Pentax-DA* 50-135mm F2.8 Review

Autofocus Performance

The Pentax-DA* 50-135mm F2.8 will use the body's screw drive autofocus mechanism or the internal SDM motor depending on the body used. All modern DSLRs will use SDM, but if you're in doubt, refer to the bullets below.

The following cameras will utilize the lens's screwdrive mechanism:

• Pentax *ist D, *ist DS, *ist DS, *ist DS2, *ist DL2, K100D, K110D
• Pentax K10D with firmware below version 1.30
• Samsing GX-1L, Samsung GX-1S
• Samsing GX-10 with firmware below version 1.30

The following cameras will utilize the SDM autofocus:

• Pentax K10D with firmware version 1.30 or newer
• Samsung GX-10 with firmware version 1.30 or newer
• All other Pentax DSLRs (K100D Super or newer)
• Pentax K-01

Autofocus Tests

For this review, a Pentax K-5 with firmware version 1.13 and Pentax-DA* 50-135mm F2.8 lens made in 2007 were utilized. 

Since the available hardware and firmware only allowed the SDM method to function, our tests will measure the time that the SDM motor takes to displace the focus from the closest focusing distance (1.0 meter) to the farthest focusing distance (infinity), and vice-versa. These tests were done 5 times each, then the resulting time values were averaged to provide a better representation. The test target has a luminance equivalent to 5.3 EV, which mimics indoor shooting conditions.

We then used the Sigma 50-150mm F2.8 II EX DC APO HSM with the same K-5 to compare its speed. Note that the closest focusing distances for both lenses are equal (1 meter).

Here are the focusing speed test results:

The fact that the Pentax lens has slightly longer focus throw, and that the Sigma lens focus motor turns in the opposite direction may explain these results.  The Sigma lens felt a bit faster overall in practice, and the test results confirm this, through the different isn't very big.

AF Verdict

The autofocus speed of the Pentax 50-135mm is perhaps its weakest point.  Many will plan to use this lens for sports, and for this purpose the 50-135mm simply does not deliver.  On the other hand, for other applications you will probably find its autofocus to be sufficient.

One interesting observation we made is that there's some latency between the time that the user activates the AF and the time that the SDM motor actually starts to move, which might account for some of the perceived sluggishness.

The fact that this lens has silent autofocus to begin with, and that it supports quick-shift reduces the overall severity of the slow AF speed in our eyes, though it's still something that sports shooters should consider when deciding between this lens and other options, such as the Sigma 70-200mm OS HSM, which boasts much faster AF. 

SDM Failure

User reports as well as a survey we performed in 2012 show that the 50-135mm lens is particularly prone to failure of the SDM system compared to other Pentax lenses.  At Photokina 2012, a Pentax engineer claimed that a design issue was responsible for this, and that the rate of SDM faliure will drop significantly in all copies of the 50-135mm made during or after the second half of 2012.  We have observed a decrease in the number of reported failures on the forum since Photokina, but we unfortunately cannot make any further conclusions as to whether or not the engineer was right.

Disabling SDM to use Screwdrive

If the SDM in your 50-135mm fails, it is possible to deactivate it and force the camera to use the screwdrive mechanism.  However, if your SDM has not failed, then we do not recommend disabling it.  The screwdrive AF system is much more noisy than SDM, and user reports have shown that it is not faster in practice.