Metering confusion

I was about to do some film speed testing, and thought I would compare my two light meters prior to doing so. I set up my gray card and metered with my Pentax spot meter. I got an EV of 7, and so a shutter speed of 15 secs at F/5.6.
My Sekonic digital meter showed an EV of 8.5, close to a 30 sec exposure at F/5.6. Other readings were similar, within 1 to 2 stops.
However, using the example I gave, I set the Sekonic to give me a shutter speed based on an F stop of 5.6, and it gave me 15 secs, what the Pentax gave me.
Can't think what I may be doing wrong. Perhaps the EV function on the sekonic is not functioning correctly?

Okay, I figured it out. I am the one thats faulty. I thought both meters should give identical EV's for the same scene. Apparently not. I was taking the EV the Sekonic gave me and dialing that in using the Pentax wheel for a shutter speed. Hence the difference. An EV of say 7 on the Pentax gives a shutter speed of 30th at F/5.6, so does an EV of about 8.6 from the Sekonic.
Now the dam clouds have rolled in and will have to try the speed test tomorrow.

When the sun reappears, let us hear more about your tests.

BTW, a 1-2 stop difference makes me curious whether your Pentax or Sekonic is the more accurate. Or perhaps they’re both off? (That’s the bane of uncalibrated meters; one can’t be sure, even if it’s new and just out of the box…)

But since you have a grey card, a simple “Sunny-16” test might tell you more…

If tomorrow is a clear, sunny day, meter your card (in this light) with both meters. Let’s say you test with an ISO-100 setting. According to the famous rule, one already knows the appropriate exposure should be very near 1/125 sec @ f/16 – or an equivalent combination.

Perhaps the Pentax or Sekonic will be closer to this result…

Thanks Heroique. I may not have made myself too clear in my second post, but it seems both meters give me the same F stop / shutter combination, but each meter gives a different EV value. Pentax gives an EV of 7, and a corresponding combination of 8th of a second at F/5.6
The Sekonic gives the same combination but arrives at an EV of 8.9.
Thats what threw me.
I will do what you suggest though.
I also tooks a few shots with the digital camera, and the exposure looks spot on.

Yes, that would perplex me, too.

My Pentax digital spot meter calculates EVs with an ISO-100 assumption. I presume that’s for convenience, because EV = LV at ISO 100.

Maybe your two EV readings are different because the Sekonic is designed differently…

There was discussion several years ago that may account for the differences you noted. As I recall Sekonic meters are calibrated to a 12% (or was it 15%?) gray while Pentax is calibrated to 18% gray.

On my Sekonic (L558R) there is a means to calibrate the meter to compensate for that difference. I don't know that other Sekonic meters have this capability...they may have or may not have.

I have owned and used the Pentax digital as well. I went to the Sekonic because I now use incident metering with BTZS.

Best regards,
Donald Miller

Coops,

When testing in sunlight using the 'sunny 16' rule, be sure that there is no glare off the grey card, as this will skew the results.

When I have done testing, I pick a day when there is a consistent overcast. Yes, it results in longer exposure times, but the results are more consistent, and I don't have to worry about glare.

-Preston

Reflected-light meters aren't calibrated to percent reflection. The calibration isn't based on a gray card, either, but rather goes back to the average reflectance of typical outdoor scenes. If you really must know, I discuss this in more detail here (PDF).

There is a slight difference in calibration between Sekonic and Pentax: the former uses K = 12.5, and the latter uses K = 14. (note that these values have nothing to do with reflectance). But the difference is only 1/6 step, which can't explain the difference you're seeing. One possibility is a difference in spectral sensitivity (I don't know what the Sekonic response looks like, but the Pentax, especially unmodified, has a very broad response, far exceeding that of any film). But I wouldn't expect large differences when metering a neutral surface.

What's really surprising is getting the same combination of f-number and exposure time for significantly different EVs--this cannot be. The relationship between EV and camera settings is quite simple; I discuss it in the paper above, and it's also described fairly well in this Wikipedia article. Is there any chance your meters are set to different ISO speeds?

On thinking about it, there could be a much simpler explanation: the Pentax always indicates EV for ISO 100, and I think the Sekonic indicates the EV for film speed you have set.

Quote:

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Originally Posted by Jeff Conrad

What's really surprising is getting the same combination of f-number and exposure time for significantly different EVs--this cannot be. The relationship between EV and camera settings is quite simple;

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Well I just metered a card outside with each meter set to 400 iso. The pentax read 10EV's, the Sekonic 11.8. Each meter gives an 8th second exposure at F/22
Several other readings gave the meters identical F stop/shutter combinations, with the Sekonic giving a higher EV of about 1.8
Must just be calibrated differently, but good to know they give the same exposures

Both meters have essentially the same calibration--the indicated EVs differ because the EV indicated by the Sekonic changes with ISO speed, while that shown by the Pentax does not. EV 10 for ISO 100 is the same as ISO 12 for ISO 100. Strictly, from the calibration difference, the expected reading from the Sekonic would be 0.17 step greater (or about 12.2 in this case), but both meters are probably within calibration tolerance.

A more interesting test might be to read a blue object and a red object with each meter and see how the meters compare; I've never tried this with a Sekonic meter, but Pentax and Minolta (and presumably now Kenko) meters readings usually differ because of the differences in spectral response.