I missed this out earlier...

[venta]

Some people might remember me grumbling last year that my light meter had packed in. My camera is sufficiently antique that it has no integral meter, I had a handy little one which had formerly belonged to my Dad.

So, my parents gave me a new one for my birthday. Not new to be fair, as they're hard to find these days at sensible prices, but a professional-standard second hand one. While I was in New Zealand, it stopped working too.

So, I've gone back to using the old one (works if you thump it) instead of the new one (needle solidly refuses to move).

Last week, wimble very kindly donated me his light meter, since he has an integral one in his camera. Despite resembling an electric razor, his meter has much the nicest interface of the three. (To change scales on the old one there was a series of two buttons which interacted strangely to switch between 3 scales, on the newer one you remove a screen in lower-light situations, and adjust the dial accordingly. On wimble's there is simply one long sliding scale, and you line up a set of cross-hairs for your reading. Dead simple.)

Only, only... the first time I take it out (to Mountfitchet), it stops working in exactly the same way as the other two did - ie the needle shoots of one end of the scale, and refuses to move. What am I doing to these meters ? They often seem to start working again if I leave them alone - does anyone think they might be affected by static ? (I'm notoriously static prone, and it's the only explanation I can think of.)

Anyone got any (sensible) ideas ?

Comments

Re: Or...

[venta.livejournal.com]

Well, I'll see if I can sort my light meter problems out first. Don't want to break any more...

I'm curious though: what does it need the batteries for ?

Re: Or...

[wimble.livejournal.com]

The light meter. It's only a lithium battery, and I've never had to change it, so it's not really something that needs thinking about. A good point though, since the light meter I passed you doesn't use batteries.

Without knowing too precisely how light meters work, the old one must be using a photocell to directly drive the needle, which can then overload. Presumably photocells don't respond linearly? augeas's comment makes me think that it could change performance due to age as well.

Presumably on my SLR, the photocell is just used as the input to a battery powered amplifier. Or some such excuse.

Then again, maybe it will just use it as a self defense mechanism against errant users.

Re: Or...

[augeas.livejournal.com]

The old ones sans batteries are mostly selenium detectors. I'm not sure that ageing is a factor, other than that the older ones will have had more abuse. Since the currents are so small, the volt-meters must be pretty sensitive, and so pssible to fry them if used on the wrong scale. Leningrads IVs look cheap and cheerfull on Ebay...

Re: Or...

[venta.livejournal.com]

The thing is, though... if I'd fried 'em, surely they'd work none of the time, rather than some of the time?

I feel like I should work out what the problem is before I get another one, or I might end up just doing the same thing again.

Re: Or...

[wimble.livejournal.com]

I was thinking that they might have overloaded the detectors, and they recovered with time. Capacitors, like.

Re: Or...

[venta.livejournal.com]

I don't think they work like that, but I'm a bit hazy. Is there a physicist who answers questions on optics in the house ? davefish?

Re: Or...

[venta.livejournal.com]

Hmmm. From howstuffworks.com:

"The main component of the light meter is a panel of semi-conductor light sensors that are sensitive to light energy. These sensors express this light energy as electrical energy."

Still don't know :)

Time for

[wimble.livejournal.com]

a screwdriver investigation!

Re: Time for

[venta.livejournal.com]

To borrow someone else's pet phrase:

Ho, and indeed No.

I'm all for taking things to bits to see how they go in general, but delicate mechanisms and blundering incompetence often don't mix well :)

Re: Time for

[wimble.livejournal.com]

I'm not incompetant! I'm highly experienced in the art of dismantling delicate mechanisms.

Re: Or...

[failmaster.livejournal.com]

Well, I'm not a physicist, but if what you're basically doing in a light meter is measuring the (tiny) voltage across a photoelectric cell and you introduce a (potentially enormous) potential from your accrued static electricity, then it certainly seems possible that the delicate components might get fried.

Try using your light meter only when you're not wearing shoes...

On a more helpful note I'll try to remember to ask my (ex-professional-photographer) boss tomorrow and see if he knows anything more useful to prevent this happening...

A physicist who answers questions on optics

[davefish.livejournal.com]

Eww, you don't want one of them!

As far as actually answering questions goes, I think that the batterless ones just have some appropriate detector connected to a moving coil galvanometer.

The detector squirts out some current, and the MCG moves a needle depending on how much current is squirted out. There are likely some variable resistors in there too, so that you can change ISO values for the film. From memory it was sorta sticking in places, so I suppose that something mechanical might be sticking, or there could be some intermittent fault with the bit of circuitry for changing the ISO values.

Or at least thats my best guess for someone who is off home about now :)