Rokinon 8mm vs. 10mm vs. Sigma 8-16mm vs. Pentax 10-17mm Ultra-wide Showdown

Focusing

Now that we've scrutinized everything about the lenses we could, from the construction to various aspects of their optical qualities, our last dedicated assessment is of how the lenses acquire those images.

Rokinon Prime Lenses - Manual Focus Only

Just to reiterate what we already know, only two of the four lenses feature autofocus - the Pentax and Sigma zooms. The two Rokinons, while featuring the electrical contacts to communicate aperture and exposure control to and from the body, are manual focus only. We didn't have any issues focusing other than what was previously mentioned - at the minimum focus distance of the fisheye prime.

From a tactile perspective, both lenses have hard stops on either end of the focus distance ring (as opposed to a clutch that allows infinite twisting such as in the Sigma zoom), and the action is firm yet smooth. Neither lens had any sticking points as the focus ring traveled back and forth and yet was excellently dampened to securely maintain whatever position the focus distance was set.

Pentax and Sigma Autofocus - Technical

The Pentax zoom features the KAF mount, which means that the lens' aperture can be controlled and adjusted by the camera body (hence the lack of an aperture ring) and that it features screw-driven auto-focus. This is where the A and F letters in the KAF acronym are derived.

The screwdrive type is exactly as its name sounds - a motor in the camera body connects to the lens via a physical shaft, driving it mechanically. This causes it to spin and thus focus between the different focusing positions. The below image shows the lens mount of the smc DA 18-55 F3.5-5.6 WR (which shares the same mount except for the red weather sealing o-ring), with the female portion of the screw drive mechanism circled:

To show where the screwdrive mechanism on the lens mates to the camera body, the below .gif animation highlights the location on a Pentax K-3:

This method of autofocusing gives a very distinct whirring sound, not unlike your carpenter's screw gun (just thankfully not as loud). Because it is entirely mechanical, screwdriven lenses have proven extremely reliable. In fact, we can't think of a single incident where a lens of this type had its autofocus fail, and many of these lenses are 15 or more years old. And it's very quick, especially with Pentax's latest camera bodies that feature continuously improving motors that drive the lenses faster and slightly quieter.

The "problem" with this type of autofocus is merely an aesthetic one - frankly it's jarring to hear, especially amongst the sea of silent focus motors that have become the standard of today. Additionally, competition aside, it's especially displeasing in situations that call for the photographer to be discrete; weddings in particular come to mind.

The Sigma zoom, on the other hand, is of the KAF₃ variety, with the ₃ signifying that it is a lens devoid of screwdriven autofocus yet features an in-lens motor (Sigma's propriety HSM, akin to Pentax's SDM or DC motors) for quick and silent autofocus. Examine the Sigma mount closely and you won't any screwdrive shaft for the motor in the body to pair with, however instead you'll see two rectangular gold contacts for lens-powered silence:

In case you may find yourself looking in your DSLR's mirror box to see if the two pins are there, this is what you should be looking for (shown using a Pentax K-3):

Lastly, we should mention that if you plan to pair the Sigma with a Pentax body from before 2006, it will not autofocus as that was when those two gold pins first made their appearance.

Pentax and Sigma Autofocus - Performance and Verdict

There's not much more to say - they both were very quick and achieves accurate results. With neither lens did we experience autofocus issues. The one caveat to that is when using Live View with the Sigma at night - because it's max aperture is F4.5, it really stresses sensor-based contrast detect focusing.

Obviously the Sigma is silent, however we had no issues with the audible focusing of the Pentax Fisheye. It's so blazingly fast (especially in good light) that it seemed to cancel out how loud and jarring it might have been.

Like the Rokinons, the Pentax features a hard stop at either end of the focus ring's travel. The Sigma is the one standout from the group in this regard because it has the much more premium focus clutch that permits unrestricted twisting of the focus ring in either direction. Additionally, both zooms feature Quick Shift focus. What this means is that no matter how you have the lens or camera body set, either to Autofocus or Manual Focus mode, you are free to rotate the focus ring as you desire, manually overriding the focus of the lens by twisting it. Because of how fast and accurate both lenses were, we honestly never resorted to using it.

Lastly, the rectilinear zoom is the only optic in this group to incorporate a physical customizable feature on the lens itself - the "AF/MF" selector switch to alternate between auto and manual focus. This switch and its two positions can be clearly seen in the animation to the left. When supporting the lens in your left hand, It's positioned in the perfect spot for your thumb to toggle it, and without taking your eye away from the viewfinder to find it. We found ourselves enjoying the very reassuring click when manipulated. The 10-17mm FE features no such a selector switch and must only rely on the camera body's lever (on the right, as seen from the Pentax K-3) to alternate between focusing modes.

The penultimate page in this review, our Sample Gallery with collections from each lens, is next!