Explaining "EV" in ambient terms

Pindy · January 11, 2009

Can somebody explain to me the definition of exposure value units when referring to autofocus capability, for example. I always read that a camera can focus to, say -1EV or -0.5EV, but I don't understand relative to what. What is the scale? Where does this scale begin?

Candid Arts · January 11, 2009

Pindy wrote:

Can somebody explain to me the definition of exposure value units when referring to autofocus capability, for example. I always read that a camera can focus to, say -1EV or -0.5EV, but I don't understand relative to what. What is the scale? Where does this scale begin?

Don't quote me, as I'm not 100% sure, but I think it is the Exposure Value. The numbers represent "stops", i.e. -1 of a stop, -0.5 of a stop, etc...

I hope I'm correct, but if not, feel free to correct.

xris · January 11, 2009

Ev

Pindy wrote:

Can somebody explain to me the definition of exposure value units when referring to autofocus capability, for example. I always read that a camera can focus to, say -1EV or -0.5EV, but I don't understand relative to what. What is the scale? Where does this scale begin?

In short form, EV is a rating that combines shutter speed with aperture. So, in theory, a particular EV setting relates to the amount of light reaching the film plane without regard to f-stop of shutter speed.

F-8 at 250 would be the same EV as F-11 at 125.

Used a lot in the motion picture world.

Pindy · January 11, 2009

Candid,

No no no, I get that—I even quoted "exposure value" in my post. I'm talking about where these values come from... I could talk about degrees celsius all I want but unless you understand what the scale means, (100 is boiling, 0 is freezing, for example) it's meaningless to you and they have no reference point. At the moment, saying the 5D2 should focus to -0.5EV is completely meaningless to me. A half-a-stop below WHAT?

Pindy · January 11, 2009

xris wrote:

In short form, EV is a rating that combines shutter speed with aperture. So, in theory, a particular EV setting relates to the amount of light reaching the film plane without regard to f-stop of shutter speed.

F-8 at 250 would be the same EV as F-11 at 125.

Used a lot in the motion picture world.

Okay so, 0EV would represent... What in terms of dynamic range or something else? Is this the lowest light a sensor can read before sinking into the noise floor? See what I mean, guys?

I'll just ask the question: how much light are we talking about when we say -0.5EV?

Candid Arts · January 11, 2009

Pindy wrote:

Candid,

No no no, I get that—I even quoted "exposure value" in my post. I'm talking about where these values come from... I could talk about degrees celsius all I want but unless you understand what the scale means, (100 is boiling, 0 is freezing, for example) it's meaningless to you and they have no reference point. At the moment, saying the 5D2 should focus to -0.5EV is completely meaningless to me. A half-a-stop below WHAT?

Gotchya. Well then I guess I'm in the same boat you are. Someone explain?

0 would be a properly exposed photo (as the eye see's it?--I dunno) so anything + or - from there would be less or more light. I'm really just babbling and throwing out ideas.

xris · January 11, 2009

It's been 30 years since I've used it but, as I remember it, it's not a relative number. You set the exposure you want, then you can go up or down as many EV as you want while retaining the same exposure. If you double your shutter speed, the aperture will automatically open up one stop... (Remember, this is from the days of manual everything.)

It was handy for sports photography, for instance, because you could choose a different shutter speed on the fly while maintaining the same exposure value.

jogle · January 11, 2009

Yes the EV is relative, that's why it's expressed as + or - a number of stops. Each unit refers to doubling the amount of light.

If you increasing the exposure by one EV, twice as much light is hitting the sensor.

jogle · January 11, 2009

Pindy wrote:

Candid,

No no no, I get that—I even quoted "exposure value" in my post. I'm talking about where these values come from... I could talk about degrees celsius all I want but unless you understand what the scale means, (100 is boiling, 0 is freezing, for example) it's meaningless to you and they have no reference point. At the moment, saying the 5D2 should focus to -0.5EV is completely meaningless to me. A half-a-stop below WHAT?

