Light Metering

Hi Folks,

I just purchased a Gossen Luna Pro to use in revitalizing my interest in film photography. FYI, I purchased the correct Wein air cells to supply the proper voltage. To test it, I used a gray card and metered with the meter and Nikon FE. They came up with an identical exposure of 1/125 @ f5.6. I then took my Canon G11 and meter the same gray card reading 1/125 @ f5.0. I suspect the comparison is an apples and oranges comparison and that I should discard the digital reading. Possibly there are filters over the sensor that absorb some light?

 

What does this astute group have to say? Are their any other tests I should run against the Gossen meter? FYI, I plan to use it mainly for incident light readings. I also couldn't resist picking up a second meter on ebay for $15 and I will test that as well.

 

Mike

 

PS - I also have a vintage Nikon F that I plan to shoot with and I will test that meter when I get some more batteries.

Your third meter will probably have a different reading than the other two. You're going to drive yourself crazy.

 

I found this link to calibrate.

How to calibrate a Gossen Luna Pro

In real life the difference is irrelevant given film latitude. But in all probability there was a difference in coverage patterns accounting for the difference. Remember the meter attempts to take what it captures and turn it into approximately 18% grey, whether or not that is your intent. That is why it is so important to understand metering and its effect on exposure. A good resource is the book, Light, Science and Magic.

The difference between f/5.0 and f/5.6 is almost exactly 1/3rd of a stop, which is the recognised calibration tolerance for meters, shutters, apertures, and nominal film speeds. Forget it!

 

Where you point the meter and subject tonal content will have far more effect on exposure than a pitiful 1/3rd stop meter discrepancy.

 

BTW. Does an incident reading agree with your grey card? There should be no need to carry a grey card if you have an incident-capable meter.

 

Personally, I would trust a handheld incident meter over any 'old' centre-weighted reflective TTL meter. Plus it's film; with a latitude as wide as a barn door.

Edited July 2, 2019 by rodeo_joe|1

Besides the issues discussed above, I assume that you were meter from a very short distance. This means that the lens on your G11 may have seen a slightly different scene than the SLRs. Maybe you included a little bit of area off of the grey card. In addition, I'm sure that the meter in the newer camera may be a "smart" system which calculated a different need.

 

Of course, as also noted, 1/3 difference between meters is practically irrelevant.

Because different meters almost always meter slightly different angles of view, etc.,

you will normally have readings that do not agree. If you do about 50 readings and average them together they will start to converge on some 'norm'.

 

I usually use sunny-16 to give me an idea whether the meter is working at all - if the meter doesn't agree, I try to figure out if there is some obvious reason.....

I have the Gossen Luna-Pro SBC (uses 9-volt battery) and it is the best light meter I've ever had, but sometimes it's bigger than the camera I'm using. :|

If you are close enough, the 18% gray should cover even an averaging (wide angle) meter.

Too close, and you put a shadow on the card.

 

Averaging meters work well for average scenes, which reflect, usually not so far from, 18%.

That is why they worked well enough for years, and now still do often enough.

 

Center weighted works a little better, and spot if you have the right spot, even better.

Another ‘handy’ sanity check is to meter the palm of your hand in the same light as the subject, and open up one stop from there. Your palm reflects approximately twice as much light as a standard grey card.

I can tell you, from personal experience, that a meter with a 1 stop error will drive you crazy until you figure it out. Trust the sunny 16 rule and compare with other cameras and meters. If the new meter incident readings match other reflected readings made from a good grey card, it's probably OK.

I can tell you, from personal experience, that a meter with a 1 stop error will drive you crazy until you figure it out.

- Except it isn't a 1 stop error.

The difference between f/5.6 and f/5 is only 1/3rd of a stop, and the OP's handheld meter agrees with a Nikon FE.

 

The question really is: "Why might a digital Canon G11 disagree with two older meters by 1/3rd of a stop?"

 

All sorts of reasons, most of which have been mentioned above, but also:

1) There is no hard-and-fast ISO proposed methodology for determining the 'speed' of a digital sensor. It's a rather woolly definition - something like 'that exposure which gives the best image quality short of highlight blooming', or some equally vague definition that's open to interpretation.

2) The best exposure for a digital sensor might not place mid-grey 18% reflectance at the same level as that for film. If you have a sensor capable of capturing a 10 stop range of subject-brightness, then that can use a different amount of exposure than a 7 stop limited slide film. So if one-third of a stop more exposure (compared to an equivalent film) gives the best result, then that's how the camera's metering is going to be calibrated.

 

And 3) it's still only a very minor 1/3rd stop difference that could be accounted for in numerous other ways.

 

The question is more akin to asking why T-Max 100 is only rated at 100 ISO, while FP4+ is rated at 125.

Edited July 4, 2019 by rodeo_joe|1

Never said it was. Mine was a 1 stop error and the crazy part you can judge for yourself. I also think of digital sensors more like slide film. Blow the high end and nothing will save it. I recently got a better hand held light meter, not so much for overall exposure readings, but for lighting ratios around the subject. Absolute calibration isn't much of a concern for that.