We all know that photography is all about the light. But not just the shape of the light, but the amount of it as well. Most of us photographers strive for faster and faster lens, mostly in order to have cleaner images. Faster lens also means more bokeh. And I don’t know a portrait photographer who doesn’t love bokeh.
So you might be asking yourself, “What are the fastest lenses available? And what would be the fastest theoretical lens possible?”
Well Matt Granger has the answer for it. It all sums up to math. Following the formula
f = focal length (in mm) / aperture diameter
it is possible to go up to f/0.1—or even more, for that matter.
However, that would not be practical at all. The thing is, the faster the lens is, the smaller the field of focus. Therefore, after certain point the lens becomes practically unusable due to the fact that the field in focus is way too small. It can get to less than a millimeter.
Now on the Leica Noctilux-M 50mm f/0.95 lens at the minimum focal distance, the field in focus is barely a millimeter, which is really impractical for proper usage. Imagine what it would be on a lens which has an aperture of f/0.1.
Another problem is size. The faster the lens is, the wider the elements need to be. If you want to make a 35mm lens have f/0.1, the aperture diameter needs to be 350mm, the front element would probably be something larger than that, and the weight would be immense.
Having all that in mind—the lens being ridiculously big and heavy, with a field of focus of microscopic size—answers the question about why we don’t see many lenses that go lower than f/1.0, even though f/1.0 is also really rare.
In the end, lens manufacturers strive to produce lenses that balance practicality and price. According to Dr. Hubert Nasse, the Otus line is the best that Zeiss can make for the mass, but if money weren’t an issue, they could have made an even better lens, which would produce even sharper image. The problem there would be that it would have to be much larger than it is now.
What’s the fastest lens you own?