Exposure problem

[Cindy-Clicks]

I seems I am doing everything right but my new 70D is acting very strange.  I have it set up on a tripod to take photos of a waterfall.  It is not sunny so I don't use the neutral desity filter.  I have it set on TV and I change the setting from 1 to 2 seconds.  The first shot at 1 sec chooses an aperture of 7.1, the second and third shots of 1.6 and 2 it chooses an aperture of 20 or above and the shots come out very dark, and often blurry.  And then when I shoot hand held on P, it overexposes most, but not all, of the shots.  Why the inconsistancy??

[ebiggs1]

The variable is Tv.  The camera is still trying to set the correct exposure but with the Tv fixed it probably can't.  Using Tv, the camera expects you to provide an aperture that will have a correct setting.  If light is not sufficient, you must select another Tv value.

[Cindy-Clicks]

Well if I am increasing the shutter time, it should allow more light in.  But why does it close the aperture so much that it underexposes the shot when I increase shutter time?  I mean if I am at 2.5 sec, why does it close the aperture to f40?  It was taken at mid day 500 ISO

[ScottyP]

The aperture should narrow by one stop if you double the exposure length. In your example it moved more than one stop, more than 3 stops actually. Something in the scene was messing with your automatic metering. Backlighting from the sky? Weird reflections off of the water?

You are dealing with a tripod already. I would just shoot manual and avoid the problem rather than playing around with exposure comp or switching between spot metering and scene metering etc.

Personally I would not use as high an ISO as 500 for a long exposure on a crop sensor unless you were really set on getting a 1 second exposure here rather than a 4 second one for artistic reasons. I hate noise and lack of detail perhaps more than is warranted but if I could use ISO 100 or 200 by lengthening exposure a second or two I would.

[ebiggs1]

Cindy-Clicks,

It is all related.  You can't move one without affecting the others.  Perhaps an exposure meter would help you.

When you go to a creative mode like Tv, part of the camera is in manual mode.  It expects you to know the other settings to make a properly exposed photo.

[cale_kat]

With those types of results, I would look at where the camera is taking its light reading. Is it possible that the metering suggested a far darker image than you "visually" confirmed because it used spot metering  instead of evaluative, for example?