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General photography questions and answers: Discuss Exposure Bias versus changin shutter speed...Hi, I am trying to understand why I would want to use the exposure bias setting on my camera. Would ...
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Old 30-07-2008, 13:32   #1 (permalink)
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Exposure Bias versus changin shutter speed

Hi,

I am trying to understand why I would want to use the exposure bias setting on my camera. Would this not do the same thing as altering the shutter speed?

Thanks
Paul
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Old 30-07-2008, 13:42   #2 (permalink)
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Re: Exposure Bias versus changin shutter speed

Yes and no! As far as I know, exposure compensation will be achieved in different ways depending what mode the camera is set on.

If you are in aperture priority mode, exposure compensation will be by changing the shutter speed.

In shutter priority, by changing the aperture.

If the depth of field is important and you are in A or Av mode, this will then be maintained, and vice versa.

+ or - EV can also be by changing ISO, but not sure if it actually is done in camera, but you can certainly ajust it to give exposure compensation.

Also the exposure compensation can be done in 1/3 of a stop increments - far more control than changing the aperture or shutter speed.
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Old 30-07-2008, 15:03   #3 (permalink)
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Re: Exposure Bias versus changin shutter speed

As Graham says, it's a way of compensating for variable lighting conditions - camera takes a reading (which may be averaged over the whole image) and there may be highlights which would otherwise be burned out without exposure compensation.

Personally, I've usually some sort of compensation set (usually -ve to prevent burned highlights) which I may then alter as I take shots.

Exposure compensation is especially useful for sunrise/sets if you've no filters (like an ND Grad) and want to take a single shot. For some obsure reason Canon have decided that +/- 2.0 is the most you'd need (a right pain in rapidly changing light conditions, so manual setting is not a realistic option even if I had a seperate lightmeter - even my 3 year old Sigma SD10 has +/- 3.0!)
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Old 30-07-2008, 15:07   #4 (permalink)
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Re: Exposure Bias versus changin shutter speed

When you take a picture, provided the camera is not in full manual mode, it measures the light available and, depending on the mode, sets the shutter speed or aperture or both to generate what it "thinks" is a correctly exposed picture.

In some cases this is wrong. For example a small white flower against a dark background. The camera will tend to average the exposure over the whole scene and then the flower will be very over exposed.

Exposure compensation (I assume this is what you mean by bias) is an over ride for this automatic exposure. You can in effect to tell the camera to decrease (or increase) the calculated exposure to get the flower in the example above correctly exposed. Of course in this case the background will be very dark.

In effect this will give you some manual control over the automatic exposure system.

I have probably made that sound more complex than it really is but I hope it helps.
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Old 30-07-2008, 15:12   #5 (permalink)
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Re: Exposure Bias versus changin shutter speed

ahh, I think that makes sense then.. so its an option that would be used and Tv or AV modes. Not really that usefull in full manual as you set the shutter and appeture manually anyway.


Thanks guys, I will need to have a play with this when I get home
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Old 30-07-2008, 16:46   #6 (permalink)
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Re: Exposure Bias versus changin shutter speed

When I first bought my EOS 20D, a 10D owner told me that his camera frequently produced blown highlights so he advised permanently introducing 1/2 stop exposure compensation. I did this for a few weeks but noticed in Photoshop that all my images were underexposed by 1/2 stop so I removed this and have never used exposure compensation since.

I understand that some may use compensation for difficult lighting conditions but I always meter an appropriate object then set the exposure manually in such cases. I am sure that, once you gain experience of using exposure compensation it will be fine but I prefer spot metering and manual as I gained experienced in this method many years ago.

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Old 30-07-2008, 16:49   #7 (permalink)
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Re: Exposure Bias versus changin shutter speed

I have a 400d so I can't use the spot metering

I think that I will have a play and see what my results are
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