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Old Aug 26, 2007, 5:50 AM   #1
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Hi,

I'm quite a newbie at this dslr lark so please don't laugh.

I've been reading lens reviews and the reviewers have been talking about 'stopping down' to improve sharpness.

Now, I know they're referring to the aperture (f number), but can anyone explain why stopping down might improve sharpness?

Also, is it a matter of trial and error finding this so-called sweet spot with your lenses to find out at what aperture the they work best?

Thanks in advance
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Old Aug 26, 2007, 6:52 AM   #2
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Those are really two very different issues. Depth of field increases resulting from stopping down and occurs with all lenses. The "sweet spot" is one of the descriptive ways of talking about the quality of a lens.

You can read reviews of lenses and descriptions of depth of field (recomended), but to understand them it is a good idea to do some experiments yourself. One of the key issues in doing the experiments is to get rid of other issues that can cause blur, in particular shake. Shoot off a tripod or at least in bright sunlight so you can use a fast shutter speed. Shoot with the lowest ISO setting your camera allows to minimize noise.

To start with, ignore the "sweet spot" issue unless you are thinking of buying another lens.

To quickly get a rough idea about depth of field, shoot somethings as close as your lens will allow you with other objects at various distances behind it. A flower with other plants receeding behind it is good. Do this in A mode (aperature priority) with one as open as your lens will allow and one stoped down several stops, e.g., f/3.5 and f/11. Just looking at them on your cameras little LCD will show the difference. Do that at both ends of your zoom.
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Old Aug 26, 2007, 11:37 AM   #3
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My understanding of optics is limited, but I think stopping down improves sharpness because it blocks out the more diffuse rays of light and takes in more uniform rays. Sharpness tends to go up until you hit the so called "sweet spot" and then begins to decline as you continue stopping down.

That may be generalizing though. The most common "sweet spot" is f/8, but if a lens was almost as sharp at f/4 as it was at f/8, some might consider that the sweet spot because it's 4 times brighter.

It can be a matter of trial and error learning the sweet spot, but there are also tests that can be done on photos to determine sharpness. If you look up reviews of various lenses you can learn about and see the results of these tests.
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Old Aug 26, 2007, 2:03 PM   #4
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An ideal, but impractical,lens is the hole in a pinhole camera. This is a closed box with a sensor (e.g., a sheet of film) on the inside of one face, and a pinhole in the middle of the other end. A ray of light from each point on a distant subject passes through the pinhole and strikes the film in a very small spot (depending on the hole size). This makes an upside downimage of the subject, resized in the ratio of the box length to the subject distance.

Unfortunately, this image is too dim for practical use as a camera; sensors need more light. This is fixed by having a bigger hole, but then the spot smears out, and gives a fuzzy image. A lens fixes this by putting a bit of glass in the hole to bend the rays into a small spot again. Sadly, this can't be done perfectly (lenses all have 'aberrations') or for a big range of subject distances, even with the use of supercomputers todesign the shape.

But if you make the hole (aperture) as small as possibleby 'stopping down', you will in principle minimise the imperfections, and more closely approach the pinhole's sharpness at the expense of letting in less light.

It's a lot more complicated than this, of course, and lenses are cleverly designed as compromises over their working range, but the principle is roughly right. They generally won't be at their sharpest when at their minimum aperture, though the depth of field (range of distances over which subject is satisfactorily sharp) will be at a maximum.

Happy shooting! Alan T
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Old Aug 26, 2007, 3:05 PM   #5
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Lenses and lens elements are mostly ground as segments of a sphere, and act as magnifying or reducinglenses. Some lenses use Aspherical elements to help correct for spherical aberration.As an extreme example of this, think of a picture taken with a fisheye lens, or a magnifying glass moved away from a newspaper. The edges get distincly blurry. Stopping down a lens makes the iris smaller, using less of the outer part, where the blur is.

