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Old Mar 29, 2007, 1:43 PM   #1
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What are EV settings and under what conditions would it be benificial to change the setting? How does it affect the photo?

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BVM
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Old Mar 29, 2007, 2:04 PM   #2
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A camera has a limited dynamic range (range of bright to dark that it can capture), and the metering system makes decisions on what needs to be properly exposed.

One example of where you can run into problems may be when you've got a lot of bright sky in an image (especially shooting into the Sun). That can fool the metering system.

As an example, it may not know that you want you want people in shadows exposed properly, versus properly exposing the bright sky in an image. If it properly exposed the people, the sky could be too bright in some cases.

So, modern film and digital cameras have a feature built into them called Exposure Compensation to help you tell the metering system what to do. That's what the EV (Exposure Value) settings are for you're asking about. It lets you set + or - values to influence the exposure.

Exposure Compensation lets you alter the way a camera's autoexposure/metering algorithms expose an image (brighten or darken it compared to the way the camera metered the scene). It's one of my most frequently used settings on most cameras.

A +EV value gives you a brighter exposure. The camera uses a slower shutter speed and/or larger aperture (smaller f/stop number) to get a brighter exposure, compared to what the camera's autoexposure/metering algorithms would have selected.

A -EV value gives you a darker exposure. The camera uses a faster shutter speed and/or smaller aperture (higher f/stop number) to get a darker exposure, compared to what the camera's autoexposure/metering algorithms would have selected.

If you're in Av Mode (Aperture Priority) and use Exposure Compensation, the camera will vary the shutter speed (since you're setting the aperture). If you're using Tv (Shutter Priority) mode and use Exposure Compensation, the camera will vary the Aperture (since you're controlling the shutter speed).

If you're in Auto (or other similar modes), the camera may vary aperture or shutter speed when you use Exposure Compensation. In low light, since your aperture is already wide open, it varies shutter speed if you use a -EV setting.

If you're shooting in Manual Exposure mode, there is no Exposure Compensation (because your aperture and shutter speed settings take the place of it since you're controlling both).

Correct Exposure comes down to the amount of light, the ISO speed, the shutter speed, and the aperture. A variety of combinations will produce identical exposure.

You use Exposure Compensation if you want a brighter or darker image compared to what the camera's metering would normally give you in the same conditions.

An example of when you may want to use a +EV setting is for a backlit subject, where the subject would normally be much darker than the rest of the image. Since the camera has a limited dynamic range, it doesn't know that you want the dark subject exposed properly (at the expense of the rest of the image). So, you can make the darker subject brighter for correct exposure (which might cause the rest of the scene to be overexposed some).

If your subject is much brighter than the rest of the image, you may want to use a -EV setting for Exposure Compensation so that your subject is not overexposed (making the rest of the image darker, too).

The camera has a limited range of bright to dark that it can capture. So, it makes choices so that most of the image is correctly exposed, depending on your metering mode. Sometimes that may not be what you want. That's where exposure compensation comes in.

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Old Mar 29, 2007, 3:45 PM   #3
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An example would be shooting a snow scene. If you let the meter decide you end up with gray snow and underexposed because the meter tries to set every thing to medium gray. By applying +1 EV or so the snow comes out white and the scene is better exposed.
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Old Mar 29, 2007, 4:42 PM   #4
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Wow, this is very helpful. I will never ask my brother-in-law to explain this again!

In the past I usually tried to avoid shots where I'd expect the background to be brighter then my subject. Is selecting the correct EV setting is done by estimation and/or experience or is there a rule of thumb?

Thanks,

BVM
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Old Mar 29, 2007, 5:17 PM   #5
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Take a batch of pictures at 1/3 or 1/2 EV steps then review on your computer for the best exposure. You'll find that similar scenes will need similar correction.

The histogram may help as well. Understanding Histograms is a good primer.
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Old Mar 30, 2007, 3:58 AM   #6
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Biff vonMises wrote:
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Wow, this is very helpful. I will never ask my brother-in-law to explain this again!

In the past I usually tried to avoid shots where I'd expect the background to be brighter then my subject. Is selecting the correct EV setting is done by estimation and/or experience or is there a rule of thumb?
The other alternative if the subject is close enough is to use fill flash, that way you don't have to blow out the background. Like someone in fron't of a sunset where you really want both elements to be right.

