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11-03-2012, 04:42 PM   #1
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Can Someone Explain EV AF Issue?

There's been a lot of talk about the K-5II's ability to focus in low light. Adam's review talks about the differences between the K-5 & K-5II in regards to focusing at different EV's. Here's the part I'm talking about: Pentax K-5 II / IIs Review - Autofocus - PentaxForums.com

Can someone explain what these mean? I know EV stands for exposure value, but what are the EV -1, EV 4, etc? EV -1 is obviously lower lighting conditions, but what does that compare to? I guess what I'm trying to understand is what are these values in real world situations? If someone could post pictures, that would also be incredibly helpful. Thanks.

11-03-2012, 04:52 PM   #2
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Have a look here for a table that gives a description of typical scenes that show certain EV values.

EV-3 for example at 100 ISO corresponds to the light of a full moon, or perhaps even the light from a bright aurora borealis.
11-03-2012, 04:53 PM   #3
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The EV refers to how much light reflects off a surface. In more common terms, it's simply a measure of how much ambient light there is.

The EV number itself is calculated as a function of the base 2 log of shutter speed squared over aperture, with ISO factored in as a scalar. So if you know the shutter speed, aperture, and ISO, and get correct exposure at those settings, you can figure out the EV, as per the table below


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11-03-2012, 05:02 PM - 2 Likes   #4
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QuoteOriginally posted by reivax Quote
Can someone explain what these mean?
If you want to really explore the issue, download PhotoMe and open up some K-5 (or other camera) un-edited JPG's or PEF/DNG's and look for the AE segment graphic display.

It will show you the metered EV/LV for each main area of the scene, overlaid on top of the image itself, similar to this K-x image:



That might help visualize the issue.

11-03-2012, 05:04 PM   #5
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And this is why I often let the funky wheel gauge on my 30 year-old light meter do the conversion for me!
11-03-2012, 06:50 PM   #6
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Sorry to appear to be pedantic -
EV = Exposure Value is only the combination of Aperture and Shutter Speed to give "EV" -
without reference to any sensitivity or ISO value.

Please see the definition: Exposure value - Wikipedia, the free encyclopedia

Whereas LV = Light Value, is the EV qualified with the sensitivity like ISO100

Please see the definition: Light value - Wikipedia, the free encyclopedia

So mostly what we are talking about when it comes to the level of light is actually LV or "Light Value"

EV or "Exposure Value" may sometimes be (mistakenly) used interchangeably with LV -
but strictly speaking EV is only the exposure set on the camera -
ie: aperture and shutter speed only - without ISO -
if ISO value is taken into account -
then it actually is a LV - Light Value
11-03-2012, 07:03 PM   #7
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In all these matters, I defer to UnknownVT.
He is a more analytical and considered person. All I know is wikipedia, and flying by the seat of my pants
11-03-2012, 10:37 PM   #8
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QuoteOriginally posted by rawr Quote
If you want to really explore the issue, download PhotoMe and open up some K-5 (or other camera) un-edited JPG's or PEF/DNG's and look for the AE segment graphic display.

It will show you the metered EV/LV for each main area of the scene, overlaid on top of the image itself, similar to this K-x image:



That might help visualize the issue.
So, according to Adam's review, the K-5 would struggle to focus on the saxophone?

11-03-2012, 10:51 PM   #9
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QuoteOriginally posted by rawr Quote
If you want to really explore the issue, download PhotoMe and open up some K-5 (or other camera) un-edited JPG's or PEF/DNG's and look for the AE segment graphic display.

It will show you the metered EV/LV for each main area of the scene, overlaid on top of the image itself, similar to this K-x image:



That might help visualize the issue.
Downloaded and installed PhotoMe, but I could not figure out how to use that feature.
11-04-2012, 12:39 AM   #10
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QuoteOriginally posted by reivax Quote
So, according to Adam's review, the K-5 would struggle to focus on the saxophone?
I doubt that in real life the K-5 would stuggle with that scene at all. My K-x had no trouble with that shot, and I've shot the K-5 in equivalent or even darker scenes. The saxophone would also be pretty easy since it is quite a contrasty object, even if its immediate surroundings may be dim.

