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zoom and focus on near objects

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Offline rkomar

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zoom and focus on near objects
« on: 27 / April / 2014, 15:00:57 »
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This is probably not a CHDK problem, but I thought I'd ask here since many members have good understanding of how cameras work.  I have a couple of Canon A4000 point and shoot cameras that I'm trying to use in a book scanner.  Ideally, I'd like to zoom in just around the page to maximize the resolution of the characters.  However, when I zoom in that way, I usually can't focus on the page.  Depending on the zoom level, there seems to be some minimum focus setting that the camera won't go below.  For example, if I set the zoom level to 64, the minimum focus setting is 800mm.  For a zoom level of 114, the minimum focus setting goes up to about 1000mm.   I have the cameras set up about 800mm from the pages.  At the 64 zoom level, a typical page only takes up a small part of the image, and I'd like to zoom in further if possible.  Is there a way to get the cameras to focus under those circumstances, or is this a physical limitation of such cheap cameras' optics?
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Offline reyalp

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Re: zoom and focus on near objects
« Reply #1 on: 27 / April / 2014, 15:16:51 »
Quote from: rkomar on 27 / April / 2014, 15:00:57
I have the cameras set up about 800mm from the pages.  At the 64 zoom level, a typical page only takes up a small part of the image, and I'd like to zoom in further if possible.  Is there a way to get the cameras to focus under those circumstances, or is this a physical limitation of such cheap cameras' optics?
The minimum focus distance at a given zoom is a property of the optics, yes.

A long time ago, some people found a way to trick S3IS to get into a non-standard macro / zoom configuration  http://chdk.wikia.com/wiki/UBASIC/Scripts:_Tele-Macro-Mode (note the script there will almost certainly not do anything useful on your camera)

No idea if anything like this exists on modern cameras, and if it does, the chance of hardware damage would most likely be quite real.
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Offline rkomar

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Re: zoom and focus on near objects
« Reply #2 on: 27 / April / 2014, 15:29:43 »
Interesting.  I actually have one of those S3 cameras which I wouldn't be too worried about destroying, but I would need a pair of them for the scanner.  It's also much lower resolution than the A4000, so I'm not sure I would gain enough from this macro mode in the S3 to surpass the results from the A4000.

So, assuming that I can't do what I want with the A4000, are there other Canon cameras that are supported by CHDK that will do what I want?  What do I look for in the specifications to see if a particular model will work for me?
« Last Edit: 27 / April / 2014, 15:42:13 by rkomar »
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Offline reyalp

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Re: zoom and focus on near objects
« Reply #3 on: 27 / April / 2014, 15:58:12 »
Quote from: rkomar on 27 / April / 2014, 15:29:43
So, assuming that I can't do what I want with the A4000, are there other Canon cameras that are supported by CHDK that will do what I want?  What do I look for in the specifications to see if a particular model will work for me?
What you see in that article is all I know about the subject. Please understand this was never "supported", it was a quirk someone discovered on a particular camera. Does it exist on any other cameras? Only experimentation will find out. Will it damage your camera? Only way to know is to try.

I have to say, I'm a bit confused as to why you need this. Can't you get the book close enough to fill the FOV without needing a lot of zoom? Unless you are scanning exceptionally small books it seems like pretty much any camera should be capable of this. With my D10, a standard paperback page fills the image at about 20cm. At 3x zoom, it also fills the FOV and being 3x further away, it is still within focus distance. Moreover, with a 16 MP camera there should be plenty to OCR even if you don't completely fill the FOV. In fact, it may be better since the edges tend to be distorted.
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Offline rkomar

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Re: zoom and focus on near objects
« Reply #4 on: 27 / April / 2014, 16:52:40 »
If I tried to put the cameras close enough to fill the view with each page, they'd definitely be getting in each other's way.  They'd also get in the way of the lighting.  They would certainly show up as reflections off the glass platen.  I think I'd also have trouble getting even focus over the whole page (although I haven't tested that).

I should also clarify that what I want is "to zoom in on a page and be able to focus on it".  The tele-macro hack is only indirectly what I want, if there's no other way to do the zooming and focusing. 
« Last Edit: 27 / April / 2014, 16:57:33 by rkomar »
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