Can someone help me with an exposure calculation I can't get my head around?

Jimmy_Lemon

Jimmy_Lemon

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I cant get my head around this calculation.

I want to know what ISO I would need to use to match a shot, taken at 1000iso - f4 - 1/125, if I was shooting at f5.6, still at 1/125.

Anyone able to help with the math?

Thanks
 
You'll need an extra stop of ISO so 2000.

The ISO scale is split up into thirds, so 1000 ->[up 1/3] 1250 -> [up 2/3] 1600 -> [up 1/1] 2000 = one stop.
 
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pah! Is it that simple? .....now you say it, of course it is! D'oh
 
FruitFlakes said:
You'll need an extra stop of ISO so 2000.

The ISO scale is split up into thirds, so 1000 ->[up 1/3] 1250 -> [up 2/3] 1600 -> [up 1/1] 2000 = one stop.
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This has always kind of bugged me having to work in "third stops" - 100,200,400,800,1600, 3200, 6400 seems a lot simpler to work with
 
boliston said:
This has always kind of bugged me having to work in "third stops" - 100,200,400,800,1600, 3200, 6400 seems a lot simpler to work with
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Then ignore the third (or half) stops and just work in whole stops.


Steve.
 
chphotos said:
You can set most cameras to only use full stops.
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I was thinking more about setting your brain to only think in full stops. If you are using the camera's automation to work it out, you don't need to think about it.


Steve.
 
boliston said:
This has always kind of bugged me having to work in "third stops" - 100,200,400,800,1600, 3200, 6400 seems a lot simpler to work with
Click to expand...

Third stops don't *really* matter nowadays considering most cameras today are so forgiving in terms of exposure (as long as you don't blow out though). I've mistakenly shot at ISO400, 1/500, f/11 when I was at least 2/3 stops under for a given scene and the RAW still gave me enough to play with for a 4 stop push in Lightroom:

QqlzDLF.jpg
 
Steve Smith said:
Still better to get it right in the camera though.



Steve.
Click to expand...

That wasn't what I was saying though. It's perfectly fine to use the 100/200/400/800/1600 scale even if a technically correct exposure for a scene would be 500 since a third of a stop is nothing to sweat about.
 
FruitFlakes said:
That wasn't what I was saying though. It's perfectly fine to use the 100/200/400/800/1600 scale even if a technically correct exposure for a scene would be 500 since a third of a stop is nothing to sweat about.
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I see. That makes sense then. What I wouldn't agree with was under exposing by a few stops as in your example (it's a lot more than 2/3 of a stop*) then correcting it later. OK to rescue something which went wrong but not as a regular thing.

It's the same with negative film. One, two or even three stops over isn't going to be a problem (don't try over exposing with digital though).

(* I don't understand why you say you were 2/3 stop under but gave a four stop push - it does look like about four stops difference in your examples).


Steve.
 
Steve Smith said:
I see. That makes sense then. What I wouldn't agree with was under exposing by a few stops as in your example (it's a lot more than 2/3 of a stop*) then correcting it later. OK to rescue something which went wrong but not as a regular thing.

It's the same with negative film. One, two or even three stops over isn't going to be a problem (don't try over exposing with digital though).

(* I don't understand why you say you were 2/3 stop under but gave a four stop push - it does look like about four stops difference in your examples).


Steve.
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I wasn't clear enough that post I meant I was about 2-3 stops off, not two thirds of a stop. I'd never underexpose by that much on purpose though, I was using it as an example that the third stop values are mostly irrelevant when cameras today have so much latitude (at least in the shadows).
 
Got an iPhone? There's a brilliant app called Photocalc, that works all this out for you, plus depth of field...
 
You don't need to remember the values, a stop is always halving/doubling the amount of light. ISO 1000 to 2000 is one stop up. ISO 1000 to 500 is one stop down. ISO 1234 to 2468 (if it were possible to set such a thing) is also one stop difference. Same applies to shutter speeds (1/125 to 1/60, etc).

