DIY light meter for darkroom

John51 · Aug 22, 2019

I've been pondering how to get the most from my Ilford EM10. imo your ideas for the Pi could work well with the EM10.

With the Pi, select what Zone is wanted for the shadows. Input the scale reading from the EM10. (It goes from 0 to 100.) Do the same for the highlights and the most important midtone. The Pi can then display the contrast grade and exposure time.

radiant · Aug 22, 2019

John51 said:
I've been pondering how to get the most from my Ilford EM10. imo your ideas for the Pi could work well with the EM10.
With the Pi, select what Zone is wanted for the shadows. Input the scale reading from the EM10. (It goes from 0 to 100.) Do the same for the highlights and the most important midtone. The Pi can then display the contrast grade and exposure time.

This kind of calculator can be made fas WWW page with javascript. Works then on mobile devices - no need for Pi or computer or anything extra. Good idea!

radiant · Aug 26, 2019

First tests with my meter. I only made a reference print for exposures and then tested it quickly in action. I raised the column to random height, measured same spot that I had reference reading and added then needed exposure (1.1 stops) to my timer. I did nothing else between. And voilá - exposure is so close that atleast I'm satisfied. Measuring is bit tricky and it is easy to get variation to readings so that might explain the suddle difference in exposure. With eyes from actual prints it is hard to notice.

The larger print on left is the reference and on right is the test print. Remember that this print is only for exposure, it was done with full aperture and without any filters. Next up is measuring the contrast and using filter. And some doding and burning

radiant · Oct 24, 2019

Some update on this. The UI has refined a bit and I have been using the meter quite a lot.

The grade measuring is far most useful tool on this device. It reports me the correct filter to use and it's pretty much correct all the time.

The exposure measurement feature is a bit tricky. You can use it 100% succesfully when changing the enlargement because you have fixed reference tone on that one. However to use it on previously unprinted negative is quite difficult. How do you know what you are measuring? To clarify a bit: say you need to measure light clouds. Some negatives it is really hard to say what is that tone.

I've used the darkest parts of the negative (for example clouds) as reference but even in clouds you have lot of variance in terms of tone. And what if you don't have clouds? Also sometimes the part you would need to measure is so damn small that you cannot be sure what are you measuring..

The idea using the dark part of the negative is to get highlights correct and then adjust the blacks with contrast grade. This approach seem to work pretty well if I can get the highlights measured correctly.

I propably need to calculate the mid tone too and try to use it on negatives that don't have suitable dark or light part in negative. Dunno how that would work ..

DREW WILEY · Oct 27, 2019

For B&W printing I find simple test strips to be much more direct and efficient than any darkroom metering system. And I do own a couple of very good quite expensive easel meters, and they're way way more sensitive at low light levels than any ordinary light meter or lux meter, and should be. And yes, I do have valid applications for them. But ordinary black and white printing isn't one of them.

radiant · Oct 28, 2019

DREW WILEY said:
For B&W printing I find simple test strips to be much more direct and efficient than any darkroom metering system. And I do own a couple of very good quite expensive easel meters, and they're way way more sensitive at low light levels than any ordinary light meter or lux meter, and should be. And yes, I do have valid applications for them. But ordinary black and white printing isn't one of them.

I agree that test prints (may it be "test strips" or preliminary print) are needed. No way I could see that final exposure time could be achieved with this kind of device. I think the problem is that it is very difficult to measure accurately enough and to position the measurement device to just the correct place might be impossible on certain negatives because the "correct" density area is just too small.

My strategy in future is to get the exposure to the ball park and then use test strip to do the rest. I can do the test strip for quite detailed f-stops because I know already roughly what is the exposure. If I can get the base exposure with meter to lets say in "range" of 1/2 stops I can then do test print with 1/5 stops (times 5). I might get then to final exposure by just printing one test print because my test strip steps are so fine.

What I have found out that I have got to used to this tool and started to "know" it and I can analyze something more by just moving around and measuring the exposure in different parts. It starts to feel a bit like a glove but I have a long way to get used to it. However this is just what I'm after: a measuring tool to understand the negative.

