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Laowa 6mm f/2 Zero-D MFT - Review / Test Report - Analysis |
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Lens Reviews -
(Micro-)Four-Thirds
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Page 2 of 3
Distortion
You may have noticed the "Zero-D" in the lens name - this stands for zero-distortions. However, we wouldn't sign this off completely because a barrel distortion of ... isn't zero. However, it's close enough and quite an achievement with such a tiny lens. It raises the question on why the big boys gave up on a low-distortion design, whereas Laowa can do it throughout their lineup.
Vignetting
While the distortion figures are truly impressive, the same can't be said about vignetting. However, we need to be fair here because all ultra-wide angle lenses suffer from so-called "natural" vignetting (also called the cos4 or "cosine fourth" law of illumination falloff). Even so, the RAW vignetting is on the high side at no less than 3.1EV (f-stops) at f/2. To put things into perspective again - that's nothing special with full format systems. Stopping down to f/2.8 reduces this substantially, but it never gets better than ~1.9EV, even at smaller aperture settings. Image auto-correction can come to the rescue, though, and it cuts off about 1 EV (f-stop). You may ask yourself why Laowa didn't kill the light falloff completely, but remember that this isn't lossless because any signal amplification comes at the cost of extra sensor noise. A vignetting correction of around 1 EV (f-stop) is a quite reasonable limit.
MTF (resolution)
Ultra-wide lenses are notoriously difficult beings when it comes to resolution. The difference between the center quality and the corners tends to be stark. However, despite the extreme focal length, the Laowa 6mm f/2 Zero-D MFT does a good job here.
The center is very sharp, even at f/2. The border quality is still impressively high, whereas the corners make it across the good mark, albeit just. Stopping down to f/2.8 boosts the quality across the image field. The center quality is now truly excellent, and the outer image field reaches very good results. Stopping down further only improves the depth-of-field from here. Typical for MFT, diffraction limits the image quality from about f/5.6 - and aperture settings smaller than f/8 should be avoided.
The centering quality of the tested sample was just soso (as so often). The field curvature is reasonably low.
Please note that the MTF results are not directly comparable across the different systems!
Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure of sharpness.
If you want to know more about the MTF50 figures, you may check out the corresponding Imatest Explanations
Chromatic Aberrations (CAs)
Lateral CAs (color shadows at the image borders) are somewhat noticeable at f/2 with an average pixel width of 1.3px at the image borders (and more than that in the corners). Stopping down reduces the issue, and it's actually pretty good at medium aperture settings.
Sun Stars (experimental)
Below is s sequence of cropped images from f/2 all the way up to f/11 - illustrating the Sunstar behavior. Sunstars are an aperture effect that shows up if a bright light source is part of the scene (usually in night shots).
While most manufacturers implement 7+ rounded aperture blades in their lenses, Laowa made the deliberate decision to use just 5 straight blades. While this may result in edgy out-of-focus highlights other than at f/2, it's probably reasonable to assume that if you are after a shallow depth-of-field, you will select f/2 anyway. Thus sticking to just 5 blades makes perfect sense because sun stars show up as soon as you stop down a little. You may notice that the (magnified) highlight is just a big blob at f/2 (=perfectly round aperture), whereas sun stars are forming straight from f/2.8 already. They "grow" the further you stop down, but you can still stay in the sweet spot range (f/2.8 to f/5.6) for really nice results.
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