Testing the lightmeter accuracy on a 35mm camera

[wiltw]

Although this discussion is quite interesting (to say the least), looking at the original problem - a built-in light meter. Why not shoot a roll of film, record the exposures used, and look at the negatives? If the neg looks good then the meter is okay. If not, you can easily estimate how much the meter is off.

^
And what you suggest is precisely the basis of the steps that I outlined in post 26. And it the basis for why folks do their testing with sensitometric measurements with densitometers to determine their personal Exposure Index with their film with a different ISO rating.

 

[faberryman]

^
And what you suggest is precisely the basis of the steps that I outlined in post 26. And it the basis for why folks do their testing with sensitometric measurements with densitometers to determine their personal Exposure Index with their film with a different ISO rating.

Do you have a recommendation for a densitometer to purchase to carry out these sensitometric measurements?

 

[DWThomas]

For comparisons between cameras and/or meters in reflected mode one can buy a sheet of gray foamcore or mat board, the latter, from some makers, even comes in a "photo gray." Is it a true calibration standard, no not likely, but the stuff comes in sizes on the order of 20x30 inches or 32x40 inches which gives enough big gray area you can hardly miss. Maybe it's because I no longer shoot color transparencies, but I don't waste too much time agonizing over this question.

Heh, I have an old Waltz Coronet selenium meter that got to the point where after sitting a few weeks the mechanical pressure contacts with the selenium cell would lose connection. I would just reach around and push the plastic honeycomb with a finger and the needle would spring to life -- and empirically, the readings worked! I used it that way for quite a few years when I was young and not rolling in $$$.

 

[wiltw]

Do you have a recommendation for a densitometer to purchase to carry out these sensitometric measurements?

Not me, as I have not ever bothered with densitometry. Someone else can offer their recommendations.

Film manufacturer ISO rating has always worked for me, with standard recommended developing times with standard developers, and that works for color films sent to pro labs I have used, so I have never had reason to question the standard ISO rating.
With digital sensors now, using a 'film lightmeter' for exposing digital has always resulted in 18% grey card depicted as mid-tone tonality, with about 48 reported for R-G-B values on a scale of 0-99, so I have never had reason to question any of my handheld meters for accuracy in reading light.
And camera meters aimed at a uniformly illuminated unfeatured wall result in 'same reading' as obtained with handheld meters. So I have never understood any statements that metering for digital is any different from metering for film.
For me, exposing transparency is very much like exposing digital...overexposure = lost detail in hightlights, and the shadows go where the shadows want to go.

 

[BrianShaw]

^
And what you suggest is precisely the basis of the steps that I outlined in post 26. And it the basis for why folks do their testing with sensitometric measurements with densitometers to determine their personal Exposure Index with their film with a different ISO rating.

Yes, it is. You wrote it first; I give you that credit! But nobody seems to be paying attention to the obvious, and there could be too much distraction with words like "densitometer, light standard, personal EI, and sensitometry" so I just tried to simplify the thought.

If one doesn't know what a decent negative or transparency looks like, there is little chance that they would know how to use the complex measuring systems/equipment previously mentioned to get to a good neg or transparency. The human eye - a photographically experienced eye - is simple and would get the job done in the easiest manner to a reasonable degree of correctness. If that eye isn't available, then the whole discussion might be a moot point.

I'm assuming this is a film camera being discussed. The simple solution is to expose and process one short roll of film. One item that the OP never mentions is what makes him/her think that the in-camera meter isn't accurate. Bad negs? A quest for absolute perfection? I'd be very interested in why the question is being asked but, perhaps, it really doesn't matter.

 

[wiltw]

I'm assuming this is a film camera being discussed. The simple solution is to expose and process one short roll of film. One item that the OP never mentions is what makes him/her think that the in-camera meter isn't accurate. Bad negs? A quest for absolute perfection? I'd be very interested in why the question is being asked but, perhaps, it really doesn't matter.

Unfortunately the meter can be perfectly calibrated, but it is the uninformed use of that meter that can lead to exposure error..and given enough shots poorly exposed can result in doubt about the accuracy of that meter. I see enough poorly exposed shots posted to know this is true that it is not the meter's fault, just misguided users.

 

[JerseyDoug]

Shooting negatives is not a particularly good test because of processing variations, the inherent exposure latitude of the film and the variations of the print or other viewing medium. Slides are another matter. The processing is consistent. Positive color film has a limited dynamic range compared with negative film. Exposure is critical. A roll of color slide film with bracketed shots (perhaps +2 +1 0 -1 -2) of a subject of average contrast under different lighting conditions will tell the story.

