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View Full Version : 10D exposure metering (P vs Av)



tarfearauko
12-06-2004, 06:50 AM
OK, so I just have had some time to play with my shiny new toy. Today I have been trying out the flash assisted focusing in low light. Using the very same subject I found that P or Fully automatic programs exposes the pictures OK with 1/60 f 5.6, if I do exactly the same thing in Av (f 5.6) the shutter speed goes up to 6'' which makes the picture look OK since once the flash lights the image there is not much light to create blurryness but I am very concerned about this issue since it might mean that the metering in Av is completely wrong.

Do you have any posible explanation for this hapening?

PS: The camera is using firmware 2.0.1 and the ISO setting is 100 for both P and Av modes.

Thanks so much

tarfearauko
12-06-2004, 12:34 PM
OK, just to clarify, I have being playing more with this and it is not an issue of low lighting environment. It just happens that the P and Av metering are exactly the same whenever you keep the flash off and Av uses shutter speeds considerably longer than P mode whenever the flash is on. Might it be that the flash fires less powerfully in Av/Tv/M modes than in fully automatic/P modes? I cannot find a reason for this but, at this moment, is the only hypothesis I can think of...

Best regards

eagle17
12-06-2004, 01:10 PM
I can verify that yes it does the same thing on the 20D. with the setting to P and the flash up I am getting 1/60" shutter setting. with the setting to Av and the same aperature I am getting 1/2" exposure time. When i put the flash down I also get 1/2" time on both modes.

I can not explain why and I am unable to actually take the pictures to see what the difference will be.

I am using the latest firmware and a canon 80-200 lens.

tarfearauko
12-08-2004, 10:27 AM
I finally decided to contact Canon support, this guys were pretty kind and fast. The explanation is the following:

The Av and TV mode function the same way in regards to flash exposure.
In these modes the metering system disregards the use of flash when
deciding shutter speed and instead regulates the flash duration to match
the ambient light shutter speed selected. Unfortunately, if the ambient
light in the room is very low then the shutter speed will be very low.
In the TV mode it will choose a wide open aperture or the aperture will
blink if it can make a properly exposed image. I hope this clears up
any confusion. I am sorry for the inconvenience.
So it looks like the flash is not used to light the scene but to "assist" a bit in Av and Tv modes, if you really need to light up the scene you need to go to P mode (I am still learning, so I have used M mode only a few times :o , I promise I will be using it more though ;) ).

Thanks eagle17!

Enjoy photographing!