How do you judge the build quality of a lens?

Question

Some lenses - mostly expensive ones - are consistently praised in reviews for their great build quality, whereas others get very low credit in this regard. To be honest, I cannot tell much of a difference when I use them. A general convention seems to be that metal is better than plastic. Is there a tangible reason for that, other than the reassuring weight and feel of metal? Is it about narrow tolerances? Fine surface details perhaps?

In concrete terms, what constitutes good build quality and why?

Answer 1

Durability, weather proofing and build tolerance are probably the main three. A well constructed lens will deal with more rigorous use and last longer without losing it's precision.

The build tolerances are also far tighter on more expensive lenses. You are talking about very small amounts of difference, but you will notice it if you set your depth of field really shallow and try to adjust the focus. There is significantly more play on cheaper lenses than on expensive ones. This extends not only to the focus and zoom gears, but also to internal alignment tracks of the lenses. The more play they have, the less precise the lens can be.

Finally, weather sealing keeps moisture and dust out of the interior of lenses when used with weather sealed bodies. This is important for use in harsher conditions as well as for avoiding mold and fungus on the inside of the lens.

Owning a couple of cheaper lenses as well as several high end lenses, I can tell you there is an immense difference. If I compare the kit lens that came with my xTi with my 24-70 f/2.8L II that I use on my 5D Mark iii or even between the $600 70-300mm f/4-5.6 IS lens and my 70-200 f/2.8L IS II, there is a huge difference in terms of how tightly the zoom and focus hold and how precisely they are able to be focused. When I reverse directions of focus on the 70-300 it takes a bit of movement before the lens elements start moving. On either of the L's though, the adjustments are immediate as soon as I move in either direction with no slack.

Similarly, if I tilt the lens down, the 70-300 will sometimes stretch out and not hold the focal length due to the weight of the lens pulling it down. The 70-200 and 24-70's are solidly enough built however that they hold their position.

Answer 2

There are quite a few things that contribute to differences in lens quality. As AJ wrote, the most obvious and frequently-discussed are things like durability, weatherproofing, tolerances. You may notice the tolerance differences by wiggling the zoom or focus ring on the lens. If the lens extends when focusing or zooming, try wiggling that extended part -- you will likely notice a very clear difference in tolerance there!

A spot where lens quality is different but often goes unnoticed is the lens body itself: better lenses will have a subtle texture to them while less expensive lenses are simply smooth plastic. It's a minor thing, but if your hands are wet (form summer heat or rain, for example), you will appreciate having a little extra solid grip on the lens thanks to the texture!