LENSES

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

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THANKS, NICK

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.

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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 .

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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.

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so:

75-300 lens = slow

300mm f/4 = faster

300mm f/2.8 lens = "fast"

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.

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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 -

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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.

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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...

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.

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

together.

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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.

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

what does that mean ?

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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.

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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).

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"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.

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Matt Swope