Digital Camera World Forum

James Blonde · #1 08-01-11, 11:28 PM

I understand that when taking a photo, exposure compensation will either brighten an underexposed scene (+ev), or darken an overexposed scene (-ev) by a certain amount against what the camera has set automatically.

However how does it do this? Does it increase / decrease the ISO? The shutter speed? The aperture? Because these are the only 3 camera functions that can control exposure, yet whenever exposure compensation is mentioned - particularly in magazines, but even in the manaul - it doesn't mention that the camera is having to change something - only that the picture will become lighter / darker as if by magic.

So what is the compromise? What does change? My guess is aperture, but I'm not sure?

rbarry · #2 08-01-11, 11:53 PM

Try taking two similar shots, one without any compensation and the other with +2 ev compensation in aperture priority mode, shutter priority mode and finally program mode. The answer will be different for each mode.

I tried this myself a few years ago and if memory serves me right, the shutter speed changed in aperture priority mode, the aperture changed in shutter priority mode and both changed in program mode.

James Blonde · #3 09-01-11, 12:10 AM

I guess I wonder what the point of exposure correction is (and why its so widely promoted as a fix all), when you could just as easily (in fact more easily in many cases!) just change the aperture or shutter speed to compensate?

Cathus · #4 09-01-11, 03:11 AM

ah, but if you are on shutter or exposure priority & you just change one of the settings all you do is let the same amount of light in to the sensor because the camera automatically changes the other setting to compensate.

These modes just allow you to change one of the settings & the camera changes the other to match the correct exposure.

The only way to let more or less light in is to switch to manual & change one of the settings, then other remains the same so you therefore let more/less light in & your photo is therefore darker or lighter.

Exposure compensation does the same as what you would do in manual, it changes just one of the settings, so in AE priority it will change the speed to let more or less light in, in TV priority it will change the aperture to let more or less light in.

rbarry · #5 09-01-11, 06:44 AM

An exposure is essentially a single image. It's the result of the combined settings that were used to capture that image. That image may or may not be the desired result, it's just an image. When purchasing film you had the choice of how many exposures you wanted : 24 exposures, 36 exposures etc but it didn't mean they would all result in perfect images.

EV or exposure value is something quite different. It's setting a mathematical value for the exposure which is based on the length of time the shutter was open, the size that the aperture was set to and the sensitivity of the film (film speed) or optical sensor, known as ISO.

A good exposure can only be judged to be so, if the desired result has achieved the objective that was required, but this could be a matter of personal taste or preference.

Different camera body manufacturers use slightly different nomenclature to describe the same thing. Canon call aperture priority AV (aperture value) and shutter priority TV (time value).
As Cathus has explained, when you are using a mode other than manual, any change you make will be compensated elsewhere to hold the exposure value as metered by the camera. To overcome this you can overide the camera's brain and under or over expose an image by adjusting the EV value to anything other than zero.

For example if you were using AV priority mode and set the aperture to f8.0, the camera body metering system will work out the ambient light conditions and come up with a shutter speed value of say 1/250. If you then decide to increase the aperture value to the next stop of F5.6 giving a wider aperture, then the camera will compensate for this and increase the shutter speed to the next stop down of 1/500. It could be that you know the camera body metering system is being fooled into under exposing the image, say a snow covered scene, so you want more light to be captured for the exposure than the camera body metering system will allow in AV mode. You could try upping the ISO sensitivity, but this would just reduce the shutter speed even more in order to set the same Exposure value.

You have two choices:

1. Use Exposure compensation of +1, which will increase the F stop value by +1 . With the camera still set in AV mode and the aperture value set at f8.0, the shutter speed will now have a value of 1/125.
2. Go to manual mode and set the values as above.

Obviously you have to decide which aperture value or shutter speed you would prefer to set for the result you want to achieve. It may be that shutter priority would be better in a different instance as you want to set the time the shutter is open and all other settings must be adjusted to suit.

James Blonde · #6 09-01-11, 10:50 AM

OK - got you! I'd genuinely forgotten how Aperture and Shutter priority worked, and that one copensated for the change in the other! Blonde by name.... So it gives you an element of controlled manual override if needed in certain modes, rather than full manual control.