Calculating long exposures

Merlin5

Merlin5

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Hi guys. I watched a night time photography video where the guy was down under a bridge on the Thames embankment at night. He looked at his live view screen and said it's telling him it that at f11 and ISO 200 he needs a 60 second exposure. I was wondering how he knew, considering cameras only show shutter speeds down to 30 seconds. Then I read this article below which made me realise how he knew he needed 60 seconds,

www.richardbernabe.com

How To Calculate Long Exposures | Richard Bernabe

Here's a quick tutorial to calculate long exposures with your camera that are more than 30 seconds in length to capture streaking clouds and smooth, silky water.
www.richardbernabe.com

It says "It's easy. First, establish a base exposure by using 30 second as your shutter speed and ISO as the exposure variable for preliminary calculations. In Manual exposure mode, select 30 seconds for the shutter speed, an ISO of 100, and the appropriate f-stop for depth of field considerations. Now, simply increase the ISO in full stops until the meter indicates that the scene is exposed correctly. Count the number of increased stops. If it was two stops, for example (ISO 100 to 400) then you just add those two stops to the shutter speed (30 seconds to 2 minutes) after resetting the ISO back to 100 and the exposure mode to Bulb. These are reciprocal exposures (30 seconds and 400 ISO equals 2 minutes and 100 ISO."

So is that the rule that I should always use, add 30 seconds for every extra 100 ISO I increase by until the exposure meter is in the centre, and would I add 10 seconds for every third of a stop?

Also, the photographer in the video I mentioned who did a 60 second exposure showed his final photo at ISO 200, f11 and 60 seconds. If he started at ISO 100 at f11 and moved it up to ISO 200 to get the correct exposure and calculate the exposure time he needs in bulb mode, should he not have reset the ISO back to 100 when he went into bulb mode? But the photo came out perfectly.
 
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Merlin5 said:
So is that the rule that I should always use, add 30 seconds for every extra 100 ISO I increase by until the exposure meter is in the centre, and would I add 10 seconds for every third of a stop?
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No... you double the time for every stop of ISO increase. In your example it went from ISO 100 to ISO 400 (2 stops; 100-200-400), so the time is increased by 2 stops if ISO 100 is used; 30-60(1min)-120(2min).
Merlin5 said:
If he started at ISO 100 at f11 and moved it up to ISO 200
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He probably didn't; he must have been at ISO 400 to get a 30sec proper exposure reading... or he could have been at ISO200 f/8 30sec instead.
 
sk66 said:
No... you double the time for every stop of ISO increase. In your example it went from ISO 100 to ISO 400 (2 stops; 100-200-400), so the time is increased by 2 stops if ISO 100 is used; 30-60(1min)-120(2min).

He probably didn't; he must have been at ISO 400 to get a 30sec proper exposure reading... or he could have been at ISO200 f/8 30sec instead.
Click to expand...

Ah right, I think I see. So if I get correct exposure at say 3 stops of ISO 800 (from ISO 100), I would need to set 3 minutes in bulb? Correct exposure of 4 stops at ISO 1600 = 4 minute exposure? And any third of a stop I add 20 seconds?
 
Merlin5 said:
Ah right, I think I see. So if I get correct exposure at say 3 stops of ISO 800 (from ISO 100), I would need to set 3 minutes in bulb? Correct exposure of 4 stops at ISO 1600 = 4 minute exposure? And any third of a stop I add 20 seconds?
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Not quite... everything about exposure is logarithmic (2x). ISO and SS are easy because each stop change doubles or halves the previous value for both of them. So 3 stops from 30sec is 1 min (60sec)- 2min- 4min, not just 1 min per stop; and 4 stops would be 8min. A third of a stop between 1min and 2 min is 20sec (1/3 of the difference); and one third of a stop between 4min and 8min is 1.33 min (1min 20sec), again 1/3 of the difference.

Aperture is a bit more convoluted because it is a logarithmic (2x) change in aperture area, and not the f# that represents it... a 2x change in area is a 1.4x numerical change (rounded).
 
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sk66 said:
Not quite... everything about exposure is logarithmic (2x). ISO and SS are easy because each stop change doubles or halves the previous value for both of them. So 3 stops from 30sec is 1 min (60sec)- 2min- 4min, not just 1 min per stop; and 4 stops would be 8min. A third of a stop between 1min and 2 min is 20sec (1/3 of the difference); and one third of a stop between 4min and 8min is 1.33 min (1min 20sec), again 1/3 of the difference.

Aperture is a bit more convoluted because it is a logarithmic (2x) change in aperture area, and not the f# that represents it... a 2x change in area is a 1.4x numerical change (rounded).
Click to expand...

Ah! Now I get it. So 5 stops at ISO 3200 is 16 mins, and 5 and a third stop at ISO 4000 is 18 min and 40 seconds.
 
Merlin5 said:
Ah! Now I get it. So 5 stops at ISO 3200 is 16 mins, and 5 and a third stop at ISO 4000 is 18 min and 40 seconds.
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Almost... you're getting there. Except that 5 and 1/3 stops is 1/3 the difference between the 5th stop (16min) and the 6th stop (32min) which is an additional 5min 20 sec, because it is above the 5th; you used the difference between the 4th and 5th stops.
 
