I get asked this question (and variations on it) a lot.....
I purchased a Sony ZIU and have one question: When shooting, either in Auto or Manual iris mode, with daylight or well lit indoor environments, the image brightness 'oscillates' up and down. I have tried to lock the iris, but with very little success. The iris fluctuating up and down rapidly is getting frustrating, and I can't seem to pinpoint what the cause is. Any help would be extremely appreciated.
What you have to remember is that there are four things that affect exposure. To remember them all think SING:
Now all of these guys work together when you go totally automatic. In the video above I shot the flowers then tilted to the bright sky. If you watch the data code at the bottom of the screen you’ll see the iris, gain and shutter all changing.
The problem comes when you start to go manual. Because you can’t just go manual on one of these functions, you have to go manual on all of them – if you want full control over exposure.
So, the answer to the original problem is … You may have switched the Iris into manual mode – but what about gain and shutter?
If gain and shutter are still in auto they will try to “help” with exposure.
So, if you go manual Iris, then make sure Gain is set to manual (usually 0dB) – then set the shutter to 50 for PAL or 60 for NTSC.
Now you will be truly in manual exposure mode and in total control.....and the exposure should stop changing of it’s own accord.
That said we can make use of this help with exposure. Stills cameras have had this function for some time – it is called aperture priority and shutter priority. If you own one of the Canon cameras it is the AV and TV modes on the control dial.
In aperture priority you have control over the iris. You set the iris – not for exposure but to get the depth of field you want and the camera sets the exposure using shutter and gain with a hint if you need ND.
In Shutter priority you have control over the shutter. You set the shutter – perhaps to get clear high speed rotating helicopter blades or for that Top Gear alloy wheel effect and once again the camera sets the exposure but this time using iris and gain (or ND).