But the problem is, few of us can do this (there are other factors adding complexity, but let's leave that aside for now). So, what can we do? Simple - and yet infinitely complex: buy not in terms of absolute quality, but in terms of value. In good ol' layman's terms: best bang for the buck.
|Choosing a lens should be connected not with how great it is, but with how useful it is to you|
How to successfully judge the value of a lens you're planning to purchase
It boils down to these things:
- how often do you expect to be using this lens?
- in what kind of variety of conditions can it be used?
- is there overlap with existing lenses you have?
- how much of the money you use to buy it can you recover by selling it?
1. is obviously a very important factor - perhaps the most important. If you plan to buy a lens that will be used once per year, for a total of a few dozen photos, then it really is not worth it. If it's a situation that absolutely requires the lens in question (say, you go for a safari and you need a long tele) consider renting one. There isn't a formula to help you decide, but in very generic terms, if you plan to be using the lens for more than 30% of your photos, then it's probably worth buying it.
2. A small-maximum-aperture lens cannot be used as successfully in low light as, say, the AF-S Nikkor 50mm f/1.4 G. But, on the other hand, a 10x zoom can be more useful in other ways. Again, you have to consider the kind of photography you're interested in. If most of your shots are in low light and with your subjects in, more-or-less, a few steps' distance, then the fast prime is obviously a much better option and a very good value.
3. No point at all having lenses that overlap either in terms of focal length or aperture. If you have an 18-200 zoom, why also having an 18-140? If you have a 50mm f/1.4 prime, no reason at all to also have a 50mm f/1.8 one. More complex are the situations that you have almost overlapping lenses. For instance, if you own a 35-70 f/2.8 zoom and a 50mm f/1.8 prime, are you justified to keep both? That's up to you to decide. I wouldn't; 15-20mm of focal length and one stop of aperture isn't important enough to justify a second lens. What would be more important to you, the extra stop or the extra flexibility? Perhaps size and weight can also be a factor. Or, if you own several lenses that share the same filters, perhaps the odd one out (that is, the lens with a different filter diameter) might be the one to go.
4. A more tricky point, but certainly worth considering. If you're buying a lens the value of which is expected to drop rapidly, then that's a minus (buying new at least; buying used is another story). On the other hand, some lenses not only retain their value, but that can actually increase over the years. It does take a certain element of educated guessing, but here's a thought: if a lens is a design unlikely to receive an update, expect its value to remain high or even increase. In other words, buying lenses such as, say, the Nikkor AF DC 105mm f/2 or the Nikkor AF DC 135mm f/2 is so-so value in absolute photographic terms, but superb value in terms of investment - I sadly doubt it that Nikon will update the DC line.