Tag Archives: optical lenses

Pentax’ PLM and DC lenses explained

2 Mar

DC (“direct current”) is a kind of focus motor that has been used in Pentax lenses for some time. While it is quiet, it’s not entirely silent. Pentax’ most recent 55-300mm lens features a new type of motor branded as PLM. Here is what Pentax representatives had to say about it in an interview:

The PLM design can quickly activate and allow for faster focusing, but the lens element must be low weight due to limited power (torque).

The DC motor can generate high power (torque) with deceleration mechanisms, which is better for lenses with larger focusing elements. A DC motor can be driven at high speed, but there is an issue that a little sound is generated.

Generally, we use the what we feel is the best focusing mechanism for each specific lens design.

It would be reasonable to suspect that the 55-300mm’s new optical formula and narrower aperture were needed to allow the faster, silent motor to be used, although it should be noted that, like the recent 18-50mm kit and non-kit zoom lenses, the new 55-300mm lens is collapsible to a somewhat smaller size, with the difference between collapsed and uncollapsed size being more pronounced in the 18-50mm.

In the interview, the representatives went on to explain that they do not expect to see PLM in a large aperture lens any time soon, instead putting their money on researching other kinds of motors as well as algorithms to improve autofocus.

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Best Sigma APS-C normal lens?

19 Aug

In this post, I’ll look at the different options to get an affordable APS-C lens in the 28-35mm range from Sigma. This translates as the equivalent of 42-52mm focal length on a full frame/small format/”35mm film” camera.

I’ve populated the field with three of Sigma’s new “Art” lenses – two primes and one zoom, as well as an older 30mm Sigma prime lens that was the predecessor of the new “Art” version with the same focal length. All four lenses are HSM (hypersonic motor) lenses.

As a bonus, I threw in the 28mm f/1.8 Macro, which is not an HSM lens. Test results are from DxOMark, technical information from Sigma.

Five APS-C normal lenses from Sigma compared.

Five APS-C normal lenses from Sigma compared.

I’ll mostly let the table speak for itself – as per convention, green is good, red is bad.

What you should notice is that while the 35mm f/1.4 Art is clearly an amazing lens, the 18-35mm f/1.8 zoom trails it only slightly in distortion and chromatic aberration, while being equally sharp. At the relatively slightly lower price, this looks like an amazing deal (but yes, it’s very long for a supposedly 35mm maximum zoom lens).