Digital ND filter question xa10

[Rainer Halbich]

How exactly does a digital ND work?

Does it lower the ISO of the image sensor?

I know that it does not adjust the shutter speed or the f stop.

[Richard Stone]

As far as I know there is no such thing as a digital ND filter. I believe the XA10 has optical ND filters which are moved into the light path between f4 and f4.8 if the 'ND Filter' setting is set to automatic.

[Don Palomaki]

Digital ND filter is an interesting concept, if by digital you mean electronic. It could fuction by changing the gain of the circuits that read the sensor. Alternatively it might function a bit like an LCD display where in by changing the bias you can change the transparency. Think in terms of the photo-sun lenses used in eye glasses.

[Rainer Halbich]

Originally Posted by Richard Stone

As far as I know there is no such thing as a digital ND filter. I believe the XA10 has optical ND filters which are moved into the light path between f4 and f4.8 if the 'ND Filter' setting is set to automatic.

Impossible, if it is true what you are saying it means that this camera must have like 20 different ND filters and it manages to switch between all of them without making a single noise and without distorting the image.

Why do I say 20 ND filters? Because if it was only 3 optical ND filters the switch between them would be very obvious.

This camera has a digital ND filter not a optical ND filter.

[Rainer Halbich]

Originally Posted by Don Palomaki

Digital ND filter is an interesting concept, if by digital you mean electronic. It could fuction by changing the gain of the circuits that read the sensor. Alternatively it might function a bit like an LCD display where in by changing the bias you can change the transparency. Think in terms of the photo-sun lenses used in eye glasses.

So what you are basically saying is that the ND filter on the xa 10 is basically the same as negative gain?

Then why don't they just call it negative gain instead of a Digital ND filter?

I have 2 theories to this.

First theory, the camera lowers the ASA or the ISO sensitivity of the cmos sensor.

Second theory, it uses a special static glass which has the ability to change its Neutral Density without moving, but I am not sure if that would count as digital.

[Richard Stone]

Originally Posted by Rainer Halbich

Impossible, if it is true what you are saying it means that this camera must have like 20 different ND filters and it manages to switch between all of them without making a single noise and without distorting the image.

Why do I say 20 ND filters? Because if it was only 3 optical ND filters the switch between them would be very obvious.

This camera has a digital ND filter not a optical ND filter.

The ND filter option in the menus is only available in aperture priority and manual modes i.e. when you select the aperture (and ND filter). I don't think the camcorder ever switches ND filters in automatically during recording. As for noise, the motors that change the aperture and focus the lens are silent (at least to me!) so I don't see why the motor that controls the ND filters shouldn't be. I have read somewhere that if you shine a torch into the lens of a Sony HVR-A1 you can see the ND filters switch in when you adjust the aperture but I can't find the reference now. I think that if Canon had really invented an electronic ND filter they would list it as a feature.

[Greg Boston]

Possibly the same technology used in auto darkening welding hoods. So yes, there can be digital ND filters. The only thing is, my AD welding hoods have a greenish tint so they aren't strictly neutral in that they do affect color reproduction.

-gb-

[Don Palomaki]

So what you are basically saying is that the ND filter on the xa 10 is basically the same as negative gain?
...

No. I am speculating as to what a "digital ND filter" might be were there such a thing, considering the way marketing folks occasionally misuse technical terms.

Stop and think about light sensors (CMOS or CCD) for a minute or two. The sensor latitude is driven by its dark current and its saturation point. It cannot meaningfully "see" anything darker than its dark current or brighter than its saturation point. And further its linearity may have rate effect (slew rate) limitations as well, sort of like reciprocity with film.

Playing with gain will effect the final image brightness, but will not be the same as a true ND filter with respect to keeping the light reaching the sensor within the desired sensitivity range of the CMOS/CCD. Playing with the physical aperture woud come closer to ND-like effects with respect to exposure, but has other effects on the image (e.g., depth of field). Playing with shutter speed is somewhere in between and its effect would depend in part on how the shutter speed is implemented within the camera in question.

