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Was reading this answer (emphasis mine):

It also tells us that the lens was stopped down, as if it were wide open there would be no corners to cause diffraction, regardless of the number of aperture blades.

What does "stopped down" mean?

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5  
It's amazing how many terms I use without even thinking that to a layman would sound like nonsense! –  Matt Grum Mar 9 '11 at 17:34
    
@Matt: Hey, I work in software development. I'm used to "strange terms" on a regular basis. (Though usually our strange terms are TLAs, ETLAs, or EETLAs) (Three letter acronym, extended three letter acronym, expanded extended three letter acronym).... –  Billy ONeal Mar 9 '11 at 17:41
    
You mean, like Four Letter TLAs?? ;-) –  ysap Mar 9 '11 at 18:42
    
@ysap: Yup. TLA is a TLA. ETLA is an ETLA. EETLA is an EETLA :) –  Billy ONeal Mar 9 '11 at 18:51

5 Answers 5

up vote 11 down vote accepted

This picture shows a lens at different apertures. I used a manual focus lens (a Soviet-built 50mm f/1.9) because having aperture controls on the lens itself made it easier to take pictures of the lens at different apertures.

composite of lens at different apertures

  • In the top third, the lens is wide open, or at its maximum (widest) aperture (lowest f-stop value), in this case f/1.9. The aperture blades are fully withdrawn into the lens housing and do not restrict any light from passing through the lens.

  • In the bottom right, the lens has been stopped down about one stop to f/2.8. It is no longer at its widest aperture: you can see the aperture blades have physically extended into the lens opening slightly, to restrict some of the light coming through the lens (if I understand correctly, one stop cuts the light coming through the lens by half; this lens has been stopped down by just over one stop (from f/1.9 to f/2.8; normally one stop would be from f/2.0 to f/2.8) so the aperture blades should actually be covering just a little more than half of the area of the lens).

  • In the bottom left, the lens has been stopped all the way down to f/16, it's smallest aperture (and highest f-stop value). This lens has six aperture blades; in this portion of the picture you can see three of them meeting.

(I apologize for the different exposure of the bottom-left photo. I was shooting in Av, not Manual; perhaps the different reflection of light from the fully-stopped-down lens convinced my camera to up its exposure by 1/3 stop. I'll leave this as a reminder to readers to shoot in manual whenever you're combining multiple photos)

Here is a picture of the lens at f/16 to see what it looks fully stopped down:

same lens at f/16 only

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+1 - Ooooh. Awesome pictures! Thanks! –  Billy ONeal Mar 10 '11 at 19:24

It means the diameter of the aperture stop (a physical object in the centre of the lens which literally stops light to increase depth of field and decrease exposure) was reduced (by the photographer either via the camera body or directly on the lens) some amount from its widest setting.

See:

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@mattdm: Yay, pedantry! –  Billy ONeal Mar 9 '11 at 18:08
1  
@Billy ONeal — just trying to help. I didn't think it warranted a comment. :) –  mattdm Mar 9 '11 at 18:11
    
@mattdm: Lol -- only reason I mentioned it is that you proved me wrong :) –  Billy ONeal Mar 9 '11 at 18:15
    
Doh! I'm normally good at not using "it's" for a possessive! I really don't mind minor corrections like this, anything that makes the site easier to read and use... –  Matt Grum Mar 10 '11 at 10:37
    
I'm sorry for switching the accepted answer on ya -- but the pictures from drewbenn really illustrate the point well. –  Billy ONeal Mar 10 '11 at 19:35

"Stopped Down" simply means "with a small aperture". The aperture is expressed as a fraction, for example, f/8 or f/2 -- f/8 (the aperture is physically an eighth as wide as the focal length) is smaller than f/2 (the aperture is half as wide as the focal length)

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1  
Okay, so this has more to do with the camera than with the lens? –  Billy ONeal Mar 9 '11 at 17:34
3  
Depends on what you mean by "has more to do". You control the stopping down with the camera (at least on modern cameras/lenses), but the action primarily effects the lens. –  Craig Walker Mar 9 '11 at 17:37
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The aperture blades are part of the lens (if you have a DSLR, switch to Aperture-priority (Av) mode, set to something like f/8, hold down your Depth of Field Preview button, and look at the front of the lens: you will be able to see the aperture blades forming a circle). So it has to do with the lens, not the camera. –  drewbenn Mar 9 '11 at 17:42
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In my Pentax Auto110, the aperture stop is part of the camera body, not the tiny interchangeable lenses. (The same iris also serves as the shutter!) –  mattdm Mar 9 '11 at 18:00

It does not mean "with a small aperture"; it means that the lens is at a smaller aperture than it is capable of. f/1.8 for example is not a small aperture; a 50 mm f/1.8 would be wide open at f/1.8 but a 50mm f/1.2 would be "stopped down" at f/1.8. Conversely, f/5.6 is really a rather small aperture, but an 18-200 f/3.5-5.6 would be wide open at 200 mm f/5.6; a 70-200 f/2.8 would be stopped down by two full stops at the same setting.

(Ah, crap. Was meant as a comment to another answer. But it works as an answer in its own right too so never mind...)

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If you have a typical lens which starts with Aperture of F3.5, then any increase in F stop, say to F4.5, would mean you are stopping down the lens.

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