What kind of applications do you use the meters?

radiant · Oct 28, 2019

Mr Jeremy S Rundle said:
I appreciate that building things is great, I do but they are only £10

Can you provide link where I can buy similar enlarger meter for that price? Closest I know is about 300 euros.

DREW WILEY · Oct 28, 2019

Mine are true lab meters. I have a couple of Gossen Labosix meters, but mostly rely on a ZBE easel densitometer that would cost around $5000 US today if the price were adjusted for inflation. It reads in true log density units suitable for curve plotting and is extremely accurate at very low light levels, far lower than regular densitometers because it's designed for projected light, and is color-neutral too, so is highly accurate regardless of hue. It's in an utterly different league than Patterson or Ilford meters. I do certain specialized tasks which demand that kind of accuracy.

radiant · Nov 9, 2019

I'm bit puzzled with contrast calculations / log density range, still.

Let's assume we have a really high dynamic "scene" to measure: very dark shadows and bright highlights. Let's get some imaginary numbers: the shadow is measured as 1 and highlights measured to 256. That gives contrast of 1:256 which is in log density 2.408 and in ISO(R) 241. Here is where I loose the track: lower ISO(R) should mean higher contrast, but based on my calculations this is just the opposite. We have a high contrast scene and high ISO(R) number.

However the print grades work correctly, so if ISO(R) is over 160, we need to use 0 (or "smaller") filter which is correct when doing printing.

What did I miss?

Anon Ymous · Nov 9, 2019

I don't understand what puzzles you, you seem to get things right.

Lower ISO(R) range means higher paper contrast, useful for a negative with low density range.
Higher ISO(R) range means lower paper contrast, useful for a negative with high density range.

You don't measure a "scene", but the density range of a frame. Suppose you had a Dmax - Dmin = 1,6. Multiply this by 100 and you get 160. So, you choose the grade closest to 160. And this assumes you want a full tonal scale from white to black.

bernard_L · Nov 9, 2019

You are confused between two meanings of "contrast".

You indeed compute a high contrast scene (let's ignore the fact that scene contrast does not go directly to the darkroom paper, irrelevant detail): here "contrast" means input dynamic range.
The transfer function of the paper from light received to darkening (logE-D curve in tech terms). All paper grades from the same range (or same variable contrast paper) have the same Dmin (paper white) and Dmax (black). A high contrast paper will have a steeper curve, i.e. will span from Dmin to Dmax for a smaller range of input illuminations (logE). Here contrast means slope of transfer curve.

This said, IMO, do not hope too much from electronic measurements. Yes, they will allow you to get close, and therefore to concentrate your test strip exposures in a fine progression. And even the test strips, often, do not replace a test print, where the global tone balance can be visually evaluated, after drying, and in proper lighting.

radiant · Nov 10, 2019

Anon Ymous said:
I don't understand what puzzles you, you seem to get things right.
Lower ISO(R) range means higher paper contrast, useful for a negative with low density range.
Higher ISO(R) range means lower paper contrast, useful for a negative with high density range.

Oh man, I didn't realize that ISO(R) is paper contrast, not "scene" contrast (negative projection on eazel for example).

radiant · Nov 10, 2019

bernard_L said:
This said, IMO, do not hope too much from electronic measurements. Yes, they will allow you to get close, and therefore to concentrate your test strip exposures in a fine progression. And even the test strips, often, do not replace a test print, where the global tone balance can be visually evaluated, after drying, and in proper lighting.

.. and sometimes even not close. I'm now better in analysing the contrast grade than my meter Very often when one develops a machine to do some task, you learn from the task itself so much that you actually don't need the tool anymore And well, that's a good thing.

Point of this project is to get a device that helps me. If it is the knoweledge I have got, that's totally fine. If it is readings on screen, that's also in scope of target. I have done so many electronic projects already so point is not to make just a device / electronics. That would be useless.

