I’ve been playing around trying to repair a lovely old Weston light meter (of which more anon). I also have a variety of other old light meters, all of which read slightly differently. I do have one meter that I bought new, but it’s getting to be as old as its owner. So which one is to be the reference standard against which I can test and adjust the rest?
The simple answer would be to buy a new meter. But that’s expensive and, well, easy.
Then I had a stroke of clever. Commercial illumination meters are much cheaper than photographic light meters, but they read in Lux. A quick search online found that there is a Lux to EV conversion (Lux is two to the power of EV times 2.5 – don’t panic: clever people have done the sums already). So I splashed out on a Chinese-made luxmeter for under a tenner delivered.
Amazingly, the intercontinental postal system is up and running again. What I got was a chunky gadget about the size of a TV remote.
The meter can read from 0.1 to 200,000 Lux, which is about -4 to 16 EV. That’s a useful range. My little book of notes tells me that -4 is ‘night away from city lights or subject lit by half-moon’ and 16 is ‘subject in bright daylight on sand or snow’. EV 15 is where the sunny 16 rule applies. So basically this meter could cover everything I am likely to encounter.
My next job then was to build a Lux to EV converter. Now Lux is a logarithmic or exponential scale. We should all be familiar with exponential curves by now but what it means is that while EV 1 is only 5 Lux, EV 13 is 10,240 Lux. The meter handles this fine by switching scales but I was going to need to build a little conversion table on a card. My ideal would be a circular table like you get on an old light meter so that you can dial-up the reading and the ISO and see all the exposure combinations. The straight table to convert Lux to EV at 100 ISO is easy, as is the table that gives the options at different ISO – see lower below. The circular calculator took longer. I had to work out how many layers of disk I needed and what was on each layer. One of the scales also had to progress around the disk in the opposite direction to the others.
After a few attempts I got it right. I took a reading with the luxmeter and converted it to a shutter speed and aperture. I took a reading with my best meter, the Sekonic, at the same place and ISO. And they matched. Result! My Sonic meter is accurate, I have a tool to test the others with and I have a new digital light meter. Go me!
I will get hold of some plastic sheet and see if I can make a better version of my wheel calculator. In the meantime it’s actually easier to print a small card with the Lux to EV conversion on one side and some common starter values for each EV and ISO on the other.
The advantage of using a card is that you can also add a rangefinder to it.
I did build a better version of the paper wheel.