Tv & Av modes (G1)

[ThiTran]

I was playing around with the Tv and Av modes yesterday.
With the Tv mode, flash on, the camera gave me 1/60 sec and 2.5 for aperture.
I switched to Av mode, the aperture stayed at 2.5 but the shutter speed went
down to something like 1/6, 1/3. I would expect that for a same scene, I should
get similar numbers for each mode given that I keep one parameter the same.
Does anyone have an explanation? Thanks.

Thi

[shaunyc]

I think the G1 works like this:

Assume that the meter reading for the scene without flash would be about 1/8 second at f2. If you are in Program mode and turn the flash on, the camera changes the shutter speed to 1/60 so that you don't get light trails or blurring of background. The flash should freeze your main subject adequately. The aperture stays at f2.

If the you change to Av or Tv mode the camera will use whatever aperture or shutter speed you choose. In either case, the combination of shutter speed and aperture will be the same as the meter would give in the absence of the flash.

So, if you choose f2 in Av mode (with flash) the camera chooses 1/8 second as the shutter speed, in the example above.

The exception to this is if you set a shutter speed in excess of 1/250 in Tv mode. Because the fastest G1 sync speed is 1/250, any shutter speed in excess of that will be reduced to 1/250 when the flash is on.

If you choose 1/8 second in Tv mode the camera sets an aperture of f2.

That means that in Av and Tv mode you have a much greater ability to balance ambient and flash light, with the caveat that a tripod may be necessary to avoid blurring of the background.

The specific behaviour you observed was as a result of the current shutter speed and aperture settings when you entered the mode. So, in Tv mode you had 1/60 selected, and the camera opened the aperture as wide as it would go, knowing that the flash would compensate for any lack of light.

When you changed to Av mode, you had f2.5 selected, and the camera had much greater latitude to balance the available light - selecting a much slowing shutter speed.

If, in Tv mode, you had changed the shutter speed to a slower speed, the aperture value would have remained the same.

Hope that helps some.

Shaun