Photographic light meter (Exposure meter build)

The other day at work I browsed the local auction site and saw a Yashica C, got me thinking about my need to get into medium format photography. The thing about medium format is that you really do need a light meter....erm well if you really want to do photography you need one too, but more so with medium format cause the film is big. And local bloke on the adventure motorcycle forum offered me his Rolleiflex 2.8 Planar - drooooooool!

This got me thinking about designing a building a Arduino powered, small OLED screened, with a few dials and maybe a small toggle switch (I love old electronics with dials and toggle switches like the WW2 gear). I plan to make the dome fitting standard to some light meter I will still decide upon.

The unit will be pretty basic, should measure spot and incident light. It will have two modes (for now). First you select your film speed via a dial. Now you can either enter the shutter speed you intend to use or the aperture you intend to use (shutter priority or aperture priority). Then take the reading and it shows you the EV number, plus either your shutter value or aperture value (dependend on which prioroty you select). It should have another function which I think will be a second button where you take the reading and it shows you the EV number, shutter speed and aperture.

Thing is I can't figure out exactly (nor find info really except what type is used) what sensor they use. I have two choices (the third, selenium offers to little power but its the way most meters worked 40 odd years ago), digital or CdS (think its Cadmium-Sulfide Sensor). I don't really mind which one as long as its pretty sensitive as I want to be able to program/use the meter in very low light as well which very long (5 minutes and more) exposures.

Can anybody assist with what component I should be looking for in say the RS-catalog. I did find photo-sensors and the like but honestly I have no clue what I am looking at.

The rest is pretty much equations. Take reading, add setting(s), run equation and voalla!

Thanks to all who help.

P.S. I am looking at these parts but really I couldn't say if its what I'm looking for since I am getting to know electronics through arduino (links to the SA division)

http://za.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6548097 http://za.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0539836 http://za.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6548918

Either of the last two are human-eye response linear temperature-compensated devices and designed for the job. The first is infra-red sensitive and not suitable. If you can find non-surface-mount devices that will be easier to solder.

That seems a very interesting project to me. Have you found application notes or examples of circuits using these devices (not the IR one) ? Have you found similar devices but not for surface mount ? It will make prototyping easier. To begin I would forget about dials with F values and so on, and try to read the output, with the arduino ADC perhaps it won't be easy to read all the range. Please keep us informed of your progress.

Thanks so much for the replies. As far as surface mount, absolutely I will look for the other one, I will only go surface mount once the whole project shows its works well (was thinking of making 5 or ten ones, put them in a box and have active photographers test them along with their usual gear), and then have custom PC boards made with surface mount components.

As for application notes not much really, I saw the Medium Format camera on Thursday, thought about the light meter on Friday, and looked at components on Saturday, and Sunday I had to work again...which is terrible I know.

I didn't notice the IR component there, will throw that one out right off the bat.

Uhm frikosal, I'm not quite sure I understand what you mean by forgetting about F values and try and read the output.....

From my understanding a exposure meter....meeting....

Right where was I?

From my understanding an exposure meter measures Incident Light and Reflected light...and let me just get this right now, spotmetering. Incedent light is measured by holding the meter facing you (an exposure meter has that white ping-pong looking ball on the front which filters light to 13% grey - 18% if Kodak) thus the light that falls on the subject. Reflected light is when you point the meter at the subject. Spot metering is when you remove the ball and mount a small disk.

The advanced meters actually measure the highlights and lowlights and provide you with a mid-tone. That will be way later, I'm just looking for something simple which provide me the EV reading, and the next step will be that you will be able to set your ISO speed (I will be using this with film cameras as Digital have meters although one must not trust them blindly) and when a reading is taken it shows....and I'm repeating my first post.

I'm a bit swamped at my new job where there is very little order, no training material, hardly ANY documentation, and I'm suppose to project manage and analyse market data in Botswana and Zambia. Kinda like tying your hands, blindfolding you, throwing you in the ocean and yell as they speed off "Find Hawai you loser! HAHAHAHAHHAHAHAH"

Will keep this thread updated as I learn more about exposure meters and hopefully find info on application notes or something more in depth regarding the meters.

