Fast Sports Zoom Lenses for Pentax

Other Features

We've already touched on the Pentax power zoom and the Sigma HSM, so all that's left is the Sigma's "OS" in-lens optical stabilization system, the Tamron's macro functionality, and the Pentax's aperture ring.

Having been designed in the ninties, the Pentax features an aperture ring which maximizes compatibility with film bodies and allows you to comfortably use manual teleconverters with it. While the aperture ring is no longer a necessity on modern lenses, it never hurts to have one.

One feature that Tamron is very proud of in its 70-200mm is its macro capability. Both the Sigma and the Pentax have a minimum focusing distance of 1.4m (which is actually somewhat burdensome), while the Tamron's is 0.95m throughout the zoom range. This leads to an effective magnification of up to 0.33x and is quite remarkable for such a long zoom lens! This gives the Tamron an edge in terms of usability, and makes up slightly for its other handling drawbacks. However, since the lens is mostly going to be used for action/telephoto photography, we aren't giving Tamron too much credit for this. True macro photography requires an effective magnification of at least 0.50x.

This Sigma 70-200mm lens is among the first Pentax K-mount lenses to feature image stabilization, so there was quite some hype surrounding this when the lens was first launched. Pictured below is the OS control switch on the lens- it features an "Off", "Weak" (1), and "Strong" (2) setting.

As you already know, every Pentax DSLR since 2006 includes in-camera image stabilization known as "Shake Reduction". This system has been improved from body to body, and in the K-5 it can compensate for movement in 2 axes. We did a quick comparison between the in-lens OS and the in-camera SR at 200mm, with the results shown below. The test was repeated three times to minimize the effects of sample variation.

It's fairly clear from the results that in-camera shake reduction is no worse than in-lens stabilization here. Perhaps at longer focal lengths in-lens stabilization might have an advantage, but not here. Since the Pentax SR system knows what focal lengths it's dealing with, its intensity is automatically adjusted to fit the lens. Thus we can conclude that the entire OS system is dead weight, and because it requires you to disable in-camera SR and manually operate the switch on the lens, you might even end up forgetting to re-enable SR when swapping lenses (side note: your camera's USER mode might come in handy for use with OS lenses, should you opt to use the in-lens stabilization).

One last thing to note about in-lens stabilization is that its effects can be viewed directly in the viewfinder. This gave us a small headache, as it sometimes generates an illusion of a lag effect in the viewfinder. Finally, we noticed that the Sigma lens has 4 more element groups than the Pentax or Tamron - this turned out to be necessary to facilitate the OS element, but also adds weight and (unnecessarily) complicates the optical design.

Other Minor Quirks

The Pentax has a narrower field of view (by 4 degrees) at the wide end than the other two lenses, and it is easier to scratch due to its silver finish and metal barrel. It's quite front-heavy, making it harder to carry around with a neck strap. However, this doesn't affect one's ability to hold the lens, due to the generous size of the focusing ring in the front. Finally, (and strangely) its zoom ring clutch isn't as snappy as the focusing ring clutch, but fortunately it isn't used as much.

The Tamron's tripod collar has no notches. Unlike the Pentax's, it doesn't snap into place at the center and at +/- 90 degrees, forcing you to look away from the viewfinder when rotating the lens. Also, its rear cap is very difficult to put on compared to the Pentax cap, as it must be lined up to a single spot.

The Sigma's tripod collar is very loose when unlocked and also lacks notches. Its rear cap, like the Tamron's, is a pain to put on.