What is the lowest ‘f-number’ a lens can have? Can it go below f/1? A famous misconception about ‘f-number’ of lenses among many, is that it cannot go below f/1, in other words, the lowest f-number that can be achieved is f/1. According to that belief, if the f-number of the lens is f/1, it means the brightness of the image is 100% the same as the brightness of actual sight. Apparently, it is not what it means.
Before coming to f-number, an important concept to be understood is the ‘relative aperture’ of a lens. Relative aperture is identified as the light-gathering power of an optical device such as lens, telescope, binocular, microscope, etc. In lenses, relative aperture is defined as the ratio of aperture diameter to the focal length.
The focal length is also considered as a factor to determine the relative aperture of a lens, other than the obvious aperture diameter. It is because the intensity of light of the image created, is also depending on the focal length.
When the focal length of a lens is low, the objects in the image it creates is small. And when the focal length increases, the objects in the image gets larger. If the amount of light that comes into the lens remains the same, the intensity or the brightness of the image reduces when it’s objects becomes larger. It is because the same amount of light has to spread on more area when the objects become larger. That is why the focal length has an inverse relationship with the relative aperture.
OK, that is relative aperture. Now, what does the f-number means? F-number is defined as the multiplicative inverse of the relative aperture. In other words, f-number is the ratio of the focal length to the aperture diameter.
Obviously, that is why f-number has a ‘direct relationship’ with aperture diameter and an ‘inverse relationship’ with focal length.
Now we can figure out how the f-number of a lens can be less than f/1. It can happen when the aperture diameter is larger than the focal length. As an example, take a 50mm prime lens with a maximum aperture diameter of 52mm. If these values are applied to the above formula, we will have the lens’s lowest f-number as f/0.96.
Only a very few number of such kind of lenses have ever come to the market. It might be due to the practical difficulties of giving that much of large aperture to a lens. Some of those lenses are,
- Leica 50mm f/0.95 Noctilux-M ASPH
- Canon 50mm f/0.95
- Carl Zeiss Planar 50mm f/0.7
- Angénieux Paris 50mm f/0.95 Type M1.
Interestingly, almost all the lenses which have f-number below f/1 are 50mm lenses. Nikon has gone further with their 58mm lens. It might be practically impossible to give f-number below f/1 to lenses with longer focal lengths.
Hope this article clears up any misconceptions about the lowest possible f-number. If you have any doubt, feel free to leave a comment below.