[Soheil]

Hi, I want to test/calibrate a spotmeter. What is the best way to test the accuracy?

[steephill]

Calibration is a job for a professional with the right equipment, it isn't something the average consumer can do. You can check whether it responds appropriately to differing light sources and compare the results to other light meters you may have around, either in camera or stand alone, but that isn't anywhere near good enough for calibration.

[dms]

A quick test is to meter the blue cloud free sky well away from the sun during the summer. It should yield 1/iso seconds exposure at f/16.

BTW Dunn in the "Exposure Manual" (a standard reference on exposure) says "a practical and sufficiently accurate way to establish the right correction ..." is to meter as I said, although he does not say in summer,** and he says 1/125 s at f/8 and ASA [iso] 25--which would be 1/30s at f/16 (vs my saying 1/iso, which would be 1/25s, this being 1/3 stop different).
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** Dunn also says the sun should be above 45 degree elevation, and with this proviso it matters not what season it is.

[Soheil]

Thank you for sharing the tip. Actually I compared it with 3 digital cameras. (Nikon D7000, canon EOSm and a Fuji X100F. And surprisingly all 3 cameras’s meter are also different from each other and different from the spot meter. But the spot meter is way more different. The eos m and fujix100 are about 1/3 stop different from each other but about 1 to 1.5 different from the spot meter. The Nikon D7000 is about 2/3 stop different from the eos/m and Fuji x100 and 2/3 stop different from the spot meter. So the result was actually more confusing than I expected.

[steephill]

What were you taking a reading from and what lenses were you using? If it was a large patch of blue sky as described above then all the meters should be fairly close. However if you are using a smaller target then the focal length of the lens used will probably change the size of the spot metered section and give you different results.

[TonyW]

You do not say which spotmeter you have but it is probably best left to professional calibration as it should be tested at both low and high levels of lighting and the internals adjusted to bring it into specification. The following may be of help in understanding why you are getting different reading from your varied systems

Comparing a spotmeter with a camera you must be sure that the you set the camera to spot reading and also be sure that you know the area covered by the spot metering - it may be 10%, 20% or 30% of the image area and that area will change depending on lens used and zoom ratio etc. Your spot meter is most likely a 1 deg spot.

You should also know the manufacturers calibration point for the camera meters or work it out yourself by testing. Calibration points for meters usually fall within ISO standards ranges, for arguments sake lets say the ranges are from 10% to 14%. - there are ISO standards for camera meters and hand held meters

For digital you may be concerned to find the highlight clipping point i.e. the point after which you will have reached sensel saturation some quick indicators so you will need to find the calibration point of your metering systems:

For a system calibrated to 10% the saturation point will be = +3.3 EV from the metered ROI
12.5% = +3 EV
14% = +2.84 EV
18% = +2.47 EV Note this percentage has never been (AFAIK) any camera or meter manufacturers calibration point. Most likely point is around 12.5%, note the difference of about 1/2 stop between 18% mid grey and 12.5%

The attachment attempts to portray the difference in how the varied calibration points affect the density and also approximate positions on the camera LCD histogram display