What constitutes a stop?

Have never officially known if it is the difference between F2.8 - F3.5 type thing or every full increment like F2.0 - F3.0 "if that number even exist on any lenses".

For apertures, a stop up or down from a given aperture can be determined by either multiplying (stop down) or dividing (a stop up) the aperture by the square root of 2 (approximately 1.414).

For shutter speeds and ISO, you would multiply or divide by 2.

So full 'stops' are 1-1.4-2-2.8-4-5.6-8-11-16 and so on...

Best regards,
Andy

http://www.dpreview.co​m …/Exposure/Apert​ure_01.htm

Elie

Think of a 'full stop' as the change required in shutter speed, aperture or ISO that either halves or doubles the amount of light entering the camera.

Thanks to all, I am clear on this now!

Here's a very good article about f-stops:

http://www.uscoles.com​/fstop.htm

My camera is set to 1/3 stop increments, so 1 stop of aperture is 3 clicks of the wheel, then halve or double the time for exposure to compensate.

This to me would fly in the face of the other information shown here, is this accurate??? or am I missing something?

If one "click" of the wheel changes the aperture by 1/3 stop, then it makes sense that three "clicks" would adjust the aperture setting by one full stop. If this is true, then halving or doubling the shutter speed (the direction based on the direction of aperture adjustment) would maintain the same total exposure.

If I am using aperture control and am set at F2.8 then click the wheel once I go to F3.2, next click is F3.5, then F4.0. As I understand it that is 3 stops. I can’t see how F2.8 to F4.0 is only 1 stop "because I clicked the wheel 3 times"

...

each click is 1/3 of a stop. thats all you need to know.

3 clicks is a stop. so, in manual mode, 3 clicks to the left with aperture, and 3 clicks to the right with shutterspeed WILL give you the same exposure.

i think on your 30d the ISO can also be adjusted in 1/3rd increments (as opposed to my 350d where i can only adjust the ISO by a stop each time - ie 100, 200, 400... etc.. see how it doubles each time? your 30d has values in between mine to choose from giving you more precise control over exposure).

The values you quoted are 1/3 stop increments, not full stops.

To go from one stop to another (in full stop increments), you need to multiply or divide the first f-stop number by 1.414. The multiplying is for "stopping down" or closing the lens down to a smaller aperture. The dividing is for opening the lens aperture.

The conventional series of full-stop settings (with numbers rounded to normally seen values) is: f/1.0 (seldom seen), f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/11, f/16, f/22, f/32.

Thanks for the help guys, I am clear on this again now. I think I was reading to much into some of the answers but it does make sense now.

It took me a while to figure out how to put what i was reading into practice, it was hard to glean the correct info from some of the posts i'd seen, so i had to play with the camera a bit then put it into layman's terms, so i could actually use it lol!