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Question about depth of field calculator.

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Question about depth of field calculator.
« on: 03 / April / 2011, 01:38:48 »
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I have an A570 and SD400, both with CHDK, but I'm struggling a bit with the depth of field calculator.  The values just seem erratic and don't necessarily match what I think they should.

For example, when I focus on my keyboard that is only about 1 foot away, I get a subject distance value of like .9 or something, when it should be about 1/3 of a meter.  When I focus on something about 1 1/2 feet away, I get a value of like 1.3 or something, when it should be more like .5 meters.  Many times if i point at a wall that is actually about a meter away, it will give the distance as inf.

I know there are multiple settings in the manual that change the way things are calculated, such as "use exif", "dist as near limit", "subject distance from lens".  But there isn't a whole lot of info about them.  Isn't there any way to make it display a more accurate distance?  So if the subject is 1/2 meter from the lens, it will say .5.  Or if they are 2 meters from the lens, it will say 2?

Re: Question about depth of field calculator.
« Reply #1 on: 03 / April / 2011, 06:11:51 »
Isn't there any way to make it display a more accurate distance? 

With CHDK, no.

With SDM, there is a built-in rangefinder that allows you to determine near and far distances more accurately.
Useful for studio photography but I doubt that anyone uses it outdoors.

In my opinion, the DOF calculator is of no use.

First of all, you need to measure nearpoint accurately.
Small-sensor cameras with very short focal-length lenses have inherently great DOF.
That makes measurement of distance very inaccurate because if it is based on the focus position that gives maximum image contrast there will be very little change, especially if the camera has an aperture and is stopped down.

Pointing at a wall one metre away could give adequate focus all the way to infinity, if you are not zoomed to telephoto.

Re: Question about depth of field calculator.
« Reply #2 on: 03 / April / 2011, 12:41:32 »
So is there any accurate way to lock the focus to a single spot using a combination of the depth of field calculator, and the subject distance override?

I'm doing a lot of high speed and other creative shooting where many shots are taken, but it is difficult to achieve focus, either because it is dark, or the subject that needs to be focused on is out of the frame and only passes through it briefly, or other reasons.  I'd like to find a way to only need to determine focus once, and then lock it in.  Perhaps using the auto focus once, documenting the number (even though it does fluctuate), and putting it into the override subject distance.  Does that work well?


Re: Question about depth of field calculator.
« Reply #3 on: 04 / April / 2011, 05:22:40 »
Does that work well?


I think you can answer that.
It should with SDM but I have not used CHDK.


 

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