Anyways I wouldn't do anything without my meter, even if it doesn't give me "straight" results, it's a good tool for analysing and understanding.

radiant · Dec 22, 2019

I've found a great use for the meter. I use it when making contact sheets! I use clear acetat archive pocket sheet for my film and expose the contact sheets directly through the pocket without removing film from the sheet.

I place the pocket sheet under enlarger light and measure the blacks thru the sheet from a clear part of developed film. I've noticed that correctly exposed & developed film should print this as pure black on paper. I calculate the exposure needed for pure black. And voilá - correctly exposed contact sheet with the clear part of film being black.

The great thing in this is that all my contact sheets are exposed in similar way and I can see under/over-exposed film straight away. For example I've been testing pushing Foma 400 to 1600 and can see right away that it is underexposed if exposed as ISO 1600 (developed in xtol 1+1 for 27minutes).

DMJ · Apr 17, 2021

I've been following this thread and the "printalyzer" by Derek Konigs:
https://www.photrio.com/forum/threa...mer-exposure-meter.180377/page-5#post-2409172

I recently developed an incident light meter for Android (actually my second one) and works very well. The Android SDK only provides integer lux values so a phone cannot be used for an enlarging meter. Now I'm trying to build a bluetooth enlarging meter that can send data to an Android device. I'm using a TI development kit that has a light sensor that filters infrared. I'm getting lux values between 0 and 1 under the enlarger but I wonder if it will work. Is it necessary a sensor that gives uLux and reads also infrared like the one the OP is using? How do you get the time in milliseconds given lux and aperture? I'm thinking on using machine learning to "train" the device to select the paper or filter according to the density of the negatives.

koraks · Apr 18, 2021

DMJ said:
I'm getting lux values between 0 and 1 under the enlarger but I wonder if it will work.

Looking at the datasheet of the opt3001 it looks like it might work OK for b&w alright.

DMJ said:
Is it necessary a sensor that gives uLux and reads also infrared like the one the OP is using?

No, your sensor should be fine in this respect. The only thing you need to get sorted is if the useful resolution at these low light levels is sufficient. The device looks pretty linear especially at low sample speeds/long integration times, so that's promising!

DMJ said:
How do you get the time in milliseconds given lux and aperture

Aperture is the easy bit; just set it at an f stop that gives useful readings from your sensor and then compute exposure time. As to the latter, it's a matter of establishing the sensitivity of the paper, which you'll need to calibrate (for each paper used of course, although some papers may have a pretty similar response). The same with contrast grades, but you can use the paper manufacturer's ISO-R specification as a starting point.
Machine learning will of course also work but requires the effort of developing a usable algorithm and training it, which may be more work than a more regular parametric approach.

radiant · Apr 18, 2021

DMJ said:
I'm getting lux values between 0 and 1 under the enlarger but I wonder if it will work.

Most of the readings I've got under enlarger are under 1. On really dense negative the highlights are so dim that my sensor cannot read them. So I would say the more sensitive, the better. Dense negatives might give problems on most of the sensors?

DMJ said:
How do you get the time in milliseconds given lux and aperture? I'm thinking on using machine learning to "train" the device to select the paper or filter according to the density of the negatives.

You need to think through f-stops. Convert the lux to f-stops and then calibrate your highlights / shadows to used paper, so finding out what f-stop gives dMax (most black) and dMin (point where paper white is slightly brighter than exposed area).

My mistake was that I was thinking enlarger meter giving same readings as "normal" light meter. Well, it does but still you want to calibrate and it is pretty easy to do anyways.

Maybe I was repeating a bit what Koraks said (and he was actually my teacher on this, see first page of this thread).

koraks · Apr 18, 2021

vedostuu said:
and he was actually my teacher on this, see first page of this thread

Too much honor

It's important to realize the paper doesn't "know" about f/stops. It just sees a certain amount of light over a certain period of time.

radiant · Apr 18, 2021

koraks said:
Too much honor

It's important to realize the paper doesn't "know" about f/stops. It just sees a certain amount of light over a certain period of time.

