Auto iris adjustment for Cinelike D gamma to avoid overexposure

M

Mike Sertic

Member
So I am still experimenting with my scene file setup on the HMC40, using the Cinelike D gamma setting. One of my primary uses for the camera will be shooting surfing in bright daylight, which can be challenging in terms of preventing blown highlights in the whitewater (big problem) or surface reflections (small problem).

For exposure, I have been able to get good results by using zebras and setting the iris manually, but I am kind of struggling to get proper exposure using the auto modes (manual exposure only is not an option for me because I will soon be using the camera in a housing which does not have an iris control and at times I will have inexperienced filmers manning the camera and need an idiot mode).

First question, I have Barry Green's HMC150 book and it mentions that the A.Iris Level setting in the scene file will bias the exposure, and that each increment on the setting is equivalent to approximately 1/6 of a stop, is that the same in the HMC40? It has the same -10 to 10 range as Barry discusses for the HMC150.

Second, while reading the Panasonic scene file white paper for the HMC150, they discuss that when using the Cinelike gammas you should "adjust the iris to reduce the video level to approximately 1/2 of the normal video level". Does anyone have any idea what that A.Iris Level setting that would correspond to?
 
I'm sure Barry can answer your questions, from the technical perspective, far better than I could.

From a real world perspective, if at all practical, I'd suggest you simply take the camera out and experiment with adjusting the A.Iris Level, shooting what it is you want to be able to set the camera up for, so it will shoot as optimally as possible in auto modes. It would be good to know what the steps (on the -10 to + 10 range) translate to, but you still need to actually go out and find the sweet spot under the actual conditions you want to prepare the cam for when you hand it to someone else. I'd also suggest that you err towards underexposure - which can be compensated for (with reason) in post, whereas blown out footage is unrecoverable.
 
Put another way, take the cam out to the beach and shoot 5 minutes of surfing, with the camera controlling the iris, but without making adjustment to A.Iris Level, then back the A.Iris Level off one point (and announce that vocally so it's recorded for reference later) and shoot for another 5 minutes, then back it off another point (again talking to the cam), etc. When you get home, take a good close look at the footage, and see at what A.Iris Level setting the highlights became consistently acceptable.
 
One thing, also, are you going to be using any filters, such as a polarizing filter? I generally don't shoot in bright daylight without one. If you are, take that into consideration when setting your Iris level. If you aren't, give it a serious consideration (especially if you are shooting against water).
 
Do you really need one of the Cinelike Gammas? I would try to use one of the Gamma curves that allowed me to crank the knee up to high. If you're shooting snow even with A.IRIS adjusted you will either over-expose the snow or under-expose everything else. The knee should help knock the snow down.

And I don't normally use DRS but you might try giving that a try too.
 
I think I am ever more confused about the scene files. In Barry's HMC 150 book, page 199, last paragraph has really thrown me. "To get the full effect of these scene files, set the camera in manual white balance mode and use the auto iris feature. The scene files make extensive use of the A.IRIS and COLOR TEMP menu settings, and those settings only take effect if you use auto-iris and manual white balance." Is Barry saying that if you use the standard scene files you MUST use the auto iris mode (besides using a manual white balance)?
 
I'm saying that if you want the full effect of what the scene files I put in the book deliver, then you'd have to let auto-iris set the exposure so that it can take into effect the a.iris menu setting. Once it's set what it thinks the proper exposure is, then you can switch to manual and take over, and adjust to taste.

But if you want the footage to impart the changes that the scene files in the book demonstrate, then you'd have to let the a.iris menu setting work.
 
Thanks Barry. If I understand you correctly, it sounds like if I am using even the standard scene files built into the camera, I need to start with a manual white balance AND the A.IRIS so the camera sets itself up properly, then I can put the iris back into manual mode and tinker with the iris--but at that point I am changing the default scen file setting, corect? Thank you.
 
The A.Iris setting, if it's present in the scene file, is intended to adjust the exposure away from what the default exposure would normally be. The only way it can do that is if you use auto-iris to set where the exposure point should be. Now, you don't have to accept the auto-iris' suggestion, you can always change it afterwards, but it at least gives you a starting point. So yes, on the stock scene files or on any that you modify or load in, the only way a changed a.iris level setting in the scene file can be actually utilized is to let the auto-iris set the exposure for you.
 
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