Pentax-DA 50mm F1.8

Focusing

The DA 50mm is a budget lens. As such, it does not feature the newest, silent SDM or DC motors. Instead, it relies on the screw-drive of the camera for autofocus. A screw-drive means that the lens is compatible with every legacy camera model. However, screw-drive focusing is considerably louder than in-lens focusing.

Autofocus

The AF delivers a surprising amount of noise, and not in a good way.  The DA 50mm is among the loudest screw-drive prime lenses that we have tested. Other lenses of comparable size and design, such as the FA 50mm F1.4, F 50mm F1.7, are quieter. For such a small and light lens, the level of noise is a disappointment. We would only expect this level of noise with larger screw-driven lenses in which there is a lot of glass to move back and forth.

The following video records the autofocus noise of the lens when used in live view with a Pentax K-3 in a low light environment (note that this is a worst case scenario).

Fortunately, the AF is generally fast and very accurate.  With the latest Pentax bodies, focus errors during our tests were almost nonexistent, even in very low light. Small AF corrections were almost instantaneous, and the noise was much less bothersome in that case. The lens' large aperture helps the camera’s autofocus system lock focus more easily.

Note the Pentax K-3 (launched in 2015) and all newer Pentax DSLRs feature a more powerful screw-drive motor compared to prior bodies.  This motor speeds up focusing speeds by up to 50%, but it may make focusing more noisy.

Live View

Focus speed is only marginally faster when using the viewfinder instead of live view. Screw-drive lenses tend to perform poorly with live view, so the AF performance of the lens in that case is actually better than average. The same level of accuracy was obtained with live view than with the viewfinder.  However, in low light the lens tends to overshoot in live view and move the AF point far beyond the target before correcting back. This results in an increased focus delay and a high level of noise. By comparison, viewfinder AF tends to make several micro-adjustments in low light, and is both faster and much quieter.

Manual Focus

Manual focus on the lens is surprisingly pleasant. The lens offers a very long throw (180°, or a half-turn) and strong dampening of the focus ring movement. This is commendable and makes manual focus much more enjoyable than with most autofocus lenses. By comparison, older AF 50mm lenses have a lifeless feel to the focus ring. The ring is positioned well and easily falls under the hand. We never observed any creeping of the focus ring.

The lens does not offer quick-shift; manual focus requires the AF/MF switch on the camera to be moved to the MF position. Note that with sufficient force, the ring can be turned manually even when AF is engaged, but this could result in damage to the AF mechanism in both the lens and the camera.  We do not recommend that this be done under any circumastances.

AF Speed

We tested the AF speed with a Pentax K-3, using both live view and the standard PDAF (phase detection AF, through the viewfinder). The subject was a black cross on a white background, about two meters in front of the camera. We used the central focus point. The lens was set at infinity before each test, and three measurements were averaged for each data point. Measurements were performed by recording the AF noise, at various levels of ambient light.

For reference, the Pentax K-3's autofocus sensor is rated for ambient light levels as low as -3EV.

This chart illustrates how performances can vary with illumination. What is somewhat surprising is that contrast AF (using live view) performs better than many other screw-drive lenses. While always slower than phase AF (via the viewfinder), it is still reliable and reasonably fast.

Verdict

The DA 50mm's AF speed won’t break any records, but is not a problem either. Accuracy is very high. The level of noise is the main problem, especially when using live view. Still, it’s hard to expect anything else from a budget lens, and thus we are satisfied with its performance.