Aperture. Lenses and Sharpness - Beginner Question

[RegRoy]

I've read that the sharpest photos that can be taken from a lens are generally two f-stops down from wide open. If this is the case, then why are lenses that have a fixed aperture over the course of the focal length so valuable?
Thanks!

 

[Compaq]

Good lenses are satisfactorly sharp over the entire frame, but uber sharp in the middle. Now, look at Canon 70-200mm f/2.8. In most situations, you'll have the subject in the middle of the frame, and the rest out of focus. Perfect sharpness in the corners and edges aren't as important as centre sharpness, especially in portraits, bird shoots, panning etc. Having a fast lens lets us keep that low ISO and rather open up the diaphragm. More light hitting the sensor is a good thing. Low light situations, require a large aperture.

Tokina 11-16mm f/2.8. let's say you're shooting stars. 2.8 vs 4 might be the difference between 30 minutes shutter speed and one hour shutter speed, not to mention 2.8 vs 5.6. huge difference!

Also, lenses with a constant aperture are very often better built than those whose aperture vary. Cost more too.

 

[kundalini]

If you're talking about zoom lenses, then you hit the nail on the head...... constant aperture. That doesn't mean it has to (or should) stay wide open. It means you're beyond the consumer phase and have the ability to stay constant with your aperture throughout the zoom range of the lens. f/3.5-f/5.6 sucks because....... well, just because.

 

[RegRoy]

So it the point that basically, once you are at the level of using a fixed-aperture lens, then messing with aperture for sharpness is something you don't need to really worry about because of the inherent higher quality of the lens?
Thanks.

 

[Quentin_Moyer]

Exactly. f/5.6 can be useless sometimes, even more useless when you have to stop down from there.
If Nikon had an AF-S 70-200mm f/4 I'd be all over that.

 

[joealcantar]

Because they can get you "the shot".
Say I was shooting in a church with a 70-200 2.8
and another lens 55-200mm f/4-5.6
The 1st lens allows more light to hit the sensor even at it's full extended position, whereas the 2nd lens only allows you to shoot at 5.6 fully extended. That difference alone can be the difference in needing a flash or no flash. Handheld or needing a tripod.
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Keep in mind that even these lenses have a sweet spot as every lens does.
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Shoot well, Joe

 

[KmH]

So it the point that basically, once you are at the level of using a fixed-aperture lens, then messing with aperture for sharpness is something you don't need to really worry about because of the inherent higher quality of the lens?
Thanks.

FWIW, it is constant aperture, not fixed aperture. Constant aperture is still adjustable from wide open to the smallest aperture the aperture mechanism can acheive.

Fixed aperture means the lens doesn't have an adjustable aperture mechanism.

A variable aperture lens changes the maximum aperture as the lens is zoomed, a constant aperture lens does not. Consequently that part of depth-of-field (DOF) related to lens aperutre, and the amount of light being let in, changes as a variable aperture lens set to an aperture value less than the maximum aperture at the longest focal length the lens, is zoomed and the aperture size changes.

A constant aperture lens lets in the same amount of light when zoomed, and the only change to DOF is that part affected by the lens focal length.

The cat's-ass is a constant aperture lens that is also capable of parfocus.

 

[Compaq]

To set things in perspective, f/2.8 lets in four times as much light as f/5.6, and 64 times as much light as f/22.

f/2 lets in 256 times as much light as f/32.

 

[Kerbouchard]

Yeah, pretty much what has already been said, but let's take your two stops from wide open example. If you are shooting at 200mm, would you rather be sharpest at 5.6 or f/11? The 2.8 constant aperture allows you to choose. The variable zoom forces you to be at f/11 which is pretty much a non-starter indoors for any subject that is moving.

The other thing a lot of people seem to gloss over when it comes to these situations is focus and the viewfinder. While looking through the viewfinder, the lens is at it's max aperture. This effects how bright your viewfinder is and how well you can see your subject in the first place. Also, your focus module uses the information it recieves to lock in focus or for more advanced cameras, predictive focus. Again, this is all taking place at the max aperture of your lens, so a 2.8 lens will generally be able to focus faster and more accurately than a variable aperture lens.

The final thing is that this is all marketing. Camera manufacturers don't have a lot of reason to build variable aperture lenses as well as they can. Heck, they are consumer lenses and most people wouldn't know the difference. Also, consumers typically prefer light lenses. They also know that somebody who is going to buy a more expensive 2.8 lens is going to want better build quality and won't mind the weight. The 2.8 lenses are marketed for pros and enthusiasts and are built for it. The 3.5/5.6 lenses are marketed towards amatures and are built accordingly.

