"stop down" "2 stops past" etc.

docphotog · Apr 28, 2009

I was wondering if someone can explain what all this means. I've read it in many places and I believe that it concerns aperature size but I don't understand much more to it than that. Thanks for the help!

rufus5150 · Apr 28, 2009

A stop is a measurement of light. Each stop up is twice as much light, each stop down is half as much light.

Changing the aperture from f/4 to f/5.6 decreases the amount of light to the sensor when the shutter is release by half. Changing the aperture from f/4 to f/2.8 doubles the amount of light to the sensor when the shutter is released. Shutter speed and ISO are pretty easy to do the math on (an ISO 100 lets in half as much light as ISO 800, a shutter speed of 1/100 lets in twice as much light as a shutter speed of 1/200) but with aperture numbers involve square roots and all sorts of fun (you can read about it in many places), so it's usually just easier to remember the scale (f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22...).

It doesn't necessarily mean aperture, though. Often you'll hear talk about stopping up or down studio lights, or certain filters will modify light in stops without adjusting the aperture, or you'll hear talk about using exposure bias to stop up or down. Exposure bias will adjust aperture, shutter speed or both depending on what mode you're in (do any adjust ISO this way? I've no idea.)

Most cameras will allow you to adjust it in half or third stops (some may do others? I'm not sure), many studio lights have smooth adjustments or 1/10th adjustments.

docphotog · Apr 28, 2009

wow Rufus, thanks for the explanation it definitely helped quite a bit!

So, to recapitulate:
- If I were to move 2 stops UP from f/2.8 I would be going to f/1.4 and I would therefore let in 4x as much light during that transition?
- Vice versa, if I were to move 2 stops DOWN from f/2.8 then I would be going to f/5.6 and would be quartering the amount of light that I let in?

That's a pretty big jump! Thanks, that really puts different lenses in perspective when you compare their F#'s... does anyone know off the top of their head how to do the calculation? I've always wondered what the big fuss is from f/1.4 to f/1.8... thanks!

ANDS! · Apr 28, 2009

1.4 to 1.8 is 2/3rds a stop. Which isn't anything to sneeze at. If you were in Aperture priority mode, and your shutter speed read 1/60th at F/1.8, going down to F/1.4 would get you at 1/100th of a second, which is quite a bit of difference camera steadiness wise.

As Rufus said, all the three mains of exposure operate in "stops", but they each have their pros and cons: Generally people will save ISO stops as a last resort, as going up introduces noise, while increasing the stops in shutter speed will just freeze/blur action and an aperture stop will bring you depth of field increases/decreases (while also increasing the amount of "light" the sensor grabs).