EDIT [9th December 2018]: ZEISS has informed that they are working on a software update that will allow users to work within a larger focus range with a wide-open aperture. This means that when working at close distances the aperture will close down less than what is described here, reducing also the visibility of nonagons. The update will be available to download for free from the ZEISS website in early 2019. ZEISS will notify all registered customers via email. In others words, the information presented here outdated and the Batis 2/40 CF will work a bit differently with a new FW-update available in early 2019
All right then, I had an idea that on a second day I would introduce myself, write something about my photography and do a some sort of selfie. Well, that idea is now thrown out of the window because the whole world is asking me about the aperture blades of the Batis 2/40 CF and how work at close focus distances, hah! Well, I might as well lose the selfie and share my experience and thoughts about aperture blades. But first I need to remind you that 'Roger’s Law of New Product Introduction' still holds true even at year 2018, maybe even more so. The complex structures of the universe determines that there will always be some kind of panic going on with the new product introductions. It's 'pdaf-striping', 'mechanical noises of focus motors', 'banding', 'one card slot' – you know the thrill and even the mighty ZEISS is no different. So we need a law of 'The Sense of Complex Proportions' to make it thru here.
Ok. First, let's lay down some facts I can come up just by using the lens. Yes, the aperture closes a bit when you focus on the objects at minimum focus distance at 24 cm (9.4in). Some have claimed at the forums that the aperture stays fully open at f/2 only when focusing at infinity. This is not true. The aperture stays at f/2 when you focus on, for example, objects that are 3 or 4 meters away. In fact, I've tried to examine how the aperture works in detail and here is my estimation. Please pay attention that these are not measured values, I've just looked into the lens and compared the position of aperture blades with normal shooting conditions where lens is focused on infinity.
Set aperture | Focus distance | 'True' aperture
f/2 > 1m (39in) f/2
f/2 1 m (39in)
f/2 0,4 m (15.7in)
f/2 0,24 m (9.4in)
Ok, then some speculation of what might be a reason for this. First of all, it's not unusual for a lens to close down a bit when it is focused on something near at minimum focusing distance. Some lenses do this (Sony lenses too) while others don't – it depends on optical design. Is it 'a design compromise'? Well, yes this would be one way to put it. But I know from the experience that ZEISS does very little 'design compromises' if one uses these words to describe 'flaws' or 'unintended side effects' that can be found from cheaper consumer lenses. On the contrary this company is known for its attention for perfection, so I'm willing to think that this behavior is there to optimize the optical performance (at close distances). In fact, couple of sentences caught my attention when ZEISS announced this lens: 'this lens incorporates the superior optical performance' and 'floating lens design for consistently high image quality across the entire focus range'. Now, these might be just marketing words, but during the years I've collaborated with the company, I've learned that you need to read their press releases very carefully as they usually contain hints about the technical decisions they have chosen. This talk about 'high image quality across entire focus range' first came with the Otus lenses and I know it has been a difficult challenge, especially with the superior Otus lenses, as most lenses in the market achieve their maximum optical performance only at the certain focusing distance. With this kind of snippets of information, I'm willing to bet that the Batis 2/40 aperture works the way it does because it is there to maximize the optical performance (throughout the focus range). I know that ZEISS is really rigorous for maintaining a very high optical performance even though they don't often promote the used technologies or chosen design paths. I love that company ethos and that's one reason you don't see hi-tech advertisement videos or abstract abbreviations like VC, HSM, etc. in their lenses. Well, there are couple of older designs in Milvus family that aren't so obsessed with optical performance and the 'CF' abbreviation with the Batis 2/40 was a bit surprise for me too.
But let's get back to the aperture thing. How does it affect shooting? Well for sure, it means less light for the sensor and that's something one just have to accept if using Batis 2/40 for close up shots. If you are running out of light, it means higher ISOs. From the aesthetic point of view I'm not sure if it is as big deal as it sounds. First of all, the focused area in an image is very thin at minimum focusing distance and changing aperture from f/2 to f/4 doesn't change it much (if you have done any macro work before you surely know this). The background bokeh is of course affected, but in most situations it is still very blurry, and again, I'm not sure if having lens at f/2 or f/4 changes it much. Don't get me wrong, it does change a bit, but like I said most of the time the background is far enough to become very blurry (because you are closer to subject and it's the ratio between foreground and background that matters). If this ensures higher optical performance, I don't see it as a very bad compromise. At least in practice I haven't experienced it as a compromise at all, in fact, I've started to examine it only after hearing about it.
How about the nonagonal bokeh balls then, do they exist (hmm, is nonagonal a word?). I say they do exist because the aperture is closed down a bit, but apart from theory I haven't really experienced them in real life. I've done some close focus work with the lens and I haven't seen them – but of course in right situation they can be introduced. I tried to find some examples of close focus shots, in a bit of a ad-hoc manner and pretty random stuff really as I wasn't prepared to this, but here are some from my little buffer. As you can see, the background looks mighty fine in all of them. In some shots you can see a bit of angular shapes, but definitely not enough to ruin the shot. I have to explore this a bit more, but for now I would say it's not a problem.
So with this post I've tried to explore how the aperture works in Batis 2/40, but I want to mention that you shouldn't just take my word for it (I've no measurements or real information about the optical design and how it works). To get some real information I also tried to contact Oberkochen, but I only got a short reply that they aware of this discussion and are preparing something for it. So for now, I would not surrender for the 'forum panic attack' or jump into generalizations. I would just wait a bit for some real information. Remember the Lensrentals 'Roger’s Law of New Product Introduction'? We are getting into the release phase. The laws of Quantum Commentary demonstrate that is possible that a product follows both paths, good and bad, simultaneously.