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01-28-2009, 02:42 PM   #16
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QuoteOriginally posted by vizjerei Quote
I remember I heard something that by fast lens which has large aperture, not only allow you faster shutter speed, but also Faster Auto Focus, since our lens are at wide open during the metering and focusing, fast lens will provide more light to the AF senor..
The term "fast lens" has been around since well before the days of autofocus. It may well be that a lens with a wider maximum aperture autofocuses faster, but that's not why it's called a "fast" lens.

01-28-2009, 02:57 PM   #17
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Thanks all, especially for not making me feel my questions were too dumb to ask. You all were very clear and even my husband understands it.
Susan
01-28-2009, 03:22 PM   #18
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Heres a thought for you, get hold of one of the fast 50's that we all talk about on here and try it for yourself.

You will soon see what all the above replies mean and you'll have bought a cracking lens to use for not much money.
01-28-2009, 03:38 PM   #19
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QuoteOriginally posted by vizjerei Quote
I remember I heard something that by fast lens which has large aperture, not only allow you faster shutter speed, but also Faster Auto Focus, since our lens are at wide open during the metering and focusing, fast lens will provide more light to the AF senor..
This is true to a certain extent but this is not the 'fast' that fast glass refers to. Just because glass is fast doesn't necessarily make it an autofocus superstar. (Particularly if it is manual focus only.) Macro lenses are often 'fast' f/2.8 but slow because they are optimized for fine manual focusing with extra-long focus paths. Sometimes it might actually suffer a little, perhaps due to reduced contrast wide open--I think the Pentax FA50/1.4 is an example of this. That being said, in very low light, it may be able to eventually get a lock where a slower lens might fail. But in good light, other more contrasty lenses (when wide open) might acquire AF lock more positively.

The camera body autofocus logic is designed to work well with the typical f/4-5.6 that many users require so is not necessarily optimized for faster apertures. I recall that some high-end Canon bodies may have some of its numerous AF points/sensors optimized for f/2.8 glass.

01-28-2009, 06:02 PM   #20
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all about focussing

QuoteOriginally posted by vizjerei Quote
I remember I heard something that by fast lens which has large aperture, not only allow you faster shutter speed, but also Faster Auto Focus, since our lens are at wide open during the metering and focusing, fast lens will provide more light to the AF senor..

You have just hit the nail on the head.

Focussing is the real point of fast lenses.

Modern cameras allow you to focus with the lens wide open, even if the quality of the lens is not good wide open, the advantage of being able to focus more quickly and more accurately, far outweighs all other advantages.

Many so-called fast lenses do not perform so well wide open, the ones which do are usually very expensive and quickly become established as classics. Some develop cult status which drives up their second hand value. Try to buy a Minolta Rokkor 58mm F/1.2, or SMC (A) 50mm F/1.2 on fleabay, and you will see this in evidence. Many other classic fast lenses exist, finding one at a bargain price is like discovering buried treasure.

Last edited by keithlester; 01-28-2009 at 06:04 PM. Reason: oops, typo
01-28-2009, 08:48 PM   #21
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QuoteOriginally posted by keithlester Quote
Focussing is the real point of fast lenses.

Modern cameras allow you to focus with the lens wide open, even if the quality of the lens is not good wide open, the advantage of being able to focus more quickly and more accurately, far outweighs all other advantages.
I think that's going a bit overboard. It's nice, but it's hardly more important in general than getting fast shutter speed in low light or shallow DOF when desired.

And of course, as others have pointed out, it also has nothing to do with the *reason* they're called "fast".

QuoteQuote:
Try to buy a Minolta Rokkor 58mm F/1.2, or SMC (A) 50mm F/1.2 on fleabay
Ironic that you're talking about MF lenses here. Also, with many DSLR focus screens, the difference in viewfinder brightness between and f/1.2 lens and an f/2 one is probably less than it is with many older film bodies.
01-28-2009, 08:57 PM   #22
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QuoteOriginally posted by keithlester Quote
. . .
Many so-called fast lenses do not perform so well wide open, the ones which do are usually very expensive and quickly become established as classics. Some develop cult status which drives up their second hand value. Try to buy a Minolta Rokkor 58mm F/1.2, or SMC (A) 50mm F/1.2 on fleabay, and you will see this in evidence. Many other classic fast lenses exist, finding one at a bargain price is like discovering buried treasure.
Funny that you mention the Rokkor. There is a 50mm f1.2 that has been converted to m42 in the Marketplace with an asking price for $600 IIRC.
01-29-2009, 07:53 AM   #23
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Have you ever used a Canon 85mm f/1.2?


QuoteOriginally posted by keithlester Quote
You have just hit the nail on the head.

Focussing is the real point of fast lenses.

Modern cameras allow you to focus with the lens wide open, even if the quality of the lens is not good wide open, the advantage of being able to focus more quickly and more accurately, far outweighs all other advantages.
I hardly think the AF "speed" of the 85mm f/1.2 outweighs the "speed" of the large maximum aperture

01-29-2009, 08:42 AM   #24
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truths, half truths and lies

there is a lot of good data in the thread, but there is also some bad data.

I will try, to consolodate all of it.

Truths

-a "fast" lens is one with a larger maximum apature.
- Larger maximum apatures result in brighter view-finders
- f8 is always f8

1/2 truths
- "fast" lenses improve AF speed/accuracy While increased light does help with AF, as you go to extremely wide apatures, the image can becomes a little soft making AF not as accurate or lock in as fast
- "fast" lenses are not the sharpest wide open, this is true especially for portrait lenses, such as F1.2 50mm and F1.4 85mm lenses, but there are some lenses that are specifically designed to perform very well wide open, specifically very fast telephoto lenses.
- fast lenses need big front elements. This is only true for telephoto lenses, where the apature (focal length / diameter) dictates the element diameter. For wide angle lenses the front element and specifically filter ring diameter are very big to accomodate the field of view without vignetting. on a 10mm lens, F2.8 theoretically needs a little over 4 mm in diameter.
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