# Thread: how to count "bulb" speed?

1. ## how to count "bulb" speed?

hi guys

lets say, i use my camera metering to meter a scenery.. its says for perfect exposure i need to expose with 30secs f/2.8.. (maybe its very dark at midnight)
then i wanna put an nd400 filter and use f/22 instead of f/2.8.. so i gotta use bulb..
then come the question.. how long should i expose for it?

wondering if there is a kinda formula to count this?

thankss

2. ## Re: how to count "bulb" speed?

A shutter speed's stop is in steps of *2. Compensate for the difference in Aperture stops and multiply it to the time. And technically u would have the same exposure. Correct me if i am mistaken =)

3. ## Re: how to count "bulb" speed?

Originally Posted by tirza
hi guys

lets say, i use my camera metering to meter a scenery.. its says for perfect exposure i need to expose with 30secs f/2.8.. (maybe its very dark at midnight)
then i wanna put an nd400 filter and use f/22 instead of f/2.8.. so i gotta use bulb..
then come the question.. how long should i expose for it?

wondering if there is a kinda formula to count this?

thankss
f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
f/2.8 to f/22 is 6 stops

ND400 is 9 stops ND filter

So 6 + 9 stops = 15 stops

30sec, 1 min, 2 min, 4 min , 8min, _________________

4. ## Re: how to count "bulb" speed?

Originally Posted by megaweb
f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
f/2.8 to f/22 is 6 stops

ND400 is 9 stops ND filter

So 6 + 9 stops = 15 stops

30sec, 1 min, 2 min, 4 min , 8min, _________________
I've never done suoer long exposure in digital, but does reciprocity failure apply to digital?

5. ## Re: how to count "bulb" speed?

Originally Posted by Dream Merchant
I've never done suoer long exposure in digital, but does reciprocity failure apply to digital?
Don't think it will. Astrophotography uses very long exposure too.

6. ## Re: how to count "bulb" speed?

Originally Posted by Dream Merchant
I've never done suoer long exposure in digital, but does reciprocity failure apply to digital?
Don't think reciprocity failure is much of an issue as compared to noise. In astro, splitting up the exposure and then stacking usually gives a more controllable result unless the subject is very very dim.

7. ## Re: how to count "bulb" speed?

Originally Posted by Dream Merchant
I've never done suoer long exposure in digital, but does reciprocity failure apply to digital?
No reciprocity failure, thank goodness. You do have to worry about noise though.

Samuel

8. ## Re: how to count "bulb" speed?

Originally Posted by tirza
hi guys

lets say, i use my camera metering to meter a scenery.. its says for perfect exposure i need to expose with 30secs f/2.8.. (maybe its very dark at midnight)
then i wanna put an nd400 filter and use f/22 instead of f/2.8.. so i gotta use bulb..
then come the question.. how long should i expose for it?

wondering if there is a kinda formula to count this?

thankss
In the first place, why would you want to add a ND400 filter when it's already so dark!

f/2.8 is 6 stops from f/22, so simply add 6 stops to the shutter speed. By adding the ND400, you need to decrease the shutter speed even further, leading to hours long exposure even...

Samuel

9. ## Re: how to count "bulb" speed?

Originally Posted by megaweb
f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
f/2.8 to f/22 is 6 stops

ND400 is 9 stops ND filter

So 6 + 9 stops = 15 stops

30sec, 1 min, 2 min, 4 min , 8min, _________________
and god, its 240hr, i think either ur sensor will just go burst first or u run out of juice.
to ur poor camera.

10. ## Re: how to count "bulb" speed?

Originally Posted by Dream Merchant
I've never done suoer long exposure in digital, but does reciprocity failure apply to digital?
no reciprocity failure on digital.

11. ## Re: how to count "bulb" speed?

Originally Posted by megaweb
f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
f/2.8 to f/22 is 6 stops

ND400 is 9 stops ND filter

So 6 + 9 stops = 15 stops

30sec, 1 min, 2 min, 4 min , 8min, _________________
oww i think i get it
so one change of aperture, one change of stop..
and when multiplying the speed by 2.. it is also one stop change..
izit sumthing like that?

- alternatve n ExplorerZ, haha this is just an example.. wont try it to my camera though.. the answer is 240hours??

btw thanks for all the replies

12. ## Re: how to count "bulb" speed?

I think TS was just asking, as an example, of how to calculate ... not really thinking of putting a 9-stop ND filter on.

Thanks guys, for answering ab reciprocity law failure in digital. Exposure stacking does sounds interesting! Time to do more research!

13. ## Re: how to count "bulb" speed?

Originally Posted by Dream Merchant
I think TS was just asking, as an example, of how to calculate ... not really thinking of putting a 9-stop ND filter on.

Thanks guys, for answering ab reciprocity law failure in digital. Exposure stacking does sounds interesting! Time to do more research!
i think most of us were just kidding about it, don even think he can even see anything thru his VF with that kind of lighting+nd400.

14. ## Re: how to count "bulb" speed?

Originally Posted by ExplorerZ
i think most of us were just kidding about it, don even think he can even see anything thru his VF with that kind of lighting+nd400.
Hmm ... yu'll be surprised what some people would do ... just for the sake of experimentation! Faded memories of climbing up on other people's roof-tops and taking one hundred and twenty minute exposures in the dead of night while walking up and down the entire block of houses out of sheer boredom and 'exploring' other neighbor's gardens and houses from a different perspective ... hahahaha!

15. ## Re: how to count "bulb" speed?

Get an old analogue meter which measures till very low EV. I have an old Gossen Luna-Pro SBC.

Else, print n keep an exposure calculator with ya.

http://www.fredparker.com/ultexp1.htm

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