Same exposure @ different zoom levels...

[Josh66]

Well after all this, can I say that if I was shooting a plane horizon over an empty sea and a clear sky right after the sunset. And the camera was on a tripod and the horizon line was in the middle, then in that case I would probably not see any change on the shutter speed. Am I right?

Here's an easier test:

Aim you camera at a white wall with nothing on it. Zoom in/out. The settings should be identical.

If they ARE identical, the differences are due to metering.

If they ARE NOT identical, the differences are due to something else (?).

 

[iskoos]

Well after all this, can I say that if I was shooting a plane horizon over an empty sea and a clear sky right after the sunset. And the camera was on a tripod and the horizon line was in the middle, then in that case I would probably not see any change on the shutter speed. Am I right?

Here's an easier test:

Aim you camera at a white wall with nothing on it. Zoom in/out. The settings should be identical.

If they ARE identical, the differences are due to metering.

If they ARE NOT identical, the differences are due to something else (?).

Thought about that already but the camera would have a hard time to focus on. But now I am thinking, it doesn't need to focus to meter. Or I can just manually focus...
So I guess white wall test should work...

 

[Josh66]

Yeah, focus shouldn't matter. Just set it to manual focus so it doesn't hunt forever.

 

[Hamtastic]

Besides the stuff mentioned above, check out the difference between f/stops and t-stops. F/stops are pretty darn close to good enough, but the fact is that they are determined by mathematical calculation and not by actually measuring the light allowed through the lens. They aren't always perfect. To get absolutely the same exposure throughout the range of apertures you need a lens with t-stops, which as far as I know they mainly only make for motion picture cameras.

 

[Mike_E]

IMHO Hamtastic is on to it. As the lens (variable aperture) zooms it needs more light to make the same exposure regardless of the aperture. As there are few simple answers you should take this into account as well as the metering issue and remember that the camera changes the shutter speed in half stops (third stops depending) so if you take a shot at the low end of the zoom and then the high end of the zoom at the same shutter speed and aperture you might well be nearly a half stop off on the second shot.

 

[pbelarge]

I had noticed this when shooting with my camera as well.

It is sort of like a see-saw.

The fixed portion is the aperture. The 2 sides of the see-saw are light and shutter. When one goes in one direction, the other changes at the same time. When it goes in another direction, it changes again.
...something like that.:mrgreen: