ND filter swinging in even when "ND filter state" is "Out"

Mattttteo

4

ND filter swinging in even when "ND filter state" is "Out"

« on: 14 / August / 2013, 06:12:21 »

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Hi everybody,

I have an IXUS 220 HS (= Canon Powershot ELPH 300 HS) with the Firmware 1.01G. I have a problem with the ND filter swinging in when I don’t want it to.

I use the override shutter speed and I set the ND filter state on "out" to work in manual mode with maximum shutter speed (I make photos of very bright scenes).
I usually do a serie of 20 to 100 pictures that I want to assemble, so I need all the pictures to have the same settings. The problem is that the ND filter automatically swings in on some pictures (usually the pictures where the global scene is a little bit brighter) and the picture is clearly underexposed and different from the rest of the serie.

Does anyone have faced the same issue ? Is there a way to prevent the ND filter to swing in ? I’ve tried almost all the combinations I could imagine and it still swings in sometimes.

Thanks in advance for any help and by the way, thanks a lot to the CHDK community for making my powershot a racing monster.

Matteo

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srsa_4c

4451

Re: ND filter swinging in even when "ND filter state" is "Out"

« Reply #1 on: 14 / August / 2013, 14:53:33 »

I have bad memories about this camera...
See for example why dark frame subtraction control is (AFAIK) still unimplemented: http://chdk.setepontos.com/index.php?topic=6341.msg82448#msg82448

ND filter override should be applied with the rest of the photo overrides (aperture, ISO, shutter speed). If it doesn't work, it's possible that none of the overrides actually works. Can you check that?

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Mattttteo

4

Re: ND filter swinging in even when "ND filter state" is "Out"

« Reply #2 on: 20 / August / 2013, 05:28:15 »

Hi,
Thank you for your answer and sorry for mine coming so late, I was away for a while…

The other overrides work fine (ISO and shutter speed ; no aperture because these cameras don’t have anything else than the ND filter) and the raw is well recorded.

Except when ND filter interferes, all the pictures look like they have exactly the same settings. I’m not sure how CHDK affects the Exif data, but the ISO and shutter speed is constant on it. Actually, it is often wrong about “aperture” : the Exif data can say f/2.8 and f/7.7 both when the filter is in or out.

I thought I found a way to avoid the interfering of the ND filter, but it’s not working well either: by changing the light metering in Canon’s functional settings on Center Weighted Avg, I could diminish the amount of dark pictures but not completely avoid the problem.

Any idea how to stop this inopportune ND filter?

Cheers, Matteo

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waterwingz

12140

Re: ND filter swinging in even when "ND filter state" is "Out"

« Reply #3 on: 20 / August / 2013, 06:20:14 »

This might sound a little strange at first, but its quite possible that the exposure changes you are seeing are not caused by the ND filter not being where you commanded it to be.

Its not clear exactly how you are overriding the shutter speed (script or CHDK menu) but when you do so, then you also need to correct the ISO setting. On cameras without adjustable aperture, correct exposure is determined by the combination of ISO sensitivity, shutter speed, and neutral density filter position. When CHDK overrides are used, the Canon firmware does not know they are being used and calculates ISO, shutter and ND settings to suit the scene brightness level. If you just override the shutter speed, then you get whatever ISO and ND setting the camera would have used combined with your shutter speed. So to get the correct exposure, you need to calculate and set the correct ISO setting for the scene brightness at the shutter speed you picked and aslo command the ND filter where you want it.

This link will explain the math - http://dougkerr.net/pumpkin/articles/APEX.pdf . I can probably come up with a demonstration script that shows how to do this if you are interested.

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Ported : A1200 SD940 G10 Powershot N G16

Mattttteo

4

Re: ND filter swinging in even when "ND filter state" is "Out"

« Reply #4 on: 20 / August / 2013, 10:25:26 »

Thank you for that link. This document is very interesting: it gives the right words to the concepts and explains them well! I’ll use it in the end of my post.

I’m gonna give some precision about the pictures I do. I use a little drone to make aerial photography. I use it in the same incident light conditions (direct sunlight in the middle of the day) with the purpose to make a map. So I’m looking for pictures that have the same exposure.

Here is how I proceed (all in the CHDK menu):
-   If the weather is sunny, I set the ISO value on 50 ISO (=market value; the real ISO is 30; the setting is 3 x 30). I want the best quality.
-   I set the ND filter state on Out
-   I take a picture of a printed checked pattern and set the shutter speed according to the histogram (I don’t have any exposure meter…), so I avoid over- or under-exposure. Usually, it is around 1/2000 s. I need a high shutter speed to avoid motion blur. From 1/800, it seems to be enough to avoid it.
-   I take some pictures around to check the settings and I mount the camera on the drone. Then the drone can make 2 things: turning the camera on/off and taking pictures. Until now, all the pictures have the same exposures and are excellent, except when the surface is a little bit brighter (like drier grass or a clear road): in these cases, the neutral density filter swings in and the exposure is far to be correct… (with my setting, it would have been bright, but not over-exposed at all).

So if I check my exposure with the equation in your document, waterwingz, we have:
Av + Tv = Iv + Sv
and using the tables on p. 3 to 5 to transform my usual settings:
3 (=F/2.8 ) + 11 (=1/2000) = 10 (=direct sunlight, according to Wikipedia) + 3 (=30 ISO)
which means the exposure is almost correct, a little bit too dark.

