Canon G9 DOF discrepancy - Hello, I'm a NEWBIE - HELP!! (Newbies assistance, User Guides and thank you notes) - CHDK Forum
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Canon G9 DOF discrepancy

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Offline Fatorisha

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Canon G9 DOF discrepancy
« on: 09 / November / 2008, 12:40:52 »
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Following a link from the Flickr G9 group, I downloaded and installed CHDK. I have no idea how perfected the version I have used is and I still don't see the G9 as a supported camera.
When I use the DOF function in CHDK, with the lens zoomed to its wide-angle setting (7.4 mm focal length) and  a subject distance of 1.01 meters I get

"Near Limit: .666", "Far Limit: 2.14", "DOF: 1.47".

If I check this here Online Depth of Field Calculator by entering the same focal length and subject distance, I get a near limit of 0.77 and a far limit of 1.47 meters, which is not at all the same.

(Aperture f2.8 in both cases).

Is this normal ? Perhaps there is something I haven't understood here (probably...)?

Is it normal that the CHDK DOF figures do not change as the lens is stopped down? I assume that it only shows DOF at the widest aperture setting.

Apologies if these questions are stupid: I'm having a little trouble navigating around the available information...

Many thanks!

PS Thank you! It was worth installing simply for the grid function...

« Last Edit: 09 / November / 2008, 12:42:37 by Fatorisha »
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Offline Hacki

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Re: Canon G9 DOF discrepancy
« Reply #1 on: 09 / November / 2008, 15:43:23 »
On my SX100, the values shown in the DOF calculator differ by 0.1 cm from the values your online calculator shows..

My guess: The focal lenght values for your G9 arent yet found out. CHDK does not detect them on runtime, they need to be entered into the sourcecode.

Someone needs to take a picture with a G9 for each zoom step, and write down the focal lenght values from the exif data.
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Offline Fatorisha

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Re: Canon G9 DOF discrepancy
« Reply #2 on: 09 / November / 2008, 16:55:06 »
Thank you, that woud make perfect sense: I assumed there was some kind of real-time communication between CHDK and the camera settings...
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Offline dzsemx

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Re: Canon G9 DOF discrepancy
« Reply #3 on: 10 / November / 2008, 17:50:53 »
no, it's just an estimation, an aid
btw, DOF depends by the resolution, because the per pixel sharpness gets worse (if it's out of the exact focus plane) as the resolution is getting higher
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Offline bongo_bingo

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Re: Canon G9 DOF discrepancy
« Reply #4 on: 30 / November / 2008, 04:32:36 »
Quote from: Hacki on 09 / November / 2008, 15:43:23
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Someone needs to take a picture with a G9 for each zoom step, and write down the focal lenght values from the exif data.

can you explain the correct way to do this?

« Last Edit: 30 / November / 2008, 04:54:00 by bongo_bingo »
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Offline Hacki

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Re: Canon G9 DOF discrepancy
« Reply #5 on: 30 / November / 2008, 07:33:06 »
Well..

1.) Take a picture at the wide angle focal range.
2.) Tip the zoom rocker so it zooms in by one step.
3.) Take another picture.
4.) Repeat 2 & 3 until you are at full tele.

Then transfer the pictures to your pc, and look at the exif data. you should get a list of focal ranges like 6mm, 6.7mm, 7.3mm .. and so on.. (g9 focal length starts at 7.4 mm)

BUT: From what i can tell, someone already did that. at least the fl_tbl is defined, and filled with fitting values. (Starting at 7.4mm, ending at 44.4) You can of course still check if those values are right. (platform/g9/main.c, line 67)
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Offline bongo_bingo

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Re: Canon G9 DOF discrepancy
« Reply #6 on: 30 / November / 2008, 10:18:29 »
Quote from: Hacki on 30 / November / 2008, 07:33:06
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BUT: From what i can tell, someone already did that. at least the fl_tbl is defined, and filled with fitting values. (Starting at 7.4mm, ending at 44.4) You can of course still check if those values are right. (platform/g9/main.c, line 67)

You were right, the current values are right!
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