slow/fast lens ?

Hi
I just want to know what'S the meaning of a slow lens and a fast lens....

Thank you

Still learning and exploring
Dany

I think if it as the effect of the lens on shutter speed.
A fast lens has a wide aperture (f/2.8 or wider) and so you can use a faster shutter speed.
A slow lens has a relatively small aperture (f/5.6 is common) and in the same light you have to use slower shutter speed.

Faster lenses let in more light, have bigger glass, are heavier, and more expensive.

Faster lens also are generally better optically, and can produce narrower depth of field (at the wider aperture).

Ok I see!

Thnak you!

Dany

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Originally Posted by Anbesol

Faster lens also are generally better optically, and can produce narrower depth of field (at the wider aperture).

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Actually, faster lenses are NOT better optically, usually producing infrior quality to slower lenses. That is why lenses capable of producing the highest resolution, such as copying lenses usually have a maximum aperture in the F8 bracket.

You are quite correct about the reduced depth of field of the faster lenses though.

The optimum aperture for quality on most lenses tends to be around F5.6

Dave

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Actually, faster lenses are NOT better optically, usually producing infrior quality to slower lenses

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Maybe there is a misunderstanding here. Take two lens, one that max at f/5.6 and one at f/2.8. The quality of say, 50mm at f/5.6 on the slower lens will be better than the quality of 50mm at f/2.8 on the other. But, stop the faster one down to f/5.6 and it will produce better optical characteristics than the 50mm at the same f-stop.

My 50mm f/1.7 can run circles around the quality of any other lens I have at 50mm, at f-stops f/3.2 and slower. This is an extreme example, as I'm comparing a prime to zooms, but the principle still stands.

But really, its not an absolute statement, just a general one. Just because one lens is faster than another doesnt mean its automatically better, it just typically is.

By the way the only time I hear of 'copying' lens is in reference to macros, of which I can only think of prime lens and the very slowest I can recall is f/4, most are typically f/2-2.8, but I've certainly never seen an f/8 macro. The only f/8's I've seen are mirrored lens, which are not famous for optical quality, but rather for size/telephoto convenience.

To quote Andreas Feininger, Staff photographer for Time Life magazine and arguably one of the finest photographers ever, Fast lenses are a compromise. Compromising image quality for sheer speed. And the finest quality lenses, used primarily for copying have high minimum apertures, because the quality of the lens cannot be recreated in fast glass. This is why most lenses need to be stopped down to produce better image quality.

Stopping down a lens will indeed produce a better image quality. Part of the compromise of speed versus quality, the argument that a faster lens produces better image quality is not true.

Dave

Understandably stopping a faster lens down does involve utilizing aperture blades as opposed to the simple optical construction, things like 9-bladed circular apertures work well to minimize the difference created.

The simple fact of the matter is, in actual practice, faster lens produce better image quality. I'm not talking about the theoretical science of optics, I'm talking about the practical reality of it. Take your garden variety 70-300 f/4-5.6, compare it to a 70-200 f/4 or f/2.8. The 70-200's will be a significant optical improvement over the 70-300, easily. Whether or not you CAN build a 200mm f/8 that works much better than a 200mm f/2.8 is irrelevant, because looking at the lens actually on the market right now, the general rule that remains true is that faster lens typically produce better IQ than slower lens.

Can you reference me a 'copy lens' (I guess an unusual way to reference macro), that has such a narrow maximum aperture? I can point you to the 50mm f/2.8, 60mm f/2, 100mm f/2.8, 90mm f/2.5, 105mm f/2.8, etc. I can't think of a single macro thats slower than f/4.

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This is why most lenses need to be stopped down to produce better image quality.

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Actually this is incorrect, stopping lens down produces better IQ because it uses the sweet spot of the lens elements, uses less of the borders and outsides of the lens elements, and because it bends light at sharper angles.

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Originally Posted by Anbesol

Understandably stopping a faster lens down does involve utilizing aperture blades as opposed to the simple optical construction, things like 9-bladed circular apertures work well to minimize the difference created.

The simple fact of the matter is, in actual practice, faster lens produce better image quality. I'm not talking about the theoretical science of optics, I'm talking about the practical reality of it. Take your garden variety 70-300 f/4-5.6, compare it to a 70-200 f/4 or f/2.8. The 70-200's will be a significant optical improvement over the 70-300, easily. Whether or not you CAN build a 200mm f/8 that works much better than a 200mm f/2.8 is irrelevant, because looking at the lens actually on the market right now, the general rule that remains true is that faster lens typically produce better IQ than slower lens.

