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View Full Version : Why do lenses get "sharper" as you stop them down?


snatiep
5th of April 2008 (Sat), 20:24
Hello Everyone!

Why do lenses get sharper as you stop them down? I have read quite a few posts here that lenses are not as sharp wide open.

Take my Sigma 17-70 for example; I get my sharpest images out of this lens at around f/11. I hesitate to do much shooting wide open. Certainly not to say that the images I take at f/2.8 are by any means poor.

If this is the case, whats the point in buying a fast f/2.8 lens?

Thank you for any insight.

djeuch
5th of April 2008 (Sat), 20:30
Has to do with how light is gathered and re-focused. A point in the distance on a wide open lens has to refocus all that light from that point back to one point on your sensor. When you stop down, it is forced to go through a much smaller portion of your lens, and less likely to have divergence.

That's brief explanation, but basically that's why.

Rankinia
5th of April 2008 (Sat), 20:34
The reason youd buy a faster lens varies in its use. I dont imagine many people would be able to do their type of photography using only f11 with either the depth of field or shutter speed. Personally I need high shutter speeds and for all but landscapes I will never use over f8. Sharpness is only one aspect of a photograph (albeit to many of us an important one).

90c4
5th of April 2008 (Sat), 20:40
The point of a fast lens is you can maintain a faster shutter speed in lower light.

To simulate what the aperture on your camera does, pinch your forefinger to your thumb on both hands, then bring your hands together so your fingernails are touching and you have a small diamond shaped opening. If you look through this hole - or aperture - at something far away, it will appear much sharper the smaller you make it, but it will also be darker. And of course less light requires a faster shutter speed to freeze motion. Photography is all about trade-offs.

gasrocks
5th of April 2008 (Sat), 20:46
Suppose you were taking a pix where you were only interested in the center part? Edge sharpness is always a concern. Regular lenses get sharper as you close them down since you then are not using the glass at the edges. It is hard to make the glass good across the whole lens. There are some expensive lenses that are sharp wide open.

snatiep
6th of April 2008 (Sun), 13:27
Thanks gasrocks! That makes sense!

So I shouldn't be concerned that my Sigma 17-70 is sharper at f/11 than f/2.8?

PaulB
6th of April 2008 (Sun), 13:39
Thanks gasrocks! That makes sense!

So I shouldn't be concerned that my Sigma 17-70 is sharper at f/11 than f/2.8?

'Most' lenses perform at their best stopped down two or three stops from maximum aperture, so f2.8 down to f5.6 or f8 would be the expected sweet spot. Is f11 the best aperture at all focal lengths? - longer focal lengths 'usually' need less stopping down than short focal lengths to get the best performance.

wallybud
6th of April 2008 (Sun), 14:15
Id expect these are basically the answers your looking for based on general "understanding" of tech talk ha. If you seek true explanations and visuals of light entering the lens element and what happens to it at different apertures maybe someone can post a link to some chart and such, I have never wondered the answer to the question but have seen in explained and it can be a lot to take in a certain points =)))))

Steve-R
6th of April 2008 (Sun), 17:08
From what I gather, sharpness decreases with larger apertures due to increasing optical path difference - the diffence in path length between the light rays travelling to a given point on the image. I'm not completely sure, but I assume that the resulting phase difference degrades sharpness. The main point is that even with perfectly shaped lens elements, there will still be a loss of sharpness with larger apertures (not to mention the difraction effect with smaller apertures).

djeuch
6th of April 2008 (Sun), 21:27
Well, a perfect lens would perfectly focus all the light rays from a distance to the focal point, so it would be perfectly sharp regardless of aperture....

But physically to design that is impossible!

TeamSpeed
6th of April 2008 (Sun), 21:41
Kinda like why more of the world seems sharper when you squint... You are basically doing the same thing as the aperture in a lens.

Perry Ge
7th of April 2008 (Mon), 01:06
Kinda like why more of the world seems sharper when you squint... You are basically doing the same thing as the aperture in a lens.
That's basically the closest explanation here. Gasrocks' answer is not 100% accurate. It's not quite correct to say that the edge of the glass corresponds to the edges of the image produced. The entire lens does the focusing, so even on a cropped camera, or with the lens stopped down, the edges of the glass still 'contribute' to the bending of the light.

I'd draw you an optical diagram, but I'm lazy and that takes too long :lol:.

gofer
7th of April 2008 (Mon), 02:29
Hello Everyone!

Why do lenses get sharper as you stop them down? I have read quite a few posts here that lenses are not as sharp wide open.

Take my Sigma 17-70 for example; I get my sharpest images out of this lens at around f/11. I hesitate to do much shooting wide open. Certainly not to say that the images I take at f/2.8 are by any means poor.

If this is the case, whats the point in buying a fast f/2.8 lens?

Thank you for any insight.

As an aside, it is possible your Sigma 17-70mm appears sharper at f11 than the wider apertures because the lens is not focusing correctly. If the lens for instance has a slight back or front focus stopping down to f11 will make the out of focus less noticeable due the increased DoF.

foxbat
7th of April 2008 (Mon), 03:21
Regular lenses get sharper as you close them down since you then are not using the glass at the edges.Are you sure about that? You are implying that the field of view changes as you stop down.

Steve-R
7th of April 2008 (Mon), 12:17
Well, a perfect lens would perfectly focus all the light rays from a distance to the focal point, so it would be perfectly sharp regardless of aperture....

But physically to design that is impossible!


No, actually a perfect lens will not perfectly focus all the light. There is a theoretical limit on sharpness - that's where the optical path difference comes in (for large apertures) and diffraction (for small apertures).

vpnd
7th of April 2008 (Mon), 13:41
think of it this way... pretend a lens at 2.8 is a big fat magic marker... then that same lens stopped down to f8 or 11 is a smaller magic marker... you can draw faster with less detail with the big marker and way more detail but slower with the small one. Circles of light is another way to put it.