Cottage Tip: Building a small exposure meter-Part II

I got (almost) all I need to build the actual meter (save the voltmeter, it should take quite longer to come, not really sure why). I got the LDR sensor, so the first thing I did was to build some kind of enclosure and light diffusor around it. I just recycled a piece of black plastics for the base and the middle of the (Fuji) film canister cap for the diffusor. I drilled two holes in the base for the sensor leads and painted the inside of the cap with white opaque nail polish (taking care to make an even layer). When the nail polish dried, I glued the sensor on the base and then glued the white-painted cap onto. I know, it is not exactly a dome-shaped diffusor like in commercial meters, but probably (hopefully) will do the job more or less in the same way. It is more like a »hybrid incident light adapter« between the dome-shaped and the flat diffusor (the ones used to asses the contrast ratio). See photos below.

Starting materials for the sensor: light-dependent resistor (LDR), black plastics for the base, white nail polish and a Fuji film canister cap.

LDR glued to the base, and the cut mid-section of canister cap painted inside.

The sensor assembled on the breadbord (don't mind the resistors nearby, they remained from a previous project).

The so-prepared sensor was ready for testing! Unfortunately I came home quite late, so I catched the last sun rays. There wasn't a 15 EV intensity anymore, but only about 13.5 EV.  Then, I measured the response-resistance down to about 4 EV at different values.  I then plotted the dependence of LDR resistance against light intensity (EV). The outcome was quite a nice exponential curve (as it should be) with a very good correlationship.

The testing rig: multimeter measuring sensor's resistance and the Minolta exposure meter for getting the actual EV value.

I then used the obtained formula of the curve equation to calculated the predicted resistance at a given EV value (also for the points I did not measure). Then, the calculated resistance values served to calculate an appropriate series of voltage values to be obtained between Rx and R1 (voltage drop across R1; see the previous post). For that purpose I used the first part of the formula:

U_OUT1= (U_Z2 * R1)/(Rx+R1)  

Where U_Z2 is the voltage of the Zener diode (supplying the voltage to Rx and R1), Rx is the value of the LDR and R1 is the chosen resistor value.

Now, I must confess, I wasn't really picky about the Uz and R1 values, but I tried to match them to what I have at hand (and/or combining various values), but anyway, I wanted to get satisfactory results, at least. So for Uz I chose a Zener diode with voltage drop of 3 V and for R1 I chose the value of 3200 ohms (3k+2x100 ohm).

I got this, quite a linear curve:

The curve equation now tells me that if I want to get the output of about 10 mV/EV I first need to add (offset) 1290 mV to this (voltage) signal and then divide it by a factor of about 28.4. Very luckily to me,  1290 mV is quite exactly the voltage drop of 2 regular diodes connected in series(cca 1.3V)! This is not necessarily the case, but luckily for me, it was. Otherwise, I would need to use another Zener diode and a trimmer to adjust the offset voltage, in a slightly different circuit arrangement. Using a different LDR  and light diffuser would certainly yield different values and curves. For the voltage divider I didn't use exactly the factor of 28.4, since the calculation  gave too much shift from the theoretical values, especially at high EVs (where the meter is used mostly). Given my resistor choices, I opted to use 1267 ohms for R2 (1k+220+47 ohm) and 47 ohms for R3. This gives a ratio of 1:29.57. By applying this ratio and the voltage bias of 1300 mV (two diodes), it gave me the following (calculated) measurement error at different values:

At first, it doesn't look like nice. But, we seldom use a meter below 6 EV (very dim light), and the error of around 0.3 EV is totally acceptable in practice for cameras and meters alike. Only between 11 EV and 13 EV the error is quite large, but as long as we know how much the error is, we can always correct for it. But clearly, all this is still theory only. The practical measurements will tell how good or bad the meter is.

