A few days ago I had the chance to test this lens, the Sony FE 55mm f/1.8 ZA Carl Zeiss Sonnar T*  for the Sony full frame E-mount, which a very good friend of mine, Giuseppe Milo (check out his website), recently bought. Initially it was a test to compare that lens with my copy of the Fujinon XF 35mm f/1.4 R, something similar to my previous test with the Canon 50mm f/1.4.
But unfortunately the lack of focus distance scale of the Fujifilm lens sometimes is a problem, and I didn’t notice that with the fuji I wasn’t on infinite focus, but it was focusing a little bit earlier, not by much, but still it wasn’t a reliable test anymore, and it has to be re-done (I know, it was very dumb, but that’s the price you pay when you do things on a rush).
Anyway, the test with the Sony Sonnar 55mm was perfect, and here I’m going to write my impressions, and a sharpness test.
To be clear, the test has been done on a Sony A7 camera, on a tripod and using a remote shutter to avoid any possible shakes, and I’ve done multiple shots from for any single aperture step, starting from f/1.8 and then passing for any of the standard single stops (f/2, f/2.8, f/4 and so on until f/22 which is the minimum aperture possible with this lens).
Here you can find a summary of the specs for this lens:

Build Quality

I have to say that this lens, with the lens hood on it, it’s quite long, maybe not as long as the Sigma 50mm f/1.4, it’s 71mm long, and with the cap it arrives to 87.7mm, so not exactly a pancake lens. On the other side is quite lightweight for its size, but at the same time if feels very sturdy, for what I could see was build with metal, no plastic here, not even on the focus ring, and if you take in account that, it’s not heavy at all overall, there’s no wobbling whatsoever in this lens, the focus ring runs very smoothly, and you can tell why this lens has the Carl Zeiss logo on it, it’s insanely well build.
The filter diameter it’s only 49mm, which is a good thing if you tend to use filters on these type of lenses (I personally don’t, but I can see somebody using a polarizer on it for some reasons), a good thing simply because smaller filter size = smaller prices for filters.

Autofocus

I was impressed, really. It was fast, maybe still not as fast as a DSLR can be, but very, very close, it focused very quickly, even trying several time to focus close subjects and to infinite, and it was always quick, maybe not 100% accurate, but none really is, and again I did test it on the Sony A7, the first full frame mirrorless camera made by Sony, and I can assume that on newer body, like the A7 II would be just better. The other thing it really impressed me, it was the fact that it was completely silent. I couldn’t hear a single tiny noise coming from it. Now, all the lenses I have on my Canon body have USM, which is the Ultrasonic Motor, and while you can’t hear the noise made by the motor that moves the lens inside, you can still hear a bit of noise made by the components moving inside, that’s normal and all the lenses I ever own, Canon or Fujifilm, they do this tiny noise. With the Sony Sonnar T* FE f/1.8 ZA I couldn’t hear absolutely anything. I have no idea how they achieve that, but well done Sony/ Carl Zeiss.

Image Quality

Here I have good news and bad news, and to be honest one of the bad news I’m not sure if it’s caused by the lens or by the camera (I really suspect the second one). But lets start with the right foot as we say in Italy, so good news first.
Just to be clear, as usual I took a crop from the center of the frame, and from one of the corners, in this case the right top corner, as shown in the photo below:

The good news
It’s very, very sharp, and on the center frame is already very good in terms of sharpness straight from f/1.8, and nowadays that’s what you would expect with a modern lens, especially from a lens with the Carl Zeiss logo on it, and the Sony Sonnar T* FE f/1.8 ZA, and on the corners it’s a bit soft, but it’s still quite good, and to be honest if you shoot wide open the 95% of the times you don’t care that much about the corners as it will be out of focus anyway, so in terms of sharpness at the aperture set wide hope we already have our thumbs up. Below you can see the first crop of the center and corner of the frame at f/1.8.

Center & Corner @ f/1.8

Now, I'll skip the shot taken at f/2 as the difference between f/1.8 and f/2 are marginal they look exactly the same, and you wouldn't expect much differences from only 1/3 of a stop, but you will find the high resolution file at the end of the article if you really want to be super picky and check the shot at f/2.8.
For me is pointless, and I'll go straight to f/2.8, which it makes more sense.

Center & Corner @ f/2.8

After one stop and 1/3  the sharpness improves a lot, especially on the corners we can see a dramatic increase in sharpness, still not perfect, but a very good result and definitely better than many other lenses I've seen in the past (definitely better than the Canon EF 50mm f/1.4 USM). The center frame is already very sharp and improved, but not as dramatically as it was already good from the start.

Center & Corner at f/4

Obviously one stop down and it gets better, well, the corner was already very good and improves slightly, essentially the corners just get better and better everytime we stop down, and they already super sharp.

Center & corner at f/5.6 and f/8

I've grouped f/5.6 and f/8 simply because they looked the same to me, I really couldn't see the differences between these two apertures, and I've checked three times to be sure I wasn't looking at the wrong files, but nope, exact same results. And that's the sweet spot of this lens, between these two aperture stops is where this lens gets the best results, tack sharp from corner to corner, very impressive results.

Center & Corner @ f/11

At f/11 you start to see a tiny little decrease in sharpness, but it's so tiny I had to flick between the photos several times to see the difference in sharpness, it's still tack sharp from corner to corner and I can't see any hint of diffraction, which is a very good thing.

Center & Corner @ f/16

At f/16 diffraction kicks in, there's nothing you can do about it apart to limit the damage, and I have to say that Sony / Carl Zeiss did a very good job here, it's not as sharp as before obviously, but still very sharp and definitely very usable shot, none of my Canon lenses are that good at f/16.

Center & Corner @ f/22

At f/22 the sharpness goes down, diffraction is at his maximum, but overall it's not that bad actually, it's au pair with the sharpness at f/2.8, which is not bad, possibly the center frame is the part who suffers the most decrease in sharpness, but again, overall the diffraction is very well controlled.

The bad news
Somebody of you might already spotted what's the main problem of this lens: chromatic aberration.
I didn't want to say anything on the tests above, but this lens suffers quite a lot of chromatic aberration or color fringing if you prefer this term. Now, almost any lenses have this problem, especially in very contrasty photos, like in our case, but generally once you stop down a couple of stops it goes away. But the problem here is that you have to stop down to f/11 to have them completely gone, which is rather bad in my opinion. Now, it's not the end of the world, as most of the times you fix it with one click on Lightroom, but from a lens with the Carl Zeiss name on it I would honestly expected a bit better.
They did a great job to have a very sharp lens, and for what I could see vignetting is noticeable only wide open, which is normal, and once you stop down by a stop it's completely gone, and again, it's something you can fix in a second in post production. So, seeing that much chromatic aberration is a bit of a let down, but again, it's easy to fix, and I wouldn't worry about it that much to be honest.

The other big issue is the amount of ghosting, just look at the photo below (click to zoom):

A detail of the area interested by the ghosting (clilck to zoom):

Essentially the sun is reflecting on the white surface of the building, and it's generating quite a lot of ghosting, but if I have to be honest I'm not sure this is a lens issue, but a sensor issue, as my friend Giuseppe showed me in the past in several occasion, the Sony A7 sensor suffers from heavy ghosting, especially visible when you photograph at night and you have various light sources like street lights for example, which is rather surprising to see an issue like that to the manifacturer that makes the 40% of sensors in the world, they should know how to make a sensor, and you don't see any of these problems in other brands like Nikon or Fujifilm for example (the latter one use a non-Bayer filter, but behind the filter there's a sensor made by Sony).
Down here you can see the same scene, shoot a minute before we took the photos with the Sony, but photographed with my Fuji X-T1, I know it's a bit out of focus, but I just wanted to show that there's no ghosting there (click to zoom):

Absolutely nothing, that's what I wanted to expect from the Sony A7. And bear in mind that that's visible at any aperture, from f/1.8 to f/22, the only difference is that the ghosting becomes more defined the more you stop down the aperture.

So I would really like to see if somebody can test the same lens but on a different body, like the more recent Sony A7/A7S II, just to be sure that it's 100% a sensor issue and the lens have nothing or little to do with that.

Bokeh

The bokeh is a rather essential factor for a lens like this one, and I have to say that it's very good on this lens, thanks to the 9 blades, it's very smooth and creamy, and I can't really complain on the quality here, who loves to shoot wide open to separate the subject as much as possible from the background will be very pleased by the bokeh of this lens, again, thumbs up.

Pro

• Very sharp lens once you stop down a bit, and still decent even wide open
• Lightweight
• Very good bokeh
• Extremely well build
• Nice color rendition
• AF is super fast and insanely silent

Cons

• Chromatic aberration till f/8
• Expensive

Conclusions

Now, I know I pointed out a few problems of this lens, but as I said, one it's easy to fix in post production (the chromatic aberration), and the other issue I guess it has to be done with an issue with the Sony A7 sensor itself. Apart from those "issues", the only other downside I can find is the price, as the price tag is at around $999/€1049, which is not low, but it's rather hard to find cheap Carl Zeiss lenses around, and this one is certainly not one of them.
But, it's a very well build lens, very sharp, the bokeh is beautiful, and not much vignetting, it's lightweight, the Autofocus is blazing fast and it does not make any single tiny noise, so I think the price is almost fair, and I'm pretty sure the price will probably come down a bit in the next months.

High Resolution Files

Special thanks to my friend Giuseppe Milo who let me test his lens for this review, please check his website and his flickr page :-)