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02-03-2011, 02:39 AM   #1
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Calculating EV

Hi,

Just curious with all the low light focus/metering talk right now, how does one calculate EV?

Specifically, my k7 will meter down to 0 EV and focus down to -1 EV, so how do you know you're at -1 EV, or 1 EV, or 4 EV?


Thanks,

Bill

02-03-2011, 03:12 AM   #2
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Hi Snydly,

I was just about to start explaining, but I found an article on wikipedia that's doing it much better than me I think... Here is the link!
I found the tables more practical than the formulas (look at EV as an indicator of camera settings and Tabulated exposure values)

LeFanch

Last edited by LeFanch; 02-03-2011 at 03:59 AM. Reason: Added information, spelling
02-03-2011, 03:37 AM - 1 Like   #3
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Excel EV calculator

You can use my Excel calculator, here:
http://www-user.uni-bremen.de/~wie/EV.xls

Last edited by blende8; 02-03-2011 at 04:30 AM.
02-03-2011, 03:46 AM   #4
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Hi Blende8, when I open the file you attached it looks like it's empty...

02-03-2011, 04:04 AM   #5
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A formula: EV = log2((f^2) / t) - log2(iso/100), this is handy enough with a calculator, without one can work the EV out by keeping in mind that EV 0 is f=1 t=1.0s @ISO 100 by definition, 1 EV unit = 1 stop and that higher values mean more light. For example f=2.8 t=0.5s is 3 + 1 = 4 stops brighter than the baseline or EV 4 (assuming ISO 100).
02-03-2011, 04:23 AM   #6
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Well, seems like Zip Files do not work.

Here's the direct link to the Excel file:
http://www-user.uni-bremen.de/~wie/EV.xls

Last edited by blende8; 02-03-2011 at 04:29 AM.
02-03-2011, 04:25 AM   #7
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Can this be used? If you have Android... Exposure Calculator - Android
02-03-2011, 04:58 AM   #8
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QuoteOriginally posted by blende8 Quote
Here's the direct link to the Excel file:
http://www-user.uni-bremen.de/~wie/EV.xls
Thanks blende8!

02-03-2011, 06:38 AM   #9
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QuoteOriginally posted by Snydly Quote
Hi,

Just curious with all the low light focus/metering talk right now, how does one calculate EV?

Specifically, my k7 will meter down to 0 EV and focus down to -1 EV, so how do you know you're at -1 EV, or 1 EV, or 4 EV?


Thanks,

Bill
for knowing where you are with low EV values, usually when AF stops working or changing aperture no longer changes shutter speed

BUT

try this link. pretty good explanation of not only EV but lighting levels.

Note EV is a relitive value, so you need to know the basis, i.e. ISO

Exposure value - Wikipedia, the free encyclopedia
02-03-2011, 07:07 AM   #10
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You can read the 'Effective LV' field in Manufacturer Notes from the Exif with PhotoME.

dave
02-03-2011, 08:43 AM   #11
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The key concept..

QuoteOriginally posted by jolepp Quote
A formula: EV = log2((f^2) / t) - log2(iso/100), this is handy enough with a calculator, without one can work the EV out by keeping in mind that EV 0 is f=1 t=1.0s @ISO 100 by definition, 1 EV unit = 1 stop and that higher values mean more light. For example f=2.8 t=0.5s is 3 + 1 = 4 stops brighter than the baseline or EV 4 (assuming ISO 100).

Jolepp (as is not uncommon) said a good thing...easy to remember & to work from..to wit:
EV 0 is f=1 t=1.0s @ISO 100 by definition - count f-stops up (brighter scene) and others down (dimmer scene).

Dave

PS it is easier for me to think in terms of speed rather than time (increase f-stops and/or speed-stops to admit less light.)

PPS I now wish the ISO people had started with ISO = 1 rather than ISO = 100, but at the time it was good to be consistent with the ASA definition.
02-03-2011, 09:02 AM   #12
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Excellent.

Lowell, you're right in that I am the type that needs to know where something came from to use it. Once I know how something is developed I'll always remember it, or at least how to derive it.

Jolepp, so is this an artificial value developed to aid photographers like a light meter, or is it to evaluate post-shot what you had for light when you took the photo?

When is it important? Is it something any of you use to take photo's or is it an analysis tool?

Lefanch, thanks for the link, I'll check it out...

GrinMode, thanks for the link, I just got an android phone. Just to let you know, initially the link stated the program was no longer available, but I clicked on the developers name which led to another path to the exposure calculator and was able to download it. Not completely sure how to use it yet, but I'll play with it.

Blende8, got the excel file. Thanks, it will let me break down the formula to get a better understanding.

Thanks again everyone,

Bill
02-03-2011, 09:11 AM   #13
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Thank, comes in very handy.

What is to be considdered as dark or low light situation?
02-03-2011, 11:14 AM   #14
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QuoteOriginally posted by Snydly Quote
...

Jolepp, so is this an artificial value developed to aid photographers like a light meter, or is it to evaluate post-shot what you had for light when you took the photo?
...
I suppose it is a way to express all shutter speed and aperture combinations resulting in the same exposure as a single number. Taking ISO into account and assuming an "optimal" exposure it is also close enough to serve as a practical measure of available light - strictly speaking it is not that, as the absolute light intensity at the film/sensor varies with different lenses at the same aperture setting, also what is an optimal exposure can be debated. I guess it would be useful as a light meter scale, with an EV reading one could look up, say, a shutter time for an aperture or vice versa from a table (I barely remember seeing / handing a handheld meter as a kid some +30 years back ).
02-03-2011, 11:57 AM   #15
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QuoteOriginally posted by Lowell Goudge Quote
Note EV is a relitive value, so you need to know the basis, i.e. ISO

Exposure value - Wikipedia, the free encyclopedia
This is correct -

For clarity -
EV is commonly confused with LV (Light Value)
- LV I believe is what most people actually want to know-
LV is synonymously the EV @ ISO100

Whereas EV is the camera settings (Aperture and Shutter Speed) ONLY.
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