davesrose wrote in post #17722390
But if all you do is zoom out, you're recomposing the image. With re-composition there can be a change in how much bright sky (vs dark horizon) the image has. EV does change in that situation.
But it's not his meter reading changing that he is complaining about. When he has a manual exposure level set, at 35mm which correctly exposes certain highlight areas when an image is exposed, those SAME highlights are then overexposed when using the same f stop, shutter speed, and ISO to take the image at 16mm.
Saying the above, when the OP says manual mode, that is assuming a full manual mode. If he were using Auto ISO he would no longer be in manual mode, and the camera's metering system would adjust the ISO to compensate. Anyway if the camera were working correctly, with Auto ISO set, it would reduce the ISO value as the metering changes on zooming out, and actually bring down the level of the highlights that were present at 35mm.
In a constant aperture zoom lens, the physical diameter to the aperture opening remains constant as you zoom, just like in a variable aperture zoom. What is different is that the optics are set up so that the apparent size of the aperture is also zoomed optically, maintaining the same apparent f ratio. As this doesn't rely on any mechanical changes to the aperture mechanism to maintain the f ratio, just the movement of the optical elements, I would strongly suspect a serious issue with the lens.