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01-04-2009, 11:45 AM   #1
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What is "Fast" about the fast Fifties...

Pardon my ignorance, but I have several A-series "fast fifties", which I absolutely love. It occurred to me that I don't really know what is fast about them, however. I mean, the shutter speed is set by you or the camera, so I assume that it has something to do with the aperture blades closing down, but I am unclear how a lens can be so much faster than another. It would seem that a really slow lens would really be detrimental to an image. I guess I really don't know the process, and would love some illumination. Jess

01-04-2009, 11:48 AM   #2
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They are fast because they allow more light to hit the sensor/film allowing for faster shutter speeds. 1.2>1.4>1.7 etc. The larger the maximum aperature the more light can get through thus a "fast" lens. I'm sure the others here can give you the exact technical reasons but that is how it was explained to me.
01-04-2009, 12:16 PM   #3
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Like Apollinax said it is "fast" because of the ability to get wide aperture. Also the fact that sharpness generally increases when the lens is slightly stopped down (when using a 50/1.2, stopping down to 2 would probably be sharp, while on a 18-50/3.5 stopping down to 5.6 would probably be sharp). The thing that still puzzles me though is that this fastness is only displayed when the photograph is made using a wide aperture (1.2-4). After 4, they are just as "fast" as low-end zooms, are they not? I personally think a more appropriate tag would be "wide" fifty rather than "fast" since going fast and sacrificing DOF is a choice made for a particular shot rather than a constant feature throughout the aperture range.

Just my 2c.
01-04-2009, 12:35 PM   #4
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The term "fast" is old-school photo jargon. It refers to any lens with a large aperture. The term is analogous to a "fast" film (high ISO). As noted above, a "fast" film or lens allows a higher (faster) shutter speed.

Steve

01-04-2009, 01:36 PM   #5
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They are "fast" because every time they are out in public they are flashing their legs, which explains all of the catcalls, wolf whistles, hooting, and hollering that goes on whenever a Pentaxian takes their "Fast" Fifty out of the house..
01-04-2009, 01:46 PM   #6
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If you could manual focus fast, the brighter aperture can really make an experienced photographers fast in snapping events and candids.
01-04-2009, 02:12 PM   #7
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It's like wot they all said.

For me and gig photography, it allows me to see the subject and take photographs with shutter speeds that are still usable.
01-04-2009, 02:25 PM   #8
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They're fast because they come with a pair of sneakers and they allow you to run after your subject really fast, thus keeping it in focus at all times.

01-04-2009, 02:27 PM   #9
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Another way to think of it: a lens is "faster" if it has a wider aperture (ie, f/1.4 is wider than f/2.0) because it allows you to use a faster shutter speed (ie, 1/15 is faster than 1/8).
01-04-2009, 03:16 PM   #10
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QuoteOriginally posted by jess Quote
Pardon my ignorance, but I have several A-series "fast fifties", which I absolutely love. It occurred to me that I don't really know what is fast about them, however. I mean, the shutter speed is set by you or the camera, so I assume that it has something to do with the aperture blades closing down, but I am unclear how a lens can be so much faster than another. It would seem that a really slow lens would really be detrimental to an image. I guess I really don't know the process, and would love some illumination. Jess
This basically echos what others have already said, but here goes:

I'm just making these number up, but let's say we're going to take a shot and it's correctly exposed at these settings:

f8 @ 1/60

Think of the aperture and the shutter as two parts of a balancing scale. In this case we're balancing light. As the physical size of the aperture increases (decreasing f number) we've let in too much light, the scale is off balance and, therefore, must be compensated with less light from the shutter and, therefore, a faster shutter speed.

f1.4 @ 1/500"


As the size of the aperture decreases (increasing f number) the scale tips the opposite way and the shutter must be compensated with a slower number.

f22 @ .5"

How does this effect you and your pictures? It isn't that a certain aperture or a "slow lens" is inherently "detrimental to an image". Two things will change in an image with the numbers given in the example:

Depth of field - the area in focus narrows with wider apertures
Motion - Because the smaller aperture needs to compensate with more light through time (shutter speed), anything moving will be blurred or even non-existent in the image. Consequently, with a wider aperture you'll be able to use those faster shutter speeds and stop motion.

You could look at images and read more words, but I say take your camera out and try it for yourself. Put your "fast fifty" on and take images with different aperture settings while noting the changes in shutter speed and in the image.
01-04-2009, 05:15 PM   #11
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Thanks guys,
I really appreciate the explanations, and I knew most of this already, although it must be said that in the past I have been incredibly lucky because using manual focus and manual control, I find myself worrying a lot more about exposure than aperature. My pics usually turn out alright, however. I guess it was just the fast terminology that had me second-guessing myself. Thanks as always for the helpful advice. Jess
01-04-2009, 05:18 PM   #12
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Just think "fast" = "bright".

Faster lenses are brighter lenses. They have wider apertures and let in more light. They are better suited to low-light shooting. They offer more range for creatively blurring your background. They are easier to focus because of the greater amount of light coming through the viewfinder.
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