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Old Jul 22, 2009, 1:19 PM   #1
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Default Shutter/Aperture & EV difference...?

Newbie question.

I understand the concept of Shutter and Aperture (along with ISO) - the combination of the above gives the Exposure Value.

But what is the difference when you adjust EV manually through the EV setting? I mean the Shutter speed and Aperture stay the same, so what does the EV adjustment do?

I hope I have made my question clear.

Thanks
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Old Jul 22, 2009, 1:30 PM   #2
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As long as the ISO speed is set to a specific speed, it should be varying either the Aperture or the Shutter speed if you're referring to using Exposure Compensation with a + or - EV setting.

If you're in Av (Aperture Value, a.k.a., Aperture Priority) mode, when you spin the control dial, you're changing the Aperture Value (expressed as f/stop). When you vary the aperture, you're controlling the iris in the lens (which like a pupil in your eye, can be opened up to let in more light or closed down to let less light in). When you use Av mode, you select the desired aperture. Then, the camera selects the correct shutter speed for proper exposure, based on how it's metering the scene.

If the camera is metering the scene where you end up with a darker or brighter exposure than desired, you can use Exposure Compensation to change it's behavior. If you use a -EV setting (needle to the left of center), you'll have a darker exposure than the camera would have used. It gives you a darker exposure by using a faster shutter speed with the selected Aperture. If you use a +EV setting (needle to the right of center), you'll have a brighter exposure than the camera would have used. It gives you a brighter exposure by using a slower shutter speed with the selected Aperture.

If you go to Shutter Priority (where you select the Shutter Speed and the camera selects the Aperture needed), it does the same thing (brighter or darker exposure with Exposure Compensation) by varying the Aperture instead. If you shoot in Programmed Auto, it may vary either one (or both) to change the exposure.

If you're using manual exposure, there is no Exposure Compensation. You're controlling the variables by setting the Aperture and Shutter speed (using the meter as a guide, with the needle to the left of center giving you a darker exposure, and the needle to the right of center giving you a brighter exposure, as compared to the way the camera metered the scene).

The selected Aperture impacts the shutter speeds you'll need for proper exposure. For example, an aperture setting of f/2.8 (if you're using a brighter lens with that aperture available) will allow you to use shutter speeds 4 times as fast as one as you could use at f/5.6 for a given lighting condition and ISO Speed. Aperture also impacts Depth of Field (wider apertures will allow for a shallower depth of field when desired to help subjects stand out from distracting backgrounds).

The aperture scale in one stop increments (with larger than f/1 apertures possible but very rare in lenses) goes f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22... With each one stop move to a smaller aperture (represented by higher f/stop numbers), you will need shutter speeds twice as long for proper exposure for the same lighting and ISO speed (only half the light gets through compared to a one stop larger aperture).

There are only 4 main variables involved for proper exposure. We've already discussed Aperture. The other variables are Lighting (typically measured in EV, which stands for Exposure Value), ISO speed and Shutter speed.

ISO speed is how sensitive the film or sensor is to light and is the same thing as the older ASA rating for film. The higher the ISO speed, the faster you can expose it (each time you double the ISO speed, you can use shutter speeds twice as fast for the same lighting and aperture.

Shutter Speed is how long the camera's shutter stays open to expose the film or sensor.

IOW, it all boils down to how much light you have, how sensitive the film or sensor is to light (which you control via the ISO speed), and how much light you need to let it see to "expose" the image (which you control via the aperture opening size and shutter speed). Here are some exposure calculators that you may find of interest.

http://www.robert-barrett.com/photo/...alculator.html

http://www.photonhead.com/simcam/shutteraperture.php
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Old Jul 22, 2009, 2:06 PM   #3
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Thank you Jim,

I was indeed referring to Exposure compensation, which I worded EV.

And now that you answered, I did notice when I change the +/- Exposure compensation the shutter speed (in this case) changes.

I just thought that Exposure compensation did something different / additional that adjusting shutter/aperture.

So in other words, Exposure compensation is the "easy way" to let the camera decide? For example, if Exposure compensation -2 adjusts the shutter speed from 1/20 to 1/60, why would I need it if I can adjust the shutter speed manually anyway to 1/60?

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Old Jul 22, 2009, 2:17 PM   #4
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Convenience and speed. ;-)

It's faster to let the camera decide at least one of the variables versus watching a meter and tweaking settings (for example, using either Aperture Priority where the camera selects the Shutter speed; or Shutter Priority where the camera selects the Aperture), especially in rapidly changing lighting.

That reduces what you need to worry about setting, and Exposure Compensation lets you quickly change it's behavior if conditions warrant a darker or lighter exposure.

For example, if you're shooting a backlit scene without a flash, you may want to use a +EV setting for a brighter exposure if the metering is being fooled by the brighter background; or if the camera is overexposing because of a lot of dark areas in a scene, you could easily use a -EV setting to get a darker exposure without worrying about as many details (like setting both the Shutter Speed and Aperture using Manual Exposure).
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Old Jul 22, 2009, 2:24 PM   #5
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Quote:
Originally Posted by evangelos K View Post
For example, if Exposure compensation -2 adjusts the shutter speed from 1/20 to 1/60...
BTW, a -2 EV setting using Exposure Compensation when shooting Aperture Priority would increase shutter speed to 1/80 second if the camera was metering the scene where it selected 1/20 second without any Exposure Compensation.

-1 EV (1 stop darker) would give you shutter speeds twice as fast, or 1/40 second in that example.

-2 EV (2 stops darker) would give you shutter speeds 4 times as fast, or 1/80 second in that example.
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