I have to admit that I've been really fascinated about focal lengths after I explored the Batis 2/40 CF's relationship with other ZEISS lenses (see previous post here). 'The standard focal length', now that's something I've been wondering while trying to position the ZEISS Batis 2/40 CF on my mental map. Like many of you already know the 35mm and 50mm lenses are most often considered to be 'standard focal lengths', meaning that their image projection looks about the same as what we see with the naked eye, or so they say. Some go even further and claim that in terms of field of view the human eye is practically a 35mm or a 50mm lens – which one it is, is of course an opportunity for a great dispute, but I guess the 50mm focal length is mentioned more often. In a way, there exists an idea of 'a standard focal length' that is based on a conception that human eye works similar way as a lens, and this very idea has been guiding many photographers for decades.
If you dig this idea little deeper, you will soon find out that human vision doesn't actually work as a lens or camera at all. As a matter of fact, the whole eye-to-camera analogy represents 19th century mechanical thinking when the human vision was not yet understood very well. Today we of course know much more and the whole concept of human vision has become a much more complex issue.
First of all, we don’t see 'a field of view', we only see a very small sharp area in the very centre of our vision and the rest is pretty blurry. There’s of course a peripheral vision, the things we see in the corner of our eye, which is about 160 degrees, but the brain reconstructs the whole image from eye scanning the scene in smaller sections. Then there’s also pattern, color and movement recognition that happens underneath the conscious mind and the weird part is that all this 'raw data' is mixed with unconscious subjective psychology that guides our seeing. The human eye (iris and all) can of course be measured and if one follows this train of mechanical thinking, it can be said the human eye is roughly a full frame equivalent of a 43mm lens with aperture of f/3.5 or so – it’s just that this snippet of information doesn’t have much to do with the complexity of human vision. Now, that 43mm focal length of the human eye would be a great pro argument for the new ZEISS Batis 2/40 CF, 'more standard than the standard' – except it wouldn’t be honest.
When biological reasons for 'the standard focal length' fall short, one has to look for technical reasons. Interestingly it turns out that the physical size of the celluloid film has much more to do with 'the standard focal length' than the biology of the human eye. For the traditional 35mm film camera, a 50mm lens was an effective approximation of the focal length necessary to fill the diagonal dimensions of the 35mm film negative – an ideal physical correspondence with the optics and film size. In addition, 50mm lenses could be manufactured with the famous double-gauss design which meant that they were easy to produce due to relatively simple optical concept. I should add that in practice the 50mm focal length was also a easy focal length to use and even beginners could get successful images with it without too much technical knowledge. In other words there are many technical reasons (related to physical size of the celluloid film and manufacturing limitations) why 50mm lens became to known as 'the standard focal length' and why so many early consumer cameras came with the 50mm lens.
In the past the celluloid film presented a manufacturing standard to which 'the standard focal length' was tied to, but today it’s different because we have many different sensor sizes. For example, ever wondered why many smartphones have lenses around 28mm focal length (full frame equivalent) which is quite far from that other 'standard focal length'? Well, they have small sensors and for those the 28mm seems to be preferred focal length as it is probably easier to manufacture lens with necessary specifications to match the small sensors. Well, to be honest, one also have to add consumer demand into equation ie. many people want to capture casual shots as well as landscape shots with the same phone. But in effect it turns out that the whole idea of 'the standard focal length' changes with time and technical application, and it’s also a question of taste as many photographers seem to associate it the 35mm lenses. In other words, there is no real standard focal length which could be based on biology of human vision or even on technical reasons. Kind of interesting isn’t it? Well, I have to make one small migitation: extreme focal lengths surely seem odd to human vision (with perspective distortions and everything) and because of that it is legitimate to argue that some focal length might be more natural than some other, but nevertheless there doesn’t exist an exact focal length that would correspond to our vision.
So, now that we have 'deconstructed' the idea of 'the standard focal length' there still remains one last question: who actually invented this original idea that has intrigued so many photographers through the decades? Well, it turns out that it was a German optician and businessman who lived in Jena and had a name you might have heard before: Carl Zeiss (1816-1888). It was ZEISS's advertising in the late 19th century which cultured a belief that things like human eye and vision could be measured scientifically and that this information could be translated to lens manufacturing. Carl Zeiss standardized the manufacturing of lenses, but also as a kind of 'a side-effect' he also categorized focal lengths into various applications that included things like 'portrait lenses', 'group shot lenses', 'studio lenses', etc., which you can find from early Carl Zeiss catalogues. Before Carl Zeiss the focal length was just an objective physical measurement without meaningful categories, a sequence of numbers that formed a continuum. Zeiss carefully educated his customers and created 'standard' categories for photographic applications as he also created 'standards' for the lens manufacturing (but to be honest he didn't do this entirely by himself, but together with other actors in his time). In other words, the idea of 'the standard focal length' traces back to Carl Zeiss and the history of photographic lenses. It’s a fascinating history and this is just one example of what ZEISS has brought to photographic world, but I personally find it to be one of the most intriguing one. It makes you think when you attach that 50mm, 35mm or 40mm lens to your camera: 'what was it again? Why am I using this particular focal length and not something else?'
Ps. There are some flares in the picture coming from the lights. These are actually sensor reflections coming from the A7-sensor (known first generation problem). Do not mistake them with the Batis 2/40 CF.