Exposure numbers confuse me, help!

[julie32]

hey guys
if you're shooting at an ISO 100 an aperture of f4 and shutterspeed of 1/500 for a correct exposure.. and you cut the lens opening to f5.6, why is it called INCREASING the shutterspeed if you go to 1/250s? Isn't that actually slowing it down?
I'm getting caught up in numbers rather than logic.
Can you please explain this to me in layman terms?

Thanks in advance.

 

[dbrandon]

Hmm, well you're increasing the time the shutter is open, but you're using a slower shutter speed.

I don't think you'd call that increasing the shutter speed

 

[julie32]

that's where I get confused. i'm looking at Bryan Peterson's understand exposure book and verbatim this is what it says:

"You need to increase your shutter speed a full stop from 1/250 to 1/125"

What confuses me is the term "increase." Is it just "increasing the amount of light that's meant here?"

 

[dbrandon]

Ahh right, yeah it must just mean the amount of light. But then i don't know really, never thought about it until now !

It kind of makes more sense that he said 'increase your shutter speed a full stop from 1/250 to 1/125', rather than just 'increase your shutter speed from 1/250 to 1/125'. You'd imagine the term increase is refering to the amount of light by including 'a full stop'.

Don't know though to be honnest !

 

[julie32]

your response makes sense!

Thanks so much.

 

[Mav]

Try not to get too caught up in the terminology because "increase" can mean different things depending on the context or just what the author happened to be thinking of.

If you close the lens aperture down from f/4 to f/5.6 then you're cutting in half the amount of light that's being let in. To maintain the same exposure you need to double the length of time that the shutter is open from 1/500s to 1/250s.

0.5 x 2 = 1 for the same exposure.

"Increase" could mean increase the amount of time that the shutter is open, or it could mean increase the speed at which the shutter opens and closes (faster, open for less time). I'm really not sure which way is correct to describe it, but just pay close attention to the context.