the whole STOPS business?

[Yella Fella]

right, just to clarify...

on my standard setting on the 20D, i can under/over expose at 1/3 stops

so i assume when people say stop up/down at one stop its literally -1 or +1 stop rather than -0.3, -0.7 at 1/3's?

Again with the aperture, going from 4 > 5.6 etc that is one stop, and again from iso 100 > 200 etc is another stop?

What about speeds? or is that measured differently?

[cyber_m0nkey]

Same...

x = 1/100
+1 = 1/50
-1 = 1/200

[chris d]

so i assume when people say stop up/down at one stop its literally -1 or +1 stop rather than -0.3, -0.7 at 1/3's?

Yes, it's literally -1 or +1 stop. Either one half the amount of light or twice the amount of light. That was the standard on film cameras/lenses. Your 20D permits you to further divide the stop into thirds, so you can exercise greater control over the exposure.

Again with the aperture, going from 4 > 5.6 etc that is one stop, and again from iso 100 > 200 etc is another stop?

Yes again. Going from f/4 to f/5.6 halves the amount of light hitting your sensor. Going from iso 100 to 200 doubles, in effect, the amount of light.

What about speeds? or is that measured differently?

No, they are also traditionally set in terms of halving/doubling. However, the 20D also permits you to further divide the traditional stop into thirds. For example, a traditional pairing of shutter speeds was 1/125 (of a second) and 1/250 (of a second). On the 20D the speeds are 1/125, 1/160, 1/200 and 1/250. I assume most digital SLRs are the same.

Ciao.

[Sean-Mcr]

The ISO is the sensitivity to light, it's not actually the same or as desirable as being able to control the light with the shutter & aperture

[PhotosGuy]

The ISO is the sensitivity to light, it's not actually the same or as desirable as being able to control the light with the shutter & aperture They all work together. First pick the speed & aperture you need. Then the ISO to get the "right" exposure.

[blue_max]

I liked the Hasselblad system of EV (exposure value). That amounts to how much light is required. Being as it is made up of size of aperture and length of time open, it makes much more sense. The f-stop figures are like learning another language.

Graham

[Sean-Mcr]

They all work together. First pick the speed & aperture you need. Then the ISO to get the "right" exposure.


I realise that they work together i've shot in almost nothing but manual for the last eight months, it's just that doubling light sensitivity is not the same as doubling the amount of light coming in to the aperture.

My post was in response to what Chris said

" Going from iso 100 to 200 doubles, in effect, the amount of light"

It doubles the sensitivity to the light, not the amount