At a 60i frame rate there is plenty of time to drop a small physical ND filter into the light path between successive fields on demand, all it needs to be is like a partly transparent between the lens leaf shutter blade.

[Rainer Halbich]

Originally Posted by Don Palomaki

At a 60i frame rate there is plenty of time to drop a small physical ND filter into the light path between successive fields on demand, all it needs to be is like a partly transparent between the lens leaf shutter blade.

I really doubt that, I have the PAL version and I record mostly 25p.

Keep in mind that the iris is not a replacement for the spinning shutter on a film camera.

I know that 1/50 shutter speed is considered as the standard SS when you record 25p, this is because a real film camera with a 180 degree shutter will have a shutter speed of 1/50 if you shoot at 25p.

The thing is, a film camera needs a rotating shutter to flick to the next film slide and a digital camera doesn't. This is why I mostly record with a 1/25 Shutter speed.

Lets do some basic maths: 1/25 x 25 frames = 25/25 = 1 second of footage with CONSTANT exposure to the CMOS sensor, if you go any slower the image will start to jitter because it has to duplicate the frames.

This means that if there is really a optical ND filter in the xa10, you should be able to see it in your footage when you change the ND filter while recording at 25p with a SS of 1/25.

I tried it out and I can't see any distorted frames or images in my shots.

Myth Busted...

[Jeff Harper]

Specs for the camera say it has a "built-in gradation filter". That sounds like an actual physical filter.

[Rainer Halbich]

Originally Posted by Jeff Harper

Specs for the camera say it has a "built-in gradation filter".

A built in graduation filter can also be a built in "Digital" graduation filter.

[Don Palomaki]

PAL, with the slower frame rate, may actually give a bit more time in which to drop a physical filter into the light path unnoticed.

Considering the speed of small mechanical shutters, it should be possible to drop a filter into place in a small fraction of a field duration. For example, the rather large focal plane shutter in a 35mm still camera can move the full frame width in less than the maximum flash synch speed, on the order of 1/250 (0.004 seconds) with good current SLRs. Think how fast it can move the much smaller distance require if in the camcorder lens.

The physics of photon-to-charge convesion in CCD/CMOS argue against using gain as a viable ND filter mechanism, although gain can be used to adjust image brightness. Keep in mind that a ND filter is intended to adjust the brightness of the light reaching the CCD/CMOS (or film), not how the light reaching the CCD/CMOS is interprreted.

[Jeff Harper]

Originally Posted by Rainer Halbich

A built in graduation filter can also be a built in "Digital" graduation filter.

It could mean that, but seems unlikely. I can find no reference anywhere on the web to a digital ND filter. Do you know of other cameras that use one?

[Rainer Halbich]

Originally Posted by Don Palomaki

PAL, with the slower frame rate, may actually give a bit more time in which to drop a physical filter into the light path unnoticed.

You missed my point.

What I am saying is if you record with a 1/25 shutter speed @ 25p, you Sensor will have "CONSTANT" exposure.

When I say CONSTANT I mean, the sensor makes use of 100% of the possible exposure time without duplicating any frames. In other words, there is NO TIME for a ND filter to jump into frame without seeing it in the footage.



Let me try again with two examples.

First example (Max shutter speed for a film camera with rotating shutter) (1/50shuttersped @ 25p)

1/50 x 25 = 0.5 seconds of expose for one second of footage.
(The other 50% of the light goes through the viewfinder.)


Second example: (Max shutter speed for a DIGITAL camera WITHOUT a spinning shutter, just like the xa10 or any other digital video camera)

1/25 x 25 = 1 second of exposure for ONE second of footage. And 0 time for a optical ND filter to jump into frame.

[Rainer Halbich]

Originally Posted by Jeff Harper

It could mean that, but seems unlikely. I can find no reference anywhere on the web to a digital ND filter. Do you know of other cameras that use one?

Yes,go to camcorder reviews/tests and comparison with test images and technical data and select the Canon xa10, scroll down to ND filter.

The canon xf100 is also Digital.