In general, the 'sweet spot' of a lens occurs about two sfull stops down from maximum aperture. So, for a f/2.8 lens, the sweet spot would be about f/5.6 or so. It varies. The term 'sweet spot' is generally, but not always used to indicate where the lens is sharpest overall, from center to edge. Some people who are obsessed with this sort of thing, spend their time taking pictures of brick walls, and macros of window screens, then blowing them up to compare. Many arguments are generated, and much bandwith is wasted in these games.

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Old Aug 26, 2007, 7:31 PM   #6
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Each optical element in a lens has two surfaces that must be ground properly. Any imperfections in either surface of any optical element degrades the quality of the image.

When the aperture of a lens is wide open, every square millimeter of both surfaces of every optical element is used to convey light through the lens to the image sensor. (If this were not true, then a lens would contain glass that was not being used for anything, and that would just make the lens heavier for no good reason.)

When the aperture is reduced by adjusting the diaphragm, less of the surfaces of the optical elements are being used. This means that there is less chance that imperfections in the surfaces of these optical elements will degrade image quality.

But when the apertureis reduced too much, any single imperfection will have a greater chance of degrading image quality.

So as you reduce the aperture ('stop down'), less area of each the surfaces of all the optical elements is used, so there are fewer imperfections to degrade the image, but if you 'stop down' too far, any single imperfection will have a greater impact on the image, degrading it more.

The 'sweet spot' is where the aperture is closed down enough so that imperfections on the outer surfaces of the optical elements are blocked by the diaphragm, but not so far that any single imperfection near the center of an optical element will have a significant impact all by itself.

Well made lenses have no significant imperfections, but neither surface of anyoptical element in anylens is ever ground perfectly, so there is no way to avoid this effect, but some of the better lenses have wider sweet spots because there are fewer imperfections and/or the imperfections are not significant. The more optical elements and the more moving parts a lens contains, the more likely that imperfections will appear.

And, sometimes, these imperfections aren't errors in the manufacturing process, butcompromises in the design.All lens designs contain compromises of one sort or another. Sometimes, a lens manufacturer may design a lens and is willing to compromise sharpness at a particular aperture and/or focal lengthin favor of ease of manufacture. (The difference between a $100 lens and a $10,000 lens is, mostly,ease of manufacture.)

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Old Aug 27, 2007, 6:44 PM   #7
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My $.02. The result of most aberrations is a failure to bring all parts of the imageand all wavelengths into focus at the same time (and the same place). As reducing the aperture (stopping down - using a numerically higher aperture) increases the depth of field in front of the lens it also increases the depth of focus behind the lens thus masking aberrations.

This continues until any additional sharpness becomes less than the steadily increasing blur circle caused by the reduced aperture. That is the definition "sweet spot"- that point at which as much aberration correction is applied before diffraction again "softens" the image.

The maximum a "perfect" lens can be stopped down before diffraction overrides depends on the sensor size. Edward Weston could use f64-f90 because he used 8x10 plates, a very large sensor. A P&S camera with a 1/2.5 sensor will become diffraction limitedaround f4.
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Old Aug 28, 2007, 10:59 AM   #8
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Beejygirl wrote:
Quote:
Stopping down
Can anyone explain in basic English?
So, have you gotten a basic English explanation yet? [suB]:-)[/suB]
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Old Aug 28, 2007, 3:52 PM   #9
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Thanks all for your responses. I THINK I know what you are all saying :-)

Makes sense really.

So many questions to ask - I'm wondering whether I should have stayed with a basic P&S!!!

But it's fun working it all out. If only I had the money to get the expensive glassware.:?
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Old Aug 28, 2007, 4:27 PM   #10
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Beejygirl wrote:
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So many questions to ask - I'm wondering whether I should have stayed with a basic P&S!!!
This applies to P&S digicams too! It's just that no one ever mentions it when talking about a digicam, because you don't have much contol over it.
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