Fast Sports Zoom Lenses for Pentax

Handling

How do these lenses handle? Let's take a look.

Focusing Mechanisms

Let's shift gears and take a look at some more-practical matters. As we mentioned earlier, all three lenses feature autofocus and internal focusing; however, their focusing mechanisms differ.

The Pentax features a screwdrive autofocus mechanism and a KAF2 lens mount. This means that the lens can be used on any Pentax camera with autofocus and it'll work. This also means that autofocusing is somewhat noisy, although not unbearable.

The Tamron features the same screwdrive mechanism and lens mount, with one small catch that we'll touch on later on. As far as the autofocusing itself goes, however, it is about as fast as that of the Pentax, and slightly noisier.

The Sigma is high-tech. It features HSM focusing technology, which is Sigma's implementation of the Pentax SDM "Supersonic Drive Motor". This means that the lens focuses almost inaudibly and that its focusing performance is slightly faster than that of the other two lenses. Unfortunately, HSM also greatly reduces compatibility. The Sigma lens uses a KAF3 mount, which means that it can only autofocus on Pentax DSLRs newer than and including the K10D.

The Sigma is probably the best of the three in this category, as its focusing technology is both quieter and faster, and the reduced compatibility doesn't affect most everyday photographers.

Manual Focusing

The Pentax's focusing ring features an innovative AF/MF clutch which when pulled back allows you to focus manually by rotating the focusing ring. When pushed forward and locked in the autofocus position, the lens will focus automatically and the focusing ring will not rotate. This system is quite ingenious, as it allows you to steadily hold the lens while being able to quickly switch to manual focus when needed.

The Tamron really drops the ball compared to the Pentax in this respect, unfortunately. Its focusing ring uses a sliding clutch mechanism just like the Pentax, but it has two fundamental flaws! First, when the ring is slid to the manual-focus position, the lens does not automatically disconnect the screwdrive lever from the camera. This means that even if you have the lens in manual-focus mode, you won't be able to freely focus manually unless the camera is also set to manual focus. This added step could cost you some precious moments on the field, as it requires you to move your hand or even look away from the viewfinder! Second, the focusing ring rotates freely when locked in the autofocus position, making it very difficult to steadily hold the lens (considering how much space the focusing ring takes up).

The Sigma, again, is high-tech. But Sigma got it all wrong and for some reason reversed both the order and the sliding direction of the focusing and zoom rings, making it hard for a Pentaxian to get used to them. With that aside, the Sigma adopts Pentax's "quick-shift" system which lets one adjust the focus freely at any time, even in autofocus mode. This feature is very handy, and due to the fact that the Sigma's focusing ring has a larger throw than the internal focusing mechanism (gears are used), the focus can be adjusted very precisely. Unfortunately, this also means that if you want to quickly move from infinity to minimum focus, you might have to take your hand off the ring and rotate it twice. To enable full manual focusing on the Sigma, it is enough to toggle the AF/MF switch found directly above the OS switch on the left side of the lens.

Concluding, we think that the Pentax deserves to win this category, as its clutch mechanism lets you hold the lens very steadily. The Sigma comes second, although its reversed rings may take a while to get used to. Last comes the Tamron, as it's design is simply flawed. We found that this is its major drawback overall.

Zooming

All three lenses feature a generously-sized zoom ring as well as internal zooming. The only thing really worth mentioning is that the Pentax and Sigma rings move very smoothly, while the Tamron ring feels a bit loose halfway through the lens. Also, the Sigma zoom ring is at the end of the lens instead of in the middle, which makes it a little hard to get used to and also prevents you from getting a perfect grip. However, this isn't as much of an inconvenience as the Tamron's freely-rotating focusing ring. Lastly, the Pentax lens features "Power Zoom", which is in fact faster and more controllable than conventional zooming, but seen by many as a useless feature. We'll leave it at that; Pentax has abandoned Power Zoom since the FA series and doesn't support all its advanced features on DSLRs. The Pentax zoom ring uses a clutch mechanism just like the focusing ring; when slid out, conventional zooming can be carried out.

Weight

Weight is always an important consideration, as heavier lenses often pose a large inconvenience and don't make it into your bag as often. The Tamron lens wins this category, weighing in at only 1150g. The Sigma weighs 1430g due to the presence of the HSM motor and OS technology, while the Pentax is also heavy at 1510g. In its case, the power zoom mechanism and metal finish are to blame. The large difference in weight between the Sigma and Tamron can certainly be a big decision factor when choosing between the two, especially for those who do a lot of backpacking and wildlife photography.