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01-28-2009, 09:00 AM   #1
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I almost hate to ask this

because I feel so stupid that I don't know. What makes a fast lens fast? I thought it meant you could use it in low light situations without using a flash in more situations than a slower lens. Does a fast lens allow more light in via a larger Av setting or Tv setting than a slower lens would? Seems like if I tried to take a picture at f8 1/60sec with a slow lens the shutter speed would be faster for a fast lens. Is this correct.
Thanks for helping me out with this. Susan

01-28-2009, 09:11 AM   #2
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QuoteOriginally posted by slowpez Quote
Seems like if I tried to take a picture at f8 1/60sec with a slow lens the shutter speed would be faster for a fast lens. Is this correct.
Correct. The term "fast lens" is a misnomer and has nothing to do with lenses, but with shutter speeds you can use in combination with a particular lens.

Each additional f-stop of light will allow you to go one shutter speed faster. For example, DA 18-55 at 50mm is f/5.6 max. In given conditions it may require, say, 1/4s which is very slow and may result in blurry shots even with SR on, and will certainly result in motion blur. But if you use FA 50/1.4 @ f/1.4 which will allow you to shoot with faster shutter speeds. How faster? Easy to calculate: f/1.4 to f/5.6 is four f-stops difference (f/5.6 -> f/4 -> f/2.8 -> f/2 -> f/1.4) which will allow you to use faster shutter speeds by exactly four f-stops and that would be 1/60s (1/4s -> 1/8s -> 1/15s -> 1/30s -> 1/60s).

So to get exactly the same exposure f/5.6 + 1/4s = f/1.4 + 1/60s, meaning that f/1.4 lens will allow you to shoot with "faster" shutter speeds compared to f/5.6 lens, assuming identical lighting conditions of course.

Last edited by Ivan Glisin; 01-28-2009 at 09:24 AM. Reason: Added example
01-28-2009, 09:14 AM   #3
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QuoteOriginally posted by slowpez Quote
Seems like if I tried to take a picture at f8 1/60sec with a slow lens the shutter speed would be faster for a fast lens. Is this correct.
Thanks for helping me out with this. Susan
You are correct in everything but this statement. If you set f/8 1/60 on any lens you will get the same exposure. A "fast" lens refers to the maximum aperture of the lens. It is the number written on the front of the lens. The smaller the number, the larger the maximum aperture. So if you have one lens that is an f/4 and another that is an f/2.8, the f/2.8 is the faster lens (one whole "stop" faster, or double the light gathering ability). Lets say your exposure calls for f/4 at 1/15sec. With the "slow" lens you are stuck with the 1/15sec shutter speed. With the fast lens you could increase your shutter speed to 1/30sec because you can increase your aperture to f/2.8.

Last edited by PentaxPoke; 01-28-2009 at 09:22 AM.
01-28-2009, 09:19 AM   #4
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QuoteOriginally posted by slowpez Quote
because I feel so stupid that I don't know. What makes a fast lens fast? I thought it meant you could use it in low light situations without using a flash in more situations than a slower lens. Does a fast lens allow more light in via a larger Av setting or Tv setting than a slower lens would? Seems like if I tried to take a picture at f8 1/60sec with a slow lens the shutter speed would be faster for a fast lens. Is this correct.
Thanks for helping me out with this. Susan

Kind of... The first part is right

'Fast lens' means it has a larger maximum Aperture which will let in more light, giving you a faster shutter speed.

But the shutterspeed at F8 is going to be the same if you have a fast or slow lens. It is only when you open one lens up (smaller F number) that you will see the faster lens begin to use faster shutter speeds.

01-28-2009, 10:56 AM   #5
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Right - f/8 is f/8 regardless of lens. What makes one lens faster than another is its *maximum* aperture, not what it does at f/8. Pretyt much all lenses can do f/8, or f/5.6 for that matter. Most can do f/4, but for zooms, maybe only at the short end. Lenses that can do f/2.8 are faster, ones that can do f/2 are faster still. The maximum aperture is usally stated clearly in the name of the lens, as in "Pentax-DA 1:2.8 40mm", usually shorted to just DA40/2.8.
01-28-2009, 11:32 AM   #6
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One other point is that the longer a lens is, the greater the difficulty in designing them with a large aperture such as 2 or 2.8. This holds somewhat true for some of the ultra-wide angle lenses as well. That's why even though the A 400mm f5.6 is a hoss of a lenses, the A* 400mm f2.8 is huge.
01-28-2009, 11:42 AM   #7
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QuoteOriginally posted by Blue Quote
One other point is that the longer a lens is, the greater the difficulty in designing them with a large aperture such as 2 or 2.8. This holds somewhat true for some of the ultra-wide angle lenses as well. That's why even though the A 400mm f5.6 is a hoss of a lenses, the A* 400mm f2.8 is huge.
Excellent point, and the reason they are so difficult to design is that the max f number you see on the lens, is the focal length divided by the number. That is why it is written f/#. So the 400 f/5.6 has a max aperture of 400/5.6=71.4mm The 400 f/2.8 has a max aperture of 400/2.8=143mm. That is 5.6 inches! (for those of us in the US ) Or approximately 25 square inches of glass compared to 6 square inches. That is a big, heavy, expensive "hoss" of glass (as Blue says.) Simply doubling a lens' maximum aperture, increases the amount of glass by a factor of 4. This is why "fast glass" is heavy and expensive.

