|
Sony FE 14mm f/1.8 GM (SEL14F18GM) - Review / Test Report - Analysis |
|
Lens Reviews -
Sony Alpha (Full Format)
|
|
Page 2 of 3
Distortion
The lens produces a native distortion of just ~1.5% which is very low for a lens in this class. The "GM" designation is well deserved here. The remaining traces can be eliminated via auto-correction with minimal impact on the image quality from here.
Vignetting
Ultra-wide lenses have a degree of "natural vignetting" anyway and it shows. At f/1.8 it is very heavy with a light-falloff of ~3.4 EV (f-stops). Stopping down to f/2.8 reduces this to just over 2 EV. Thus image auto-correction should come to the rescue here. It reduces the vignetting to an acceptable ~1.7EV at maximum aperture and a very manageable 0.8EV from f/2.8. This increases the sensor noise in the outer image field accordingly.
MTF (resolution)
The resolution characteristic of the Sony FE 14mm f/1.8 is "mixed" as you might expect from such a lens. The center performance is pretty much breathtaking straight from f/1.8 - it exceeds the capabilities of the 42mp sensor used for testing by quite a margin here. The near center is also very good to excellent in the relevant aperture range. The outer image region sees a noticeable drop in quality down to good levels at f/1.8. For best results stop down to f/5.6 where the resolution is good to very good. Please note that the borders/corners "look" mediocre in the chart because of the excessive center resolution making the difference starker than it really is. Diffraction has a more pronounced impact from f/11. Also, note that the MTFs were taken at a fairly close focus distance - test charts only go so far in terms of size and ultra-wides are ... wide. You can expect slightly better border/corner quality at "infinity" shots.
The field curvature is on a medium level and noticeable. The centering quality of the tested sample was good.
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 reasonably well controlled with an average CA pixel width of around ~1.3px. As always, image auto-correction can help here without a loss in image quality.
Bokeh
Formal bokeh tests of ultra-wide lenses are notoriously difficult. However, based on field images, we'd say that the rendering is very pleasing. Wide-angle lenses tend to have a very busy bokeh but this doesn't seem to be the case here. The background blur is smooth and even the highlights are fine as you can see below.
Sun-Stars
Another aspect that is often considered to be important is "sun stars". Sun stars are a diffraction effect at aperture edges. They are created around spot highlights at smaller aperture settings. The rendering can vary from spikey rays to diffuse blades. The latter is usually considered to be less pleasing but your mileage may vary here. The Sony FE 14mm f/1.8 GM falls into the first category as you can see below. The effect starts to show up at f/5.6 with more obvious rays at f/8. For best results, you should stop down to f/11. If you count the rays you will notice that there are 18 of them due to 9 aperture blades. You may also notice some color fringing - but keep in mind that this is an extreme example with the sun in the early afternoon here.
Note: These are 100% crops.
Flare
Due to the immense field of view, extreme ultra-wides are very prone to having strong light sources within the image field. Lenses with low-quality coating and internal reflections can produce A LOT of flare and ghosting.
However, the Sony FE 14mm f/1.8 GM is pretty efficient in this respect. The scene below is an extreme situation - and you'd normally not take such a shot anyway - but the Sony lens manages this quite nicely. There are color fringing artifacts around the sun and marginal ghosting that invades the rest of the scene. The contrast level remains very high. The ghostings and fringing increase at f/16 though - it's best to stay around f/8 to minimize the effect if needed.
|
