ISO and Exposure query

Mystery57

Mystery57

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Andrew
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After reading many items I have now got myself confused !!!

Not hard with the amount I have learnt in the past few months though.

How do I calculate an appx exposure time with a different ISO.

For example

If an ISO 6400 at 2 seconds gave a good exposure

How many seconds would I need at say ISO 400


My logic being that image noise should be less at ISO 400

thanks
Andrew
 
I make the difference between 400 and 6400 ISO to be four stops - so you'd need to increase your exposure by four stop - which would be a 30 seconds exposure.

Edit: I used the tables on this web page
 
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Yep, image noise will be lower.

Photography works in 'stops'

ISO stops go:

50, 100, 200, 400, 800, 1600, 3200, 6400, 12800

i.e. one stop increase is double the ISO, one stop less is half.

So, ISO 400 is 4 stops less than 6400 so you need to double the shutter speed 4 times:

2s -> 4s, 8s, 16s, 32s.

edit: which, as said, will equate to 30s on the camera. The exposure won't be identical but close enough that you couldn't notice.
 
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Whilst it is true that lower ISO gives lower noise, be aware that long exposures can also introduce noise as the sensor heats up.
 
Mystery57 said:
Following on from the above am I right in thinking that if an ND filter is used, each number value on it represents one stop in the time calculations above ?
Click to expand...

Yes, but not always. Filters are marked in various ways, like x2, x4, x8, x16 are one, two, three and four stops. But could be marked 0.3, 0.6, 0.9 and 1.2 in optical density. Ten stops is x1024 or 3.0.

All exposure settings work on a halving/doubling principle, referred to as 'stops'. Shutter speeds are easy, and 1/125sec is clearly double 1/250sec, one stop difference. ISO is straightforward too, like ISO100 is half as sensitive to light as ISO200 etc.

The tricky one is lens apertures. They are also linked by the halving/doubling thing, but involve a square root (1.414 is square root of 2) and are further confused by high numbers passing less light. Full f/number settings run f/1.0, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 etc, with the lower number passing twice as much light as the next one up. You also get inbetween settings like f/3.5 or f/6.3. You just have to learn this stuff.
 
There is a great little AP for smart phones on exposure, it allows you to see the impact on the ISO shutter speed or aperture by adjusting any one setting
 
Nawty said:
So, ISO 400 is 4 stops less than 6400 so you need to double the shutter speed 4 times:

2s -> 4s, 8s, 16s, 32s.
Click to expand...

I know what you mean, but not strictly correct :)

You need to halve the shutter speed four times ... or double the time the shutter is open four times :)

Okay, I'll take my pedant a55 out of here now :wave:
 
Bristolian said:
I know what you mean, but not strictly correct :)

You need to halve the shutter speed four times ... or double the time the shutter is open four times :)

Okay, I'll take my pedant a55 out of here now :wave:
Click to expand...

My shutter opens and closes at the same speed every time - but the length of time between it opening and closing does vary :D
 
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