Light meter accuracy over time

Do light meters loose accuracy over time? I guess some technologies might be affected more than others.

 

To be more specific:

 

I was considering to buy a Sekonic with spot meter functionality. A model of the L-758 family would be perfect for my needs. I get that every tool will eventually stops working one day. But my question is whether it's relatively safe to assume that those light meters (which aren't cheap also on the used market) are working fine if their aesthetic conditions appear to be good.

 

Is there any evidence that those light meters might be affected systematically by some decay in terms of accuracy as time passes, maybe be because of the specific technology they use or because of some known weak spot?

I'd think any meter that uses a photodiode would be extremely stable. Meters with the old Weston cells, not so much. Don't know about CdS but they have their own issues.

+1 for Conrad Hoffman's advice--I have a 30 year old Sekonic L 518 flash meter that still works like new, and a 20+ year old Sekonic L 718 that I use regularly. They are still as accurate as the day I bought them. CdS meters usually work best with mercury batteries that are no longer available, although there are substitutes. Bear in mind that most selenium cell meters and CdS meters will be approaching at least 40 yers old if not more, so take that into account when buying.

Ideally, any measuring instrument should be calibrated periodically.

 

I'm not sure how you check the calibration of a light meter?

 

(Other than the obvious of seeing if it gives a correctly exposed photo, what I mean is, there must a procedure for this)

Selenium cells are only as stable as the 'hermetic' varnish that protects the light-sensitive surface. Hence they are very variable in their longevity and accuracy over time. A quick test is to set the 'Lo' range and find a light level that indicates the highest reading on that range - but not over scale. Switch to the 'Hi' range and check that the same value is indicated. If not, then suspect a duff selenium cell.

 

Least reliable meters, IME, are Westons from the model IV onwards, while older model IIIs are usually working and accurate. (The selenium cell construction and quality obviously changed between the model III and IV.)

Model II and previous versions aren't calibrated in ASA/ISO speeds.

 

Many CdS meters use a mercury cell as their power source. These are best avoided, since the 'batteries' are no longer available, or have to be imported. While substitute batteries are expensive or short-lived, or both. Conversion and/or re-calibration to readily available batteries is expensive or time-consuming too.

 

FWIW. I recalibrated my Gossen Lunarsix 3 to take silver-oxide cells. It required a complete dismantle and several hours tweaking the internal potentiometers to get the readings accurate and in balance across the two ranges.

 

As previously said; silicon cell meters are usually free of any of the above shortcomings, and are your best bet for being accurate, stable and taking a readily-available power source. However any type of meter can suffer from failure of a component, 'dry' solder joint, contact corrosion, etc. So it's advisable to check the accuracy from time to time.

 

Unlike the old saying "a man with two watches never knows the right time" - a person with two lightmeters has a better chance of getting a correct exposure.

 

I was considering to buy a Sekonic with spot meter functionality. A model of the L-758 family would be perfect for my needs.

You could buy a used digital camera that would be just as accurate and give a zoom spotmeter function for less. It might even be more compact!

 

Remember that a one stop change in exposure doubles or halves the amount of light. That's a target that's two barn-doors wide, and requires a very sloppy level of accuracy to get right.

Edited July 13, 2020 by rodeo_joe|1

That's a target that's two barn-doors wide, and requires a very sloppy level of accuracy to get right.

 

I didn't really get what you meant here, probably due to the fact that I'm not a native speaker. I might be missing the metaphor.

 

You could buy a used digital camera that would be just as accurate and give a zoom spotmeter function for less. It might even be more compact!

 

Thanks for your suggestions. If I were to follow this advice, I wouldn't even need to buy any digital camera as I own three.

But I generally prefer to leave them at home whenever I decide to pick up my Bronica GS-1, hence my desire to get a dedicated light meter.

 

Plus, there is a (probably silly) reason behind me willing to invest on something like that: it's an intangible benefit, a good feeling of actively learning something with the right tool. It even gives me the motivation to learn something new I'd probably discard if it wasn't for that "cool factor" :)

 

Thanks everyone for your advice. I learned something for sure. It is my understanding that the model I had in mind (Sekonic L-758) uses Silicon cells, so – if anything – it should be among the ones with the best possibility to still be accurate over time.