Last edited by PentaxPoke; 01-28-2009 at 12:01 PM.
01-28-2009, 12:00 PM   #8
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QuoteOriginally posted by PentaxPoke Quote
You are correct in everything but this statement. If you set f/8 1/60 on any lens you will get the same exposure.
Well that's almost true

The maximum aperture is a ratio between the focal and the diameter of the front element of the lens. That's why a bigger aperture mean a bigger glass. So the bigger the aperture, the bigger the amount of light that is gathered and illuminates the sensor.

But some light is always absorbed by the glass that make the lens. Depending of the design of the lens, light will go through a different thickness of glass. Typically a zoom will have move glass than a prime at the same focal lens.

So in pratical term, the light that hits the sensor can vary of a full stop on 2 different lens at same aperture.

Cinema technicians prefer to speak of Transmittance value (or T) instead of Aperture (or f), this is just a standardized value for aperture and was adopted at the time it was really important to get the same exposure on the film whatever was the lens used.

Hope this help to understand and don't bring too much confusion.

Best regards,
Guillaume

01-28-2009, 01:12 PM   #9
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OK, I'm beginning to understand but now of course I have another question. Does the faster lens mean you have to give up DOF? Seems like if I have to opt for f1.4 vs. f5.6 I'm going to have a narrower DOF.
Thanks again for all the help. Susan
01-28-2009, 01:16 PM   #10
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YES

You are correct, the wider the aperture the less the DOF... that becomes part of the balancing act with low light shooting.
01-28-2009, 01:16 PM   #11
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You are correct. The wider the aperture the thinner DOF you will start to have. Here is a great link I use to get a better grasp on DOF as it is affected by aperture and distance from your subject:

Online Depth of Field Calculator
01-28-2009, 01:22 PM   #12
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QuoteOriginally posted by slowpez Quote
OK, I'm beginning to understand but now of course I have another question. Does the faster lens mean you have to give up DOF? Seems like if I have to opt for f1.4 vs. f5.6 I'm going to have a narrower DOF.
Thanks again for all the help. Susan
susan,

"a fast lens" refers to a lens that has a large maximum aperture.

any lens that has an aperture of 2.8 or less (smaller number) is considered to be "a fast lens"

that is all.

fast lens = large maximum aperture

fast lens = big hole

done.

=====================

part 2

depth of field.

how you USE your lens is up to you

if you shoot your FA50 F1.4 at F1.4 then yes, your depth of field will be shallow, but allow you to increase shutter speed, assuming ISO is unchanged

if you shoot your FA50 F1.4 at F5.6 then your depth of field will not be shallow, but your shutter speed will decrease, assuming ISO is unchanged

however if you take a 50 F2.0, a 50F1.4 and a 50 F4.0 lens and shoot all 3 at F5.6, they will all yield the same depth of field, at the same shutter speeds, assuming ISO is identical.

so, how you USE your lens is entirely up to you.

no one is forcing you to shoot at wide apertures, wider aperture means less depth of field, it is there for artistic purposes.


==================

part 3

"fast lens and low light"

people often say that a fast lens is good for low light,

true, but that is not why its there

lenses shot wide open suffer from reduced sharpness, people who shoot wide open in low light conditions trying to attain a shot that would benefit from large depth of field are sacrificing

they would rather have a fuzzy, well exposed shot with high shutter speed rather than a blurry shot
01-28-2009, 02:01 PM   #13
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QuoteOriginally posted by slowpez Quote
OK, I'm beginning to understand but now of course I have another question. Does the faster lens mean you have to give up DOF?
Merely *having* the fast lens doesn't mean giving up DOF - we're talking *maximum* aperture here. No one is forcing use to go to the maximum all the time. An f/1.4 lens is just as capable of shooting at f/8 as a slower lens is, and DOF will be the same. But sure, on the occasions when you *choose* to use f/1.4, DOF is very shallow. That's part of why I find f/2.8 to be an acceptable maximum aperture in practice. Even on a lens where I *can* go below f/2.8, I seldom do except when I have absolutely no choice (and by absolutely no choice I mean, I'm already at ISO 1600 and underexposing a stop) or when I *want* the shallow DOF.
01-28-2009, 02:09 PM   #14
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Many lenses also get sharper as you increase the f-stop setting. So if you have an f4 lens, you might need to set it to 5.6 or 7 to get a sharp image...and that also means you need more light or a slower shutter. With a "fast" lens you start at a much lower number and often have a sharper image at a relatively lower f-stop.
01-28-2009, 02:25 PM   #15
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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..
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