What was confusing for me that we know papers have ISO range of 3-6. You can expose with that sensitivity as paper negatives. But then you measure the lux on easel and it is completely different value there You are basically shining a scene from negative to paper, same way as it is done when doing paper negatives.

That was my head-hits-wall moment when I tried to use this analog (no pun!) to darkroom light meter

koraks · Apr 18, 2021

vedostuu said:
You can expose with that sensitivity as paper negatives. But then you measure the lux on easel and it is completely different value there

Yes that was a confusing moment, I agree. It then helps to draw a parallel with normal photography, where the light meter is not positioned at the film plane (or if it is, its readings are compensated for it!)

DMJ · Apr 18, 2021

koraks said:
Yes that was a confusing moment, I agree. It then helps to draw a parallel with normal photography, where the light meter is not positioned at the film plane (or if it is, its readings are compensated for it!)

Thank you guys for the replies, your comments are very helpful. I'm going to look into Ilford's RC paper specs and see if I can get useful times.

albada · Apr 18, 2021

I recently purchased this enlarger meter for under US$100:
http://www.darkroomautomation.com/em.htm
I like it!
I measures EV, where EV 0 is defined as the exposure that produces D-max on Ilford RC IV paper when exposed for one minute. I suspect this is close to the minimum sensitivity of its sensor.
It also has a relative mode (called "delta mode") letting you make relative measurements. I use normal mode to determine exposure, and relative mode to estimate grade.
For calibrating paper's response, I printed near-black, mid-tone, and skin-tone at 8 seconds, and then metered those spots on the negative (Stouffer wedge in my case), giving me the meter-values for those tones at 8 seconds. Knowing those calibration numbers, I can meter any negative at any aperture-setting and at any head-height, and I will know how to adjust exposure (aperture and/or time and/or LED-power) to obtain the desired tone. This method is simple and works well.

DMJ · Apr 18, 2021

albada said:
I recently purchased this enlarger meter for under US$100:
http://www.darkroomautomation.com/em.htm
I like it!
I measures EV, where EV 0 is defined as the exposure that produces D-max on Ilford RC IV paper when exposed for one minute. I suspect this is close to the minimum sensitivity of its sensor.
It also has a relative mode (called "delta mode") letting you make relative measurements. I use normal mode to determine exposure, and relative mode to estimate grade.
For calibrating paper's response, I printed near-black, mid-tone, and skin-tone at 8 seconds, and then metered those spots on the negative (Stouffer wedge in my case), giving me the meter-values for those tones at 8 seconds. Knowing those calibration numbers, I can meter any negative at any aperture-setting and at any head-height, and I will know how to adjust exposure (aperture and/or time and/or LED-power) to obtain the desired tone. This method is simple and works well.

I'm glad you mention it. I knew about this device from the description in Derek Konigsberg's video:
I want to get it but I was reluctant because they use an order form. How was your buying experience/shipping times ? They do have a lot of good documentation in their support page.

mshchem · Apr 18, 2021

albada said:
I recently purchased this enlarger meter for under US$100:
http://www.darkroomautomation.com/em.htm
I like it!
I measures EV, where EV 0 is defined as the exposure that produces D-max on Ilford RC IV paper when exposed for one minute. I suspect this is close to the minimum sensitivity of its sensor.
It also has a relative mode (called "delta mode") letting you make relative measurements. I use normal mode to determine exposure, and relative mode to estimate grade.
For calibrating paper's response, I printed near-black, mid-tone, and skin-tone at 8 seconds, and then metered those spots on the negative (Stouffer wedge in my case), giving me the meter-values for those tones at 8 seconds. Knowing those calibration numbers, I can meter any negative at any aperture-setting and at any head-height, and I will know how to adjust exposure (aperture and/or time and/or LED-power) to obtain the desired tone. This method is simple and works well.

This company has been around a long time. They made enlarger timers IIRC

MattKing · Apr 18, 2021

Nicholas Lindan, the owner of Darkroom Automation, is a long time member here, and has recently been posting again regularly.

DIY light meter for darkroom

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