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Autofocus

In the tested lens you can find the linear HLA motor (High-response Linear Actuator), that is supposed to combine increased speed with an exceptional accuracy.

We decided to check these declaration by joining the Sigma with the Sony A7R IIIa and the A7R V. In both cases the autofocus was completely noiseless. When it comes to the speed, a lot depends on the settings as it often happens with the Sigmas. There are two modes, AF-S and AF-C. In case of the first one the mechanism is not that fast in the full range of distances, and focusing often lasts even 0.8-0.9 of a second. What's more, the mechanism can very often move in the wrong direction and momentarily also waver. All this changes completely when you pass to the AF-C mode. The lens transforms into a rocket that is able to cover the whole range and confirm the right focal point in 0.3-0.4 of a second. This time is so short that you don't have to use any range limiter but if you decide to do so you are able to shorten this time even further, to less than 0.2 of a second. These are excellent results.

Still, it is important that the accuracy of the mechanism is very good. It didn't change no matter what combinations we tried in our test so it didn't depend on the body we attached the lens to. It didn't matter whether we worked in our studio or outside. I admit the working culture of the autofocus is worth an expensive, top-of-the-range prime.

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When it comes to the front or back focus tendency we didn't notice its traces no matter what body we used in our test.

A7R IIIa, f/2.0

A7R V, f/2.0

Focus breathing

Focus breathing tests show reframing images as you oversharp them. We conduct the test by manually passing from the minimum focusing distance to infinity with the aperture stopped down; then we check how the field of view of the lens changed as a result.

After conducting a significant number of tests now we think we are also able to determine some reference points. A frame change ranging from 0 to 5% we consider to be low. Between 5 and 10% you can speak about medium levels. Usually such values constitute also the maximum efficiency level of any breathing compensation algorithms, present in some bodies. Between 10 and 15% focus breathing is high, above 15% its level can be called very high.

The test video of the Sigma lens is presented below:

On the basis of the recording above, comparing freeze-frames before and after oversharpening, we can estimate that the breathing of the tested lens is 31%, a really high value, not conforming to a lens that might be used by both filmmakers and photographers.

The Sigma broke its own disreputable record, broken by the S 500 mm f/5.6 DG DN OS model (28%), and we can only propound a hypotesis that Aizu engineers should take a remedial course in breathing correction of lenses with longer focal lengths or at least prioritize this issue differently.

At the end we should add that, with the present level of focus breathing at the minimum focusing distance the effective focal length of the tested lens increases to near 260 mm and the aperture speed decreases to f/2.6.