Monday, October 18, 2010

Tamron 28-75: An Introduction for DX Use

The Tamron 28-75, in it's various incarnations, has a well deserved reputation for quality optics at an affordable price. It's even more affordable if you find one on Craigslist, which is not that difficult a proposition considering how popular this lens is. Most would compare it to the Nikon 24-70 AF-S, but it's really apples to oranges. The Tamron is very competent on full frame, but the Nikon is better... albeit, much, much more expensive.

Mounted on a D80, D90 or the new D7000, the weight and size of the 28-75 balance nicely. It's a little bit longer and heavier than the Nikon consumer lenses in this range, but it's much shorter and lighter than the Nikon equivalent. The barrel is plastic, and not metal like a professional lens, but it's build quality is in keeping with the cameras in this price range. The older screw-drive versions are more desirable than the ones with built in focusing motors (BIM). The older lenses auto-focus faster; when Tamron added built in motors to their lenses, it was pretty much a kludge on top of the existing lens design...  in other words, the BIM versions aren't a true implementation of AF-S focusing.

One aspect of this lens that doesn't get mentioned much on internet forums is how nice it is to manually focus. The focus ring is nice and wide, unlike the rubber-band like afterthought on the 18-105, and it has a nice smooth dampened feeling... not quite as good as the old fluid-dampened manually focused lenses, but much nicer than the majority of the lenses out there.

On DX, the 28-75 is a bit of an awkward focal length, as it is the equivalent of 42-112mm. The limiting factor is the wide end... to be honest, if you keep this lens in your bag, you will need something wider to supplement. This usually isn't a limiting factor except in the tightest of places. It's still a great landscape lens, especially if you do stitching. To be honest, this is a lens that you tend to use a lot of at 28mm, as it's closer to the natural viewing angle of human vision than the so called 50mm "normal" (A topic for another day....)
However, the extra length that the DX-crop affords does make this a nice portrait lens. If you like shooting headshots with a 50mm wide open, try this lens at 75mm and f/2.8... you don't get the extreme subject isolation that you would get with a f/1.4 aperture, but you do make up for it with the longer focal length (which enhances foreground background isolation). The perspective at 75mm is pleasing, and at f/2.8, you have more depth of field than with the 50mm's wide open. Think of it as a flexible compromise for the 85mm lenses... not as good as what they do at this focal length, but with the added flexibility of zoom.

The constant f/2.8 is a breath of fresh air, especially if you've only shot with variable aperture lenses. However, don't be tricked into thinking that f/2.8 zooms are low-light lenses... they're not, they're just better than the consumer lenses. In most indoor situations, at f/2.8 the light still required a fair bit of ISO boost. What the f/2.8 means is that the long end of the lens is more usable than a variable aperture lens. If you stop down, this lens gets really good at f/4 and basically sits on a plateau of goodness up to f11.

If you are paying attention to your safe shutter speed (the minimum shutter speed for acceptable hand-held shots, usually defined as the inverse of the effective focal length, or 1.5x your focal length on DX), you'll notice that you run out of safe shutter speed pretty quickly on a variable aperture lens. Say you are using the venerable 18-70... Your safe shutter speed at 70mm is 1/135 seconds. However, that lens is f/4.5 at the long end. In comparison, the 28-75 is still f/2.8 at the long end... meaning that at the same ISO, the Tamron will be able to use a minimum shutter speed that is about 2.6 times faster than the Nikon 18-70.

Fans (or people who spent a lot of money) of the Nikon 24-70 like to point out that the Tamron 28-75 has only average resolution wide open. This is a given, but to be honest, the Nikon is not at it's best wide open lens is. At f/2.8 the Tamron is reasonable sharp in the center and average in the corners. If you've ever used a 50mm AF-D, the resolving power is similar. In fact, if you look at the MTF curves, the Nikon 50 AF-D from f/1.4 to f/2.8 looks like a continuation of the Tamron's MTF curve. In other words, it's quite serviceable, it's just not exemplary

One final note: In many ways, the Tamron 17-50 is the DX version of this lens. Tamron has recently updated it with their VC image stabilization technology, and by all accounts it's brilliant (provided you get a decent copy, but  quality control is another post). My bet is that the 28-75 will get stabilization before the Nikon  does.

1 comment: