Lens Review - Carl Zeiss Sonnar FE 55mm f/1.8

For over a year I have been using the full spectrum A7 with only manual lenses (Nikon and various other legacy glass). I've hugely appreciated this camera's ability to use any SLR lenses and the results have been fantastic, but I have missed the convenience of autofocus. I decided to see what all the fuss was about with the 55mm f/1.8 Zeiss lens. There are a lot of reviews for this lens around, but info on its IR performance is practically non-existent. Since I shoot so much infra-red it made me rather nervous, but I eventually decided to get it anyway. Can its IR ability live up to the staggeringly positive consensus on its colour performance?.. Let's find out.

Note: This review marks the start of a new format for me. I will do my best to talk specifically about how each lens copes with capturing invisible light.

      Stats
● Mount: Sony FE
● Adapts to DSLR: No
● Adapts to (other) ILC: No
● Focal Length (FF): 55mm 
● Field of view (FF): 43.7°
● Lens Elements: 7
● Lens Groups: 5
● Aspherical Elements: 3
● Aperture range (f stop): 1.8 - 22
● F-stop increments: 1/3
● Aperture Blades: 9 (curved)
● Aperture Ring: No
● Autofocus: Yes (silent)
● Manual Focus Ring: By Wire
● Min. Focus Distance (cm): 50
● Max Reproduction Ratio: 1:7.14
● Internal Focus: Yes
● DoF Scale: No
● IR offset markings: No
● Body Material: Metal
● Filter Thread Material: Plastic
● Filter thread: 49mm
● Static Filter Rotation: Yes
● Dimensions (mm): 64.4 x 70.5
● Weight (g): 281
● Dust / Moisture Sealed: Yes
● Mount Seal: No
● Manufacture Country: Japan
● Manufacturing Span: 2013 >
● Price New: £860
● Price Second Hand: £400-500
● Accessories (Included): Hood / Leather Case

     Quick IR Performance Scores

● Pure IR HotSpot (Lack of): 9/10

● Colour IR HotSpot (Lack of): 10/10

● IR Sharpness (Middle): 10/10

● IR Sharpness (Edges @ f/8): 9.5/10

● IR Sharpness (Edges @ f/1.8): 9.5/10

IR Quality / Hot Spot

Here's a new set of images, a test for IR hot spot that I'm trying out (see more about it here). Basically what you're looking for here is a clean dark sky, with no bright spots. Now, this lens does vignette badly and that rather looks like a large hotspot at f/1.8, but it probably isn't. That would be worse on smaller apertures (more towards the bottom right image). Although vignette itself rarely bothers me, if you want rid of it that process can add noise in the corners when pushing exposures. I also found Photoshop's lens profile didn't fully remove it. I hope this is because it's not properly calibrated, rather that a filter issue.

For pure infra-red shooting this lens produces some of the cleanest results that I've seen. In this worst case scenario for IR hot spots it's near perfect right up to f/22. After testing the 35mm f/2.8 lens from Zeiss I honestly expected there to be more issues here. It does start to produce a light disc opposite bright areas during the last two stops, here's a boosted example to better illustrate the issue:

However, it occurred to me that the cause of this problem may not be internal at all. Perhaps this lens is so good at bouncing stray light back out to the world that it's getting caught by the flat surface of the filter and coming back to the sensor, at angles it can't be designed to cope with. Here's an image (the normal one of which you can find in the samples below), boosted in Photoshop to show a reflected image of the bright building on the opposite side of the frame:

This suggests to me that any brightness I am getting here at all is filter related. Also the super dark corners look like the filter itself might be causing some of the vignetting. I will test that on a full spectrum image soon. If only I had a 850nm converted IR Sony A7 as well I could tell for certain if the external filter is the main issue. I will do some more tests and come back with some more info as soon as I can.

This next set was shot using a Hoya R25A filter. On a full spectrum camera this is very similar to a 590nm conversion. Things get even cleaner still here, in fact the only issue I see to any degree this time around is vignetting. This is also true of the 850nm B&W test, but on the more neutrally toned sky it's a bit more obvious. It's also affecting the colours in the corner, which can cause issues with channel swapping false colour infra-red processing.

Here is the same +3EV boosted f/22 shot (this time in colour). As you can see the bright disc is gone. All that's left is natural haze and a bit of pesky vignetting (yes, even at f/22).

