Not really clear on EV steps...
Ok, I have a question. Let's say I have a light meter, camera in manual mode, and a filter on my lens. That filter is +1.3 stops (or whatever). I use the light meter to find the correct exposure. Can I correct for the filter by setting my camera +1.3 EV steps?
Errr... wouldn't you subtract the EV? I mean, aren't you loosing the light rather than gaining it?
Guess it's all how you phrase it...
Open up 1.3 stops
Reduce metered amount by 1.3 EV
Lower your ASA by a factor 1.3 or about 80%
All the same thing.
Last edited by Kino; 06-11-2008 at 12:37 AM. Click to view previous post history.
I always find exposure compensation/ film speed adjustments etc terminology very confusing. The key thing to remember is the filter reduces the amount of light entering the camera, so you want to compensate for this by allowing more light in so, yes, setting auto exposure comp to + 1.3 would be perfect, or slowing down the film speed by 1.3 stops to force a slower speed would also work fine.
whoops - I wrote this not realising I've echoed what Kino and Matt have said - I think matt puts it best.
How do you calculate it as a factor of film speed (ISO/ASA)?
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You can lower your film speed rating by the filter factor when metering. If you're using 400 speed film, and usually meter at 400, then you can set your meter to 160 (with your 1.3 stop example) to get the proper exposure.
Powers of 2 are your friend. " ^ " means "raise to the power of" in this example.
Originally Posted by AutumnJazz
Film Speed / (2 ^ factor) = new speed
factor = 1.3
Film Speed = 100
new speed = 100 / (2 ^ 1.3) = 40.61
But as you may have noticed, you don't have 40.61 as an option for film speed. Set to the closest value on your dial and be happy, probably 40.
You can carry around a cheap (seen them for $5) calculator, or make up some cheat sheets (Excel / OpenOffice) or remember a few "tricks" and simply count 1/3 or 1/2 stops on the dials.
That's probably where we are differing; I am assuming a change of EV value on a hand held meter and I think (correct me if I am wrong) you are speaking of exposure compensation adjustments on a camera; right?
Originally Posted by Leon
And, you are right, it is always confusing...
Kino - I assumed the OP was referring to camera auto compensation, so that is what i was referring to, but probably just clouded matters more. this kind of thing can drive me nuts! recently, fellow apugger Stoo Batchelor came round for a cup-a-tea and we were trying to work out matching his spot meter readings with a 1.3 stop auto compensation on his camera - a simple thing on the face of it, but it took us ages to work out that what he was doing was right in the first place. Bloody Nightmare
It's not hard at all once you learn to "think in f-stops." The first thing to learn is the film speeds, which progress in 1/3 stop increments, as follows, and starting from 25:
25 32 40 50 64 80 100 125 160 200 250 320 400 500 650 800 1000 1250 1600 etc.
The numbers are easy to remember once you notice that each third number is doubled: 25 50 100 200; 32 64 125 250; 40 80 160 320
And of course, each third film speed is a full stop faster or slower, depending on the direction you move.
Full f-stops are also a numerical progression; every other one is doubled:
f1.4 2.0 2.8 4 5.6 8 11 16 22 45 etc.
To apply compensation, all you have to know is whether you are opening up or stopping down, and how many stops.