Help me figure out what's wrong with my light meter?

dcy

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After some nudging from this forum, I have acquired a handheld light meter: a Gossen Luna Pro F. However, I'm not getting sensible results from it. I am hoping that this is just user error.

Batteries: I inserted a 9V battery that is not "fresh" from the store, but has been stored under good conditions (20°C , low humidity) for 1 year and my battery tester says it's close to full charge. Furthermore, when I test the battery with the light meter's own internal battery tester, that too says that the battery is fine (the needle quickly goes to the "Over" side of the scale).

Usage: I watched this tutorial by the Film Photography Project. Here is a link to the manual.

Zero Position: As per the manual, I checked that when the meter is off and horizontal, the needle is covering the green line. I adjusted the needle by the tiniest amount using the adjustment screw on the back.

The Test: I walked outside my office under beautiful sunny conditions --- precisely the kind where you should be able to use the Sunny 16 rule. I picked a scene and first checked it with the light meter app on my phone. The app recommended shooting at ISO 200, F/16, 1/180" --- confirming the Sunny 16 lighting conditions. Then I grabbed the Gossen light meter, I set the ASA to 200, I moved the plastic orb thingy to the side so it's measuring reflected light --- so it should be doing the same thing as the phone's light meter --- and pressed the button. The reading is way off. It says I should shoot at something like F/8 and 1/4". The images below show both my phone's light meter app (including an image of the scene) and the Gossen light meter.

Worse yet, the readings are inconsistent. While writing this message, I decided to go back outside and try again, and now it reads the scene even darker than before. Now it's telling me to shoot at F/2 and 1/4".

Any idea what's wrong?


 

BobUK

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Instead of pointing the camera at the scene with lots of dark shadow, and bright skies. Try pointing the meters in turn at a bland scene. Such as a sheet of news print taped to a wall. Or if possible a plain dull painted wall.
The meters may be getting confused because of different angles of view between the cell phone sensor and the Gossen meter.

The cell phone image does not appear to display if you are in incident or reflected mode. Check both phone and Gossen are set in the same mode.
 

mshchem

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Either you have a bad battery or a defective meter. I gave up on Luna Pro meters 40 years ago. Elegant devices, but I suspect this needs service. Like I said it's been decades. I hope you can get it going.
 
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dcy

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Tried a bland scene. Doesn't help.

The phone light meter doesn't have a mode setting. It's just using the phone's camera, so it can't really be anything other than reflected mode.
 
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dcy

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Chan Tran said:
Sorry but I must ask. Is the meter in ambient or flash mode?
Click to expand...

No no, I'm glad you asked. I think you got it!

I think I misunderstood the flash setting and I had it backwards. I thought that if the button is "up" then it's normal mode, and "down" is flash mode. So I had it "up". But I just toggled the button and the needle shot to the right.

I cannot test the same conditions anymore (sun has gone down) but I tested again on the hallway, and now the Gossen meter is just over 1 stop away from the camera's light meter. The Gossen meter says to shoot at F/8 and 1/15" for ISO 200, while the app on my phone says to shoot at F/8 and 1/6". Hopefully the 1.3 stop difference can be explained by the two meters weighing the scene differently.


 

MattKing

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You are not the first to make that mistake!
Try the comparison in better/more illumination, before you draw any conclusions.
 

mshchem

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Glad to hear you figured it out. I was watching the video tutorial, the fellow had to go back and release the flash button. The beauty of an all analog meter! Good shooting!
 

wiltw

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Or the phone light meter simply is not right! https://www.photrio.com/forum/threads/stability-of-sunny-16.116696/post-2906014
And the sky affecting the phone reading is often brighter than Sunny 16. https://www.photrio.com/forum/threads/ettr-expose-to-the-right.214791/post-2923931
And sometimes the ambient light is closer to Sunny 15.5 or Sunny 16.4 https://www.photrio.com/forum/threa...shooting-color-print-film.184916/post-2888518

I have and old phone and a current phone, and they do not agree with each other (in spite of the fact that they are simply successive phones from the same manufacturer, both using the same app), and neither agree with a know-accurate Minolta light meter that agrees with my dSLR meter.

...all of which can mislead your conclusion about the so-called inaccuracy of your Gossen. Expose some color transparency film per the Gossen, or test against a known good photographic meter, not a phone 'meter'.
 
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dcy

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Well... The whole reason for buying a light meter is the implicit expectation that it will perform better than a phone. When it arrived, I tested it and I could tell intuitively that the readings were way off. I waited for sunny conditions to test again with better lighting. I reported Sunny 16 and the phone as two independent measures --- my subjective assessment + the phone.

As limited as a phone meter is, it should not have a 7.5-stop discrepancy with the Gossen. So it is good enough to detect a cathedral-sized error in the Gossen. Especially if the phone agrees with what I judge with my eyes.

There was a recent thread that started out with me asking about color films, and ended with some encouragement that I avail myself of a handheld light meter. I am happy to report that in that thread I did not mention my phone app because I understood that it's not really a replacement for a real light meter.
 

