Need advice for Spotmeter calibration

Oblidor

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Hi

I have a Minolta Spotmeter F. I recently bought a Sekonic Litemaster L-478DR. Now that I can meter with both, I notice that my Spotmeter seems to underexpose one stop or there abouts compared the the Sekonic and digital cameras. It has a small adjustment regulator in the battery compartment. So I thought to try to calibrate it. However, I need some controlled environment. I was wondering if it made sense to use my Kaiser light table and use the Sekonic to measure the light from it inside a tube (not to get light from surroundings and then measure with the spotmatic in the same way. I could use ND filter to get multiple measuring points.

Is this a way to do it, or is there a better way? I am also to modify some of my cameras with Schottky diods to use SR44 batteries, would need a method for calibrations for those too.

Any hints much appreciated.
 

Chan Tran

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You would need the 5 degree spot attachment to make the comparison because without it your 478DR doesn't do reflected light reading. You can't not compare reading from an incident to a reflected light meter (which the spotmeter F is) without using something like a good gray card and the reading angle is critical so it's not very reliable.
 

koraks

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@Chan Tran makes a valid point.

I'd be inclined to take any modern camera that has a spot/partial meter (can be digital) and then use that as a benchmark for the old Minolta. I generally use my digital Canon SLR for this.
 

benjiboy

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A man with one clock knows the time, if he has two or more he's never sure
 

Chan Tran

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benjiboy said:
A man with one clock knows the time, if he has two or more he's never sure
Click to expand...

Nothing is perfectly accurate but you should know for sure within a tolerance. You can have large or small tolerance depending how critical you are. If you have one clock and you believe your clock is absolutely accurate then you're absolutely wrong.
 

Ian C

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Incident readings and narrow-angle reflected-light readings (spotmeter) readings are almost never the same, even when using the SAME meter. This assumes that both meters are in good condition and close to their original adjustments from the maker.

For example, I use a Minolta Flash Meter IV with and without the 5º spotmeter attachment. I also use a Sekonic L-508 which has both functions built-in.

In most situations, incident and reflected reading don’t agree (but in some situations they do). There is nothing wrong with either meter. There are many factors influencing this: the tone and reflectivity of the surface, its angle relative to the main light source, the angle relative to the meter’s lens or opening to the subject surface, and so forth. Of course, these play no role with incident-light readings, which are inherently more accurate and consistent.

The price of the meter doesn’t seem to matter. Both of my meters were expensive and highly regarded by professional photographers. Provided that there is nothing out of adjustment with your meter (which is probably the case), making any adjustment is unwarranted and will most likely mis-adjust what is a correctly working meter.

The following is a link to the user’s manual for Kodak Gray Cards. This clarifies their use.

https://docs.acescentral.com/assets/pdfs/kodak-grey-card.pdf
 

ic-racer

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Calibrating a spot meter with a "Personal Exposure Index" is as old as the zone system. Look up "Zone I Test" or get a copy of Fred Picker's Zone VI Workshop.
 
OP
OP

Oblidor

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I see. Thanks all

I thought that if I had a fixed light source and pointed the incident meter towards it, it would read the same as if I use a 1° spot meter at the same light. But I guess that depends on how the meter reads.

I'll try to compare digital camera to the spotmeter reading the Kaiser lighttable so see if it is good enough or not.
 

wiltw

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You could use an 18% grey card and meter that with your spotmeter, and compare that reading with incident light reading at the grey card taken with your Sekonic.
When I just did that with my Minolta Autometer Vf and Minolta Spotmeter F, the incident meter yielded a reading which was +0.1EV compared to the spotmeter.
 

MattKing

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If you point a reflected light meter at your light source, it will give you an incorrect reading.
The reflected light meter design assumes a certain average subject reflectance, and measures the light reflecting off the subject you are pointing at.
An incident meter actually meters the light hitting it. Frequently that meter includes some method of integrating the light from several directions - a dome, as an example.
 

Sirius Glass

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The only way to get the two meters to agree is to calibrate them to a know source. Since most of us have the resources, equipment and training, the most expeditious way to do that is to send them to a qualified facility that with the resources and staff, a laboratory that specializes in calibrating light meters. That would save time and money, and allow you to go back to doing what you do best, taking photographs.
 

BrianShaw

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Is there is a “don’t” missing?

Where, pray tell? This advice keeps getting repeatedly repeated without mention of who this qualified facility might be. Generally correct but not implementable without further guidance.

It’s likely that I’m not the only person who is truly interested.
 

Taylor Nankervis

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How old is the Spotmeter F, compared to the Sekonic (the newer) L478DR? My guess — quite a few years, and drift in the accuracy of the older (or any) meter could be expected.

