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Weird border noise with CHDK using exp time override

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bugmenot

Weird border noise with CHDK using exp time override
« on: 01 / November / 2008, 16:28:25 »
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Hi guys

I've been playing around with long exposure times and CHDK and i've hit this weird noise bug. Please take a look at the noise around the margins of those shots. Why is it there?

First picture is a 45 sec exposure

Second picture is a normal 15 sec exposure
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Offline hotvedt

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Re: Weird border noise with CHDK using exp time override
« Reply #1 on: 01 / November / 2008, 16:39:00 »
This is not a bug, this is noise from the sensor itself.
Varmer areas of the sensor gives more noise.
If you take a black-frame you can use it to reduce this.
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Offline fe50

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Re: Weird border noise with CHDK using exp time override
« Reply #2 on: 01 / November / 2008, 16:39:48 »
Hi bugmenot,

on long exposures you should enable the Noise Reduction in the CHDK menu.

the purple artefacts in your 45s exp pic is sensor noise, with the noise reduction a dark frame ist captured after each shot and this 'noise' get filtered out, see Wikipedia: Dark frame subtraction.
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bugmenot

Re: Weird border noise with CHDK using exp time override
« Reply #3 on: 01 / November / 2008, 17:13:02 »
Hi

Thanks for the quick response. So this means that I have to shoot in RAW and set Dark frame substraction to ON or auto?

Does this work for jpegs too?

Do I have to turn on Bad pixel too? and if so how should i set it? average or Raw Convert?

Many thanks for the response guys
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Offline fudgey

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Re: Weird border noise with CHDK using exp time override
« Reply #4 on: 02 / November / 2008, 06:03:47 »
Quote from: bugmenot on 01 / November / 2008, 17:13:02
Thanks for the quick response. So this means that I have to shoot in RAW and set Dark frame substraction to ON or auto?

Does this work for jpegs too?

Do I have to turn on Bad pixel too? and if so how should i set it? average or Raw Convert?

Without CHDK, your camera always uses dark frame subraction for long exposures. With CHDK you can turn it off if you like, but by default it's still "AUTO" i.e. the camera decides whether to use it like it would without CHDK.

So no, you don't need to shoot RAW. This option just happens to be located in the RAW menu. Bad pixel removal is complete separate from this, no need to touch it.

But that purple coloring you're seeing in the corner may not go away completely with dark frame removal, because the sensor temperature changes during the exposures. Maybe your shots were actually taken with dark frame removal and turning it OFF will reveal you the extent of the problem (some of these artificial software limits we break with CHDK really aren't there just to sell more DSLRs...without 15 second Tv limit there would probably be a number of people who thought their camera was broken when it's just not good enough for the job).

You know for sure dark frame removal is active if your long exposures take twice the Tv.

An easy solution around this is to shoot wide angle and crop the bad area away in post processing. It's always the same corner that will go bad first.
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bugmenot

Re: Weird border noise with CHDK using exp time override
« Reply #5 on: 02 / November / 2008, 07:40:47 »
i got it fudgey. now i get it

My last question (i've read the manual and faq but i didn't find any info) is what do Average and Raw Convert mean in the Bad pixel setting?

Also i remember on one of the older builds that i had a field called ND filter. Does this still exist in the current builds as i didn't see it and if it's still there, does this work as a real ND filter? I'm planing to buy one as i have a 58 mm adaptor. Or better, how does this ND software filter really works

Sorry to bug you off with all this questions, but I'm really fond of CHDK and i want to maximize my usage of it

have a nice weekend
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Offline fudgey

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Re: Weird border noise with CHDK using exp time override
« Reply #6 on: 02 / November / 2008, 08:22:44 »
Quote from: bugmenot on 02 / November / 2008, 07:40:47
My last question (i've read the manual and faq but i didn't find any info) is what do Average and Raw Convert mean in the Bad pixel setting?

TBH I don't know, I've never looked into those. But try searching around the forum, they have been discussed before. Probably in the RAW shooting board.

Quote from: bugmenot on 02 / November / 2008, 07:40:47
Also i remember on one of the older builds that i had a field called ND filter. Does this still exist in the current builds as i didn't see it and if it's still there, does this work as a real ND filter? I'm planing to buy one as i have a 58 mm adaptor. Or better, how does this ND software filter really works

There is no software ND filter.

Many cameras have a hardware ND filter (usually instead of an iris to block light to allow shooting in bright daylight, but some cameras have both an iris and an ND filter) and CHDK is able to force it in and out. It's possible that an early build for your camera had that option because the developers didn't know whether the camera had one and the option has since been removed because it had no effect.
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