Can someone explain EV to me?

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shufflemoomin

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Hey everyone,

I'm about to pick up a Canon 6D and I repeatedly see reviews going on about how great it is that the centre AF point can focus down to -3EV. I've been shooting quite a few years and I'm afraid to say that my understanding of this term is non-existent and a Google search hasn't turned up anything. Is there anyone here who understands this rating and can explain it in basic terms? I mean, I understand that it means that it can focus in very low light, but I don't really understand the rating beyond that. I've seen people refer to things such as bringing up exposure by one stop as +1EV but I'm not sure what -3EV means to me. What's -3 below what? :)

Any help or information would be greatly appreciated.
 
You can dial in a -EV to stop the blowing of highlights, or if you're shooting a dark coloured bird for eg, against a bright sky, you can dial in some +EV to expose the bird better.
 
i'm not even sure that makes any sense to me. EV - Exposure Value - is completely relative. -3EV on it's own doesn't really mean anything useful.

Maybe they just mean the centre point is 3 stops more sensitive than the other focus points?
 
As I said in my comment, I understand the term as in -EV and +EV meaning stops in exposure, but I don't understand what they mean but the AF can work in -3EV. What are they comparing that too? Three stops under what baseline?
 
http://en.wikipedia.org/wiki/Exposure_value#Exposure_compensation_in_EV

Many current cameras allow for exposure compensation, and usually state it in terms of EV (Ray 2000, 316). In this context, EV refers to the difference between the indicated and set exposures. For example, an exposure compensation of +1 EV (or +1 step) means to increase exposure, by using either a longer exposure time or a smaller -number.

The sense of exposure compensation is opposite that of the EV scale itself. An increase in exposure corresponds to a decrease in EV, so an exposure compensation of +1 EV results in a smaller EV; conversely, an exposure compensation of −1 EV results in a greater EV. For example, if a meter reading of a lighter-than-normal subject indicates EV 16, and an exposure compensation of +1 EV is applied to render the subject appropriately, the final camera settings will correspond to EV 15.
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(Googled "ev photography definition")

EDIT: Reading your clarification I'm not sure the Wiki link will help, but maybe one of the others from the search will.

EDIT2: Is it talking about using one of the AF points as a metering point?
 
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Here's a quote from one of the reviews:

"However, the 6D's trump card is the ability to focus in extremely low light levels; right down to a claimed -3 EV – a full stop dimmer than the 5D Mark III. This figure might not mean much to you, but for reference, -3 EV is roughly equivalent to the light cast by a full moon."

You can see here that they're saying "-3 EV" is comparable to the light from a full moon, but I'm still now sure what the baseline level is. They're talking in stops of light but what is that compared to? A stop of light is as meaningless as a percentage. It doesn't mean anything without something to compare it to. If this is such a common statement in regards to camera specs, surely it shouldn't be so hard to understand but I'm truly struggling.
 
EV stands for Exposure Value and is measured on a logaritmic scale, therefore the difference between EV1 and EV2 is 1 stop. The scale is designed so that EV0 is the light level that gives an exposure of 1 second at f1, or 2 seconds at f1.4 or 4 seconds at f2 etc. Thus EV 2 = 1/2 sec at f1, and EV12 is 1/4000 sec at f1. EV-3 is therefore the light level that gives an exposure of 8 seconds at f1 (or 30 seconds at f2).
 
I read that as the single stop being the equivalent of the light cast by a full moon, rather than the -3. And it's an offset of the standard exposure of whatever camera is in use I would think rather than an absolute value?
 
Brachytron said:
EV stands for Exposure Value and is measured on a logaritmic scale, therefore the difference between EV1 and EV2 is 1 stop. The scale is designed so that EV0 is the light level that gives an exposure of 1 second at f1, or 2 seconds at f1.4 or 4 seconds at f2 etc. Thus EV 2 = 1/2 sec at f1, and EV12 is 1/4000 sec at f1. EV-3 is therefore the light level that gives an exposure of 8 seconds at f1 (or 30 seconds at f2).
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Thank you. That's crystal clear. So the 6D centre point can focus in light that would require 8 second shutter speed at an aperture of f1 for a proper exposure? I guess that clears up my question. Thanks again. :)
 
shufflemoomin said:
Thank you. That's crystal clear. So the 6D centre point can focus in light that would require 8 second shutter speed at an aperture of f1 for a proper exposure? I guess that clears up my question. Thanks again. :)
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Yes. When EV is used in camera specs like that, it means at ISO100. Sometimes expressed as EV/100 or similar, but the 100 bit often gets missed off.

Same applies to flash guide numbers, almost always meaning at ISO100 with distance measured in metres, but might not always say so.
 
HoppyUK said:
Yes. When EV is used in camera specs like that, it means at ISO100. Sometimes expressed as EV/100 or similar, but the 100 bit often gets missed off.

Same applies to flash guide numbers, almost always meaning at ISO100 with distance measured in metres, but might not always say so.
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It is often incorrectly used as a measure of light level when LV (Light Value) should be used instead.


At ISO 100 the numbers are the same but EV is purely a function of shutter speed and aperture.


Steve.
 
HoppyUK said:
Yes. When EV is used in camera specs like that, it means at ISO100. Sometimes expressed as EV/100 or similar, but the 100 bit often gets missed off.
Click to expand...

It is often incorrectly used as a measure of light level when LV (Light Value) shoukd be used instead.


At ISO 100 the numbers are the same but EV is purely a function of shutter speed and aperture.


Steve.
 
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