How do I calculate exposure times for night photography?

Question

My cameras inbuilt exposure meter only calculates shutter speeds up to 30 seconds long. Anything longer and you have to use bulb mode.

What I want to do is set my camera meter to a very high iso such as 3200 so that I can get a shutter reading for the night scene, and then use this shutter speed to calculate a shutter speed for ISO 100 film.

How do I calculate this?

I hope you understand what I am saying!

thanks

Answer 1

Hypothetical meter readings beginning at ISO 3200 to illustrate the relationship:

3200, f8 @ 20 seconds
1600, f8 @ 40 seconds
800, f8 @ 80 seconds
400, f8 @ 160 seconds
200, f8 @ 320 seconds
100, f8 @ 640 seconds

3200, f5.6 @ 10 seconds
1600, f5.6 @ 20 seconds
800, f5.6 @ 40 seconds
400, f5.6 @ 80 seconds
200, f5.6 @ 160 seconds
100, f5.6 @ 320 seconds

3200, f4 @ 5 seconds
1600, f4 @ 10 seconds
800, f4 @ 30 seconds
400, f4 @ 60 seconds
200, f4 @ 120 seconds
100, f4 @ 240 seconds

3200, f2.8 @ 2.5 seconds
1600, f2.8 @ 5 seconds
800, f2.8 @ 10 seconds
400, f2.8 @ 20 seconds
200, f2.8 @ 40 seconds
100, f2.8 @ 80 seconds

Since ISO is a measure of the sensitivity to light of a light-sensitive surface, either film or sensor, each step down the ISO scale means that the sensitivity of our film or sensor has been reduced by 1/2. Since the shutter speed determines how long our light-sensitive subject is exposed to light, our shutter must stay open longer for a correct exposure at a given f-stop.

As you change f-stops you have to remember that f2.8 admits twice as much light as f4 and f4 admits twice as much light as f5.6. The inverse is that f5.6 admits 1/2 as much light as f4 and f4 admits 1/2 as much light as f2.8.

Answer 2

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Answer 3

Take the basic shutter speeds (not the ones on the digital camera, the ones on a traditional, ie. 1sec, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500), then take your traditional ISO (100, 200, 400, 800, 1600, 3200)

For each ISO you move down, you need to move your shutter speed up. So, if you are at 1/250 @ 3200iso then you would be at 1 second for 100iso

It's just like f/stops and shutter speeds.

Answer 4

To elaborate. Each ISO step is a full f-stop. Many cameras go by 1/3 stops or 1/2 stops so it can get confusing. ISO 3200 is 5 full stops faster than ISO 100.

Heres a link that will be of help if you can absorb this much information http://www.fredparker.com/ultexp1.htm#EXPOSURE%20FACTOR%20RELATIONSHIP%20CHART%20B
Scroll down to the "Exposure Value Chart" and try to decifer what your situation is. Then scroll to the next chart "Exposure Factor Relationship Chart B" and find the correct EV number from the first chart, match it to your ISO your using and find the f-stop and shutter speed from that.

Hope this helps

Answer 5

Not helpful at all, really. There is no single right exposure for every situation, which is why any table like yours is just as guide. It can help get you in the ballpark, but the final exposure is what the photographer wants, not what your table says. Taking multiple shots, and bracketing those shots, will continue to be the way to get what you want out of your camera.

Answer 6

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