Camera Lenses: a Crash Course


Yet another design challenge for lenses is usability. The ultimate lens might be something like a 10-400mm-e f/1.4 zoom. So far as anyone knows it would be impossible to even design such a lens without atrocious optical problems like the ones just described. But even ignoring all that, the lens would probably weigh 10 lbs and totally dwarf the camera in size. So keeping a lens light enough in weight and small enough in size for general portability is yet another challenge, and the answer involves compromises none of us really like, including using plastics instead of metal for the housing, skimping on the tightness and smoothness of the way the lens feels as you turn the zoom and focus rings, and of course accepting a slower maximum aperture, as discussed above.

Weather sealing: I don't know how much it adds to weight, but another usability issue is weather sealing, meaning water-proofing. If you shoot outdoors in the rain or in significant spray, such as when rafting, you'll either need to buy a more expensive weather sealed lens, or tape a clear plastic bag around the barrel of the lens and the camera, leaving the front open. This is obviously going to be a nuisance if you have to rotate the zoom ring or focus ring on the barrel.

What to buy?

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Fig. 14. It all comes down to capturing the moment

Now that you've plowed through all this detail (and it's only a decent beginning as to what there is to know) you have enough background to start thinking about what should be the first lens you're going to buy.

The obvious first choice has got to be the manufacturer's kit lens for the camera body you've opted for. The kit lens is rarely the best optical performer available, either in sharpness or lack of distortions. It was designed to be small, light, and inexpensive above all else, while still weighing in as optically decent, if not stellar. In fact, the manufacturer may well be taking a loss on it or barely breaking even. Given that the latest cameras are above 10 megapixels even at the entry end, the kit lens will probably not really do the camera justice, except perhaps at the middle apertures. But it takes the disciplined technique of an expert photographer to take advantage of a better lens; and that expert photographer may well own a kit lens to use when hiking or traveling and minimum weight overrides all other considerations.

There is an alternative approach that many serious photographers will recommend: buy a prime. We've looked at the merits of primes in size, weight, and optical goodness above. A prime enthusiast will urge you to buy a moderate wide angle or normal focal length prime, such as 35mm-e or 50mm-e, with your camera, then add more as time passes. You now know enough to understand the limitations of having just one focal length available, but it's also true that you can't take every picture in the world, and learning to make maximal use a single focal length can pay dividends down the road.

If you're buying the camera for a specific purpose, such as birding or sports or macro or astronomy, you may well have very specific advice from an expert in your area of interest. For bird photography, for example, you'd be hard-put to have a long enough lens, and you may have been urged to buy a 300mm or 400mm f/4. Don't hesitate to follow such advice. A kit lens is a general-purpose tool, not a be-all-and-end-all tool.

Further reading

Hopefully, the above has given you a good general idea of the things you need to know in order to make lens buying decisions. For more detail (mostly technical) here are some other pages available on the web:

1. Decoding those cryptic lens barrel markings:

2. Wikipedia and Camerapedia lens entries:

3. Cambridge in Colour article on lenses and optics:

4. More about depth of field: