DIY incident light meter lens cap

Weston invercone modified

I always wanted one of those nice lens caps that allegedly let you take incident exposure readings with your camera, without having to lug around an actual external light meter.

If you carry something with you, after all, better be useful in more than one way, right? This is basic backpacking philosophy.

What thrown me back was the fact that these things – basically a filter mount with a piece of white plastic on top – can cost up to 60 euro…somehow it didn’t seem right.

And no, I’m pretty sure that a white ping pong ball will not give you the same results. If you go all the trouble to take an incident reading you do it because what you want is precision, not to screw up your exposure cheaping out on a vital piece of equipment. By the way, this is the same motive I don’t trust the 1$ chinese knock-offs.

Moreover, all those cheap alternatives are sold mostly as a mean to set the white balance – I shoot in raw, so I tend to correct it in post, so I don’t care. What I wanted, I repeat, was the possibility to use the camera like an incident meter, without having to carry one.

Fast forward till this month: in addition to a Rolleicord I bought last year from eBay – a camera claimed to be in “superb state” that required some serious full disassebly and CLA job to became operational… – I received a Weston Invercone.

If you are asking yourself “what the **** he’s blabbing about?” check the following link:

From what I gather it was an accessory for the Master line of Weston exposure meters that gave the possibility to take incident readings, so the plastic is of the right kind and shape for the job.

Actually, according to the aforementioned website, “this little chunk of white plastic […] is in fact an extraordinary piece of design which has made the Weston the most accurate incident light meter in the world”.

The Weston meters are, I think, most if not all selenium based ones, meaning that even if you can find one operational it will probably be out of calibration or about to die.

But the Invercone happens to sit pretty nicely inside the 49mm ring of a beaten up old filter! You will just have to:

– cut or file a bit of the corners at the flat base on the back of the Invercone, tearing away the little metal strip at the same time

– glue together the two halves of the Invercone – front and back – that will almost certainly come apart once you tore away the metal strip; just a dot or two will do the trick

– unscrew or pop out – depending on the brand – the glass from its filter metal ring

– glue the Invercone inside the filter ring (cyanoacrylate works like a charm), shielding the slight gap that will remain at the base with a piece of something neutrally colored. I used a piece of camera insulation self-adhesive foam, but you can use also, for example, the foam that comes inside pen and flashlight boxes or whatever you have at your disposal, as long it is gray or, better still, black.

Weston invercone inside part

Congratulations, now you have a fully functional, professionally looking, incident meter lens cap! Just point it towards the direction from which the light is coming – not towards the subject! – and take a reading for the exposure. All set.

Obviously you can use a different filter diameter, as long as it fits the Invercone. For example I’m quite sure that with a 52mm filter you could even avoid filing down the corners. I used a 49mm filter just because I had it, and as it happens is the same diameter of some of my most used lenses.

Now for the prices: my Invercone came with other stuff, like I said, but I see them on eBay all the time going for like 8 bucks top – only the cone, without a meter.

If you have to spring cash because you don’t have an old filter in the correct size buy a step up ring and not a filter, so you will not have to unscrew the glass; even simpler. This will set you back another 1 or 2 euro.

Does it pay in the end going all this trouble and taking incident readings? Well, it depends.

The differences between my readings – (spot, interpreted by my own brain, not by a camera chip) – and the ones taken with the help of the Invercone are usually within 1-2 third of a stop. However I grow up spending my weekly allowance on professional slide film I almost cannot afford at the time, so I had to learn pretty quickly to expose correctly. As they say: your mileage may vary!


If you shoot jpeg

Yes it is worth the trouble, because, given your files have less latitude in post to recover from mistakes, whatever you can use to nail the exposure is more than welcome. And as a bonus you will be able to nail the white balance as well.

If you shoot raw

It might be useful, especially in reportage situations when you can not miss the picture (obviously taking a reading of the scene, and then sticking with it).

In more relaxed settings – say landscape – I don’t think so. The problem is that with raw the smart thing to do, if one has time, is to expose to maximize the amount of light information collected – so not to expose for the correct visual rendering of the scene; it will be in post, comfortably sitting in front of a big calibrated display, that you will choose the right look for each image.

Happy pictures!