Fractional Measurements on a Light Meter

I am new to light meters and just bought a Polaris SPD100 and trying to figure out how to use it. I don't understand the analog scale at the bottom of the LCD screen that shows fractional measurements in 1/10's with a scale of 10.

What is this scale and do I need to worry about it when setting my f/stop, shutter speed combination? Thanks for your help.

Here is the scale.

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Without having access to the meter or the manual, I would assume that scale is showing how close the power is to the next displayable stop.
The main question here is what stops does your meter actually display? Does it just display full stops, 5.6, 8, 11...? If so than that scale would be showing fractional stops with marks for 1/3 stops. In your image it would be displaying a required aperture value of 7.1 (5.6 + 2/3 stops.)

John,
I looked at the manual which was on amazon, and it would appear that R-C-G is correct. What you have displayed in your post would be = f/5.66. The capture I show from the manual in this post would be called f/5.64. Read this way because 4 of the little bars are lit up. The bigger hash marks as RCG said are reference for 1/3 of a stop, then 2/3 stop. Once you would get up to where nine of the little marks would be lit, you'd call that f/5.69, which is 1/10 of a stop under f/8.
Not to try to confuse this, but on your Canon camera if you have it set to display in one third stop increments, then what your meter is showing as:

f/5.63 ( 3 bars ) would show in your camera as f/6.3

and a polaris meter reading of:

f/5.66 ( 6 bars ) would show in your camera as f/7.1


I hope this didn't just serve to confuse you. Let me also include a screenshot of the Sekonic meter and how it displays when set to 1/10 stop mode. You can see it is shown reading f/5.65, which your camera would display as f/6.7, because 6.7 is half way between 5.6 and 8.0. If this is all just clear as mud, just say so, and I'll try to explain it a different way, or perhaps someone else will chime in and make it simpler !

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Thanks for your responses. So what I'm beginning to understand is that my light meter is very accurate, measuring in full stops, but then 1/10 stops on the analog scale at the bottom. In essence, the light meter is more accurate than my camera, since I have it set to 1/3 stop increments. So, I basically need to pay attention to the 1/3 hash marks on the analog scale (since I cannot adjust the camera in 1/10 increments). So on the picture indicated, I would set my camera to f/5.6 and then move the scroll wheel on the back of my camera two clicks,1/3 stops (that is a total of 2/3 stop). This should make a perfect exposure. If this is not correct, please let me know. Again, thanks for your help R-C-G and George M W.

Hi John,

It may just be semantics, but I don't think "accurate" is necessarily the right word. Maybe "precise"? That is; if your meter is out of calibration, it would still display these fractional stops, but the readings would still be incorrect or "inaccurate".

Didn't want you to come away with the impression that meter readings were absolute.

You are using manual mode. With the meter reading you posted, you end up with setting the shutter to 1/125 and the aperture to f/7.1.

Also, as noted above, the meter itself can be calibrated if you think that the reported aperture is not the "correct" exposure. Before you assume that the meter is inaccurate, you need to insure that you are taking the readings properly and that the subject is not moving forward or backward after the meter reading is performed. If the lights are close to a person, even a slight tilt toward the camera after the meter reading will make a difference.

A light meter is a measuring device. As such it has to be calibrated. As with any measuring device repeatability is an important consideration.

There are two ways to calibrate a light meter. The easiest is to note that a measurement it provides is consistantly a certain amount high or low. Knowing the "factor" one can mentally make the adjustment.
The other is to find out how to adjust the reading so that it provides a reading that is accurate.

Comparing one light meter to another light meter does not deliver an accurate result. It only demonstrates the relative readings, both of which may be different than the "real" exposure.

With today's digital SLRs the best way to determine proper exposure it using the histogram, a color/exposure target and RGB value readings in post processing software such as CS3/4 or Lightroom. Then if you want the light meter to read the accurately adjusting it to match the correct exposure as determined in the software.

i.e. The white square on the XRite Color Checker should read 238 when properly exposed. Thus, if your lightmeter says F8 and you make an exposure at F8 (persuming proper ISO) and the square reads something other than 238 move the exposure slider until 238 is reached. Say the result is 1/3 stop more exposure. Now adjust the meter so that it reads F8 minus 3/10s under the same conditions.
Now adjust the light so the meter gets an F8 reading. Check that exposure in the software. the white square should now read 238.

The same kind of testing can be done with ambient light to validate the accuracy of the in-camera meter as well.

John E wrote in post #9256390
Thanks for your responses. So what I'm beginning to understand is that my light meter is very accurate, measuring in full stops, but then 1/10 stops on the analog scale at the bottom. In essence, the light meter is more accurate than my camera, since I have it set to 1/3 stop increments. So, I basically need to pay attention to the 1/3 hash marks on the analog scale (since I cannot adjust the camera in 1/10 increments). So on the picture indicated, I would set my camera to f/5.6 and then move the scroll wheel on the back of my camera two clicks,1/3 stops (that is a total of 2/3 stop). This should make a perfect exposure. If this is not correct, please let me know. Again, thanks for your help R-C-G and George M W.

Your hand held meter is more precise than your camera meter. Its accuracy is unknown without professional calibration to a known intensity light source.

I think you may be able to calibrate your meter. If it's the same polaris that i have there is a button in the battery compartment for that. See the user's manual for details. As has been said the reading along the bottom is in fact a 1/10 stop scale. I would point out that the two small marks above the scale along with the two long ones at the end divide the ten tick scale into thirds. Which is the incriment the camera is set to.

Matching the meter to a known intenstity light source is in theory a good idea. I'm sure what you really want is something calibrated to your specific camera. That would be most useful. What you need to do is set up a bench test with a repeatable light source at a fixed distance from an average target. Take a meter reading and compare it with a perfectly exposed image of the test target. If they're the same then super. (In all likelyhood they won't be) If they aren't the same, then you adjust the meter till it reads the same f-stop as the camera was set when the perfect test shot was taken.

What to use as a test target?

See my sig...

Thanks to everyone for your input! My original question was not so much about calibration as it was trying to figure out the hash marks at the bottom of the screen. With your help, I think I've got it now! Today I had a chance to try it out and it works like a charm. Why didn't I get one of these things a long time ago. Would have saved a lot of headaches

John E wrote in post #9264561
Thanks to everyone for your input! My original question was not so much about calibration as it was trying to figure out the hash marks at the bottom of the screen. With your help, I think I've got it now! Today I had a chance to try it out and it works like a charm. Why didn't I get one of these things a long time ago. Would have saved a lot of headaches

Hi John,

Sorry. It seems I was the one who started the "accurate/precise" thought line. I was not calling the accuracy of your meter into question. But was instead suggesting to you how to think about its functions.

Glad you've got a handle on it.