You may sometimes have a choice between two similar lenses; one with a fixed aperture, the other with a variable aperture, priced higher and lower respectively. Which is the better buy?

Firstly let me say that there is no simple answer to the question of whether it is better to buy a cheaper variable-aperture lens or a more costly fixed-aperture lens. If price is a prime concern then the less costly lens will always look more appealing – and it may not be significantly lower quality than the more expensive lens if used appropriately.

Let’s take a specific example using two lenses that I have reviewed recently for What Digital Camera magazine. The lenses in question are both Nikkors with a zoom range of 24-120mm. The variable-aperture lens is rated at f/3.5-5.6 and the fixed-aperture lens is f/4. The guide prices are £550 and £1050 respectively, so that’s roughly a doubling in price for the fixed-aperture lens.

Of course there’s more to the differences than price alone, as the table below shows.

Lens comparison table

Clearly the more expensive lens is bigger and heavier; it contains more glass and benefits from a more sophisticated anti-shake (Vibration Reduction) system. It is also Nano-Crystal Coated and in tests to date all lenses that benefit from this new Nikon technology have proved to be excellent performers all-round.

MTF graph for a fixed-aperture Nikkor.

MTF graph for a variable-aperture Nikkor.

The two lens-test charts accompanying this article illustrate the technical performance of these two zooms. The cheaper lens was tested on a DX-format body (it was previously tested on a full-frame D700 and didn’t perform quite as well so the results shown here are probably the best that could be expected from the f/3.5-5.6 lens). The pro-spec lens was tested on a full-frame Nikon D700 and has produced an outstanding set of MTF curves.

So what does this tell us? Clearly the more expensive lens turns-in a better technical performance, which is a good thing – but is it twice as good? Not numerically but if every ounce of quality is important then the extra performance is worth having. If it were my money then I would save a bit longer and buy the f/4 fixed-aperture lens rather than rushing out to acquire the cheaper lens sooner. But I certainly wouldn’t dismiss the variable-aperture lens either. They’re both good zooms and the final choice must come down to each individual’s financial wealth and photographic demands.