Lens Review - Carl Zeiss Sonnar T* FE 35mm f/2.8

This was one of the first lenses to come out for the FE mount & A7 series cameras. It typifies the dream of having a really small full frame camera that's also able to pull off some impressive image quality.

For a relatively normal fov prime with a lacklustre aperture it actually isn't that small. What's worse is that it costs a lot considering those specifications. For colour use it mostly justifies the badge price with some generally stellar sharpness and decently fast auto focus. Can it pull a similar rabbit out of its hat for infra-red though? Let's see...

      Stats
● Mount: Sony FE
● Adapts to DSLR: No
● Adapts to (other) ILC: No
● Focal Length (FF): 35mm 
● Field of view (FF): 63.4°
● Lens Elements: 7
● Lens Groups: 5
● Aspherical Elements: 3
● Aperture range (f stop): 2.8 - 22
● F-stop increments: 1/3
● Aperture Blades: 7 (curved)
● Aperture Ring: No
● Autofocus: Yes (silent)
● Manual Focus Ring: By Wire
● Min. Focus Distance (cm): 35
● Max Reproduction Ratio: 1:8.33
● 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): 61.5 x 36.5
● Weight (g): 120
● Dust / Moisture Sealed: Yes
● Mount Seal: No
● Manufacture Country: Japan
● Manufacturing Span: 2013 >
● Price New: £550 - 700
● Price Second Hand: £350 - 400
● Accessories (Included): Hood (ALC-SH129)

     Quick IR Performance Scores

● Pure IR HotSpot (Lack of): 2/10

● Colour IR HotSpot (Lack of): 2/10

● IR Sharpness (Middle): 8/10

● IR Sharpness (Edges @ f/8): 5/10

● IR Sharpness (Edges @ f/2): 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.

Like most lenses, this one vignettes noticeably when wide open and that can look like a large hotspot. Here there actually is a large hot spot at f/2.8, but usually its vignetting that's largely responcible for this confusion. With this lens the opposite illusion occurs. It almost looks like a perpetual vignetting, all the way up to f/22. You'll still notice the hot spot is more obvious closer to the bottom right of the set. There is so much stray reflection here that it's difficult to see where the true value of the clear sky actually is, even with access to the same scene in every aperture. The hotspot will always be present, even if its hidden in detail.

Another sign that the hot spot is bad is being able to tell how many aperture blades the lens has from the reflected light. In a traditional lens flare that's quite normal, but with hot spot it's on the disturbing side.

This video helps illustrate the hot spot performance at f/22. As well as the strong central circular hot spot, the reflection issues can continue outwards to the very edge depending on where the bright areas are in the scene. Although the main spot is concentrated when fully stopped down (f/22) it's often noticeable and annoying at f/8. These issues never fully go away, even wide open (f/2.8) and that can make it very difficult to use in general.

This colour IR set was shot using a Hoya R25A filter (which is roughly 590nm). Usually when using this colour infrared filter most of the hot spot issues improve or go away entirely, but here it's so extreme I didnt even need to shoot a clear sky to show how bad it is. This example also helps to illustrate how the hot spot effects the colour of the overal scene, because the reflection is concentrated to the blue channel. This makes colour very difficult or impossible to process.

Lens Flare

Here are some samples at different wavelengths to show how it copes with unavoidable lens flare in each. These are all shot at f/8. As you can see the more infra-red light that comes through the filter the more obvious the flare becomes. It's actually very good in colour and even when external filters are used like this. In IR however it looks downright horrible. I wouldn't advise shooting into the sun when taking infra-red photography on this lens.

Kolari Vision Hot Mirror (Colour)

Hoya R25A (Red / 590nm)

B+W 093 (850nm)

Sharpness

The sharpness of the Zeiss 35/2.8 is pretty good in the middle. I'll give it its due credit and say it almost lives up to the badge & price here, but on the outer edges of its full frame I'd label it as wholy average. There's nothing to write home about here and it left me rather disappointed.

Competition

There are now two Zeiss 35mm lenses for the FE mount. The newer f/1.4 version is the size and weight of the moon however, so is in no way (shape or form) comparable to this, almost pancake-like, lens. If you want something like this on a tighter budget there are other options. Not much can rival the small size of the Zeiss (and certainly not the weight), but there are some interesting alternatives.

On the left here we have a Canon 40mm f/2.8 STM lens with an auto focus adapter. Unfortunately this isn't a fast focusing system on the first generation A7 bodies, but it's performance is uncomfortably close to the Zeiss for colour photography. For infrared photography however things change completely. The Canon is a superb performer in IR, producing very little hot spot, even at f/22. Considering this costs less than a 1/3 of what the Zeiss does (including the adapter) it's extremely good value for money. I will be reviewing this lens in more detail soon.

On the right we have a Nikon 35mm f/2 AF-D lens. As yet there is no way to focus this automatically with the A7 cameras. Although it looks like an AF adapter won't be too far off. The adapter you see here allows filters to be mounted behind the lens, which has huge image quality implications for full spectrum users like myself. I also feel the need to point out here that without the adapter this lens is roughly the same size as the Zeiss, while being a whole stop faster. Clearly Sony & Zeiss have issues making truly compact lenses with the short flange distance of the A7 cameras. When it comes to infrared image quality and lack of hot spot this Nikon lens also hits it out of the park. At under half the cost of the Zeiss (again, including an adapter) this also puts the Zeiss to shame, even if it's more for infra-red and subject isolation than sharpness.