I didn't really get what you meant here, probably due to the fact that I'm not a native speaker. I might be missing the metaphor.

I meant that when you're dealing with a tolerance of - 50% to +100%, which is what a 1 stop change in exposure is; then it doesn't take much of a meter to be well within that degree of accuracy.

 

Compared to most scientific measurement standards, it's as easy as hitting a very big target from a very close distance with a shotgun.

 

I really don't know that many people willing to pay the new price of a Sekonic L758 just for a lightmeter, or to carry such a bulky item about in order to do the fairly simple task of measuring light, which is why I suggested the cheaper option of a digital camera.

Edited July 14, 2020 by rodeo_joe|1

I have a Sekonic L-558 spot/incident/flash meter purchased in 1999. The only problem I have is that the lettering has largely worn off, since it is only printed on the shell. Most DSLR's have a spot function in name only. The L-558 is variable from 1 to 3 degrees. I use it for years when I mostly used an Hasselblad system. I continue to use it to set up studio flash units.

 

Matrix metering is a lot smarter now than with early Nikon DSLRs. I found the incident measurements very useful for closeups, flash and product photography.

Edited July 15, 2020 by Ed_Ingold

Most DSLR's have a spot function in name only.

Yes, but the difference between that and a handheld spotmeter is that the DSLR spot zooms with the lens in use, and the TTL spot also automatically takes into account the flare-factor of the taking lens; something that a handheld meter can never do. A handheld spotmeter will have its own flare (impossible to completely blacken or baffle the sensor cell from adjacent light), which is unknown and unlikely to match that of the camera.

 

A handheld incident meter/flashmeter is extremely useful, but my cheap, and years old Shepherd FM-1000 gives exactly the same reading as meters costing several times as much.

Edited July 15, 2020 by rodeo_joe|1

A hand-held spot meter makes it easy to survey the field of view and make adjustments to keep necessary highlights from overexposing. In order to get the same FOV with my Sony, I would need to switch to a 200 mm lens, then back to a more appropriate focal length for shooting. How would that preserve the "flare" factor, or the "T" factor for that matter? That said, I regularly use the camera "spot" function to properly expose faces in plays and concerts (i.e., spotlights and black clothing). I make the measurement and set the camera to manual mode.

 

I use the incident-flash mode to survey the stage when setting up for a large group photo. There is a PC port on the L-558 which triggers the flash with a radio transmitter.

 

You look through the same lens used for spot measurement in the L-558. If there were flare, I would notice it. In fact, it does not occur in practical use. If your lenses are prone to flare, perhaps there are better lenses. Alternately, you can shoot anyway and call it "art." Flare is a problem with lenses designed for SLRs and film, but mostly negligible in premium lenses designed for mirrorless.

All lenses and cameras have flare Ed. It doesn't have to be noticeable spots or glare. It just lowers the maximum contrast that can be obtained, by evenly spreading light from the highlights into the deepest shadows.

 

The issue with spotmeters is that the dark-chamber holding the sensor cell is tiny compared to almost any camera, and the lens used is quite crude (compared to a camera lens), of long focus and often not multi-coated. The aperture is also fixed at quite a wide value in order to get good sensitivity. All this adds up to a high flare factor that's not in line with most cameras and lenses.

 

It's well known that the popular Pentax spotmeter, and its Soligor/Elicar clones, aren't well baffled to exclude adjacent highlights from affecting shadow readings. People pay quite a lot of money to have these instruments modified to improve this defect. Something they wouldn't do if there was no practical benefit.

 

DSLR 'spot' meters aren't that great either, since they meter wide-open (flare factor will change with aperture) and often after the image has passed through the viewing screen (!!) further modifying the flare factor from that which hits the sensor or film. However LiveView or mirrorless camera metering should largely do away with this discrepancy.

