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07-27-2008, 11:13 PM   #1
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The meaning of "faster lens"

I have been wondering for a while on how fast a fast lens actually is. Say, I currently own the DA 50-200, and I read about people saying how the DA* 50-135 is a "faster lens" at f/2.8. My problem with this is that if I shoot both at f/8, will the DA* produce a faster shutter speed? As I understand it right now, a "faster lens" is simply faster because you can step up the aperture to a smaller f number, so you're sacrificing depth of field (and sharpness) due to stepping up from the sweet f/8 spot.

I am expecting a "it depends" answer, but please enlighten me =x

07-28-2008, 02:16 AM   #2
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Andi

the term "faster" when applied to a lens is really only applied to the maximum apature.

if you take 2 lenses and shoot them both at F8 there is no change to your shutter speed.

The reason "faster" lenses are sought after is that they afford faster shutter speeds when used at maximum apature, result in brighter viewfinder images (because focusing etc are done with the lens wide open), allow more control of depth of field and separation of foreground and back ground when used wide open.

They are generally also better lenses in terms of image quality, and can produce (again generally) a better sharper image at larager apatures. Most lenses produce their best images stopped down several stops, therefore an F2.8 lens may produce its best image at F5.6 where as an F5.6 lens may be at its best at F11. The result is that the best image setting may also be faster but this si a generalization not a hard and fast rule
07-28-2008, 02:49 AM   #3
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Lowell is correct. With very few exceptions lenses are softer wide open, so-so stepped down one step and sharp stepped down two steps. Hence the fast lens with a max apperture at for example 1.8 will be sharp already at 2.8, while a lens with the same focal length but a max apperture at 3.5 will not be sharp until 5.6. There are to various degrees exceptions to this and with some lenses the constructors tried to break this rule (for example some of the Vivitar series 1 if I remember correctly).

For example in portrait work, people often seek a shallow DOF (depth of focus) to isolate the face/body from the background (to get a soft out of focus background), and this is difficult/impossible to do with a slow lens since the larger aperture results in a larger DOF, hence portrait lenses are wanted with a large maximum aperture.

In macro photo on objects that does not move, you often want to maximize your DOF to get as much as possible of the object (flower for example) in focus, you work with a tripod, so you can use long exposure times to maximize your f value, and most macro lenses perform best at f8-16, and many of them are relatively slow, see for example the older Pentax takumar/K/M 50 and 100 mm macro lenses with a max aperture at 4.

Then again, if you are shooting bugs that crawl, or even worse, fly, you often have to work hand-held, yourself crawling or running after them, you will need your macro lens to be fast.

For a long telephoto, you will also want fastness, since the rule is that you need an exposure time corresponding to the focal length (for example 1/500 sec for 500mm) to be able to get a sharp hand-held photo, so you cannot work with too slow lenses, especially since you don't want to use the lens full open anyway to avoid the less good optical properties which this reveal. Monopods/tripods/picture stabilizing change this, but basically the same applies but they extend the limit a bit to your advantage.

Maybe speed is least important for wide-angle photographing.

So the importance of speed depends on your application.
07-28-2008, 03:23 AM   #4
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Ah, I see. So if my concern is to keep the lens at its sharpest spot and I dont mind the shallower DOF for some shots, using a faster lens will allow me to use is at a fast shutter speed and therefore may free me of the tripod.

Thanks alot for the explanation =)

07-28-2008, 05:32 AM   #5
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QuoteOriginally posted by Andi Lo Quote
I have been wondering for a while on how fast a fast lens actually is. Say, I currently own the DA 50-200, and I read about people saying how the DA* 50-135 is a "faster lens" at f/2.8. My problem with this is that if I shoot both at f/8, will the DA* produce a faster shutter speed?

No. The faster lens will not give you higher shutter speed than DA50-200. The word 'faster' is not related to the actual shutter speed.


QuoteQuote:
Lowell said it well:
the term "faster" when applied to a lens is really only applied to the maximum apature.
Besides that faster lens allows you to have a brighter view at the viewfinder as the aperture can be opened larger with more light coming in


Daniel
07-28-2008, 05:46 AM   #6
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QuoteOriginally posted by Andi Lo Quote
I have been wondering for a while on how fast a fast lens actually is. Say, I currently own the DA 50-200, and I read about people saying how the DA* 50-135 is a "faster lens" at f/2.8. My problem with this is that if I shoot both at f/8, will the DA* produce a faster shutter speed? As I understand it right now, a "faster lens" is simply faster because you can step up the aperture to a smaller f number, so you're sacrificing depth of field (and sharpness) due to stepping up from the sweet f/8 spot.

I am expecting a "it depends" answer, but please enlighten me =x

If you are shooting at long distances, the depth of field is going to expand, and at infinity, the aperture is of less concern if you don't have anything in the foreground.

You gain flexibility, and you can typically open the aperture wider and still have acceptable sharpness as compared to the slower lens. IE the f2.8 lens will be sharper at f4 than the f4 lens is at f4. Most lenses are not at optimal sharpness wide open.
07-28-2008, 05:47 AM   #7
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Means brighter aperture.
07-28-2008, 06:27 AM   #8
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Regarding max aperture being faster / brighter: it makes focusing easier, whether manual or auto. The other main variable being contrast and resolution, of course... a bright but non-contrasty image isn't as easy to latch onto as a contrasty but slightly darker one.

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