QuoteOriginally posted by reivax Quote
Downloaded and installed PhotoMe, but I could not figure out how to use that feature.
- Start PhotoMe
- Open an un-modified image that hasn't had the EXIF stripped out of it
- Click the 'Manufacturers Notes' tab
- Scroll down a bit to 'AE Metering Segment'
- Next to AE Metering Segment it should say <Graphic>
- Click <Graphic> to see a graphic display of AE metering segments.
11-04-2012, 12:41 AM   #11
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QuoteOriginally posted by reivax Quote
Downloaded and installed PhotoMe, but I could not figure out how to use that feature.
Open the photo with PhotoME
use the Find (ctrl-F) and type in "seg"
and the screen should show as this
and the feature is highlighted in Red -
click on the blue <Graphic> link
11-04-2012, 09:22 AM   #12
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That is pretty cool. Thanks.
11-04-2012, 12:48 PM   #13
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QuoteOriginally posted by reivax Quote
So, according to Adam's review, the K-5 would struggle to focus on the saxophone?
Well, since it's around -1, it should be able to do it.

Being in the US I would take advantage of B&H or Adorama's generous return policies. Order the K-5 II, try it out for 30 days, and if the AF isn't what you were expecting, you can always send it back for a full refund as long as you don't damage the camera

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11-05-2012, 06:45 PM   #14
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QuoteOriginally posted by Adam Quote
The EV refers to how much light reflects off a surface.
Thanks.

QuoteOriginally posted by rawr Quote
If you want to really explore the issue, download PhotoMe and open up some K-5 (or other camera) un-edited JPG's or PEF/DNG's and look for the AE segment graphic display.
That's a trap.

If you point the camera towards a white wall, the camera will expose under the assumption that it is 18% gray, leading to about 2EV underexposure. I.e., the EV readings from the metering or AE segment display are typically 2EV larger than they really are (they are only about correct for white patches appearing bright white in the final image, possibly but not necessarily requiring a dose of EV compensation for the shot).

Or in other words, what reads -1EV for Pentax may read +1EV for you in more practical terms.
11-05-2012, 09:04 PM   #15
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QuoteOriginally posted by falconeye Quote
That's a trap.

If you point the camera towards a white wall, the camera will expose under the assumption that it is 18% gray, leading to about 2EV underexposure. I.e., the EV readings from the metering or AE segment display are typically 2EV larger than they really are (they are only about correct for white patches appearing bright white in the final image, possibly but not necessarily requiring a dose of EV compensation for the shot).

Or in other words, what reads -1EV for Pentax may read +1EV for you in more practical terms.
I believe you are trying to point out there may be a difference between incident, reflected light, and "correct" exposure.

The "correct" exposure to a greater or lesser degree is dependent on subjective interpretation -
your example of a white wall was good - normally people do want it to look "white" -
but there may be the occasion when the exposure may not want it to be in zone X or white -
- thus open to subjective interpretation -
but I do get your point the camera will see it as 18% gray, and will underexposure it by several stops -
if the subjective interpretation was to place it in zone X or total white.

Incident light meter reading will normally help prevent this - as it measures the light falling on the scene.

Whereas reflected light meter reading - does what it says reads the reflected light from the scene.

However for the LV - Light Value that determines the AF limits -
it is reflected light that matters -
because that is what the camera "sees" -
and not necessarily the incident (or the amount of) light falling on the scene.

For an average 18% gray scene there is no difference between reflected and incident light meter readings.

As a possibly silly, example -
if a scene is a total light absorbent black -
a light that would say produce close to daylight exposures on an average 18% gray scene -
would probably be below the AF and even metering limit of the camera when pointed at that scene.

So for measurement of the AF light level limit it is the reflected light that is relevant
and not the incident light.

rawr's example of using the metering segments is very relevant to see the reflected LV or light levels captured.

Last edited by UnknownVT; 11-05-2012 at 09:23 PM.
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