Competition

Currently there are no other autofocus 50mm lenses for the Sony A7 series, but I will find something. Technically you could use a Canon 50mm f/1.8 STM with an AF adapter (and I will soon), but it will be a mostly frustratingly experience, at least on the first generation A7 bodies. On the mkII models it will work a lot better because of the added support for phase detect AF. However if you can afford one of those bodies you'll probably also want the resolving power of a lens like this, which is similar to what you get with lenses like the Sigma 50mm Art or the Zeiss Otus. This kind of quality doesn't come cheap or compact (for their respective speed) though and it can often destroy the pretty bokeh aesthetics too. That does happen here, but there is still the typical 3D pop feeling with this lens. For example - close portraits at f/2.5-2.8 can be extremely stunning.

If you're not interested in eye-bleeding sharpness or autofocus then I highly suggest looking elsewhere. You could pick up something like a 50mm f/1.7 from Pentax of Konica. It's a smaller package, even with the metal adapter (although slightly heavier), but how does a lens like this compare to the Zeiss optically... in infrared of course?

Click the image to see it full size

The above samples are from both lenses at their widest apertures. The corner performance differs wildly here due to haze. This issue affects many fast 50mm lenses, when shooting at greater distances and can start from just 1m. It's an issue that is alleviated greatly by stopping the lens down, sometimes one stop is enough and sometimes not. In this regard the Zeiss performs ridiculously well, showing almost no haze at f/1.8. However, if you stop both lenses down to f/8 the sharpness levels between the two are pretty much indistinguishable and right to the corners too. This is a testament to the fantastic value Konica lens, but it is true that many 50mm lenses perform well at middle apertures due to the focal length being a relatively easy one to manufacture.

Conclusions 

Although the last 2 full stops of aperture (f/16 - f/22) are capable of causing hot-spot related issues, it’s only true of pure IR (850nm), in extremely rare circumstances and so subtle you might never notice it anyway. At these settings diffraction is a bigger problem and that's not specific to infra-red. To all intents and purposes this lens' IR performance is extremely good! It's some of the best I've seen on any lens, since that's getting increasingly rare on more complex modern optics it makes this lens extra special for multi spectrum use.

A slightly bigger issue (but still nit picking for most people) is the vignetting. Now I wouldn't normally worry about this, but with IR it can cause a couple of problems when extreme. Corner specific noise when trying to remove vignetting can crop up due to how far IR values sometimes need to be pushed. It can also throw off the colours when pushing them around after channel swapping colour infra-reds (560-720nm). Although vignetting happens on all lenses I really expected better results from such an expensive and slow lens. f/1.8 almost seems like a joke at this price for a 50mm. Several cheap lenses from the 70s perform better and especially when stopped down. This is still a pretty minor issue, it mostly bugs me for it's price point, like the plastic filter thread.

While you could argue that this lens is a little overpriced for a 55 f/1.8, despite its amazing image quality, if you're an IR shooter as well it's an easy choice if you can afford it.

General Pros and Cons

    Pros (rated out of 10 for how awesome they are)
(10) - Sharpness and contrast is simply stunning all round (just like colour)
(9) - Accurate and silent auto focus
(7) - Good construction (metal... mostly) and great balance
(7) - Included lens hood is very nice & comes with a nice case

    Cons (rated out of 10 for how annoying they are)
(8) - Plastic filter thread (A common trend on new lenses, but unforgivable at this price)
(5) - DMF focusing is great, but inconsistent speed and lack of feedback make it difficult to use
(5) - No Aperture ring & 1/3 stops make scanning through the range painful
(2) - Strength of vignette removal often adds noise to IR

Overall Score (Colour IR): 9.5/10

Overall Score (pure IR): 9.5/10
Extremely Highly Recommended

   IR Samples

Here are some b&w infra-red images, taken with the B+W 093 filter (on the full spectrum Sony A7). This filter has a 50% transmission at 850nm (the amount of visible light that passes through this filter is nominal).

 f/1.8

 f/5.6

 f/8

f/5.6

f/2.5

Here are some colour infra-red photos taken with the Hoya R25A (Red) filter (on the full spectrum Sony A7). This is equivalent to about a 590nm conversion. All images (accept tge cellist) are processed using the 'false colour' technique:

 f/8

 f/7.1

 f/7.1

f/5.6 (fake tilt-shift)

 f/5.6

f/1.8

Bonus - Multi-Spectrum Sample

Here's an example of this lens performing in UV, colour and IR. This is using the following filters (from left to right):

Baader U
Kolari Vision Hot Mirror
B+W 093

I have put the exposure values below so you can see how much light the lens lets in for each (mostly interesting for UV - seemingly worse than the Sony 28mm f/2 lens - shocking). These were landscape images of my brother in-law, but I cropped them to portrait so that they better fitted into a single image.