Is there any way to take away the guess work from long exposures

The issue I was having was one in which my A7 display reads bulb indicating an exposure of over 30 seconds is required but fails to indicate the length of exposure- which in a scene consisting of next to no light could be anything plus the time it takes to process. Once you need to go above 2 minutes it becomes a pain.

I suppose the other way to do this would be to take the exposure when there still remains a little bit of light to work with. However some Olympus cameras have a function called live bulb mode but the Sony doesn't making things a little harder.
 
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Merlin5 said:
I don't know if this will help you but I taught myself about long exposure times a few weeks ago. Basically keep increasing the ISO until the exposure meter is correct, then once you know how long the exposure should be, put the ISO back to 100 and then take the picture.

Below I've pasted my own words that I wrote into my photography notes on my phone to remind myself how it works. Then based on those notes I calculated bulb exposure times for every ISO and wrote out my own cheat sheet which will save me having to calculate when I'm actually out and set up with the camera on a tripod. I don't know about with ND filters, so these calculations are mostly for night time with no filter. I hope my notes below make sense :p


MY NOTES

'To calculate exposures longer than 30 seconds (mostly at night) so that the photo is not under or overexposed:-

Set 1SO to 100 and shutter speed to 30 seconds. It should be underexposed at this point. For each full stop of ISO I increase by, double the shutter time.

So 200 iso is 1 full stop and is doubled from 30 secs to 1 minute. 400 iso is 2 stops and doubles from 1 minute to 2 minutes. 800 iso is 3 stops and doubles from 2 minutes to 4 minutes. 1600 iso is 4 stops and doubles to 8 minutes. 3200 iso is 5 stops and doubles to 16 minutes, etc.

So if for example my exposure meter was correct at 3 stops (iso 800), I'd set bulb timer to 4 minutes. I'd then bring the ISO back down to 100 before starting the 4 minute exposure.

For thirds of a stop, take the difference between stops and divide by 3. So for example, the difference between 400 and 800 iso is 4 minutes minus 2 minutes = 2 minutes.
2 minutes × 60 = 120 seconds.
120 seconds ÷ 3 = 40 seconds.
So 2 and a third stops is 2 min 40.'

..............

My ISO CHART BULB TIMES

100 - 30 secs
125 - 40 secs
160 - 50 secs

One stop and thirds
200 - 1 minute
250 - 1 min 20 secs
320 - 1 min 40 secs

Two stops and thirds
400 - 2 minute
500 - 2 mins 40 secs
640 - 3 mins 20 secs

Three stops and thirds
800 - 4 minutes
1000 - 5 min 20 secs
1250 - 6 min 40 secs

Four stops and thirds
1600 - 8 minutes
2000 - 10 min 40 secs
2500 - 13 min 20 secs

Five stops and thirds
3200 - 16 minutes
4000 - 21 min 20 secs
5000 - 26 min 40 secs

Six stops and thirds
6400 - 32 minutes
8000 - 42 mins 40 secs
10,000 - 53 mins 20 secs

Seven stops
12,500 - 64 minutes
Click to expand...

interesting. Thank you.
 
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