Can you reference me a 'copy lens' (I guess an unusual way to reference macro), that has such a narrow maximum aperture? I can point you to the 50mm f/2.8, 60mm f/2, 100mm f/2.8, 90mm f/2.5, 105mm f/2.8, etc. I can't think of a single macro thats slower than f/4.


Actually this is incorrect, stopping lens down produces better IQ because it uses the sweet spot of the lens elements, uses less of the borders and outsides of the lens elements, and because it bends light at sharper angles.

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"Take your garden variety 70-300 f/4-5.6, compare it to a 70-200 f/4 or f/2.8. The 70-200's will be a significant optical improvement over the 70-300, easily." Aren't you just saying that a $2000 lens has better IQ than a $200 lens? Fast lenses generally are made better (and cost way more), but I wouldn't call the better IQ a characteristic of being fast.

"...stopping lens down produces better IQ because it uses the sweet spot of the lens elements, uses less of the borders and outsides of the lens elements..." If this were true, wouldn't you get a 'cropped' image?

TF

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but I wouldn't call the better IQ a characteristic of being fast.

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Right, the argument sort of went on a tangent as Dave failed to read the proper context of my original statement, which I simply said 'faster lens are generally better optically', which is true. Its also a practical characteristic in f-stop range considering the sweet spot rule which favors faster lens. Albeit, the point at which light bends can go through a perfect circle at maximum f-stop in a given lens, whereas aperture blades stopped down do not produce the same perfect circle, the difference at this point, especially with 9-bladed circular apertures, is somewhere between minuscule and nonexistent. Even assuming there is a difference, the sweet spot rule benefit would outweigh the perfectly circular point of infraction anyway.

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If this were true, wouldn't you get a 'cropped' image?

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no, you actually do use more of the inner circle of the lens elements when you stop an aperture down, or, I should say - less of the outer edges. Due to the angle that light bends at the aperture. You use less light, and you use less of the elements. Think of the optics immediately ahead of the aperture blades, on say a f/1.4 lens, stopped down to f/8. The light passes through a more cone-like shape, instead of straight through as a cylinder. Not using those two terms scientifically, I'm just trying to illustrate in practical expression.

I love to read and try to understand 'theory' as much as anybody and sometimes I even understand it.
However, I would like to let Dany, a beginner who is "still learning and exploring" know that a fast lens IS a lens with a wide aperture, usually thought of as f/2.8 or wider. It is fast because it allows for faster shutter speeds.

The theories and opinions stated above are interesting Dany, but I wouldn't stress my mind about it. Fast lenses usually do have better quality but are not always necessary for 'good' quality. The Tokina lens I have with nothing wider than f/4 is one of the most excellent lenses I have.

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Originally Posted by Anbesol

Right, the argument sort of went on a tangent as Dave failed to read the proper context of my original statement, which I simply said 'faster lens are generally better optically', which is true. Its also a practical characteristic in f-stop range considering the sweet spot rule which favors faster lens. Albeit, the point at which light bends can go through a perfect circle at maximum f-stop in a given lens, whereas aperture blades stopped down do not produce the same perfect circle, the difference at this point, especially with 9-bladed circular apertures, is somewhere between minuscule and nonexistent. Even assuming there is a difference, the sweet spot rule benefit would outweigh the perfectly circular point of infraction anyway.


no, you actually do use more of the inner circle of the lens elements when you stop an aperture down, or, I should say - less of the outer edges. Due to the angle that light bends at the aperture. You use less light, and you use less of the elements. Think of the optics immediately ahead of the aperture blades, on say a f/1.4 lens, stopped down to f/8. The light passes through a more cone-like shape, instead of straight through as a cylinder. Not using those two terms scientifically, I'm just trying to illustrate in practical expression.

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Agree that it is more cone-like, but it is still coming from the edges of the lens. - TF

Slow = struggles to focus in low light = lower cost = frustration
Fast = focuses well in low light = expensive = happiness
Advise - Read reviews and ask questions before purchase

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Originally Posted by OldClicker

Agree that it is more cone-like, but it is still coming from the edges of the lens. - TF

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It comes *less* from the edges, particularly in regards to the inner lens elements.

Thanks Frog for resuming everything!
He he he! I Read everything... As I said, I'm still learning and exploring... it's the same for my english... (sorry if I do some mistakes in my writing...)