Myself, to check the exposure metering or anything else about a new camera I shoot half a roll of film with the subject camera, remove the film, put it in my Nikon F6*, repeat the same shots, develop the film, scan it with fixed settings of 5he digital camera I use for scanning, invert the scans with Affinity Photo but no other processing, and compare the images in the computer screen.

* The F6 has never let me down in anything but stupidly high contrast situations that should never have been photographed in the first place.

 

[BrianShaw]

Unfortunately the meter can be perfectly calibrated, but it is the uninformed use of that meter that can lead to exposure error..and given enough shots poorly exposed can result in doubt about the accuracy of that meter. I see enough poorly exposed shots posted to know this is true that it is not the meter's fault, just misguided users.

That, sir, is a whole other, and equally valid, issue.

 

[Sirius Glass]

Unfortunately the meter can be perfectly calibrated, but it is the uninformed use of that meter that can lead to exposure error..and given enough shots poorly exposed can result in doubt about the accuracy of that meter. I see enough poorly exposed shots posted to know this is true that it is not the meter's fault, just misguided users.

For example taking readings with the sky or sun in the light meter field of view.

 

[Bill Burk]

Do you have a recommendation for a densitometer to purchase to carry out these sensitometric measurements?

Get me started and it will be hard to stop.

 

[unwantedfocus]

One item that the OP never mentions is what makes him/her think that the in-camera meter isn't accurate. Bad negs? A quest for absolute perfection? I'd be very interested in why the question is being asked but, perhaps, it really doesn't matter.

Let me start from the beginning, I shoot with Pentax KM at the moment because I really like the focussing screen, I prefer that type of screen compared to a split prism. I still had a Minolta SRT 303b with a faulty light meter. I saw this deal online, a Minolta SRT 303 and a Minolta SRT 101 with 4 lenses very cheap, I jumped the gun. Everything arrived and I tested the lightmeter on both cameras by pointing it against my iMac screen which is very bright, the Minolta 303 had a 6 stop difference compared to the Minolta 101.

Side note when I picked up the 101 and saw that this model doesn't have a split prism and I think the screen is even a little bit better then on the KM, also one lens that came with it the MC Rokkor-pf 1.7 has the smoothest focusing ring I've ever held in my hands so far just amazing.

After comparing both Minoltas I put my Pentax KM into the mix the minolta 101 and the pentax Km both had the same value difference was half a stop or less so the lightmeter on the 303 was completely off. I was wondering if there was a DIY method to test the lightmeter and adjust it myself. Some of you might argue against these kind of repairs and rather let a professional handle it but I thought there might be a method that is affordable since there are a very accurate shutter testers on the market that got DIY'd and work very well. Some device that I can test with that gives me that little certainty that everything is okay, you know? Also found 3 minolta SRT 101's 2 days ago for 30 bucks and bought them....would be good to have something I can test them with and later give them away.

 

[Moose22]

That, sir, is a whole other, and equally valid, issue.

I prefer to blame my equipment. Even when I don't use a meter and quick take a shot before setting my aperture I'll blame the meter for the bad exposure. Or when I switch film backs and forget to set the meter to the correct ISO before I use the Auto mode... yeah, that's the meters fault, too.

 

[DWThomas]

(Scanning through this again ... ) Are any of these items old enough to have been designed for mercury batteries? In that case, using alkaline cells might throw things off. (In some designs it does, others use a bridge circuit, or regulator that is less sensitive to power supply voltage.)

 

[Bill Burk]

Ok! Using the Pentax KM let’s make a calibrated reference light source!

Start with a box having a light in it, like an old contact printer. Add a dimmer and replace the clear glass with a frosted or opal glass.

We can talk about what f/stop shutter speed and film speed to pick. Then brighten/dim the lightbox until the Pentax says it’s good.

Then take a Minolta apart and make your adjustments until it matches.

With this general setup you can try several “ranges”, though we might have to add something like an 80B blue filter to the mix.

Something that can “klunk” into the light path when you flip a lever, (like a couple layers of window screen in a frame) can be built into the box to adjust range without affecting color temperature.

 

[BrianShaw]

the Minolta 303 had a 6 stop difference compared to the Minolta 101

The 303, clearly, has a problem. Was it over or under by 6 stops. Are you sure that all else was the same... ISO setting, etc? If so, something is really wrong with the metering. There is no need for fancy measurement techniques to know that.

Fixing it... that's a whole different problem that might not be easy. You might want to invest just a little more to find out wha might be involved in making a repair:

https://learncamerarepair.com/product.php?product=363

 

[BrianShaw]

I prefer to blame my equipment. Even when I don't use a meter and quick take a shot before setting my aperture I'll blame the meter for the bad exposure. Or when I switch film backs and forget to set the meter to the correct ISO before I use the Auto mode... yeah, that's the meters fault, too.

