Focus lock and Zooming

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Focus lock and Zooming
« on: 23 / August / 2015, 02:18:52 »
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Browsing the forum for scripting ideas, I see threads from people trying to focus their camera only once, then zoom and shoot more photos, saving the time required by refocusing.  It usually doesn't work, not because of some scripting or CHDK problem, but because you're asking the optics to do the impossible.
When I bought my Nikon 80-200mm zoom lens in 1974, it was one of the highest-rated zoom lenses on the market, partly because it managed to maintain a constant focus throughout its 2.5X zoom range.  Many other lenses on the market were actually "multifocal" lenses, the difference being that a multifocal lens can be zoomed, but must be refocused after zooming.  A true zoom lens gets extra bragging rights (and money) for holding its focus.
My Powershot S3IS's 12x zoom seems to act like a true zoom, but only because the camera automatically focuses each shot.  It's asking a lot just to get pictures with minimal color fringing from one end of the zoom range to the other; holding the same focus distance would be an optical miracle.  It's still very impressive for a complete camera that costs less than that Nikon lens did.
I didn't need CHDK to test my S3; I just manually focused on something with sharp edges 4 feet away, then zoomed way in.  The viewfinder blurred completely.  I was able to refocus on my subject, but it took a major change in the focus setting.  The same test with the lens focused at infinity might come closer, but I seriously doubt you'll find optical perfection when it's so much easier for the manufacturer to program the camera to focus every shot, and 99.9% of consumers won't know the difference.
If you're trying to write a script that focuses only once before shooting over a wide zoom range, you're going to be disappointed, unless your optics were built by wizards.  Try the simple test above, and I think you'll decide to focus after every zoom change.
S3 IS with LensMate filter/hood adapter

Re: Focus lock and Zooming
« Reply #1 on: 23 / August / 2015, 02:29:13 »
More evidence of magic:  my 80 - 200mm lens in 1974 could focus down to about 4 feet at 80mm zoom.  My S3's 36 - 432mm lens can lock focus on the dust on its UV filter!  Ain't that miraculous enough?
S3 IS with LensMate filter/hood adapter

 

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