Is it possible to read and write the position of the focus lense? I would like to write a distance measuring tool and thought that as part of it I would give the option of using the focus position as an indicator of the distance to the object, and thus of the (object plane) pixel size. (Obviously it will yield poor accuracy, but in some cases it might be enough.)

Not sure but Fcam allowing you to see how far the focal point is.

FCamera does it but it's poor:

a) It does 5,10, etc centimeters. From there it goes every 10 or so, then 1 meter, then over 5m.

b) Never twice same focus. for 60 cm, I got 30, under 5, then over 1 meter. When shooting, speed beats precision.

c) Phone is correctly and happily locking to 5cm or infinity when unsure or close enough.

The only measurements you are likely to get consistently is "under 10 cm", "about a meter" and "way out there". You could use manual on it, but IMO you're better off guessing or using your hands.

Ok. Thanks. You've persuaded me to scrap the focus idea. I'll use manual or automatic calibration with a known target instead. E.g. a coin is a simple and easily available target for determining the pixel size.

Easier to use a large ruler or a calibrated 20 cm object. (hehe).

You need perspective correction. But it's doable for good ratios, like a known window dimension to estimate building size.

That's one of the nice properties of a fixed zoom camera, its distortion field is static, and once you map it you can just solve a few linear equations to estimate things like perspective transformations.

I thought about the window size idea, or of pasting an A4 sheet paper on the side of a building or the trunk of a tree.

But because of its auto focus the n900 is usable to measure the size of small objects as well.

What does autofocus have to do with it? Zoom affects parralax calculatons, not focus. You should be able to use it as a fixed camera.

I just meant that because the camera is not fixed focus like in cheap cell phones, you can also measure small objects and not just buildings. E.g. lets say that you are curious what is the size of the font in a book you are reading. With the application that I have in mind, you pull out your favorite coin and put it on the page, take a snapshot with the N900, calibrate the pixelsize manually (or automatically) by locating the coin, and then measure the font size from the image.