Beginner EV when speaking of autofocus?

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Conan
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#1
Hi everyone, I hope you are all OK.

I have a question in regards to the wording on Nikon's website for the D500, which states the AF works in near darkness, down to -4EV.
I am a beginner still, but this has me confused.

I have used exposure compensation in AP and SP mode when I have needed to expose the main subject correctly, as the camera may choose a setting that is much too dark, or the opposite.

I thought that say +1EV compensation would be a extra stop brighter, -2 2 stops under and so on.

So, with the mention of autofocus working in near dark conditions at -4EV, exactly WHAT is this being measured from? How dark exactly is -4EV, or does it simply mean that the AF still works if I set the camera to -4EV?

Thank you all for your help and advice so far, Conan.
 
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conanthewarrior
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Conan
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#3
Thank you. That explained it nicely for me, I must have been using a long combination of words instead of simple exposure value Doh!

Even my camera down to -1 EV is quite impressive when I see it written out. My widest aperture is f3.5, at f4 that is a 30 second exposure to get an image that is 'correct'.
 
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Phil
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#4
Thank you. That explained it nicely for me, I must have been using a long combination of words instead of simple exposure value Doh!

Even my camera down to -1 EV is quite impressive when I see it written out. My widest aperture is f3.5, at f4 that is a 30 second exposure to get an image that is 'correct'.
The article didn't help then. :)
In terms of the spec used to measure AF, EV is a constant, prior to DSLR's being good at this, video cameras used to use 'candles' as the measure. IIRC 1EV is a single candle light - sometimes 1 LUX - though my numbers could be off, I'm not checking, I've had wine
Your exp comp shows your compensation relative to the metered reading in stops of EV, it's not related to the fixed EV standard.

It's like Nikon are saying the camera can 'see' at 0-4cm (a constant) and you're trying to equate that to 1 metre - 4cm, or -1m -4cm (a variable) if that makes sense.
 
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Ned
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#5
Thank you. That explained it nicely for me, I must have been using a long combination of words instead of simple exposure value Doh!

Even my camera down to -1 EV is quite impressive when I see it written out. My widest aperture is f3.5, at f4 that is a 30 second exposure to get an image that is 'correct'.

You're confusing exposure and AF sensitivity.

Exposure is exposure, but with regards to AF it is easier to forget about the absolute values or frames of reference and just realise that -4EV is better than -3EV is better than -2EV... The lower the number the darker light the AF will be able to work
 
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StewartR

Efrem Zimbalist Jr
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Stewart
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#7
How dark exactly is -4EV, or does it simply mean that the AF still works if I set the camera to -4EV?
EV is a combination of shutter speed and aperture, it is not an indication of light level.
The correct term is LV. LV = EV at ISO 100.
What he said. You might also see it written as EV100, which is not quite so correct but has the advantage of being easy to understand.

Anyway an LV or an EV100 of -4 is pretty gloomy, 60 seconds at f/2, about the level of illumination you'd get from a bright aurora or a 3/4 moon. Your camera's ability to autofocus is dependent on the level of illumination, not the camera settings.
 
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#8
Another way to think about is is that LV 15 (or EV 15 at ISO 100) is the Sunny 16 bright sun setting. So -4 is 19 stops down from that, or 1/524,288 of the bright sunny day light. One more stop and it's one millionth, which is reckoned to be the amount of light the moon reflects from the sun. So LV -4 would be just a little bit brighter than anything lit by a full moon.


Steve.
 
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