The only one that gets slightly complicated is the aperture, because that is affected by the area of the hole but quoted as the diameter. f/2.8 (f is the focal length of the lens) has twice the area of f/4 but half the area of f/2. You could work it out using A=pi*r^2 but I'd recommend just learning the whole stop numbers instead!

In answer to your original question, f/4 to f/5.6 is one stop so you'd either have to double the ISO to 2000 or double the exposure time to 1/60 to maintain the same exposure.
 
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Mr Will said:
You could work it out using A=pi*r^2 but I'd recommend just learning the whole stop numbers instead!
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Or just learn two adjacent stops then do the doubling thing. Start with f/2 and f2.8 and double....

f2..............f4....................f8...............f16..................f32
.......f2.8..............f5.6...............f11................f22..............

So f11 is one stop slower than f8 and one stop faster than f16.
 
Have a look for the wristbands that show the shutter speed to aperture to ISO settings on it and then wear it when you go out with the camera. I got one when I started out using 1/3 stops as I kept getting confused and it was really helpful to have a quick glance to make sure the settings were right. I found mine on ebay a while ago but unfortunately I snapped it a few weeks ago.
 
I am amazed that no one suggested simply adjusting the iso up until it reaches proper exposure for that fstop and shutter speed. Seems a lot easier then doing math, resetting camera adjustments or memorizing how the stops are counted. I can tell you first hand that it really doesn't matter. I don't know what a full stop, half stop or quarter stop is on my camera but I fully understand how to expose a picture. So my advise to the op is to set your camera to manual set your aperture and shutter and turn up your iso until it reaches proper exposure. You will not only answer your own question but you can see first hand how these adjustments effect your exposure. I hope that helps.

:canon:
 
Never understood why people need gadgets and apps to work these things out. If you can't remember the numbers (I think the common apertures and shutter-speeds were stuck in my head after a couple of weeks of owning a camera) the two times table is pretty straightforward. Or you can just count the clicks on the setting wheels as you change it; or use your meter as Shayne suggests.
 
hollis_f said:
Or just learn two adjacent stops then do the doubling thing. Start with f/2 and f2.8 and double....

f2..............f4....................f8...............f16..................f32
.......f2.8..............f5.6...............f11................f22..............

So f11 is one stop slower than f8 and one stop faster than f16.
Click to expand...
That's pretty awesome actually, can't believe I never noticed it before!
 
I, everyone new to photography ,I have a basic understanding on camera settings on my dslr, but I,m better with hands on instructions, when I started reading this blog on calculations,& maths .i was just about to get my unused camera and jump up and down on it because of the great dread it caused me , until I saw shayne,s reply,why do people make thing so complicated to us lesser mortals, ps, I,m (crap at maths) is photography this complicated or can it still be a fun hobby.:banghead::runaway:
 
shaylou said:
I am amazed that no one suggested simply adjusting the iso up until it reaches proper exposure for that fstop and shutter speed. Seems a lot easier then doing math, resetting camera adjustments or memorizing how the stops are counted. I can tell you first hand that it really doesn't matter. I don't know what a full stop, half stop or quarter stop is on my camera but I fully understand how to expose a picture. So my advise to the op is to set your camera to manual set your aperture and shutter and turn up your iso until it reaches proper exposure.
Click to expand...

But as you up the ISO you introduce noise, so that's not a great general purpose rule, so it does matter, depending on what and where you are shooting.
 
Byker28i said:
But as you up the ISO you introduce noise, so that's not a great general purpose rule, so it does matter, depending on what and where you are shooting.
Click to expand...


How does that matter? Noise has nothing to do with this question. The op wanted a calculation for what iso they would be using under certain parameters. According to the setting shown the iso has to go up. We all know that the noise will follow but once again that has no relevance to the question and the suggested solution that I provided.


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