Thanks for the interest and I apologize for my lengthy posts.

I was trying to say (I'm not English speaker) that the first think I would do is the circuit to read the meter, then the rest will come.

From my understanding an exposure meter measures Incident Light and Reflected light

Hand held maters do this.

Incedent light is measured by holding the meter facing you (an exposure meter has that white ping-pong looking ball on the front which filters light to 13% grey - 18% if Kodak) thus the light that falls on the subject. Reflected light is when you point the meter at the subject. Spot metering is when you remove the ball and mount a small disk.

Pretty true. With incident you hold the meter at the subject and point it at the camera.

(I love old electronics with dials and toggle switches like the WW2 gear)

Why not use PWM to drive an old needle-style volt meter?

should measure spot and incident light.

Spot is 1 degree, that might be hard to do. And then you heed a method of aiming it. With a real spot meter you look through it and there's a tiny circle in the centre of the viewfinder, that's what is being metered.

very low light as well which very long (5 minutes and more) exposures.

I'm not sure that's practical. Also if you are using film you have to allow for reciprocity failure, for example IIRC a metered exposure of 30sec with TMAX resulted in a 3-minute exposure. And it's not linear, so even if you can meter down to 5 mins the corrected exposure would be about three weeks :)


Rob

Inprogress:
From my understanding an exposure meter measures Incident Light and Reflected light…and let me just get this right now, spotmetering. Incedent light is measured by holding the meter facing you (an exposure meter has that white ping-pong looking ball on the front which filters light to 13% grey - 18% if Kodak) thus the light that falls on the subject. Reflected light is when you point the meter at the subject. Spot metering is when you remove the ball and mount a small disk.

Incident metering is essentially reading the amount of light hitting the meter’s sensor. Reflected metering (of which spot metering is one type) is what your more modern camera with built in metering does, which is the EV of the subject itself with the light reflecting off it and bouncing back to the camera. So, really you’d want some kind of little camera on there to read the scene, and an LCD to show you what you’re pointing at - otherwise, you only have a vague idea what you’re metering and it would be like your standard scene metering mode where it looks at the entire scene and averages it out (which isn’t always necessarily what you want) - that is if you actually want spot metering.

If you have a look at the Sekonic L-758, those two nobbly bits on the side are a viewfinder and lens for spot metering, because you really need to be able to see what it is you’re metering as it’s such a small area to look at.

http://www.sekonic.com/products/Sekonic%20L-758DR%20DIGITALMASTER.asp

Having a camera and an LCD and a little push button to toggle through modes, you’d be able to go between spot, center and scene metering modes easily. You essentially have your software adjusting the camera until it gets the right exposure for the metering type you’ve chosen and then have it spit out the settings it thinks is right (there’s an free iPhone app that works this way if you have an iPhone and want to have a play - Pocket Light Meter)

I think your main problem though is going to be calibration. Spot, center-weighted and scene metering should be easy enough. Just take along any modern DSLR, and compare the output with what the camera meters it as at a given ISO. Calibrating an incident meter will probably require you to actually borrow a real light meter for a few days (probably longer) to compare the output from the two so you can get yours as accurate as possible.

I picked up a 1932 Voigtlander Brilliant TLR a few months ago. Haven’t had a proper chance to test it out yet, but I picked up about 25 or so rolls of Ilford FP-4 120 film, from a friend who just got rid of all his medium format gear last year, so I can have a play. I’m going to be cheating though. I’m just going to take along a DSLR and use that to meter, and I’m only going to be using spot metering. No center weighted or 3D Matrix metering.

But I’m looking to build up an incedent meter myself at some point for an intervalometer I’m working on, so that it can control the shutter speed of the camera as it gets darker for day-to-night transitions, so I’ll be keeping an eye on the project and see how you do. :slight_smile:

WOW Kaouthia, now that is WAY more than I had in mind. I was really only thinking about Reflected and Incident metering, and maybe single spot-metering as an addition....maybe. Was thinking that I could program it such that you take a number of spot readings across the scene (log the readings) and then when you are done, it uses the readings and gives you the average.

I was thinking about an analogue needle as Graynomad mentioned when I read up about light meters and how the Gossen Profisix works with the Null System based on Adam Ansels Zone System.

Also there is a nice Wikipedia entry that explains the formulae and gives the K and C values as used in the formulae for direct sunlight reading and reflected light reading based on the 12% gray baseline. Interestingly there was also a nice argument about Kodak and the fact that their 18% gray card isn't actually 18% gray but closer to 12%. That is a side note.

With this info (not that I wont compare readings with a commercial exposure meter) leads me to the conclusion that the light meter only needs a photoresistor(transistor) which will give me a value (resistance I' sure). Now that value needs to be aligned to Luminance and Illuminance - I'm sure I will find a factory spec somewhere. I was thinking about including a spot meter with some lens configuration - program records the spotmeter readings you take across the scene and then calculates the average tone of the scene. But this will be MUCH later when the exposure meter actually works as intended with direct- and reflected lighting. Was also thinking of using two photoresistors or a combination of resistor and transistor and through some calculation get an average of the two in hopes to make the reading more accurate.

I will try and keep momentum on this project as its almost April and I have vowed that I will take part in Script Frenzy which challenges you to write 100 pages in 30 days of a script. Good fun!

Kaouthia, I think I will get me either a Mayima C220 or C330 of ebay or the like. I like the macro photography option. Of course that Rolleiflex that was offered to me does look absolutely stunning. :sweat_smile:

frikosal: I was trying to say (I'm not English speaker) that the first think I would do is the circuit to read the meter, then the rest will come.

Oh now I understand. Its good that you mention it cause I do get impatient sometimes and with this I think I will have to take it one step at a time. Sort out the actually taking of the readings first and simply display the reading in EV format on my LCD screen. Once that is done I will start adding features like the ISO speed setting and aperture setting and the like.

Back to work.

Thanks for the interest and contributions. I appreciate all your input.

Inprogress: With this info (not that I wont compare readings with a commercial exposure meter) leads me to the conclusion that the light meter only needs a photoresistor(transistor) which will give me a value (resistance I' sure). Now that value needs to be aligned to Luminance and Illuminance - I'm sure I will find a factory spec somewhere. I was thinking about including a spot meter with some lens configuration - program records the spotmeter readings you take across the scene and then calculates the average tone of the scene. But this will be MUCH later when the exposure meter actually works as intended with direct- and reflected lighting. Was also thinking of using two photoresistors or a combination of resistor and transistor and through some calculation get an average of the two in hopes to make the reading more accurate.

A photoresistor for ambient (incident) light readings was my idea too, but I hadn't considered using some kind of lens. If, even on the brighest sunniest days, your photoresistor isn't using its full range, it could be handy to help focus the light and give it a bit more resolution - a neat idea.

The tough bit with spot metering though is still how you're going to know what you're spot metering. When you do it on an SLR, you set your metering mode to spot, you look through the viewfinder, and whatever's bang in the center of it is what's being metered and conformed to 18% grey. Without having either some form of optical viewfinder that you and your device are both looking through simultaneously, or a miniature camera and a display, you're never going to be quite sure what it's looking at (because the target on a spot meter is just so damn tiny). On a landscape, panning the camera just a few mm can be a 10 or more stop difference if you were metering a mountain in shadow and now you're facing the clear bright sky.

But, I wish you luck and can't wait to see what you eventually come up with. :)

Inprogress: Kaouthia, I think I will get me either a Mayima C220 or C330 of ebay or the like. I like the macro photography option. Of course that Rolleiflex that was offered to me does look absolutely stunning. :sweat_smile:

Rolleis are nice, my dad used to have one, and the fella that I got the Ilford FP-4 from just sold two C330s last year when they were worth nothing. Now they're all fashionable again, lol.

I used to teach large-format B&W landscape photography and the Zone system in a previous life.

With film (negative) you "expose for the shadows and process for the highlights", meaning that it's critical to get the exposure correct for the shadows because that information cannot be retrieved if you get it wrong. Back in the darkroom you can adjust the film density to make sure you don't lose the highlights.

With digital the reverse is the case, it's important to get the histogram as far to the right as possible, so you expose for the brightest part of a scene (specular highlights excluded) and if necessary shoot again and use HDR techniques to get the shadows.

I would argue that you do not want the "average", you need to know the darkest part and the lightest part of a scene. With a scene lit by average light an average reading will be fine, but most good photos aren't taken with average light, quite the opposite.

To do this a spot meter is required (unless you can use a large CCD and do it in software), but spot meters are a two-edged sword. Point it at the wrong place and you can be several stops out.

Aah those were the days, schlepping 20kgs of camera and dark slides around the bush, with enough film for 5 or maybe 10 photos over several days. Now I rattle off 300 in an afternoon.


Rob

Kaouthia: Rolleis are nice, my dad used to have one, and the fella that I got the Ilford FP-4 from just sold two C330s last year when they were worth nothing. Now they're all fashionable again, lol.

You see, again I get into the game to late, and its usually when I want to use the things when others start to like them as fashion statements. Thankfully, though not that it helps me much, South Africa is pretty much a market that follows the international trend. Telling someone that I will be shooting film makes their eyes glaze over, hence I should be able to find quite a few TLR camera's, although they will be in pawn shops across the country and old folk who forgot they kept theirs in their cupboard. There is that Yashica C for auction..... :|

I still need to learn a few things about metering when it comes to landscapes. I thought Spot for a landscape is a waste of time since its a landscape. I thought one will probably use incident light and be done with it, MAYBE spot a section of the scene far off, say a shadowy mountain that you want to make the subject.

Regarding the spot meter I will definitely include some kind of look-through lens/scope like the Minolta's do. But like I said, way way into the future if I see I need it, which also means I might just get me a proper meter by then. This project is more the average metering needs. I might still get me a Sekonic L208 in the mean time cause I actually don't have a meter. I have a Ashai Pentax SP500 with meter which I thought I could just carry around with me which kills two flies at once: Quick photography with the 35mm, and more setup type of photography with the 120 film. Yet 35mm film isn't that much fun for me anymore. They payoff just isn't there for me, I think 35mm will die eventually since DSLR's do it quite well now in that format. 120 film not so much yet cause digital backs are unbelievably expensive. :fearful: I was looking at the Pentax Kx for an upgrade to go with my M42 Takumar lenses, not that it will fit, just cause I seem to like Pentax, but then budget wise the Canon's are really accessible over here - and in DSLR I just want to start playing around with wide-angle shots - thinking of the Zenith lens as a start.

I like the idea of a camera in the meter on a project basis, but I think its overkill for what it will be. I think I should maybe just stick to the basics and even leave the spot-metering alone - or like I said, basic spotmetering with a viewfinder of some sort - as an attachment.

Chat again.

Graynomad: Aah those were the days, schlepping 20kgs of camera and dark slides around the bush, with enough film for 5 or maybe 10 photos over several days. Now I rattle off 300 in an afternoon.


Rob

That is something that gets my jogs in a knot....do you actually review all the snaps you have afterwards?

Nothing against digital though, want to get me a DSLR.

300 is a bit extreme but I have done that. And yes I review every single one.

The main reason I can take so many these days is I mostly shoot birds and bugs and in both cases they move so fast you sometimes just shoot and hope like hell that some are in focus and usable. It's a totally different way of working to landscapes, where as I said I sometimes only had enough film for 10 photos and that would last me for days on a walk.

With landscapes there's no reason that pretty much every photo should not be a keeper, with bugs I'd guess that 1 in 10 is technically good and maybe 1 in 50 is aesthetically a good photo. Thank God for digital, no way I could use film for this type of photography.

But even with landscapes you can use take a lot of exposures.

In an attempt to relive my large-format days I also do a lot of panoramas these days, now there's another way to burn "film". A typical pano may be 5 shots wide and 2 high, that's 10 exposures. I often bracket a stop either side so that's 30. Then if the light is fading or changing I may do this 4 or 5 times, that's 150 exposures. The end result? ONE photograph.

Check out my website (www.robgray.com) if you want to see some examples.