Many cameras also offer (Enven some of the upper end P&S) auto bracketing where is will take 3 shots in quck sucession, usually one under, one as metered, and one over. Some allow you to make bracket all multiple step over or under .

But yeah EV is sort of a learned guesswork.... as suggested just go out and shoot some test shots and see how much things change.
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Old Apr 2, 2007, 10:08 PM   #7
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Biff vonMises wrote:
Quote:
What are EV settings and under what conditions would it be benificial to change the setting? How does it affect the photo?

Thanks,

BVM
In very bright Sun light, I use EV-5 if not using zoom. However, if you want
to keep the shutter speed and aperture, reduce the ISO if you want a darker image.

try it, and you will note a difference between 1/400 using ISO200 & ISO50.
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Old Apr 3, 2007, 1:07 AM   #8
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Biff vonMises wrote:
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What are EV settings and under what conditions would it be benificial to change the setting? How does it affect the photo?
Well here is a prime example.... far closer to sunseet than it looks but -2/EV stops under... (might be -1.5)

EV has very much to do with just what proportion of your frame you want exposed optimally. Also bring things away from an all over middle grey level AE often goes for when it shouldn't (like in snow)

Might be nice to have trees/sky a little brighter (and could do that post VERY easily) but moon was the essential object.

As a little white spot ANY even 11 point averageing AE system would have said NOT the object of interest.... but you'd have the nice leaves and away from sunset twilight sky... but a blown out white bullet hole for the moon. Even spot metering probaby wouldn't get it right
And actually shot both ways seeing how far to back off EV.... I could just composite them given the issolated moon... and never mess with the more intricate layered level adjustments.
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Old Apr 3, 2007, 5:30 PM   #9
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Hayward wrote"
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Well here is a prime example.... far closer to sunseet than it looks but -2/EV stops under... (might be -1.5)
The EXIF tags say -2. Being able to review these tags has really helped. Using the EV option was one of my most used ones this winter as I was out and about on the ice and snow. I would make the EV change and then once back home and after the photos had been moved to the hard-drive I would check/compare each one and look at the EV change that I had set.

And, always looking at the histogram both right after I took the photo and then again on the hard-drive. Now if I could just get them to look like the ones I am envious of.


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Old Apr 3, 2007, 11:08 PM   #10
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There's really a misnomer involved here. EV means "exposure value" and is a convenient photographic scale expressing the absolute amount of light measured on a scene. One could equally measure the amount of light in "candlepower" or "lumens," but EV is a convenient linear scale for photographers, since each increase of integer represents a doubling of the amount of light (the same as moving between whole f-stops or shutter speeds, or ISO settings).

That's why most light meters give the option of indicating their measurements in EV (and sometimes in candlepower or lumens), in addition to the more common combination of ISO, shutter speed, and aperture. For any given amount of light on a scene (the EV value), there are several different ways of capturing the same exposure. For example, if the light meter says there is "EV 15" amount of light shining on a scene, you can capture the same exposure with any of the following combinations of camera settings:

ISO 100, 1/250s @ f11
ISO 100, 1/1000s @ f 5.6
ISO 400, 1/2000s @ f8.0
(see table below)



On the other hand, what everyone here is talking about is "exposure compensation," which is also measured in "EV steps." Example: your camera is set to ISO 100 and the camera's light meter tells you that the current lighting on a scene requires an exposure of 1/250s at f11. But let's say that, based on previous experience, you know that your camera tends to overexpose scenes of this type, so you want to set the camera to underexpose the scene in order to compensate for that tendency. That's when you use the "EC" (exposure compensation) setting on the camera. You set it to -.7 or -1 or whatever value (measured in EV steps) you figure is necessary to compensate for the camera's natural tendency to overexpose.
Similarly, if you're shooting against the sun or other bright backlight, your camera will likely have the tendency to see the bright light and underexpose the scene in response. So to correct for that tendency, you set your exposure compensation for +1, +2, or whatever value (EV) you figure is necessary to compensate for the bright backlighting.

Although in today's digital world the term EV is probably most commonly used in the context of exposure compensation (EC), it's actually a separate concept.





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