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08-02-2009, 04:08 PM   #1
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Speed of A Lens

I was just reading the Shutterbug magazine and in one of the lens review, there was a mention of the speed of a lens.

While I understand shutter speed but what does a fast or slow lens equate to? Is the speed referencing the AF speed?

What makes it fast or slow? Is fast better ?

I tried looking around the forum and also found a few post making similar statement on a lens.

Thanks all

08-02-2009, 04:11 PM   #2
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The smaller the f-number, the faster the lens. Or said another way, the larger the aperture, the faster the lens.

Edit: For example, a 50mm f/4 macro is a reasonably slow lens; a 50mm f/1.2 is 3-1/3 f-stops "faster."

Last edited by Duck Dodgers; 08-02-2009 at 04:16 PM.
08-02-2009, 04:20 PM   #3
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Eh... Isn't it the other way around? The smaller the number, the slower the lens. Say, f/22 is slower than f/1.4 (1/22 is smaller than 1/1.4), and f/1.4 is faster than f/22 (1/1.4 is larger than f/22).
Anyways, nitpicking aside, the faster the lens (i.e. the larger the aperture), the higher shutter speed you can use. For example, if you use your kit lens at aperture of f/4 and shutter speed of 1/500, then with a faster lens, you can use aperture of say f/2 and speed of 1/2000.
With faster lenses, you get a brighter viewfinder, more possibilities under low light, and a shallower depth of field.
08-02-2009, 04:37 PM   #4
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22 is larger than 1.4 from a layman's point. Although 22 equates to a smaller aperture it is still a larger number. Yes I understand your point pbo, but the OP has to grasp the fact that a larger number means a smaller "hole" i.e. less light on the sensor and a slower shutter speed. Point taken though!

08-02-2009, 05:53 PM   #5
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Original Poster
I do understand the relation between Shutter speed, aperture and ISO. But was confused why a large aperture (f/1.4) would be "fast" since aperture controls a certain "volume" of light while shutter speed controls the "amount of time".

From the responces and in summary, it is correct to say that a lens is considered fast provides a large aperture (f/1.4) that will allow a faster shutter speed that gives a "correct" exposure?
08-02-2009, 06:49 PM   #6
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Basically, aperture is how wide a lens can open as compared to its focal length. The faster lenses can open wider and therefore, as you say, allow a faster shutter speed. Rule of thumb is that if you open the lens one stop (say from 4 to 2.8) you can have your shutter speed twice as fast (say go from 1/60 to 1/120).
08-02-2009, 08:43 PM   #7
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QuoteQuote:
From the responces and in summary, it is correct to say that a lens is considered fast provides a large aperture (f/1.4) that will allow a faster shutter speed that gives a "correct" exposure?
Yep, that's it.
08-02-2009, 10:50 PM   #8
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So, why do they call it "fast" lens, not "bright" (or "wide" or .....)?

In the old days, films were not very sensitive. Shutter speed was in terms of minutes. Having a portrait photo taken was a major undertaking, not only for the photographer, but also for the person being photographed.

A "bright" lens allowed the photographer to take a photo faster (and make more money per day), thus the term "fast" lens.

08-03-2009, 07:23 AM   #9
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Aperture is a ratio

The aperture numbers that are being bandied around are actually ratios. Specifically, the ratio of the lens diameter to focal length.

For example, a 50mm, f/1.4 lens has a diameter of 35mm. An f/2.0 version of the same lens has a diameter of 25mm. The iris in the lens reduces the effective diameter of the lens, thereby reducing the amount of light that passes through the lens for any given time. At f/8, the aperture is only 6.25mm.

The numbers (1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22, 32) were chosen so that with each successive step, the area of the effective aperture is cut in half. This makes reciprocity calculations easy. If 1/500 at f/2.8 is a "correct" exposure, then 1/250 at f/4.0 will be the equivalent. So would 1/1000 at f/2.0.
08-03-2009, 09:43 AM   #10
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That's why it's called "f over something something"
08-03-2009, 02:49 PM   #11
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That's why a fast lens also costs a lot 'coz it is also hard to make one.
Just consider the Pentax-A 1:1.2.
I've seen these lenses listed at times for around $700-800!
That's more than a fast lens..more of like fast money to me!
08-03-2009, 03:41 PM   #12
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QuoteQuote:
That's more than a fast lens..more of like fast money to me!
Not "to you", but away from you
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