If we see only the incriminated pictures, we probably have a higher average scene luminance, which means a higher exposure value (Ev), which explains the ND filter “reflex” to avoid a picture too bright, if we see this equation: Ev = Av + Tv
The shutter speed is indeed supposed to be at its maximum, so the only thing the camera could try to change is the aperture, by using this filter.

So I tried other settings, with the ND filter in:
-   Shutter speed: 1/800 (Tv=9.5)
-   ND filter state: In (equ. F/8, Av=6)
-   Override ISO Value: 15; Value Factor: 10 (real ISO 150, Sv=5.5)
-   Illumination: direct sunlight (Iv=10)
According to the first equation, it should be the “proper exposure”. And it is the case on many pictures. But it happens the same thing in the other way: When the subject is less bright, the ND filter comes off and the picture is overexposed.

I don’t know how to use/write scripts, may be it could allow a solution… For the moment, the only way out I see is to buy an other camera that allows to really override the aperture/ND filter.

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waterwingz

12140

Re: ND filter swinging in even when "ND filter state" is "Out"

« Reply #5 on: 20 / August / 2013, 12:04:15 »

It certainly sounds like you've done things correctly. As you guessed, maybe the camera does not implement ND control properly.

Can you test the ND filter manually? Use the CHDK override to set the shutter speed to something like 1/100 and ISO to 100. Set the ND filter "in" and take a shot of at normal room daylighting levels. Then set the ND filter "out" and take a shot at the same light level. Does the brightness change?

Then set the ISO in the Canon menu as high as it will go (e.g. 1600) and the scene mode to "fireworks" if possible Then set the CHDK shutter speed override to 1/1000. Point the camera at something really bright (like a 100 watt bulb from 12" away). Test with the ND filter in & out manually.

The first test should demonstrate forcing the ND filter "in" when the Canon firmware wants it "out'. The second test should demonstrate forcing the ND filter "out" when the Canon firmware wants it "in".

On a side note, its unlikely you are getting the actual ISO to be much less than 80. I'd be very surprised if ISO 30 was possible.

« Last Edit: 20 / August / 2013, 12:48:25 by waterwingz »

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Ported : A1200 SD940 G10 Powershot N G16

philmoz

3450

Re: ND filter swinging in even when "ND filter state" is "Out"

« Reply #6 on: 20 / August / 2013, 15:49:55 »

The value for 'nrflag' in capt_seq.c does not match the value found by finsig_dryos in stubs_entry.S.

My guess is the current value in the code is wrong and the ND filter control is not working at all.

Ignore the above - the nrflag is probably wrong; but it controls noise reduction (NR) not ND.

Phil.

« Last Edit: 20 / August / 2013, 16:35:16 by philmoz »

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g12 (1.00c, 1.00e, 1.00f & 1.00g)
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Mattttteo

4

Re: ND filter swinging in even when "ND filter state" is "Out"

« Reply #7 on: 22 / August / 2013, 13:46:27 »

Quote from: waterwingz on 20 / August / 2013, 12:04:15

Can you test the ND filter manually? Use the CHDK override to set the shutter speed to something like 1/100 and ISO to 100. Set the ND filter "in" and take a shot of at normal room daylighting levels. Then set the ND filter "out" and take a shot at the same light level. Does the brightness change?

Then set the ISO in the Canon menu as high as it will go (e.g. 1600) and the scene mode to "fireworks" if possible Then set the CHDK shutter speed override to 1/1000. Point the camera at something really bright (like a 100 watt bulb from 12" away). Test with the ND filter in & out manually.

I followed your suggestions and the ND filter’s action isn’t really repeatable. It seems to be in the middle of a dilemma or something and sometimes the override wins, sometimes Canon’s firmware wins… When I take a picture in a dark place with the ND filter engaged, it’s like a fight inside the camera (LCD alternating very quickly underexposed and normal views) and in the end the filter is usually removed. When I take a picture of a bright bulb with the ND filter set out, it usually sets off. So as in my prior experiences, the ND filter override doesn’t work every time.

But eventually, I found a solution that gives me plenty of satisfaction. This is a little bit some kind of patch up monkey business (don't know how to say it in english...), but I found a very simple way to prevent the neutral density filter from engaging: I set the overexposure on +2 Ev (on Canon's Menu). As the camera doesn’t understand that the picture will have a correct exposure, I tell it that I want an overexposed picture, so it blocks the ND filter. It will probably work the same if you want the opposite (-2 Ev to force the ND filter), but I didn’t try it yet. Since that, I’ve done hundreds of pictures and they are all perfect: same settings and good exposure.

Thanks a lot to all for your help !

Matteo

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waterwingz

12140

Re: ND filter swinging in even when "ND filter state" is "Out"

« Reply #8 on: 22 / August / 2013, 14:57:00 »

Quote from: Mattttteo on 22 / August / 2013, 13:46:27

But eventually, I found a solution that gives me plenty of satisfaction. This is a little bit some kind of patch up monkey business (don't know how to say it in english...), but I found a very simple way to prevent the neutral density filter from engaging: I set the overexposure on +2 Ev (on Canon's Menu). As the camera doesn’t understand that the picture will have a correct exposure, I tell it that I want an overexposed picture, so it blocks the ND filter. It will probably work the same if you want the opposite (-2 Ev to force the ND filter), but I didn’t try it yet. Since that, I’ve done hundreds of pictures and they are all perfect: same settings and good exposure.

An interesting work around.

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Ported : A1200 SD940 G10 Powershot N G16