questions on F stops

fguihen

i was reading in a magazine about doing long exposures during the day using a 10 stop neutral density filter. it says compose the shot, add teh filter, and then calculate the shutter time needed for a 10 stop filter. My first question is

1. how do you work out how much shutter time to add/detract per stop up/down?I know the camera usually does this but id like to understand how to do it manually.

2. I have a Sigma 70-300mm F4-5.6 APO Lens. what does the F4-F5.6 mean? I know this lens can be stopped up way higher than F5.6, and lower than F4.

Paulw

fguihen wrote: »

1. how do you work out how much shutter time to add/detract per stop up/down?I know the camera usually does this but id like to understand how to do it manually.

2. I have a Sigma 70-300mm F4-5.6 APO Lens. what does the F4-F5.6 mean? I know this lens can be stopped up way higher than F5.6, and lower than F4.

Not sure about 1), but for 2) 70-300mm f/4-5.6 means that at 70mm you can get f/4 and lower, and at 300mm you can get f/5.6 and lower. You can still get f/22 (or lower) at both 70 and 300mm.

I hope that helps.

Mellor

My understanding of the first part is, for every increase in a full stop the area of the aperature doubles (the F stops are based on [sq.route]2).
So if you increase by 2 stops, the aperature increases by a factor of 4.
Basically 2^[stop increase]
It also works in reverse. So if you reduce light entering by 10 stops, the amount of light is reduced by a factor of "1024", (2^10)
So shutter speed comps for this much, roughly. If correct shutter speed was 1/200 it is now 5 seconds (1000)x(1/200)

Im open to correction on the above, its just my understanding of it, I don't shoot manual so knowledge is limited. One of our manuel shooters might know

As for second part, as Paulw said, its the max aperature possible as the top and bottom focal length limit

quilmore

shutter time goes by stops too
they are messured by EV
a 1/30 will have one more EV than 1/60
and 1/60 will have one more than 1/125
so, if without filters you get 1/1000
then you have to count 10 "stops" down
that goes:
1/500
1/250
1/125
1/60
1/30
1/15
1/8
1/4
1/2

so, a 1/1000 stopped down by 10 stops ends up being 1/2 second

fguihen

great. thanks for the answers here guys! its helped my understanding a lot.