What does 'Fast' mean?

mhowel22 · Mar 22, 2005

When I see an advert with Fast lens mean. E.g. Fast 200mm.

Hertz van Rental · Mar 22, 2005

It means a large maximum aperture - which means a large piece of glass in front - which means you pay a lot more.

tr0gd0o0r · Mar 22, 2005

It also means you can take photos in a lower light with a faster shutter speed.

Kent Frost · Mar 22, 2005

A fast 200mm lens would be something like a 200mm lens with a maximum aperture of 2.8.

One that would not be as fast would be one with a maximum aperture of, say, 4.5. As you surely already know, the smaller the aperture number, or F-Stop, the wider open it can get, allowing more light income, thus letting you shoot, as said, with a faster shutter speed, which minimizes motion blur in the image caused from camera shake.

ShutteredEye · Mar 22, 2005

How does this affect the other side of the coin? What differences are there between a "fast" and "slow" lens in the very small apertures?

Also, if my camera as an f27 setting, does that mean any lens I attach to the body will shoot at f27?

ksmattfish · Mar 22, 2005

mountainlander said:
How does this affect the other side of the coin? What differences are there between a "fast" and "slow" lens in the very small apertures?

There is usually no difference. A "fast" lens just has a larger max aperture than a "slow" lens.

willg133 · Mar 22, 2005

the way i think about it....if you have a lens that has a max aperature of 4.5 it won't perform as well at 4.5 as a lens with a max aperature of 2.8 or so....am i right?

ksmattfish · Mar 22, 2005

willg133 said:
the way i think about it....if you have a lens that has a max aperature of 4.5 it won't perform as well at 4.5 as a lens with a max aperature of 2.8 or so....am i right?

It is often said that a lens will be it's sharpest a couple of stops closed down from wide open. That is a popular generalization, but it may or may not be true, depending on the particular lens. There are always exceptions to the rule.

The Nikon 50mm f/1.8 manual focus lens is said to be sharper at f/1.8 than the Nikon 50mm f/1.4 is at f/1.8

There are folks who claim the 75mm f/3.5 planar on a Rollei TLR is sharper at f/3.5 than the 80mm f/2.8 version at f/3.5

I'm sure there are folks who would debate these examples, but there are many, many posts on the internet saying it is so.

Contra|Brett| · Mar 23, 2005

willg133 said:
the way i think about it....if you have a lens that has a max aperature of 4.5 it won't perform as well at 4.5 as a lens with a max aperature of 2.8 or so....am i right?

yes and no.

Generally the fast 2.8's are made of much higher quality glass then their 4.5 brethrin.

KizaHood · Mar 24, 2005

mountainlander said:
Also, if my camera as an f27 setting, does that mean any lens I attach to the body will shoot at f27?

I think not! If you attach some lens that has minimum aperture at f16, you won't be able to shoot at f27.

So, you have the camera with f setting on it's body? As I know, such setting may work properly only if the lens supports using of that particular feature.

For example, Nikon F-100 has f control on it's body. But if you attach for example Helios lens on F-100 body, the only way to set the aperture would be by operating aperture ring placed on the lens. If you set camera's f setting to f8 while aperture ring on Helios is set to f4, the camera will shoot at f4!

Similar thing is with autofocus - you may use the autofocus feature only if you have an AF lens attached!

ksmattfish · Mar 24, 2005

Contra|Brett| said:
Generally the fast 2.8's are made of much higher quality glass then their 4.5 brethrin.

That is definately what the salepeople are going to tell you

Their mantra is more money equals better stuff. Personal testing (or by a friend you trust) is really the only way to tell for sure.

ShutteredEye · Mar 24, 2005

KizaHood said:
I think not! If you attach some lens that has minimum aperture at f16, you won't be able to shoot at f27.

So, you have the camera with f setting on it's body? As I know, such setting may work properly only if the lens supports using of that particular feature.

For example, Nikon F-100 has f control on it's body. But if you attach for example Helios lens on F-100 body, the only way to set the aperture would be by operating aperture ring placed on the lens. If you set camera's f setting to f8 while aperture ring on Helios is set to f4, the camera will shoot at f4!

Similar thing is with autofocus - you may use the autofocus feature only if you have an AF lens attached!

Hmm, good point. I just haven't ever noticed a lense advertising its upper f settings. Maybe I better check that out on my lenses.....

ksmattfish · Mar 24, 2005

mountainlander said:
Hmm, good point. I just haven't ever noticed a lense advertising its upper f settings.

Generally the typical small aperture for 35mm lenses is f/22. You can find some wide angle lenses that only go to f/16. There are zooms that go up to f/32 or f/45 when used at longer focal lengths; this is because they are designed to go to f/22 at the shorter focal lengths, and the math changes (not the actual size of the aperture) with the longer focal lengths, resulting in f/32 or f/45.

f/# = focal length divided by size of aperture
size of aperture = focal length divided by f/#

A 50mm lens set at f/2 has an aperture size of 25mm

A 100mm lens set at f/2 has an aperture size of 50mm

A zoom lens set at 100mm and f/22 has an aperture size of approx 4.5mm

Change the focal length on the zoom lens to 200mm without adjusting the aperture size, and it's now approx f/45

Some higher end zoom lenses do adjust aperture size, so that when you change the focal length from 100 to 200, the f/# does remain the same.

You can see why longer focal length lenses usually don't open up as far as f/2, they would have to be very big.

Canon used to make a rangefinder that had a 50mm f/0.9 lens available. The lens dwarfed the camera. At f/0.9 it has a wider aperture than the focal length of the lens.