and objective.
Sorin Vacaru's Blog 