Overall Score (Colour IR): 3/10

Overall Score (pure IR): 2/10
Very Poor IR Performance

Conclusions 

As good as this lens is for colour photography it doesn't often relate to infrared. Anything shot with apertures smaller than f/5.6 are a gamble, which isn't necessarily obvious when looking through an EVF either. Although contrast is still good it's wholy dependant on hot spot, which one of the worst I've seen. Infra-red image sharpness isn't as good as it is with colour either, although it's not the extreme difference that I saw with the Sony 28mm f/2 lens.

The included lens hood is both good and bad. On the the good side; it keeps the lens really small while protecting the filter really well. On the bad side; if your filter doesn't fit inside the hood then you can't use the hood or the lens cap at all. Circular polarisers would be a pain (although they always are with lens hoods), but you'd still have no lens cap. If you're going to be brave with a hood design like this then include a second 'normal 'hood and provide a 49mm lens cap as well. This solution seems rather short sighted.

35mm is my favourite focal length for a general walk around lens and I like that it shares the same filter size as the Zeiss 55mm and Sony 28mm FE primes. Despite not being particularly fast this could have been my favourite lens for IR on the Sony A7 and that's what makes this lens such a disappointment. Unlike the Sony 28mm its mostly hot spot that kills this lens for IR use. Bright sunlight can give some decent results, just make sure you have enough central detail to cover up the hot spot and be very aware how big/bold that hot spot will be on your chosen aperture. I gave colour IR a slightly higher score for a couple of reasons. Firstly, despite some bad results, some images can look great and secondly - you can avoid the blue channel to get clean B&W IR images.

General Pros and Cons

    Pros (rated out of 10 for how awesome they are)
(9) - Fast, accurate and silent auto focus
(7) - Good construction (metal... mostly)
(6) - Extremely small & light, although not for it's specification
(4) - Included lens hood is interesting (some caveats to that though)

    Cons (rated out of 10 for how annoying they are)
(9) - Infra-red Hot Spot is extremely bad, almost makes it unusable for IR
(9) - IR colours are tricky because the hot spot is in the blue channel
(7) - Nasty looking lens flare in IR
(7) - Image quality should be better for the price (mostly corners)
(5) - Focus by wire speed is inconsistent, lack of feedback make it difficult to use
(5) - No Aperture ring
(3) - Plastic filter thread (unforgivable at this price)

   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/22

f/2.8

These two B&W samples (above) are opposite apertures. This is to show how a hot spot can be hidden inside the detail of an object. This can be acceptable if you keep it away from flat tones (like this sky), as long as your expectations are fairly low. If you're looking for a very clean result here it's likely that you'll never be happy with this lens. Even wide open (f/2.8) it produces a noticeable glow from the middle of the frame.

 

f/8

This last B&W shot illustrates how a medium sized hot spot at f/8 might not be very noticeable on it's own, but if you look at the bright grass in the middle it looks overexposed. It's actually not clipping here at all, it's just washed out by the addition of the hot spot.

Here is a colour infra-red video that I took with the 35mm lens. This has had it's motion stabilised via YouTube, so it's a bit rough, but it is better than it was.

f/5.6

These next few colour infra-red samples taken with the Hoya R25A (Red) filter (on the full spectrum Sony A7). This is roughly 590nm. All images are processed using the 'false colour' technique, unless the foliage is blue or it's been turned B&W like this next one:

f/5.6

f/5.6

f/5.6

f/5.6

f/4

f/4

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.

IR Hot Spots - Part 1

For those who don't know what an infra-red Hot Spot is, here's an example (above). This is not a normal lens flare (the sun is directly behind the camera here). It's not something that can be fixed by using a lens hood, nor is it something that changes with a different brand of filter.

     The Cause
The culprit is the lens itself and unfortunately it cannot be fixed. It's caused by bright parts of the exposure reflecting off the coating inside the lens (and/or the elements) and back to the sensor. These coating are designed to absorb light and stop it from reflecting, but how well this works for infra-red light is quite random.

It's worth noting that even when a lens is highly susceptible to this internal reflection it may only be obvious in 10% of your images or less. It's not only dependent on scene contrast & angle of light, but it shows up more clearly on dark flat tones and is deminished by wider apertures.

     The Consensus & The Problem
There are a few websites that talk about hot spots and what lenses it affects. Both Life Pixel & Kolari Vision have a list (the latter being the most extensive). These, and many other websites that mention the hot spot issue, generally categorize lenses as simply either good or bad. However, due to subjectivity and various reporters, I don't feel that this black and white evaluation is the best approach.

After shooting primarily infra-red on a mirrorless camera for more than a year, with various types of lenses (old and new) I'd like to re-evaluate how IR hot spots are reviewed. I see this issue as something that affects every lens to varying degrees (like sharpness or CA). I've used several lenses described by several sources as not suffering from hot spots and still found the issue (although they do usually suffer from it less than others) and not just when stress testing.