One test is a 'Wild West Shootout' between the two meters: use slide film, both meters set to 0.3 steps.
Meter and shoot the very same scenes (in lighting conditions that favour slide film) take detailed notes and make row-by-row comparisons from the results on the lightbox. Using handheld meters with negative film does not necessarily mean faults will be evident (with the exception of metering technique!) because the latitude of those films are far more forgiving. If one of the meters is dramatically faulting, slide film is likely to show that fault very plainly and then you can start pointing fingers and work from there.

A side tidbit...With Sekonic meters, there is a known (deliberate) variance between the incident metering value and the reflected/spot metering baseline values, which makes them perhaps not so much a first choice/go-to for comparing accuracy across different meters (ref: correspondence with Sekonic product engineer, August 2013).
In the case of the Sekonic L758D and variants (incident/multispot), this variance is 12.6% for the reflected spot and 16.8% for the incident; it's possible your own Sekonic has a variance like this even as a single-method meter (incident). The variance does not mean the meter is inaccurate in any or either method of metering.

What is a schottky diod???
 

RalphLambrecht

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If you have one meter, you always know what the exposure is. If you have two, you can never be sure. However, you can try this: read a
Graycard in bright sunlight. It should read EV15 at 100 ISO between 10 am and 4 pm.
 

koraks

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Taylor Nankervis said:
What is a schottky diod???
Click to expand...

"Schottky diode" - it's a diode, which is an electrical component that passes DC in one direction, but blocks it in the opposite direction. A Schottky diode is generally fast, and more importantly there's a small voltage drop over it in the pass direction. In this case, this voltage drop is exploited to bring down the voltage of e.g. an LR44/SR44 down to about the voltage of a PX625 battery.
 

Chan Tran

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RalphLambrecht said:
If you have one meter, you always know what the exposure is. If you have two, you can never be sure. However, you can try this: read a
Graycard in bright sunlight. It should read EV15 at 100 ISO between 10 am and 4 pm.
Click to expand...

All my meters read EV 14.7 or 14 and 2/3 at ISO 100.
 
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Chan Tran

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Now that I think about it perhaps the different is the way you meter. If you use the spotmeter and point at the face of a light skin person the exposure would be about1 stop less than what an incident meter would be.
 
OP
OP

Oblidor

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Chan Tran said:
Now that I think about it perhaps the different is the way you meter. If you use the spotmeter and point at the face of a light skin person the exposure would be about1 stop less than what an incident meter would be.
Click to expand...

Problem I measure a stop more, not less.
 

Chan Tran

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Oblidor said:
Problem I measure a stop more, not less.
Click to expand...

I said the exposure is 1 stop less because the meter reads 1 stop more. For example if you use the incident meter and it reads f/8 and you use the spotmeter on a white person face it reads f/11 and thus you will expose your film 1 stop less so you get an underexposure.
 

wiltw

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Chan Tran said:
Now that I think about it perhaps the different is the way you meter. If you use the spotmeter and point at the face of a light skin person the exposure would be about1 stop less than what an incident meter would be.
Click to expand...

It has long been the rule of thumb to measure the PALM of the hand, which is much less subject to variation due to sun tan or racial skin color, and the rule of thumb about the palm is +1EV.
Metering my own palm and comparing it to an 18% grey card on separate occasions in the past, I typically measure difference of +1.3EV. But in reviewing my posts on the subject in a span of years, I see that the range has been +1.0 to +1.5EV compared to an 18% grey card.

From an old POTN post of mine in which I quoted Kodak on the topic, in 2010 (the link stopped working a number of years ago, now that Kodak is a shadow of itself):​
​
"Substitute Readings​
What if you can't walk up to your subject to take a meter reading? For instance, suppose that you're trying to photograph a deer in sunlight at the edge of a wood. If the background is dark, a meter reading of the overall scene will give you an incorrect exposure for the deer. Obviously, if you try to take a close-up reading of the deer, you're going to lose your subject before you ever get the picture. One answer is to make a substitute reading off the palm of your hand, providing that your hand is illuminated by the same light as your subject, then use a lens opening 1 stop larger than the meter indicates. For example, if the reading off your hand is f/16, open up one stop to f/11 to get the correct exposure. The exposure increase is necessary because the meter overreacts to the brightness of your palm which is about twice as bright as an average subject. When you take the reading, be sure that the lighting on your palm is the same as on the subject. Don't shade your palm. "​


At 9:30am today my incident meter says EV14.6, but I also have posted in the past about measurements taken of EV15.2 (Fall) and EV15.7 (Summer)
And the camera-to-subject angle relative so the sun's position also matters, as evidenced by the 360 degree angle variation of reading which I mentioned in the linked post.
 
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Sirius Glass

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Great catch! Yes "don't" is definitely missing.

The only way to get the two meters to agree is to calibrate them to a know source. Since most of us don't have the resources, equipment and training, the most expeditious way to do that is to send them to a qualified facility that with the resources and staff, a laboratory that specializes in calibrating light meters. That would save time and money, and allow you to go back to doing what you do best, taking photographs.​
 
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