Originally Posted by VideoGrabber
Any idea with his iris at -12 how much headroom he still has left? I.e., how many ftL would he gain by opening it to 0?
Edit --- I would have to check the aperture f-stops, but very roughly I believe he should gain at least 10 fL (possibly a bit more).
I did not measure this myself, but am going by what some reviewers claim. There is probably an officially spec'd F-stop number for the fixed aperture, which may or may not be accurate (not sure).
F-Stops on the main lens (for the throw position / zoom) are often not that accurate because of a multitude of reasons, mainly because if going by measuring calibrated lumens, the measurement technique offsets the F-stop accuracy, but also because of coatings and calibration stuff it messes up the linearity of the F-stop curves logarithm.
In professional and commercial film making, people will often create their own offset using reference equipment and call this a T-STOP instead of an F-STOP. When you see a review site measure a specific point at a specific throw, this is essentially a T-Stop (though incomplete). If they produced enough measurements, you can derive a full T-Stop curve to make it work more like an official F-Stop curve.
For Lumens estimates, my Projector Calculator attempts to simulate a T-STOP rather than use F-Stops, though in a future version I do plan to add the ability to use F-Stops eventually as well, so you can check a box and see the difference between the T-Stop EST and the F-Stop MFR supplied #. This is because the reviewers are essentially measuring using T-Stops, so I want to match their technique if I am reporting their numbers.