WHAT DOES IT MEAN WHEN A LENS IS FAST??? WHEN A LENS IS SLOW???

THANKS, NICK

-- NICK REPASKY (bbaker3843@aol.com), February 02, 1999

Answers

These are comparable terms. A fast lens simply implies that when the lenses aperature is wide open, it lets in more light than most other lenses of the same focal length.

For example: most 75-300mm zoom lenses at 300mm have a maximum aperature of f/5.6 . You can buy a fixed 300mm lens with a maximum aperature of f/4, it will usually cost you $600-800 more, but will be "faster" than the 75-300 f/5.6 .

A 300mm f/2.8 lens is faster than either of these other two lenses, and also two to three times the cost and weight, but it is a "fast" lens indeed. To get these "fast" lenses, the lens usually has to be very large to let the extra light in, these are the big lenses you see at professional sporting events.

so: 75-300 lens = slow 300mm f/4 = faster 300mm f/2.8 lens = "fast"

-- Matt Swope (vswope@voyager.net), February 02, 1999.

I would not say that the 300f4 is a slow lens. Yes , slower by 1 stop than f2.8 but a lens opening of f4 is quite large and yet affordable and not too beefy to lug around on a tripod and can be handheld in many situations , especially Canon,n new IS lenses.

f5.6, f8 etc would be considered slow by most photographers I think. Until recent years these slow lenses were mostly made from standard optical glass and not of professional quality. Today such slow lenses are made from various superior glass and aside from only allowing a maximum of 5.6 lens opening, are as good as the big fast lenses IMHO. With a 5.6 lens, one also loses AF ability using a TC but with the advent of such cameras as the EOS 3, even this is becoming passe.

-- Jeff Hallett (franjeff@alltel.net), February 03, 1999.

Jeff -

I believe Matt's final sentence should read, "75-300 lens = slow, 300mm f/4 = faster, 300mm f/2.8 lens = 'fast'." If you read his reply, he clearly understands that speed is relative, and that an 300f/4 is, relative to the 75-300f/4-5.6, FASTER.

I wonder how many people with the wherewithal to purchase an EOS 3 would consider such a slow, consumer-grade lens to mount on it...

-- Scott (bliorg@yahoo.com), February 04, 1999.

i might buy an eos-3 soon, and i'll happily put my 100-300 4.5-5.6 on it. there's some times and places i don't want to carry huge lenses. the xx-300 lenses are pretty small and light, they have a place in my bag because of that.

-- Sean Hester (seanh@ncfweb.net), February 04, 1999.

Scott did clarify my answer. The software lumped my last sentence together.

I have talked to Nick by e-mail about this and there is an important part that I did not mention that may be usefull when this post is archived.

I mentioned that a lens is "fast" because it lets in more light, so what does that mean ?

It means that since the lens is letting in more light, we do not have to leave the shutter open as long to let light in, so we can use a "fast" shutter speed.

If the lens does not let in as much light, we have to leave the shutter open longer, (ie.."slower" shutter speed, your getting my point here).

"fast" and "slow" lenses have nothing to do with the lenses focusing motors, mount, etc..it is all about the shutter speed available when you use a "fast" (more light) or "slow" (less light) lens.

Matt Swope

-- Matt Swope (vswope@voyager.net), February 06, 1999.