How does AUTO IRIS work?

[Cranky]

First some background. I have used several consumer Canon, Sony and Panasonic camcorders. Of these, all Canon and Sony HD camcorders had a built-in variable ND filter, the goal of which was to preserve aperture somewhere around F5.6 (HV20/HV30/HV40) or F4 (HF100, HC1). The filter is fully automatic, no direct control over it whatsoever.

Canon's ND filter had 1/4-stop gradation, up to 3 stops on the HV20/HV30/HV40 or up to 4 stops on the HF100. Sony's ND filter had 1/2-stop gradations, covering the range up to 3-1/2 stops.

Now I am reading about AUTO IRIS function on the Panasonic camcorders and I am not sure how similar it is to the Canon/Sony consumer setup. I am especially interested because the new HMC40 has a built-in ND filter which, presumably, can be turned on and off, but in ON mode it is "auto iris" only, no manual control over specific f-stop.

Could anyone in the know explain how Panasonic system is similar or different to Canon/Sony?

I came across this message on another site: "Turn on the DVX100 AUTO iris function and let the internal light meter process the light. You should see the f-stop numbers on the display change. From there, the display will either read “ND 1/8” or “ND 1/64”." Umm, it seems to have only two gradations, and these gradations are pretty crude, 3 stops and 6 stops.

With Canon/Sony system, the camcorder can keep relatively even brightness when lightning changes, because the ND steps are so small, especially with Canon. With the Panasonic system it seems that I would experience a serious jumps in brightness. Or, I would experience a serious change in DOF when the iris compensates for a huge 3-stop ND engaging/disengaging.

Another question. Barry wrote: "The A.Iris menu setting in the scene file has range from -4 to +4, and each tick accounts for about 1/3 of a stop of exposure compensation, so +2 would be about 2/3 of a stop brighter than +0 would be."

What exactly does this mean? Is it the same thing that is called "AE shift" on Sony camcorders or "Automatic Exposure compensation" on still cameras? This has no direct relation to the ND filter, has it?

An explanation and advice are appreciated. Thanks!

 

[paulears]

They are a little more sneaky than that and what seems to happen is that they give you psuedo ND filters - which are not ND filters at all!

Auto exposure is what is really happening - not necessarily iris or neutral density filters in the optical path. So what happens is light comes in the front of the lens. Some of it is needed to give the correct output level from the chip, some isn't needed. The software compares the output of the chip, and then starts by closing the iris to reduce the 'real' amount of light hitting the sensor. On the other hand, it could do this electronically by limiting the amount of time that the sensor is receptive to light - rather like shortening the shutter opening time in a film camera. It can of course do both. This gives a bit of leeway on depth of field and ability to capture fast movement, and this is often then presented by the camera as an extra feature - often with 'sports' labels for these settings. Real ND filters just get put in the optical path like sunglasses. Electronic adjustment of camera gain and shutter speed serve a similar purpose. Unless a piece of optical media is put in the light path, then it isn't really a neutral density feature, just a bit of electronics to serve the same purpose.

All these methods are open to the manufacturers and all have pros and cons. If you limit the period the sensor is receptive, then the lens can open up to compensate, or you could just add some gain. The correct method is the one that works best for the particular shot. To much gain means too much grain. To wide an iris mean shallow depth of field. I took a small HD camera skiing in the new year and even with the iris closed, the light level was too high and exposure was compromised. The camera isn't clever enough to give me high shutter speed, a large f number iris and low gain all at the same time - so next time, a real ND filter will be required. Some cameras give you more control over these features than others, while some don't even really let you mess with them all.

The features Cranky mentions are all specific ways each manufacturer deals with this on more complex camera. How the cameras deal with these individual parameters, isn't that clear. I assume they have 'real' ND filters that electronically slot in and out - but unless they specifically state they are real, it's quite feasible they could be electronic versions - that don't have absolute scaling to the numbers.

If anyone cares to comment on the individual models detailed, that would be good - I only know the generic ways manufacturers handle exposure.

 

[Cranky]

Paul, I am not talking about auto exposure per se. I am not talking about changing shutter speed and/or aperture to limit exposure either. These are known ways of controlling exposure, but this is not an ND filter.

First, I think it is a known fact that Canon camcorders at least (not sure about Sony) use real opto-mechanical filters, somewhat similar to ND filter wheel. I believe I've come across a message where a user claimed that he could actually see filtering getting in and out of the light path when he looked at the lens while adjusting exposure. Also, I've come across a patent (Sony's or Canon's? Don't remember) of this feature. So it IS a real filter.

Now, I don't care about auto exposure. What built-in filters in the Canon and Sony camcorders do, they try to keep certain aperture. Say I lock shutter speed, then I turn manual exposure. The exposure slider now controls aperture/ND/gain only, but not shutter speed. Now, when I adjust exposure (manually!), the camcorder adds or removes ND filters if possible. It was too bright outside, then the camcorder sets aperture at F4, then adds ND filters, then if this is not enough, it reduces aperture. If I decide to increase exposure, the camcorder will increase aperture to F4 first, then while I am increasing exposure, the camcorder will start removing ND filters (1/4-stop on the Canons, 1/2-stop on the Sonys) keeping aperture at F4, then, after all filters are removed, it will start increasing aperture again. Notice, that I control exposure MANUALLY, but the ND filters are added/dropped AUTOMATICALLY in an effort to keep f-number at F4 (or F5.6, does not matter what a specific number is).

Now, how this is similar to Panasonic setup? Can I use MANUAL exposure control with AUTOMATIC ND filter control, or I get either full manual or full automatic? Can I get fine-grained steps like on the Canon and Sony, or all I get is crude 3-stop steps?