 

What I'm suggesting is, that the high exposure accuracy expected by using a spotmeter, might well be illusory, and that the unavoidable tolerances of shutter speed and aperture (and with film, exposure latitude) could easily swamp, mask or negate any carefully calculated exposure based on a false assumption of absolute metering accuracy.

But my question is whether it's relatively safe to assume that those light meters (which aren't cheap also on the used market) are working fine if their aesthetic conditions appear to be good.

 

It's not good practice to judge only by aesthetics. Modern digital meters are mini electronic computers with light sensors and LCD displays. It's the quality of the components and the build quality that will need considering. Positive reviews and high prices probably say how good a particular brand of meter is. Buying online can be a problem, you've just got to believe the seller, but ensure they have a return policy with refund. Generally speaking though, modern electronics are pretty good, depending on which country they were manufactured in. All my compact digital cameras from the early 2000s are still working. A digital camera is only a light meter with a shutter. Only one camera has played up, it needed some contact cleaner on a sliding switch.

Edited July 15, 2020 by kmac

It's the quality of the components and the build quality that will need considering. Positive reviews and high prices probably say how good a particular brand of meter is.

 

By 'those', I meant 'those Sekonic L-758' light meters. I believe this is a good brand.

I agree that aesthetics might not mean much. The information I was looking for was whether their technology was affected by time in a considerable way – from the discussion it seems like it isn't and that other factors play a much more relevant role in the end result.

I found my Minolta flashmeter III stayed accurate for like 30 years. It does get lower reading by 0.2 stops when the dome got yellow slightly over many years.

Most meters I find either work well, or not at all, rarely a little bit off.

 

I have a Canon selenium meter, such as for the Canon VI.

It often enough doesn't work, but a tap on the side will get it working.

As well as I know, selenium cells use mechanical (springy) contacts

onto both the front and back, which can get loose or oxidized.

 

CdS and newer meters sometimes fail due to a loose wire,

or other simple problem, though still not always easy to fix.

Most meters I find either work well, or not at all, rarely a little bit off.

 

I have a Canon selenium meter, such as for the Canon VI.

It often enough doesn't work, but a tap on the side will get it working.

As well as I know, selenium cells use mechanical (springy) contacts

onto both the front and back, which can get loose or oxidized.

 

CdS and newer meters sometimes fail due to a loose wire,

or other simple problem, though still not always easy to fix.

 

Not a bit off? I guess your bit is quite large.

Not a bit off? I guess your bit is quite large.

 

I usually test them against sunny 16, so within a stop or so.

 

Most old meters only have about one stop resolution in the first place.

(Most old meters are selenium cell. I suppose some CdS would now

count as old.). I have never had a selenium cell meter read, say,

two or three stops off the correct value. As I said, either "close enough"

or not at all. (Mostly used with old folding 120 cameras and black and

white negative film.)

 

I do have a Nikon FE2 where the meter seems to be two stops off.

I set the exposure offset to two stops, but never tried to figure out why.

I believe that one has changeable view screens, and that there is an offset

for some of them, or maybe somehow the meter was affected by changed

screens.

 

The FE2 is interesting, like the FM and FE, if the meter works, the battery

is good enough. Last time I used it, it stopped working on exposure 37 of

a 36 roll. Figuring it was about time, I just rewound the film. Later I figured

out it was just dead battery. There is no warning indicator.

I usually test them against sunny 16, so within a stop or so.

 

Most old meters only have about one stop resolution in the first place.

(Most old meters are selenium cell. I suppose some CdS would now

count as old.). I have never had a selenium cell meter read, say,

two or three stops off the correct value. As I said, either "close enough"

or not at all. (Mostly used with old folding 120 cameras and black and

white negative film.)

 

I do have a Nikon FE2 where the meter seems to be two stops off.

I set the exposure offset to two stops, but never tried to figure out why.

I believe that one has changeable view screens, and that there is an offset

for some of them, or maybe somehow the meter was affected by changed

screens.