Where does it say "5D2 should focus to -0.5EV"? Because that IS completely meaningless, To be accurate they really need to give a lens aperture (because the amount of light hitting the AF sensor changes depending of the f/stop of the lens) and a Lux rating (which is a number of lumens over a given area)

http://en.wikipedia.org/wiki/Lux

Pindy · January 11, 2009

jogle wrote:

Where does it say "5D2 should focus to -0.5EV"? Because that IS completely meaningless, To be accurate they really need to give a lens aperture (because the amount of light hitting the AF sensor changes depending of the f/stop of the lens) and a Lux rating (which is a number of lumens over a given area)

http://en.wikipedia.org/wiki/Lux

We're beginning to get somewhere. I'll hunt for that info. This figure is always given without due reference.

Back when we used to record to tape, you would calibrate your machine to record at a certain reference level for your tape formulation. People would often tell you "plus 3" or "plus 6". Unless you knew they meant "+3db over 185 nanowebers-per-meter", +3 is a totally meaningless term. Sorry for the braintanglia!

Pindy · January 11, 2009

From the 5D2 specs page at Canon:

Autofocus
Type: TTL-CT-SIR AF-dedicated CMOS sensor
AF Points: 9 AF Points (1 Cross Type) + 6 AF Assist Points
Metering Range: EV -0.5-18 (at 73°F/23°C, ISO 100)

Exposure Control
Metering Range: EV 1-20 (at 73°F/23°C with EF50mm f/1.4 USM lens, ISO 100)

ziggy53 · January 11, 2009

A pretty good explanation of "EV" (Exposure Value):

http://en.wikipedia.org/wiki/Exposure_value

Yes, the Canon 5D MKII has an autofocus sensitivity of EV -0.5. That represents light levels of around f2 and 6 seconds at ISO 100.

Pindy · January 11, 2009

Ziggy as always, my hero.

It's handy to see this table and also to know that these values are etched in stone. Thanks—a big help.

jogle · January 11, 2009

I think this section is exactly what you are looking for.

http://en.wikipedia.org/wiki/Exposure_value#EV_as_a_measure_of_luminance_and_illuminance

I don't exactly understand how the temperature at a given iso relates to a luminance value but it does and I'd need more coffee to really understand it properly

-edit: beat to it by Ziggy!

LiquidAir · January 11, 2009

EV is a way of expressing the combined effect of aperture and shutter speed. It is not really a brightness measure because it does not take ISO into account, but often EV values are used as brightness values by saying (or implying) "Can focus a properly exposed subject at EV -0.5 at ISO 100".

EV 0 is f/1 at 1 second. Every stop brighter is on EV larger, so EV 1 can be either f/1.4 at 1 second or f/1 at 1/2 second. Another reference point is "Sunny 16" which is EV 14.7 at ISO 100. Since EV doesn't take into ISO into account, it would also be EV 15.7 at ISO 200 and so forth.

Heres a link to a set of tables I put together a while back which I find handy: http://www.liquidairphoto.com/ev_table.html

Pindy · January 11, 2009

Geniuses.

A little photo theory for Dgrinners today! I realize the concept of program shift works along this principle. You rob Peter to pay Paul (SS for Aperture or vice versa) in order to achieve the same overall light passage.

Robinivich · January 11, 2009

Nice thread pindy! I'd often wondered about those quoted specs! Silly camera manufacturers using relative units to denote absolute light levels :nono

Edit: hit post a little early...

So canon's 5D II specs as an example, -0.5 ev at ISO 100 = ~2 lux = Somewhat more light than a full moon overhead...

Pindy · January 11, 2009

D700/D3 is -1EV at same specs. I only know that cause I looked them up.

NeilL · January 12, 2009

:whew that was a little i n t e n s e

Pindy · January 12, 2009

There wouldn't be the confusion if EV weren't used as both a relative and absolute value, depending on context. I don't think I'll ever look at a meter reading the same way now.

Explaining "EV" in ambient terms

Comments