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I got a used pentax digital spotmeter and found its reading one stop over that from canon 1v in spot mode(85/1.8 lens). Target is evenly lighted wall. Changing the lens to 50mm, 200mm doesn't affect the camera reading. My friend's eos5 in spot mode gave same data as my 1v. I also found that camera meter seems to be more sensitive. At very dim situation camera meter can still give a reading, for example, 3sec f2.8 at ISO200 but pentax meter just gave zero EV. Is this pentax meter defective?
Is it possible that one uses 18% grey as reference while the other uses 13%? Is there any other easy way to check the accuracy of a used meter?
I have a Pentax Digital Spotmeter, and it agrees within reasonable limits with my other meters. But I have found that it is sometimes hard to compare a spotmeter with another meter. You have to find a truly uniformly lit surface and get both meters sufficiently close to it so the readings are entirely from the surface. Sometimes you find that a spotmeter will show some significant variation over what looks like a uniformly lit field. I don't know what the angles of view of the meters are but they are unlikely to be as small as that of the Pentax.
You should also check the Pentax against other meters over a wide range of values. If there is something wrong with the meter, I would guess you won't get consistent differences. If you do get consistent differences and you believe the other meters, it is easy enough to compensate.
Do you have a fresh battery in the Pentax?
Finally, you should test the meter by taking some bw pictures containing carefully measured different intensities as shown by the spotmeter, develop in your standard way and examine the negatives. You should do that anyway to calibrate your equipment, but if the meter is a full stop off, that should be apparent in the lowest values that register.
It is entirely possible the meter needs to be calibrated. There should not be such a significant discrepancy between the two. If the difference is consistent for various lighting conditions you can probably get by making the compensation manually, otherwise send it to an authorized repair facility for CLA.
As for low light - the Pentax meter's sensitivity is EV 1-20, the 1V's meter is EV 0-18 so what you describe makes sense.
Guy
First, your spot mode in the slr is at most 3 degree (most are 5 degrees)where as the area of the Pentax is 1 degree. And the slr still averages the light from the area it sees. And an 85mm lens lets in a lot of light. And you are comparing apples and oranges. EV is not directly equivelant to fstop. Your measurement of 3 secs at f2.8 for iso 200 film is not the same. What did the 0 ev give you as exposure and fstop for that speed film? And do you know what your slr was calibrated at? Like Aaron wrote, 18% or 13%? Yes your Pentax might need to be recalibrated. But maybe your slr's meter is off. Testing is the only way to find out. Compare your Pentax against a couple other spot meters and your slr again other slr's.
There shouldn't be alot of difference but it could be your lens also. Have your lens bench tested and you might see the difference falls there. Sometimes, lens can differ from 1/2 to 1.5 stops in accuracy.
This is followup to my original post. I found the difference of the exposure value between the pentax spotmeter and my camera meter depends on the light source. Under fluorescent light, pentax spotmeter is one stop over but under incandescent or sunlight, difference thrinks to 1/3 stop. Can anyone help me confirm this?
Aaron, while I cannot confirm your results exactly, every meter has a different spectral response. That means it's sensitivity depends on the color of the light. While it seems more or less white to our eyes, especially fluorescent light can show significant differences, since it has a discrete spectrum. You will probably also see major differences between your meters at the red end of the spectrum or if some infrared light is present.
Unfortunately only few manufacturers provide data on the spectral sensitivity of their meters. Some, like Sekonic, even refuse to provide that data on request, claiming it a trade secret.