Not matching light meter values

BrianShaw

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Fatih Ayoglu said:
Camera, right in the middle of the level curve. Digital Sekonic is not too far of, Old Sekonic is 1/3 EV darker.

Pentax Digital Spotmeter which I thought would be the most accurate is 1 EV darker, which is now a bigger problem for me.
Click to expand...

Earlier there were hints alluding to apples versus oranges comparisons based on meter differences. Perhaps that is especially true with the Pentax Spot Meter at close distances. A 1-degree spot at close range is very small; much smaller than a general-coverage meter and might be more sensitive to surface irregularities and the like. Before giving up on it why not test the spot meter at “normal” distances and see if it still appears to read wrong?

And I’m sure that you have checked this but asking anyway… the Pentax is set to 200 like the other meters, right?
 

RalphLambrecht

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Fatih Ayoglu said:
Sure but not in UK. It is constantly cloudy and grey. Yes I know it is bit like a huge diffuser but I am not good at calculating that.
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You've got the same sun! You can just wait for a clear sunny day to calibrate your meter. I lived in Essex and I know you have those days too.
 

BrianShaw

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… give or take latitude and seasonal variations.
 
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Fatih Ayoglu

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Yes the ISO was set correctly. I will hopefully test this tomorrow outside where I will be a bit more careful. It might be also for some reason or by somebody Pentax Spotmeter was altered.
 
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Fatih Ayoglu

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ok so I have done more test in the house by the window, I guess using the card is sensitive of the angle of the sun vs the measurement like the instructions of the card. If I really pay attention to that, Digital Sekonic is more or less the same with Pentax Spotmeter. It's not like you can hang the card on the wall and keep taking measurement perpendicular to the card every single time (well as long as the sun is not directly behind you)
 

BrianShaw

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This study that you have done proves a very important point: there are many factors affecting light measurement.
 

Bill Burk

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So for example, with no slide this was your old Sekonic reading?
 

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Bill Burk

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Right. That must’ve turned when I held it up for the picture.
 

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Bill Burk

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I would recommend an 0.2 ND filter. Put it over the spotmeter lens when aiming at gray card that is 18% gray. The 0.2 density will bring down the reading from the spotmeter to where you can use that reading directly.

There are a couple on eBay around twelve bucks if you’re not picky about the ring size.
 
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Fatih Ayoglu

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Is this the general way how to use Spotmeter to test with gray card or is it because mine is the problem?

I’ll check eBay, I believe the filter diameter is 40.5mm
 
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Bill Burk

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Nobody else cares enough to put an 0.2 ND on their spotmeter. You would be the first. You could get any one that’s big enough and just hold it up in front of the meter.
 

Philippe-Georges

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Bill Burk said:
Right. That must’ve turned when I held it up for the picture.
Click to expand...

Please do correct my if I and wrong, and it is a so log time I haven't used my Sekonic lightmeter (I putted back in it's box years ago), but is the dome fixed correctly?
Shouldn't the white dots only be aligned when putting the dome on and then turn it clockwise to fix it?
I know, there are no electric contacts in the dome, not like the first model Variosix F, but the dome might not be parellel over the cell, or doesn't it matter?
 

Philippe-Georges

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Bill Burk said:
Nobody else cares enough to put an 0.2 ND on their spotmeter. You would be the first. You could get any one that’s big enough and just hold it up in front of the meter.
Click to expand...

Or cut a round pice out of a polyester foil 0.2 ND filter (LEE?) and attach it the ADOX way...

BTW: didn't anyone tried a smartphone lightmeter in this comparison? I have the LUXI dome attachment/app for my iPhone and it is a good backup.
 
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Bill Burk

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Philippe-Georges said:
Or cut a round pice out of a polyester foil 0.2 ND filter (LEE?) and attach it the ADOX way...

BTW: didn't anyone tried a smartphone lightmeter in this comparison? I have the LUXI dome attachment/app for my iPhone and it is a good backup.
Click to expand...

Sure LEE filters probably would do the trick.
 

Bill Burk

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You’re right the dots are in line to remove the dome, and you would want to twist to lock it. Doesn’t affect reading though.
 

Bill Burk

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Darn it. It’s an 0.15 ND filter that we’d need for the job of matching a spotmeter aimed at an 18% gray card to an incident meter.

A half stop filter is needed. They don’t make those. The 0.2 ND will be close, but 1/6 stop too far. And the point of the exercise is to demonstrate a theoretical exact correlation, so that from all the different readings you can determine which is the properly calibrated exposure meter.

Edited
—Card is bad forgot pi—

—Card is bad forgot pi—

The oldest Weston’s had all the footcandle numbers, all the f/stops and all the shutter speeds marked in third-stops on the dial. I heartily recommend getting one of these for the dial. The Weston Master III has ASA calibration. Earlier ones are off 1/3 stop from ASA (160 = 200 in this example).


Using the numbers on the Weston dial (and transcribed to the card for illustration) you can figure what would be reflected at what percent reflectance based on what’s incident on the card.
 
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Bill Burk

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The 3x5 card incident footcandles column is wrong.

The incident light is pi times what I’ve shown. Will fix that.

It was easy enough to see on the back of an old gray Gossen Luna Pro meter.

 

Bill Burk

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Relationship between incident footcandles as indicated on analog Sekonic. And reflected footcandles as indicated on Weston (after correcting Weston speed to ASA)

Edit
—Card is bad, pi was the wrong factor. It did not check at “sunny 16”—

 
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RalphLambrecht

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BrianShaw said:
… give or take latitude and seasonal variations.
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that makes surprisingly little difference. I've tested this over the last ten years multiple times and thanks to dozens of international business trips and family vacations all over the globe. Of course, a big black cloud right in front of the sun makes a difference, but every season has a clear bright day now and again. In such a case, hardly influenced by the time of day, a clear sun always measures EV15. He sun is a super-constant light source and well-suited to calibrate a lightmeter(or check it for accuracy).
 
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Fatih Ayoglu

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I’ll try as soon as rain stops and we have a clear blue sky. I assume sun should be behind me and fully exposing the light meter some.
 
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Fatih Ayoglu

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Philippe-Georges said:
Or cut a round pice out of a polyester foil 0.2 ND filter (LEE?) and attach it the ADOX way...

BTW: didn't anyone tried a smartphone lightmeter in this comparison? I have the LUXI dome attachment/app for my iPhone and it is a good backup.
Click to expand...

I have bid for one on eBay, lost it. The reflected meter of old Sekonic is not too far from Lux iOS app. I just need to angle it correctly not to have sky as the old Sekonic is seeing a much bigger area than my iphone
 

Bill Burk

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I might have bought the 0.2 ND filter you were looking at.

But really 0.15 is what we need.

I think you could make your own. Get a UV filter for the spotmeter (that’s bound to be cheap and easy to find).

Sputter a little black spray paint on it until it changes the reading exactly a half stop.

Then when you are comparing meters take the incident reading and set the same speed, f/stop, shutter on the spotmeter dial. Try to get that reading on the spotmeter needle.

If you don’t want to mess with filters you could mess with numbers instead.
 
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Fatih Ayoglu

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I meant I’ve bid for a Luma iPhone light meter on eBay,

@Bill Burk what I’m trying to understand is; is my spotmeter un calibrated or is spotmeter reading 18% gray card differently than other meters?

TBH I can just use 1/2 EV slower ISO each time instead of using ND filter. Much easier
 
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