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08-05-2012, 09:59 AM   #1
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Perfect exposure time in Bulb mode

How do you calculate the right exposure time to shot in bulb mode?
I mean in order to shot in a dark environment, you can set the highest ISO and then,decreasing this value, increase time or aperture. But what if you reach 400ISO and 30sec of exposure. How can you go on increasing time? I mean, is 31,32...or 30,40sec or something else?!
Another thing that I can't understand is: if you are in complete dark, without lighting hints, how can you set your camera? Randomly and then step by step set it exactly?

Sorry if those are stupid questions, but I can't go on!
Thank you!!

08-05-2012, 10:05 AM   #2
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You use a lightmeter.

To increase the time you use bulb mode, you either need to manually time it or use a digital remote with a timer, either way the longer the exposure the less accurate you need to be, you don't see a difference between 30 or 31 seconds but you will see a difference between 1/4000th of a second and 1/8000th of a second.

If ti's complete dark then you cant capture anything, camera capture light so no light means nothing to capture.
08-05-2012, 10:13 AM   #3
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Yes you are right, but sometimes I read "287sec exposure". Assuming he's not using filters, how can he exactly set 287seconds?

For the question about the complete dark room, I mean a place in which light doesn't allow the camera to set correct values for a shot, maybe with the highest ISO. (ok, just thinking without a lightmeter but only the one inside the camera)
08-05-2012, 11:17 AM   #4
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I'm not sure this addresses the question, are you asking how to setup a long duration exposure? I shoot my long duration exposures at ISO100. In order to know how long the exposure needs to be I boost the ISO to 400-1600 as required and take a light reading off the camera meter to get the exposure time at the higher ISO. Then set the camera back to ISO 100 and multiply the high ISO exposure time by the ratio between the high ISO and ISO100 to get my exposure time at ISO100.

For example:
At ISO100 the camera returns bulb for a given Apeture. No problem, increase my ISO to 400 and it returns an exposure time of 10s. 400/100= 4. Therefore my exposure time at ISO100 = 40s.

In this instance it would have been easier to open up the Apeture by a stop, at which point the camera would tell me that the exposure time is 20s and knowing each stop requires a doubling of exposure time then allows me to determine the required exposure time at the desired Apeture.

Basically, understanding the fundamentals of exposure + basic math = how to estimate exposure time for long duration exposures.

08-05-2012, 11:21 AM   #5
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Sorry that comes off snarky. Wasn't the intent, just making the point that understanding fundamentals of exposure allow you to take better control of your photography.
08-05-2012, 11:54 AM   #6
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Thank you very much Gareth! No, that wasn't snarky at all. I have never thought to the link between the ISO that I want and the ISO of the light metering! Now I understand.

So, if i'm correct, the general rule is that I make a metering of the light at the highest ISO that give me a non-bulb exposure. then divide the high ISO with the ISO I want and then multiply this number with the seconds found with the highest ISO. Now I can understand how the photographer I saw set exactly 288 sec (for example at 6400 ISO it gives 4.5sec and he made the pic at 100ISO).

I'm really thankful to you!!
08-05-2012, 12:12 PM   #7
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Bear in mind that at 288 seconds and accidental half stop over exposure is an additional 144 second - that's more than 2 minutes tolerance.
Also extrapolating a metered reading at ISO 12800 back to 100 does not guarantee results - in my experience using this method gives under exposed images and additional minutes are often required - not sure if this is because the sensor is not totally linear in this respect or it's an accumulation of rounding errors but the result means some inevitable trial and error.
08-05-2012, 12:32 PM   #8
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Thank you Darren for posting! But if I can ask you, how do you make long exposures?

08-05-2012, 12:57 PM   #9
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Normally I make a best metering calculation - then add half a stop. This means if I meter at ISO12800 and get 1 seconds I then set to ISO100 and add 7 stops which gets me to 128 seconds (2 mins) - add half a stop (1 minute) and shoot at 3 minutes - but It's trail and error from that point. There really is no need to worry about exact timing at these exposures.
08-05-2012, 01:30 PM   #10
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upping the iso doesnt make your light meter more sensitive, keep that in mind there is only a certain way it can go.
A fast lens would help a bit there.
08-05-2012, 01:45 PM   #11
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Thank you very much Darren! The only thing that I don't understand is why the half stop is 1 minute. Is because it is the half of the last stop?

Yes I know that a fast lens may help, but this is just to have a theorical view of the topic!

Thank you!
08-05-2012, 03:25 PM   #12
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Yes it works logarithmic , that's why between f/2.8 and f/4 is 1 stop but also f/22 and f/32 is 1 stop
or with shutterspeed 1/1000 and 1/500 is 1 stop but also 32minutes to 64 minutes is 1 stop
08-05-2012, 03:43 PM   #13
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If the OP intends to do such shots more often in the future, the acquisition of a separate light meter with extreme sensitivity could help. Unfortunately, these are not cheap. Even a nearly 25 years old Gossen Profisix (named different in US) on eBay usually goes for 100-200. But it will give very precise values even in darkness. It's not only that the scales go up to 8 hours, the values are measured and real! Even very good built-in light meters of the bodies are about 4 stops less sensitive.
08-06-2012, 02:29 AM   #14
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Thank you very much for those tips!

But let me ask you another thing about long exposures at night: I have no experience in this field, but I'm reading about star photography. What I can't understand is that people said that there is a maximum exposure time beyond that you can see the star trail. This is 600/focal lenght (on FF). So for example if i'm shooting with a 10mm, on FF it would be 10*1.5=15, and so 600/15=40sec of exposure.
Now the question: if I find that the scene is correctly exposed by for example 288sec, but I don't want star trails but just "star points", how can I combine those results? Is the only way to achive this make a double exposure, one for the sky/stars and one for the ground?

Thank you!
08-06-2012, 05:09 AM   #15
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Well you need to manual photography the stars and then put them in your other photo, shoot with the widest aperture for that for example.
It's a bit like HDR but then different

Or you low the shutterspeed to the point it's enough for the whole photo.
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