What does means EV for camera focusing?

[Ceridan]

Hi!

for example the Nikon D500 specs state, that the central focus point can focus up to -4EV. But what does that actually mean? I understand EV, as a "Exposure Value" is a relative measurement, so what does that "-4EV" in D500 relates to?

Thanks

 

[Telyx]

I believe it means it can still focus if the scene is "under-lit" by up to 4 stops. Many cameras would need a focus assist lamp for a scene that dark.

 

[meland]

I believe it means it can still focus if the scene is "under-lit" by up to 4 stops. Many cameras would need a focus assist lamp for a scene that dark.

Unfortunately that's not quite correct. It is nothing to do with being "under-lit"or underexposure but I can see where you are coming from.

There is much information about what an EV (Exposure Value) is on the internet and you can access the various articles on it with a quick search far quicker than I can regurgitate it all here.

But simply EV -4 corresponds to a Luminance Value of 0.008 cd/m2. To give you an idea of what kind of lighting level that is, it would require an exposure of 30 secs at f/1.4 at 100 ISO.

Therefore if a camera can auto focus at EV-4 the above luminance level is the lowest amount of light that would be required for the focus system to operate (assuming sufficient subject contrast, etc., etc., but we won't go into that)

 

[ajscullard]

As noted above, any given Exposure Value (EV) is an absolute value. EV -4 is a fixed measured amount of light, 4 stops less than EV 0.

What you may be thinking of is Exposure Compensation (EC) which is a positive or negative offset from the value measured by your metering system.

 

[Ceridan]

Thanks, can you point me to the article? I tried google but to no vail. Maybe just a bad luck...

 

[meland]

Thanks, can you point me to the article? I tried google but to no vail. Maybe just a bad luck...

Sorry for the delay - away for a couple of days.

Any of the following should set you on the right track:

Exposure value - Wikipedia

en.wikipedia.org

Loading…

camerapedia.wikia.com

Bear in mind that when people are talking about EV Values they are often referring to 'Exposure' values. But in this case we are using it to describe the minimum lighting level at which a focus system can theoretically operate.

 

[R Laing]

Hi!

for example the Nikon D500 specs state, that the central focus point can focus up to -4EV. But what does that actually mean? I understand EV, as a "Exposure Value" is a relative measurement, so what does that "-4EV" in D500 relates to?

Thanks

It means the sensor can see enough info at that light to focus properly.

 

[bosjohn21]

I believe it refers to the aperture at which the minimum accuracy of focus will be achieved

a very fast lens has a much narrower depth of field than a normal lens thus needs more accurate focusing

 

[VenusFivePhotoStudio]

I understand that nikon can focus at -4EV.
But using what lens? At what aperture? 2.8? 1.2? How is it calculated?

 

[lilBuddha]

I understand that nikon can focus at -4EV.
But using what lens? At what aperture? 2.8? 1.2? How is it calculated?

From Wikipedia:

EV 0 corresponds to an exposure time of 1 s and a relative aperture of f/1.0. If the EV is known, it can be used to select combinations of exposure time and f-number, as shown in Table 1.

So EV 0 is the amount of light needed for a proper exposure at 1 second if you had a lens with a 1.0 aperture.

At EV-4, the camera would need 15 seconds to capture the same. Each EV down roughly doubles the sensitivity and each up halves it.

 

[Fogel70]

Hi!

for example the Nikon D500 specs state, that the central focus point can focus up to -4EV. But what does that actually mean? I understand EV, as a "Exposure Value" is a relative measurement, so what does that "-4EV" in D500 relates to?

Thanks

As noted earlier you got this a bit wrong. If you put EV in front or after the numbers makes it mean two different things.

Nikon D500 can focus down to EV -4, which is a absolute value defined by the EV table.

-4 EV means something different, which is a relative value with 4 stops less exposure.

https://www.scantips.com/lights/evchart.html

Edit: Wow, I just realized that this is an 2.5 year old thread. Someone must have done a archaeological excavation to find this. I hope something interesting was discovered.

 

[alcelc]

Review of Imaging Resource sometimes has done low light AF testing on various models. It is under the Performance Tag and usually at the end of that section.

Sometimes IR will report a result in ev value, sometimes in terms of candle light. Might need conversion work. IR also mentions the lens it used, e.g. f/2.8 etc and on low or high contrast object for the result.

Might worth to check it out.

A general tendency is the hybrid OSPDAF need more light to work, PDAF of higher spec model can do darker, CDAF can do the darkest.

--
Albert