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Puzzling change to my standard light meter reference

Theo Sulphate

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The standard way I've been doing a quick test of in-camera and handheld meters has been -- don't laugh -- the broad white counter in my downstairs bathroom.

This "reference" evolved because I'm a Night Owl and play with cameras and meters at night. Therefore, the countertop has proven both convenient and consistent.

With the lights on, a reading off this surface at ISO 100, 1/30, is exactly f/5.6. I use a Sekonic L-308s to make (confirm) the reference reading and then compare that with what some other meter gives me. Based on that, I know how I'll need to compensate using that camera or meter.

Tonight however, the Sekonic at ISO 100, 1/30, gave me f/4 (specifically f/4.09). The batteries are at full strength, the six overhead lights are all lit, I don't understand why I'm suddenly getting a 1-stop lower reading. I doubt that the overhead lights are getting dimmer, since the only time I turn them on is for this test and even then they are on for about 20 seconds only.

Ideas?
 
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Voltage affects light output. You could improvise a light source from a regulated power supply for critical work. Having an inexpensive spare light meter is more practical. Meters can also vary over time or due to internal problems.
 
Global dimming.
 
What this has taught me is not to have a single standard reference. Either use multiple trusted meters that agree (so that if the light source, however regulated, changes, then both meters will indicate that, or use the one trusted meter on different "reference" surfaces.

I am a global dimming denier!
 
It is possible lamp output fell due to the local AC mains voltage falling. The eye is a poor light meter, dimming that the meter detects might not be perceived. Regular incandescent lamps are directly sensitive to the AC voltage. CFL and LED lamps are insensitive to the voltage. There are inexpensive instruments for checking AC voltage (and current, power, frequency etc) one is sold by homedepot called killerwatt, and one of these will tell you if the AC voltage is drooping.
 
If it was cold at the time, batteries in the light meter may be putting out a wee bit less.
 

If I am not mistaken your new reading is 4.09 means f/4 and 9/10 stop toward f/5.6. That means it's only read 1/10 of a stop darker. Well 1/10 is 1/10 it could be ageing dome if you use incident mode. In reflected light mode the counter suffers from yellowing or it's your light.
 
All common lighting devices produce less light as they age. Tungsten bulbs do it faster than most others, but none are immune. Add to that dust in the fixture, possibly fluctuating current or voltage, or malfunctioning devices such as dimmers or power supplies, the meter may be the least likely source of the change. That you use that light infrequently doesn't really change any of that.
It's probably worthwhile making comparison readings using other meters to see what's what though.
 
You probably need a "secondary" reference standard for metering. I can also report that being near the outer fringe of the distribution system for my electricity supplier, I have occasionally seen voltage fluctuations in the +/- ten or fifteen percent range. It's been better in recent years, but a decade or two back they did a "tap change" somewhere in the system around 1 AM and the line voltage would change by close to twenty volts in one quick flash.

And CFLs and LEDs may be less directly sensitive, but they do exhibit some voltage sensitivity (some are even dimmable) and they do decline with age, they just take longer.
 
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I think that Chan Tran is likely right, that this is an insignificant change of only about 1/10 stop.
 
My arithmetic may be rusty but wouldn't 4.09 round up to 4.1... One-tenth more than f/4? 4.9 would be 1/10 less than f/5.6.
 
BrianShaw said:
My arithmetic may be rusty but wouldn't 4.09 round up to 4.1... One-tenth more than f/4? 4.9 would be 1/10 less than f/5.6.
Click to expand...

Well meter readings can be sort cryptic in between the standard full stops. Amost certainly this reading was in "full (standard) f-stop increments" of which f/4.0 and 5.6 would be reported, but an "accessory digit" gives the nominal tenth stop offset.

So most likely the reported f/4.09 was actually showing f/4.0 plus the accessory tenth stop digit.
 
If that's the case, it's not 4.09... But you and Chan may be right.

I've often wondered how one writes f/5.6 and 7/10s.

This is a time when two decimal points seems appropriate : 4.0.9 and 5.6.7.
 
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You are exactly correct.

I was doing more experiments today, slowly moving the light away from a brightly lit wall, and the reading goes from 4.09 to 5.6 immediately - there is no 4.1, etc.

The digital display fooled me into thinking there was a decimal progression of 4.0, 4.1, 4.2, etc. Another advantage of analog meters!

Re-reading the manual: "When set to full-stop mode, exposures are displayed in tenth-stop increments with a repeat accuracy of +/-0.1 EV."

My thanks to all for your responses. Perhaps this thread will help someone else if they search this site and have similar issues.
 
BrianShaw said:
My arithmetic may be rusty but wouldn't 4.09 round up to 4.1... One-tenth more than f/4? 4.9 would be 1/10 less than f/5.6.
Click to expand...

All Sekonic meters display in tenths. In practical application — transferring a reading from the meter to the camera, it makes negligible difference.
 
Actually it can make a difference. I use that information from my Sekonic L-558 to round up if it seems necessary, which is usually 5/10 or greater. It's very useful information if one knows how to read and write it.
 
summer demands on power lines.

i use a similar standard and find summer rolling brown outs effect light out put. the power companies hold back very slightly. they can hold back 5% n you wont notice it at all unless you measure it... and your meter is measuring the small effect.
 
the best std reference is the sun;always EV15(nice for sunny16)if a light meter differs from that,there is something wrong with he meter.the sun has been consistent for 4 billion years and promises to be just as consistent of another billion years;hard to beat
 
The Sun may be a fine standard: the atmosphere is not.
 
Jim Jones said:
The Sun may be a fine standard: the atmosphere is not.
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As soon as I can find a scan of it, I'll post a table that verifies you thesis: change in sun based on time of day and latitude and month of year. The variation is actually greater than one would expect. I have not fully validated it ( nor will I) but the sun printing corrections I've made based on the table tends to confirm its validity. Table dates from the 1880s or so... And is presumably based on experience of the day from sun printing.
 

The Sekonic like most modern meters can and only can display in 1/10 stop increments. If you said 4.09 means f/4 and 9/100 of a stop then the resolution is way to high for an exposure meter. Although the Sekonic may display 4.09 it really meant f/4 and 9/10 stop.
 
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