Rob

Inprogress: That is something that gets my jogs in a knot....do you actually review all the snaps you have afterwards?

Nothing against digital though, want to get me a DSLR.

I think that question depends what you're shooting (and who's doing the shooting). These days I mostly shoot people, with flash. On each set, I'll fire off the few "safe" shots I would normally take had I been using film, then I'll experiment.

Most of them are location shoots too, and often outdoors, so you're prone to things like wind. I've had times I've fired off 25-30 images in a sequence and had every one almost identical except with random chunks of stray hair blowing in the wind covering the model's eyes or half her face (and each one was as technically correct as all the others). No matter how good your timing may be, sometimes you just need to get lucky.

The difference is that a lot of people will shoot hundreds of crap images in the hopes that they get a handful that might be a half-decent composition with good exposure and sharp focus (which is why DSLRs have gotten the rep they have when it comes to shooting lots of images), and some will shoot a few hundred perfectly composed & exposed technically correct images - all of which would be keepers to the majority of people out there - in order to get that handful of GREAT images.

Experimentation is key to that, imo, and that freedom to experiment you get with DSLRs is something that film just really doesn't allow you to do (without great expense).

Agree Kaouthia. Its for experimentation reasons I want a DSLR, and one day when I can afford it, a medium format Digital back. I like film and what it does teach you tough: Think before you click. Digital will eventually rule out film, and hence me being in the market for a DSLR of some sort - I am starting to relate a 35mm SLR as a "Point & Shoot" especially the DSLR's - not saying they are, just my feeling towards them, or rather what they do offer in convenience plus low cost. The medium formats for me is the one's where you do setups and more artistic things when the picture will be enlarged dramatically.Its a very loose association though.

With DSLR I will be able to start taking those action shots at the Motorcross track or enduro events, I will even try it with the Medium Format. And play around with low-light wide angle streetphotography type things. I like shooting people emotions and dog emotions...or dogs really. Fisheyes is my next venture, and Friday I will be doing my first dabble with live performance photography at a local folk music event. Was thinking of doing it all B&W film.

But now I'm completely off topic. Let me do some more research about light meters.

Hi Frants,

I'll respond to your PM soon, meanwhile here's a document I wrote several years ago about exposure control (aka the Zone system). It's largely geared towards sheet film because you really need to control the processing on a shot-by-shot basis. But there is a section on roll film.

www.robgray.com/grayoutdoors/giftshop/freestuff/pdf/exp_cont.pdf


Rob

Thanks Graynomad, appreciate it. No rush though, like I mentioned, I need to write a 100 pages of a script in April.

A quick reply. I just found a very interesting little device on the net, and given what I have read in the tutorial form Graynomad (thanks), I will probably stick to designing a Spotmeter, and once that works, I will ad the other features.

For now, I want it to be able to show you the EV number, and based on that, you use a set of calculation discs on the unit to show you your shutter speeds related to Aperture. Yes it will be nice if I can have some dials which change the settings digitally on an OLED screen.

But big design freeze is that it will be a Spotmeter because to my understanding its more useful - especially with my application of Medium Format.

For now, I want it to be able to show you the EV number, and based on that, you use a set of calculation discs on the unit to show you your shutter speeds related to Aperture

That's exactly how my Pentax spot meter works. EV reading then rotate a dial. Then on top of it I have a sticky label with the reciprocity adjustments. Very low tech but an OLED/Arduino equivalent would be fun.

The biggest problem I would think is getting a suitable lens/viewfinder.


Rob

I was pondering the lens bit and a thought came to mind to use a tube that is painted with a mat black (the same Mat black you see inside camera's and lenses). The tube will then act as a "lens" ensuring that the light falls on the photoresistor is not more than one degree. However I think the photoresistors will be available in a 1 degree sensor since some of them come in 60 degree sensor angles or 120 degree angles.

I think...I think that even if a sensor reads in larger than 60 degrees, by using the tube concept you should be able to force the sensor to only read 1 degree.

Its just an idea that needs testing. And its important thought that the paint is truly Mat Black and not the type of mat black you get in rattle cans for example.

The research continues.