I totally agree. Even including forgetting to load film. IT'S THE METER'S FAULT!

 

[Chan Tran]

Unfortunately the meter can be perfectly calibrated, but it is the uninformed use of that meter that can lead to exposure error..and given enough shots poorly exposed can result in doubt about the accuracy of that meter. I see enough poorly exposed shots posted to know this is true that it is not the meter's fault, just misguided users.

I agree with you that I would never shoot a roll of film to test the meter. The meter must be tested on its own. Film has way too much variations to use it for testing.

 

[Paul Howell]

My concern is that a 6 stop difference may mean that the 303's Cds cell is going, worse case, or maybe just a component of the the perhaps a resistor if failing? Do you have the repair manual for the 303?

 

[BrianShaw]

… or the display linkage, a thread, has malfunctioned.

 

[wiltw]

I agree with you that I would never shoot a roll of film to test the meter. The meter must be tested on its own. Film has way too much variations to use it for testing.

So just how would you test a meter with a camera like Medium Format or 135?!...it takes a roll to test!

In my test procedure I remove the error possibility of wrongly using meter, by filling the frame with

• a uniformly illuminated,
• featureless surface,

so that matrix metering or center-weighted, or spot metering or other form of biasing ...all zones are totally eliminated fas factors, via the entire unitorm frame content, so that no matter what type of meter, all readings should recommend the identical exposure, and that validates the entire system via shots at different combinations of equivalent exposure, proves out accuracey of shutter speed increments vs. f/stop increments, too!

 

[Chan Tran]

So just how would you test a meter with a camera like Medium Format or 135?!...it takes a roll to test!

In my test procedure I remove the error possibility of wrongly using meter, by filling the frame with

• a uniformly illuminated,
• featureless surface,

so that matrix metering or center-weighted, or spot metering or other form of biasing ...all zones are totally eliminated fas factors, via the entire unitorm frame content, so that no matter what type of meter, all readings should recommend the identical exposure, and that validates the entire system via shots at different combinations of equivalent exposure, proves out accuracey of shutter speed increments vs. f/stop increments, too!

The testing has to be done for the meter, for manual shutter speed and for auto operation. Testing for auto operation is most difficult as you have to test both the meter and shutter speed at the same time. As far as light source I would do the same thing as you do. I use an enlarger dichroic color head as light source. My 4x5 enlarger head has about a 6" round evenly light surface. I use a color analyzer with fiber optic probe inserted in the color head to monitor color balance. Set the filter to get a color temp approximately 4700K (I don't think this is very critical. Checking the color temp with a digital camera). Use the spot meter (I use a Minolta Flashmeter VI and Minolta Spot meter M both have spot mode) checking the brightness. Dial in more filtration, monitoring it with the color analyzer to have the same reading on 3 channel so the color temp doesn't change as the light intensity changes. I was able to get the light level of EV15 and EV9 @ ISO 100 doing this. Verifiying using the spotmeter.
First check the meter alone.
Then check manual shutter speed.
Then check the operation of auto by pointing the camera at the light source, check the readout in viewfinder and measure the exposure time at film plane.

 

[Cholentpot]

Do not forget What-Cha-Call-It.

Related the the whatchamacallit.

A dongle is actually not a generic thing, it's a specific thing. A hardware device that connects to a computer port, originally for copy protection. Now sometimes for other things, like datalogging.


And on topic, I compare camera meters by comparing cameras and my sekonic. Just point 'em at a grey card, and sometimes the sky. Also, incident here is a dead nuts match to my agave plants out front, so I sometimes cheat there mid day when I know what to expect.

One of the wiser members here warned me about green stuff, though, as some meters can be fooled by the IR reflectivity. Doesn't seem an issue with modern camera meters, but if I cared about perfect accuracy I'd always use grey cards.

Strange now that I think about it, I've done this with most of my meters and never had one really too far off. Even my Leicameter, which is 50s vintage, is close enough or black and white. I think the only one marginal was the porofinder meter for my C330, though I don't actually use that for the meter.

Maybe you're thinking of the Thagomizer named after the late Thag Simmons.

 

[Paul Howell]

OP issue is that he knows his meter is off by 6 stops, how do does he set up a stand in which a camera with it's metering circuit open for adjustment?

 

[Alan Edward Klein]

If a meter is off six stops, I'd think it was broken or the battery was no good.

 

[Bill Burk]

OP issue is that he knows his meter is off by 6 stops, how do does he set up a stand in which a camera with it's metering circuit open for adjustment?

He has a Pentax in good working order so his Minolta’s are obviously the test subjects while the Pentax establishes the baseline.