My aim is to make a new review-category for each lens that I use. Something that will better show infra-red photographers how well each lens copes with the hot spot issue and better aid them in future acquisitions.

     The Plan
The above example (from the Mitakon 50mm f/0.95 @ max aperture of f/16) is the worst I've seen from this lens, it's also one of the worst hot spot I've ever seen. There are a few reasons why this scene shows the issue so clearly and this gave me an idea. I plan to use a similar set up to measure the IR performance for each lens that I test. Let me first explain the technical details of this test...

For this test I will use an 850nm infra-red filter (B+W 093 or Kolari K850), which lets through mostly IR light and this produces better contrast than say the common Hoya R72. This helps show up the hot spot issue more clearly. Since hot spots are essentially a white blob in the middle of the frame it helps to locate them with a clean dark centre to the image. I achieve this using a clear blue sky and face away from the sun (as directly as possible). To best contrast this I fill as much of the outer frame with well lit foliage and due to facing away from the sun this should be as bright as possible.

Camera Settings - Firstly, I set the camera to auto exposure, with +1 stop compensation. Aperture is set manually. Shutter speed and iso are left to the camera as I go through the entire aperture range. For each shot the focus & exposure is set on the foliage, before moving some clear sky into middle of the frame to release the shutter. Once back on the computer I process all the images in RAW. Other than the saturation being set to 0, the only other adjustment is done in the curves to maximize the histogram and balance out any small differences in exposure.

Problem - The first complication here is that all lenses suffer from vignetting when at their widest apertures. This confuses the results a bit because a dark vignetted border looks rather like a large hot spot. I don't believe that these two issues are intrinsically linked, rather that when a lens is wide open, infra-red images are showing both vignette and a hot spot at the same time. The hot spot is just so big that the effects are amplifying each other and are difficult to separately identify. If you look at the effect (below) at f/22 and work your way back up the images (as the aperture opens), what you see is an effect that essentially blurs out. It gets bigger and less intense, but it never fully goes away. The more infrared images I analyse from different lenses the more I feel that this might be a rule.

The Results - I recently purchased two of Sony's auto focus primes for the A7. Here's how they performed in IR, using this new hot spot test scene:

      Sony 28mm f/2

This Sony 28mm lens is not the best or worst for IR, but I can still see the effect at f/2. It might be subjectively acceptable here somewhere in the middle, but if this issue really bothers you it might only seem reasonable at f/2.8, which could make it a terrible lens for you. This is where all the subjective assessments get tricky to read with Kolari's list. It's worth noting here that this is the worst case scenario. In general shooting it's mostly fine at f/11 and I've even shot images at f/22 where I've seen no hint of hot spot. It all depends on what's in the middle of the frame, what angle you shoot into the bright areas and its relative intensity to the rest of the scene.

So you've seen one bad example and one slightly annoying one, but now for an example of a good infra-red lens...

     Zeiss 55mm f/1.8

I recently bought the Carl Zeiss Sonnar 55mm f/1.8 lens for the Sony A7, but before I did I had no idea how it would perform in IR. In most circles this lens is considered to be extremely good for visible light photography, and I'd have to agree (DxO give it the highest score of any autofocus lens). In infra-red there's just no guarantee it will work well. I was well and truly crossing my fingers when I first took this lens out with the IR filter. The Zeiss 35mm f/2.8 was the only other prime lens around when the Sony A7 was launched. I borrowed that from a friend a few months back and found out that it produced quite an obvious hot spot from about f/5.6, so this made me nervous. Fortunately though, this Zeiss turned out to be not only the best performing Sony AF lens so far, but it's the best lens I've ever tested, period!

That said, although this is the cleanest looking infra-red samples I've seen, @f/22 it's not quite 100% void of issue. If you look closely you'll see that there is a large, but faint disc starting to appear at the top of the image (monitors with high gama show this more clearly). This is a mirror image of the bright foliage at the bottom, but in an even more odd pattern than the 28mm and extremely subtle. So, as close as this Zeiss lens gets to perfection, thus far I've not found a lens that produces zero hot spot effect at all. Right now I'm wondering whether any exists when subjected to this kind of intense test.

Here is the last image from the Zeiss 55mm @f/22, brightened up by several stops, to show the effect:

Don't let this pushed result fool you though, this lens' IR performance is right up there with the best of them. Well that's my current experience anyway, but I haven't tested every lens, so...

     Feedback
If you have a lens that you think performs better than the 55mm Zeiss here please let me know which they are, or better yet try to replicate this test and share the results. I appreciate any feedback you may have on my thoughts here, Especially if I have gotten anything badly wrong.

     Reviews
These two images go a long way to forming the basis of the next two reviews that I want to write (Sony 28mm / Zeiss 55mm). I will be rewriting the existing reviews in new format before I tackle those, but they will be coming soon.

     Weather
Due to the rather 'changeable' weather here it may take me some time to test all my current lenses. Clear skies are not all that common, but I will do my best to update all the old reviews and will continue to work on new ones as well.