Another strange bug with D4, possibly D800!?!?

Started Feb 5, 2013 | Discussions thread
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This is exactly as expected!
In reply to Roland Schulz, Feb 6, 2013

If you do not lock the exposure, the total scene will be measured with a specific attention to the old spot you focussed on when you press the AF-ON to achieve a focus lock. But, if you move the scene (recompose) and the scene becomes darker, the metering will change only to some extent but because the value metered unde the original focus point still influences the calculated aperture and shutter speed. This chance could very well be rather limited. However, once you let go the button, the camera now evaluates the amount of light under the current focus point in the new scene. That area may be way darker (or brighter), hence changing the values even more.

You can test this by changing the scene using your method of holfing the focus and recomposing. Go from a bright scene with a bright object under focus to a dark scene with a dark subject (without refocussing). You will see a lengthening of the shutter speed while recomposing but holding the af-on button and the shutter release button half pressed. Once you let go the button, it should drop even further. Doing it the other way around (from dark to bright), you will initially see some shortening of the exposure time, and when you let go the button, an even further shortening. If this is indeed the case, than you know why.

Let me know if it works this way.

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