The response of a Pentax Digital Spotmeter - with a nominal spot diameter of 1 degree - was measured using a high intensity fibre bundle light source against a black background at a range of 4m. Angles were measured using a scale on a Gitzo tripod to which the spotmeter was rigidly attached using the mounting screw on the base. Small angles were estimated using the 1 deg circle in the spotmeter field. Almost the full dynamic range of the meter was covered and a background level of 1.3EV was subtracted.
A Gaussian fit to the central part of the PSF gives a FWHM of 1.3 deg.
data
file PSF
plot, linear scale PSF
plot, EV scale
This is an Excel spreadsheet (SYLK format) which allows the calculation of exposure correction factors for extension tubes with lenses of different focal lengths. The lens focal length and the extension tube length can be entered and the magnifications and exposure correction factors are calculated for different lens focus positions.
The table from the Hasselblad extension tube set (Table part 1 & part 2 ) and the explanatory text are given here as well.
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