Shutter Speed in Av Mode

What determins the shutter speed when you are shooting in Av mose and what determines the Aperture size when shooting in Tv Mode. btw, im using a canon D-Rebel.

The cameras metering system is what determines this. It will try it's best, in most cases pretty darn close to correctly doing so, expose the shot properly. That is, there will be details in all ranges...photo is not "dark" or "bright" rather, it is "natural".

There are three aspects of a photograph that make up the exposure.

The ISO which is how sensitive to light the medium your using is, in this case (your Drebel) it is the sensor.

The aperture, which is the size of the opening in your lens

The shutter speed, which is how long your shutter will be opened, or...to tie everything together, how long light will pass through a certain sized hole that will be projected onto your light sensitive sensor.

Now that we've covered these basics we can dive into your question.

The idea behind AV mode is to allow the photographer to think of nothing but the ISO and aperture of the lens while forgetting about what shutter speed is needed to expose the photograph properly. If for example, you want to have a very narrow depth of field you may select "1.8" in AV mode. Based on your cameras ISO setting, and your aperture setting, the cameras metering system will select a shutter speed that will properly expose that photograph. On your camera, the range has to fall in between 30 seconds and 1/4000 of a second. If it wont your LCD in your viewfinder will blink, and you'd want to adjust your ISO.

In TV mode it is nearly the same except now your deciding a certain shutter speed you want to use. If your shooting a race car for example, you may want to have a slower shutter speed to allow you to have a motion blur effect of the background. You may want to use a shutter speed of 1/125 to achieve this however it is a bright sunny day and there are shadows and some cars are black and some cars are white and...in the end, to have all your photographs exposed the same and to maintain a certain ISO and your shutter speed of 1/125, you must make sure the aperture is at the proper setting. In TV mode, the camera metering system does this work for you.


I hope it makes a little more sense now, if not...it's 2am

If you're using flash, in Av and Tv mode the camera will meter for ambient lighting. So if you're using Av and flash, the camera will choose a long shutter time if the lighting is low.