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sk66 said:
Almost... you're getting there. Except that 5 and 1/3 stops is 1/3 the difference between the 5th stop (16min) and the 6th stop (32min) which is an additional 5min 20 sec, because it is above the 5th; you used the difference between the 4th and 5th stops.
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Thank you for this guidance, Steven. Right, here goes, I think I have it now!

6th stop (6400) would be 32 minutes, and 7th stop would be 64 minutes.

A third of a stop between 6th and 7th stop is 10 min 40 secs. So 6 and a 1/3 stops will be 42 mins 40 secs bulb time. 6 and 2/3 stops would be 53 mins 20 secs.
 
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Merlin5 said:
Thank you for this guidance, Steven. Right, here goes, I think I have it now!

6 stops (6400) would be 32 minutes.

A third of a stop between 6th and 7th stop is 10 min 40 secs. So 6 and a 1/3 stops will be 42 mins 40 secs bulb time. 6 and 2/3 stops would be 53 mins 20 secs.
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Now you've got it... you're 2/3 of the way to understanding manual exposure!

Aperture #'s can be dealt with in the same 2x log way instead of the 1.4x; you just have to separate them into two groups. E.g.
1----2----4----8----16----32
-1.4--2.8---5.6--11----22
gets you the aperture scale 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32... each one is a stop of light which can also be traded for a stop of ISO or SS (I don't remember the 1/3 or 1/2 stop numbers offhand anymore)
 
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sk66 said:
Now you've got it... you're 2/3 of the way to understanding manual exposure!

Aperture #'s can be dealt with in the same 2x log way instead of the 1.4x; you just have to separate them into two groups. E.g.
1----2----4----8----16----32
-1.4--2.8---5.6--11----22
gets you the aperture scale 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32... each one is a stop of light which can also be traded for a stop of ISO or SS (I don't remember the 1/3 or 1/2 stop numbers offhand anymore)
Click to expand...

Awesome, I would have been embarrassed if I hadn't got it right! :D Thanks for your help and I've written all this down in my phone with the rest of my photography notes.
 
Just to say, now that I understand how it works, in my phone I've written down my own ISO chart with each exposure time written next to every full stop and and every third of a stop from 100 ISO right up to 12,500 ISO. I shouldn't think I'll ever need a bulb time for beyond 2000 ISO but it means I don't have to calculate it if it's needed. Anything to make life easier! :giggle:
 
Merlin5 said:
Just to say, now that I understand how it works, in my phone I've written down my own ISO chart with each exposure time written next to every full stop and and every third of a stop from 100 ISO right up to 12,500 ISO. I shouldn't think I'll ever need a bulb time for beyond 2000 ISO but it means I don't have to calculate it if it's needed. Anything to make life easier! :giggle:
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What aperture did you base that on? ;)
 
sk66 said:
What aperture did you base that on? ;)
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Ah, now you've got me. :oops: :$ I guess I didn't understand your aperture scale. But I thought that whatever aperture I set between f1.4 and f22, I just increase the ISO until the exposure is correct and then use the appropriate bulb time associated with that ISO that I've written down?
 
My camera shows shutter speeds to 60 seconds
 
Merlin5 said:
Ah, now you've got me. :oops: :$ I guess I didn't understand your aperture scale. But I thought that whatever aperture I set between f1.4 and f22, I just increase the ISO until the exposure is correct and then use the appropriate bulb time associated with that ISO that I've written down?
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Yes, that would work; as long as the aperture is not a variable... and I do agree with choosing an aperture for a specific reason before other exposure considerations (or at least equally weighted).
 
Hi guys, just revisiting my old thread. I was standing on westminster bridge around 7.30pm as it was starting to get dark and considered taking a long exposure over the thames pointing at another bridge. I hadn't set my tripod up at this stage and in the end didn't bother trying for a photo as I wanted to come home and figure something out.

I've seen a video of someone shooting from the millennium bridge across the water with only a 30 seconds to 1 minute exposure.

So doing just a quick exposure test while handheld, I'd set F/11 (I think I put it at that aperture can't quite remember) and my camera was giving me an auto iso of 5000. Going by the ISO chart I'd created, that would mean putting my ISO to manual and back down to 100 and a bulb time of 26 min 40 seconds. That seems way too long doesn't it?
 
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Thanks for the replies guys. I'm officially an idiot lol and was getting mixed up. I've got it sorted in my brain now. The 26 minute 40 second exposure would of course only be if I was set at base ISO 100 and set to the the slowest non bulb time of 30 seconds shutter speed but needed to increase to 5000 ISO to get the correct exposure. Mind you, I've no idea idea what kind of conditions would actually ever need a 26 minute exposure!

Yeah j crossley, I had my shutter speed too high, around 1/50th hence the 5000 ISO. I tested outside my house tonight where the light conditions are about the same as on the bridge, set the ISO to 100 and kept bringing the shutter speed down where it gave me 20 seconds exposure. I should have known/remembered this as I already did 15 to 30 second nighttime light trails back in October. :facepalm: Oh well I'm happy now and can go and shoot my bridge photos with confidence. Apologies gents and thanks for the help anyway. :)
 
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