FWIW, one thing Canon does better than Nikon is provide a middle ground. I do wish Nikon would come out with some f/4 lenses.

 

[RegRoy]

Constant aperture is still adjustable from wide open to the smallest aperture the aperture mechanism can acheive.

Fixed aperture means the lens doesn't have an adjustable aperture mechanism.

A variable aperture lens changes the maximum aperture as the lens is zoomed, a constant aperture lens does not. Consequently that part of depth-of-field (DOF) related to lens aperutre, and the amount of light being let in, changes as a variable aperture lens set to an aperture value less than the maximum aperture at the longest focal length the lens, is zoomed and the aperture size changes.

A constant aperture lens lets in the same amount of light when zoomed, and the only change to DOF is that part affected by the lens focal length.

Yikes! If a lens is a f/2.8 -- constant or fixed, does that still mean that the aperture doesn't change no matter what the focal length does???? Maybe I don't get what you mean by constant aperture....

Also, when you say "variable aperture" is that the same thing as a "zoom" lens?

Thanks.

 

[Derrel]

Touching on something kerbouchard noted above: "a 2.8 lens will generally be able to focus faster and more accurately than a variable aperture lens."

Canon has several cameras now that feature what they call "double-precision autofocus" with lenses that have an f/2.8 or wider maximum aperture.

 

[Kerbouchard]

Constant aperture is still adjustable from wide open to the smallest aperture the aperture mechanism can acheive.

Fixed aperture means the lens doesn't have an adjustable aperture mechanism.

A variable aperture lens changes the maximum aperture as the lens is zoomed, a constant aperture lens does not. Consequently that part of depth-of-field (DOF) related to lens aperutre, and the amount of light being let in, changes as a variable aperture lens set to an aperture value less than the maximum aperture at the longest focal length the lens, is zoomed and the aperture size changes.

A constant aperture lens lets in the same amount of light when zoomed, and the only change to DOF is that part affected by the lens focal length.

Yikes! If a lens is a f/2.8 -- constant or fixed, does that still mean that the aperture doesn't change no matter what the focal length does???? Maybe I don't get what you mean by constant aperture....

Also, when you say "variable aperture" is that the same thing as a "zoom" lens?

Thanks.

I don't think there are any modern lenses that don't have an adjustable aperture, so when people say constant or fixed aperture, what they are referring to is a lens that doesn't have a smaller aperture at one end of the zoom range, or in other words, a constant max aperture throughout the zoom range.

A prime lens, which is a lens that does not zoom, or has only one focal length has one max aperture(macro lenses can be an excpetion to this, but that's a bit beyond the scope of this thread). You change it based on whatever your creative goals are.

For zoom lenses, there is a bit more variety. Professional lenses will have the same max aperture throughout the zoom range. Consumer lenses will usually have a smaller aperture at the high end of their zoom range. This matters because if you are using a 70-300 3.5 to 5.6 and you are shooting at 70mm at an aperture of 3.5 and your exposure is perfect and you are happy with your ISO and shutter speed, and then you decide to zoom in, your aperture will automatically change to something smaller. This will effect your exposure, or if shooting in an automatic mode, your shutter speed or ISO which could give you vastly different results. This effects the shorter end as well, since often, you will go ahead and shoot at 5.6 or smaller so that you don't have to continue to adjust your exposure every time you zoom in or out.

 

[tyler_h]

I don't think there are any modern lenses that don't have an adjustable aperture, so when people say constant or fixed aperture, what they are referring to is a lens that doesn't have a smaller aperture at one end of the zoom range, or in other words, a constant max aperture throughout the zoom range.

Not sure how you are considering modern but fixed aperture mirror lenses are around... (just throwing it out there to help with the confusion )

 

[chaosrealm93]

FWIW, one thing Canon does better than Nikon is provide a middle ground. I do wish Nikon would come out with some f/4 lenses.

but a lot of people end up buying the more expensive versions (f2.8) anyways lol

 

[ann]

Reg, your correct , a zoom lens at 2.8 will stay at 2.8 regardless of the focal length. (Of course it will also work at the fstops in the same manner)

A 3.5-5.6 lens will change aperture as you zoom out. I.E. 3.5 at 18mm 5.6 at 55, etc.
It will stay constant if you start with f8 , etc.