Digital ND filter question xa10

[Jeff Harper]

Kawika, well done sir. Excellent work.

[Tom Hardwick]

The reflection you see is very common and it's because the internal NDs aren't coated (I know not why, they're another element in the multi-element lineup and need to be the best they can be). If the iris didn't appear to change size as you varied the f stop then the ND must be varying - even if you can't see it because of the reflections.

[David Heath]

Originally Posted by Jeff Harper

Ranier, I believe that in science they say that the simplest explanation is the most likely the correct one, or something to that effect.

Almost - "the simplest explanation that fully explains the observed facts is the most likely the correct one" is how it's normally expressed, and it's Occams Razor. (See Occam's razor - Wikipedia, the free encyclopedia ) But do note the "most likely" - SOMETIMES it may let you down!

As far as this goes, then forget about "digital ND". The concept can't work beyond a small amount (which is negative gain) and I'll try to explain why.

Increase the light incident on a chip and the output signal will increase - up to a limit when saturation is reached. There will be a certain maximum output voltage that can't be exceeded no matter the light intensity. For reasons of best signal-noise it's normal to operate the chip just below this light level to normally get full amplitude. More light would mean clipping, less will mean more gain needs to be used - hence a worse s/n figure.

So, if the light intensity doubles, then adding a stop of optical ND gets you back to square one. Don't add it and we're into clipping - and there is nothing that can then be done electronically to simulate the ND filter. That's why "digital ND" is a physical impossibility.

(Practically, there is a little headroom, hence the possibility of using a little negative gain, -3 or 6dB. Even this will likely come at the expense of highlight handling though.)

[Don Palomaki]

The "Canon A1 Digital" was a Hi8 camcorder from the early 1990s. The Digital in the name was with respect to the internal DSP. The read from the 1/2" CCD was analog, the signal on tape was analog, and all outputs were analog.

In the case of the XA10, digital refers no doubt to the method of controlling the ND filter. the "digital control" allows the camcorder to control the iris up to the point where the ND filter enters, stop iris change while the ND filter is changing value,. and result iris change when the ND filter is maximized. Easy to do digitally with stepper or servo motors, not as easy with simple mechanical schemes.

The "ISO rating" determines exposure required to produce a specified image based on certain exposure processing assumptions. The digital camera revolution has dumbed things down for Joe and Jane Sixpack in the transition from chemical to electronic photography. Gain converts the latent image on the CCD/CMOS into voltage, much as processing converts the latent image on the emulsion into silver (density) in the negative. Changing gain is like push/pull processing.

If you under expose an image, but increase developing time/temperature you can get a negative that shows a "normal" density range, but the density curve and grain structure may be different (typically larger gain and poorer shadow detail). Add gain to underexposed video and what do you see - usually more image noise and less satisfactory shadow details.

However, all this is an academic exercise in self abuse. Shoot and edit video, have fun, and make your clients happy with the end result. What counts is not the specifics of the technology but the end result on the end users' display of choice.

[Kawika Ohumukini]

Here's a video I made of the action. It's at normal speed, slow and slower. I don't know exactly what it all means but here it is. Cheers.

Canon XA10 Internal ND Filter Test on Vimeo

p.s. The aperture changing size is actually me going past f/4 to f/3.7.

[Rainer Halbich]

Originally Posted by Kawika Ohumukini

I'm convinced this is a physical filter that moves completely in. I zoomed all the way in and put a rectangle HDV-Z96 LED light facing slightly off to the side facing into the lens. When I went one click past f/4 I could see a reflection of my entire rectangle light appear. When I went back a click the light reflection disappeared. It appeared to be a filter rotating from the bottom up to cover the opening completely. As I continued to increase the f-stop the iris didn't appear to change size so I'm wondering how they're handling the continuous change.

We have a WINNER! I checked it out and yes, you are right. I admit defeat, it is a rotating filter very close to the iris.

Sorry for the trouble, but it was a very interesting debate.

It is time to change the subject;
What type of internal memory does the Canon xa 10 use? is it a Hard drive or is it solid state memory?

[Rainer Halbich]

Just one last thought on the ND filter subject.

It would be nice if canon made the ND filter setting independent from the F stop value. I love to record with a open iris and a shutter speed between 1/25 and 1/120.

With the auto ND filter it is difficult to expose in very bright light conditions with a open iris and without having a ridicules shutter speed.

[Don Palomaki]

It would be nice if canon made the ND filter setting independent from the F stop value.

All it takes is a bit more money for a step-up camcorder. However, you can still add an external ND filter.

[Tom Hardwick]

Rainer, Don's correct. If you buy a camcorder with no manual ND filter switch on the body then you can bet your boots that it has undocumented, unavoidable and constantly operating internal ND. This hidden ND isn't there to confuse or distort your aperture readouts (though it does of course), it's there to give you sharper pictures, while keeping to sensible (movie-making) shutter speeds.

Of course cameras like the GoPro couldn't care less about using 'proper' shutter speeds, so they abandon the complex and expensive internal ND solution and simply control the exposure in the aperture priority mode - by varying the shutter speed per frame. In bright sunlight 1/1000th sec is not uncommon, giving the footage that staccato look.

tom.