Does a "fast" lens imply sharpness

Started Nov 3, 2012 | Questions thread
blue_cheese Senior Member • Posts: 1,839
not really

A fast lens has a larger opening (aprture) to focal lenght. as a result when the aperture is opened up it lets in more light and allows for faster shutter speed, hence the name fast. In my opinion it also results in the misconception that fast lenses are needed to shoot in dim situations. While true that as a result of the larger aperture you get more light at the same shutter speed, you give up DOF. making it more difficult to control how much of your subject is in focus. I guess that this is generally negated by the fact that wider lesnses reach a hyperfocal distance closer than longer lenses so most dont notice, but in some situations increasing ISO or using a flash is the more appriopriate way to take the photo.

What is important to note is that light falloff increases with longer lenses, so to keep the same amount of light the hole has to get bigger

an F1.8 50mm has an aperture (hole) size of 50/1.8 = 27.8m

an equally fast F1.8 105mm lens has an aperture (hole) of 105/1.8=58.3mm

This requires literally double the glass, hence fast long lenses are generally more expensive and heavier. larger hole also means larger circles of confusion resulting in less DOF and more heavily blurred background.

Sharpness is generally not related to how fast the lens is, in addition the sharpness varies with aperture. Fast lenses may be sharper than zoom lenses at the same aperture but that is not always the case, and even less often is it the case when comparing a fast prime to a slower prime ie 105 f1.8 vs 105 f2.8. There are F4 primes that are certainly not fast, but are used for sharpness and less carryaroud weight.

One other potential issue to be aware of is focus breathing. In some optical designes the focal plane will shift as the aperture opens up. Your camera's viewfinder does not open the lens to its maximum aperture while metering and focusing, it sets the aperture when taking the picture and as a result the focus may shift giving the impression that the focus is soft

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