 

The FE2 is interesting, like the FM and FE, if the meter works, the battery

is good enough. Last time I used it, it stopped working on exposure 37 of

a 36 roll. Figuring it was about time, I just rewound the film. Later I figured

out it was just dead battery. There is no warning indicator.

Yeah that is what I thought. For me 1 stop is a whole lot off. A little bit is about 1/10 or 2/10 of a stop. 1/3 of a stop is the bare minimum for me to even use the meter.

Yeah that is what I thought. For me 1 stop is a whole lot off. A little bit is about 1/10 or 2/10 of a stop. 1/3 of a stop is the bare minimum for me to even use the meter.

 

An averaging meter could be one stop off relative to the actual subject.

 

For meters with electromechanical movement pointing at exposure settings, reading better than about 1/2 stop would take a lot of luck.

 

The meter movement has to be designed to respond logarithmically to the current, which requires some complications. One stop is pretty good.

I suspect the 50 or 60 or 70 year old light meters weren't 1/10 or 2/10 stop accurate when new.

 

All electronic meters should be able to do much better. The Nikon FM meter indicates within one stop either way (two LEDs), and then within +/-0.2 stop with only the center LED.

I have never had a selenium cell meter read, say,

two or three stops off the correct value.

Then you've never used an old Weston IV, V or Euro-Master. Hardly any of them are accurate to within even two stops at the high end of the range these days.

 

With some of them you're lucky to get the needle to even a half-scale deflection.... and that's pointed straight at the sun!

Most old meters only have about one stop resolution in the first place.

Hmmm. Most of the ones I've seen or owned were marked in one-third stop intervals.

Edited August 10, 2020 by rodeo_joe|1

(snip)

 

Hmmm. Most of the ones I've seen or owned were marked in one-third stop intervals.

 

Meters that I know allow setting of the ASA value in 1/3 stop increments, but the readout

scale is marked in whole stops. With some luck, you could interpolate.

Weston III, circa 1956. Last of the Westons with a reliable selenium cell IME, and first to be calibrated in ASA speed ratings.

 

The scale is divided into half-stop intervals, but easily readable and settable to one quarter stop after the first couple of divisions on the scale.

 

Both the aperture and shutter-speed scales are divided into third-stop intervals.

 

Note the old-style invercone, that required a special ND filter to be fitted when reading the High scale. The invercone and ND filter were supplied together as a kit, and a specially made case (as shown) was also available to accommodate the invercone and filter.

Edited August 11, 2020 by rodeo_joe|1

One of the first meters I remember, as my father had one, is the GE meter:

 

James's Light Meter Collection: GE DW-58

 

which (I forgot until I looked at the picture) has a linear scale.

 

So, tiny fraction of a stop at the high end, a few stops at the low end.

(There are lines at 1, 2, 3 footcandles, but you will have a hard time

telling which one the meter needle is over.)

One of the first meters I remember, as my father had one, is the GE meter:

 

James's Light Meter Collection: GE DW-58

 

which (I forgot until I looked at the picture) has a linear scale.

But that meter only covers a readable range of about 7 stops.... and it still has one-third stop aperture divisions!

 

I didn't realise we were only looking at exposure meters that Noah might have carried on the Ark. I thought it might have included models that you can actually find in working condition today, and still use.

Well I have the Canon VI that took my baby pictures, maybe starting when

it was new, and I was one. The GE is a little older than that.

 

The Canon VI has a coupled (to the shutter speed knob), but otherwise external meter

that uses a selenium cell. Still, lots nice than the GE.

 

The original one died years ago. Not so many years ago, I decided to get a replacement.

The first one I got didn't work at all, and had the sound of things moving around inside.

 

The one I have now works about half the time, and when it doesn't usually a few taps

on the side will get it back again. The needle points to black and white bars one stop

wide, which you follow over to an aperture value.

 

So, maybe Noah's grandsons or so. Selenium cell age.

Selenium cells are on an iron substrate, and with a very thin layer of gold

on the front. It melts at 430F, so you can't solder to it. It is usually a

mechanical spring contact on the front and back, which can get loose

or oxidized.