Overruling Auto-ISO limit in Tv Mode - Creative Uses of CHDK - CHDK Forum

Overruling Auto-ISO limit in Tv Mode

  • 1 Replies
  • 335 Views
Overruling Auto-ISO limit in Tv Mode
« on: 02 / June / 2020, 08:01:14 »
Advertisements
Hi guys!

I hope this is the right section of the forum and maybe you can help me. Long explanation first: i reactivated my old Canon PowerShot SX200is and went out to shoot some nature pictures.... wildlife. But as you can imagine squirrels, woodpeckers and so on don't stand still for me. They move when they want and as fast as they want. And the sun is also not always available as i want. Longest focal length doesn't make the light better, but it's often necessary. Therefore i used the "Tv" mode to set the shutterspeed to a fixed fast value (for example 1/500s) and i used Auto-ISO. So the shutterspeed should be constantly fast enough and the camera sets the ISO automatically as high as necessary. But the problem is that the SX200is has an Auto-ISO limit of only 200 in Tv-mode. Lots of the pictures are too dark. So dark that they are almost black and useless. If i don't use Auto-ISO i can set it up manually until ISO 1600!!!! But i don't want to use too high ISO if it isn't absolutely necessary and nature and animals don't give you the time for experiments with values. When the situation is over, it' over! I mean.... that's the reason what an auto-function is made for. Unlogical for my why manually ISO 1600 is possible but automatically it isn't.... a limit of 200 especially in Tv doesn't make sense. I don't know why a shutterspeed of 1/3200 is possible here, the pictures all must be black at that speed combined with ISO 200. Of course ISO 1600 will not look very good with this small sensor, but it looks better then motion blurred pictures. By the way: Safety Shift function is deactivated!

So i thought maybe the problem can be solved with CHDK by "overruling" the Auto-ISO. First try (thanks to user erfi00 from a german CHDK forum!) was in "P"-mode using these settings...

https://i.postimg.cc/Z5fpxNFT/CHDK-Benutzer-Men.jpg

But my used values were different: minimum shutterspeed 1/500s (i tried 1/1000s too) and max.ISO-Auto 3200. Doesn't work as i want. Here the camera still overrules often (if it is too dark) CHDK and sets down the shutter speed instead setting the ISO higher. Maybe because it's still an automatic mode and safety shift can't be disabled?!?!? I don't know. But the point is: doesn't work solid when CHDK shall overrule the camera but the camera often overrules CHDK. The shutterspeed doesn't stay as i want.

So second thought is: back to Tv mode and overruling Auto-Iso there. But how? The same method as before does absolutely NOT work in this mode. Shutterspeed 1/500s and max. Auto-ISO 3200 get ignored, the camera still uses it's own automatic. I'm no expert, i have no knowledge about writing scripts for CHDK or something else. I don't know how i could solve this problem. Somebody who knows how???? Thank's in advance!!!


P.S.: Finally i want to say, i know it's an old camera. I know it's only a compact superzoom with the possibility of some manual settings and it isn't a DSLR. Yes, even modern smartphones do better. But i'm interested in WHAT is possible with this old piece and HOW. ;-)

Re: Overruling Auto-ISO limit in Tv Mode
« Reply #1 on: 02 / June / 2020, 09:10:37 »
So i thought maybe the problem can be solved with CHDK by "overruling" the Auto-ISO. First try (thanks to user erfi00 from a german CHDK forum!) was in "P"-mode using these settings...

From my google translate of the photo and German CHDK forum thread

link > CHDK DE : Iso does not go over 200 in TV mode despite automatic

it seems you are trying to use CHDK's AutoISO function?

Instructions for using that mode can be found here :

link > CHDK User Manual : Custom AutoISO

For what it's worth, the programmer who wrote that code descibed how it is supposed to work here : 

link > tsvstar's patch to autoISO


Quote
But my used values were different: minimum shutterspeed 1/500s (i tried 1/1000s too) and max.ISO-Auto 3200. Doesn't work as i want. Here the camera still overrules often (if it is too dark) CHDK and sets down the shutter speed instead setting the ISO higher. Maybe because it's still an automatic mode and safety shift can't be disabled?!?!? I don't know. But the point is: doesn't work solid when CHDK shall overrule the camera but the camera often overrules CHDK. The shutterspeed doesn't stay as i want.
A quick google search of the forum for [ site:https ://chdk.setepontos.com custom autoiso  ] will show quite a few people, myself included, struggling to use that mode.  I personally do not want to revisit the subject but you are of course most welcome to read through the threads and see if you can puzzle it out.

Quote
So second thought is: back to Tv mode and overruling Auto-Iso there. But how? The same method as before does absolutely NOT work in this mode. Shutterspeed 1/500s and max. Auto-ISO 3200 get ignored, the camera still uses it's own automatic.
The CHDK User Manual mentions that Custom AutoISO mode does not work in Tv priority mode (or M for that matter).

Quote
I'm no expert, i have no knowledge about writing scripts for CHDK or something else. I don't know how i could solve this problem. Somebody who knows how????
Take a look at this description of exposure control from a script designed for taking photos from kites and UAV drones :

link > KAP_UAV Exposure Control Script : Exposure Calculation Algorithm

It's not really setup for shooting single photos via the camera's shutter button but if you have a USB Remote it will work that way.  It could also be hacked fairly easily to use the shutter button instead.

I've always thought about adding that scripts exposure control to either replace the existing Custom AutoISO mode or be a second option.


Ported :   A1200    SD940   G10    Powershot N    G16

 

Related Topics