EV Calculation

[BillN_33]

I have put this question in this section ...... as it does not seem to fit in any of the others

I have been reading about the EV calculation - not really getting anywhere

My question is

? is there a formula that takes the shutter speed, f stop and ISO value to give an EV value?

say 1/500th sec at f4 at ISO 1000 = ?EV?

or am I wrong in thinking this way

 

[dbr1066]

Well, there is a formula, but whether it will help is down to your math skills. From Wikipedia:

Exposure value is a base-2 logarithmic scale defined by Ray (2000, p. 318):

where

• N is the relative aperture (f-number)
• t is the exposure time (“shutter speed”) in seconds

Realistically, forget using a formula, just remember that shutter speed, fstop, and ISO can all be manipulated to find a 'correct' exposure for any scene.

 

[Alastair]

It's easier to use a table. This is an old one I use when shooting medium format film (that's why it stops at 1/500 as that's the fastest shutter speed on an RB67 ProS), with practice I've found it to often be more reliable than a lightmeter..



For an in-depth description see Fred Parker - http://www.fredparker.com/ultexp1.htm

Knowing the EV is useful in very few circumstances when shooting (unless using film and without a reliable lightmeter), but I do think it's difficult to find any value in someone else's exif data unless you convert it back to an EV and a lighting situation you can relate to.

 

[Phil-D]

Not sure if this helps Bill but this exposure calculator also gives you the EV of any giving combination

http://www.scantips.com/lights/exposurecalc.html

 

[BillN_33]

Thanks guys - I came across the formula - but as you said ???

the Table is very useful, as are the links

The reason I am interested is that we had very dull light today ......... and also camera manufacturers have started to quote that there equipment will AF down to - EV 3 or what ever and whatever

I am also surprised/interested at the colours which are produced by sensors at -EV low values and wondering how they are achieved in that some seem different from "reality" ........ but I could be talking a load of rubbish as I am getting obsessed by colour cast this week!!!

 

[Steve Smith]

EV is only a function of shutter speed and aperture, it is not a measurement of light and does not depend on ISO.

LV (Light Value) is. It is the same number as EV at ISO 100.

? is there a formula that takes the shutter speed, f stop and ISO value to give an EV value?

The way I work it out is to use the sunny 16 figures of 1/125 and f16 (at ISO 100) as a starting point and work it out from there. 1/125 at f16 is EV 15

So to use your example of 1/500 and f4 - 1/500 is two stops up from 1/125 and f4 is four stops down from f16 so a total of two down. So that combination is EV 13.



Steve.