Monday, 9 November 2015

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: (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

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