MattKing

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We express concerns about the cel phone metering apps not because it is impossible to encounter a reliable and accurate one, but because it is difficult to tell whether the one in your hand is one of those, in the circumstances you are using them.
Particularly because the apps are designed to be usable on a wide variety of cel phones.
So if you have one, and reliance on it results in well exposed negatives or, even better, transparencies, or alternatively the cel phone metering app you are using closely matches another reliable metering tool - one that gives reliable and accurate real world results - then it then becomes reasonable that you also use the cel phone metering app both as a tool that supports your photography and as a comparison for another meter.
Every meter you were considering when you began considering them, was designed to be tested in real world conditions by users exposing film, and failure to provide good results when used by knowledgeable and experienced film users would have led to a market failure. But every meter you were considering also has spent many years out there, possibly being misused or exposed to poor conditions, so they need to be checked.
And that was what you have done, and while doing that, learned something important about the way to use your meter - well done.
We aren't concerned about you coming to the conclusion you did about the initial multi-stop discrepancy between the metering app results and the initial test of the Gossen meter. Our words of caution were prompted by your reaction after you figured out how to switch the ambient/flash switch, and then attempted to draw a conclusion about the much smaller discrepancy between the metering app results and the Gossen reading.
FWIW, as your familiarity with film exposure grows, so will your ability to look at a meter reading (in many lighting conditions) and say to yourself whether or not that reading makes sense and is likely to give good results. Some of that comes from familiarity with Sunny 16 and the like, but much of it also comes from just becoming more familiar with light levels.
Which is a backhanded way of saying I wouldn't have needed to check the clearly wrong initial reading against a cel phone app - I would have known it was wrong immediately.
Not so with the much more difficult low light reading.
 

Chan Tran

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I for one don't trust the phone but since the OP tested under sunny 16 condition and although the scene has influence the error was too large. That's why I thought about the flash and ambient mode.
 

BrianShaw

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Nice meter, and a great price. Congratulations!

One of the things to remember when testing/using handheld meters, especially when comparing readings between different meters, is that the specific angle of view and coverage pattern may not be the same. In fact, it's often an unknown. That's why sone folks suggest using a blank neutral wall or a gray card to do the testing... to mitigate that unknown. Another time-honored way to both test and use is to point the meter down a bit to exclude the sky. Generally that gives more consistent readings, albeit possibly a tiny bit skewed.

Enjoy the experience of learning a new photographic tool. BTW, when I looked at your first post, my initial reaction is that your phone's meter knows what it is measuring. Your light is very similar to mine.
 
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dcy

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MattKing said:
Our words of caution were prompted by your reaction after you figured out how to switch the ambient/flash switch, and then attempted to draw a conclusion about the much smaller discrepancy between the metering app results and the Gossen reading.
Click to expand...

I'm confused. What reaction are you talking about? What conclusion? I wrote only one sentence after I reported the small discrepancy:


"Hopefully the 1.3 stop difference can be explained by the two meters weighing the scene differently."


That is not a conclusion. It doesn't really say anything. I feel that you are reading something into it that I never wrote, and I can only guess what that might be.


MattKing said:
Which is a backhanded way of saying I wouldn't have needed to check the clearly wrong initial reading against a cel phone app - I would have known it was wrong immediately.
Click to expand...

You've made the incorrect assumption that I needed to check the clearly wrong initial reading with a cell phone app.

I used the app for the purpose of posting it here. When asking for help diagnosing a problem, it is best to be a bit thorough. For all you know, I might be awful at judging lighting, or maybe I do not understand Sunny 16. Had I simply said that the conditions were Sunny 16 ("trust me bro"), there might have been unproductive tangents about the conditions. The app was there to prove to you, the reader, what the conditions were.
 

djdister

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If doing a test, for reflected light point it at something middle gray that is within a foot or less to the meter. But you should also use the meter in incident metering mode with the white globe over the metering cell, and meter appropriately for an incident metering. When testing for sunny 16, stand with the sun at your back, the meter in incident mode pointing right back at your face. If testing in incident meter in full sun and you don't get within 1/2 stop of sunny 16, then the meter is no good.
 

wiltw

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dcy,
You appear to be on the defensive. What many of us posted were pointing out cautions about very common assumptions that people can make in photography, which can often be proven to be wrong rather than universally true. Awareness that these are not always true...
  1. Sunny 16
  2. blue sky brightness follows Sunny 16
  3. cell phone meters are accurate (or even consistent with each other, once installed but used without calibration adjustment)

So that you have eyes wide open about the potential wrong conclusions in continued blind belief about the 'rules', not to accuse you of anything (thereby putting you on the defensive). That, and pointing out techniques to neutralize individual things that can mislead one.
 
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wiltw

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MartinCrabtree said:
Gray card?
Click to expand...

One place to start, but one needs to have the awareness that the angle of the gray card's surface to the light source can strongly influences its 'apparent brightness', as illustrated here (shot against a background to illustrate compartive brightness to the surrounding scene, using a fixed exposure to record all shots)



...and neutralize the issue of meters having different angles of view (or even an evaluative meter biasing certain zones) by always filling each meter's angle of view with the card.
That your Gossen is not right is not disputed...the QUANTITY of error could be different from your conclusion. Film testing, in the absence of comparison against a KNOWN ACCURATE meter is needed. The error of usiing the meter in flash mode rather than ambient mode has already presented itself.
 
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Chan Tran

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Your first post surely to me that something is wrong with the Luna-Pro F and as you found out it was in the wrong mode. Now that the readings are closer to other meters you need to do a much more careful check to see if it's correct. For Sunny 16 condition it's much more reliable to check using the incident mode than the reflective mode. If one of the mode is accurate pretty much it's accurate for the other mode as I don't think you can calibrate the 2 mode separately.
 

djdister

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Alternatively, get a meter that doesn't take any batteries. I only use this one in incident mode, and have a separate 1 degree spotmeter when the situation calls for it.

 
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