n00b question: stopping down?

bwg

ok, so i'm getting comfortable with my new 20d, i understand the relationship between aperture, shutter speed and film speed (film?).

I keep seeing the phrase "stopping down" and i've yet to figure out the context. Ex: "the lens stops down when the shutter is released" or "IS allows you to stop down 2 stops".

what do it mean?

Mike Lane

bigwebguy wrote:

ok, so i'm getting comfortable with my new 20d, i understand the relationship between aperture, shutter speed and film speed (film?).

I keep seeing the phrase "stopping down" and i've yet to figure out the context. Ex: "the lens stops down when the shutter is released" or "IS allows you to stop down 2 stops".

what do it mean?

1 stop is either 1/2 the light reaching the sensor or twice the light reaching the sensor.

So a shutter speed of 2 seconds is one stop down from 4 seconds and one stop up from 1 second.

Similarly f/8.0 is one stop below f/5.6 (i.e. f/8.0 lets in half as much light as f/5.6) and one stop above f/11 (f/8.0 lets in twice as much light as f/11)

A combination of shutter speed and aperture works as well of course and your 20D likely uses 1/3 stop values for both shutter speed and aperture (unless you set it to 1/2, but why would you do that?)

Hope that helps.

pathfinder

bigwebguy wrote:

ok, so i'm getting comfortable with my new 20d, i understand the relationship between aperture, shutter speed and film speed (film?).

I keep seeing the phrase "stopping down" and i've yet to figure out the context. Ex: "the lens stops down when the shutter is released" or "IS allows you to stop down 2 stops".

what do it mean?

WHen you look through the view finder of a 20D, you are looking through the lens with the iris open to its maximum extent. SO if you are using an f2.8 200mm lens, you are viewing through f2.8 iris aperature.

BUT, when you press the shutter, the mirror clangs up out of the viewing path, the shutter curtain opens as the iris diaphragm in the lens closes down to the desired aperature, which could be any where from 2.8 to maybe f16 or f22 ( depends on the lens)

You can see this happen, without triggering the shutter, by pressing the preview button, which is the liitle spring loaded round button directly beneath the lens unlocking button on the left side of the lens breach when looking from the shooters position. If you press the preview button, the view through the viewfinder will get darker as the iris stops down, and if you look carefully, the depth of field will get larger due to the smaller aperature . Thus "The lens stops down when the shutter is released!" It will close to the aperature that you or the camera has chosen. In manual or Av you get to chose, and that is the modes I almost always use. In Tv you get to chose the shutter speed, In the Basic modes - little mountain( landscape), head (portrait) skier( sports) et all you don't get to chose - the camera choses for you. But the iris still stops down to the chosen aperature.

The phrase "IS lets you stop down two stops" means that you can hand hold the camera without movement about 2 steps of shutter speed slower without apparent blurring ie: 1/400th would be about the limit for handholding a non-IS 400mm lens, but with IS used, a 400mm lens might be shot at 1/100 th or so depending.

The second pharase is referring to the equivalence between changing an iris diaphragm one stop or a shutter speed 1 step since shutter speeds and lens stops are all based on halving or doubling exposures like Mike demonstrated in his post above.

Hope this helps.

bwg

perfect answers. thanks guys.

Andy

"stop down" also means "don't shoot wide-open (e.g. f/2.8) but rather, shoot with a narrow aperture (e.g. f/11, f/16)...

usage: I'll often shoot with this lens stopped down, for my landscapes, thus giving me max depth of field.