Anyway, at least I came up with the final version of the circuit, with resistor and diode values to test, and hopefully, solder into the circuit board. See below (this is only the signal part of the circuit). But let me stress once again: this circuit is (should be) suitable for MY very own case of sensor, not necessarily (or likely) yours!

The signal part of the circuit I came up with.

The diodes D1 and D2 are just ordinary small-signal diodes. The Zener diode, as said, has  a drop of 3 volts, while the resistor Rz has been set arbitrarily at 4.7k, just to get a current somewhat higher than 1 mA, (at high EV values current can approach about 1 mA in this configuration). Voltage drop across Rz is 7.7 V (12-3-1.3), divided by 4700 yields about 1.6 mA. The photo below shows a more general (and probably also more appropriate) case of a signal circuit-using another Zener diode and a trimmer potentiometer to adjust the bias voltage. The latter is probably the largest source of measurement error in such a meter:

A more generic signal circuit.

During the weekend I'll test the sensor and circuit on the protoboard (»breadboard«), and I am quite anxious to get the results, which I'll promptly report to you. If time will permit, I'll also get in the final construction till next time.

Silver regards

Mitja

CORRIGENDUM: While the circuit in the penultimate photo (without the trimmer pot.) is in principle OK, your restless editor forgot for a moment a basic aspect of Ohm's law, and a vital coefficient....Therefore, the correct values for R2 and R3 are 1.267 M ohm and 47 k ohm, respectively. I apologize for that.

Plustek OpticFilm 120 update (and a rant)

It was one of the first posts on the blog  about this medium format scanner. The actual market release date still seems to "float". When should this happen? Who knows, I don't. The vendors have been accepting preorders for the scanner for quite a while now. The feaures look promising, at least on paper. Not so for the price. Wex Photographic offers the scanner (in pre-order, of course) for £1999. Wow, that's serious money, 2.5k€! In these days people get a medium format camera in mint condition for some 15-30% of that amount! And you can get a (used but working, of course) Imacon or drum scanner for that sum.....I really do not understand who will ever buy such a monster (price-wise). If one needs to scan MF film for sharing online, then a Canon or Epson flatbed does the job just right, for 10% of that price. When you ever need high quality scans (exhibitions, print sales), then it's just better to pay a professional to do that. But yes, it is a problem to find a good professional these days.....I don't know for you, but I'm not gonna buy it. Not in a million years. If I ever get crazy one day, I'll get to scavenge an old drum scanner....and a SCSI adapter :)

Portfolio of the Week: 4Spo

Dear Readers,

We have moved back to Germany and feature another great photographer from this great country, 4Spo. Of course, 4Spo is not his real name, as he wanted to preserve his privacy, but that's his username on Flickr. And that's OK, we respect your privacy-it's your work that counts. He is with no doubt a modest guy, even though his work tells another story, and he mainly defines himself as a fan of Peter Lindbergh.  No doubt he is a die-hard guy as well! After a few years, he happily came back to film, using the many fine cameras in his possession: Leica, Plaubel, Hasselblad, Nikon etc. He has no particular preferences for film brand or type, but he likes very much expired film for its unpredictability and the effects it gives. Possibly with the lens wide open-another bokeh fetishist, I would say. He states that he likes the subtle eroticism in photography, with a nice bokeh, with the great help of film medium. And he obviously makes his job right! Also in the fashion/editorial field.  Enjoy his work! 4Spo's work can be found of Flickr on his website, along with his colleague work.

Mitja

All photos copyright: 4Spo

Monday Column: Analogue Photography as Escape from Digital World - Part II

Last Monday column I wrote about analogue photography as escape from digital world.  In this column I will tell about things in analogue photography that differentiate analogue photography from modern digital world and which I love.

 

You are already familiar with all sorts of digital and electronic helps and shortcuts found in modern digital and not so modern analogue cameras. A few classical photographic electronic helps found already in cameras made in 70-ties and before. We are all familiar with metering in our cameras, P, A, T (S), and M modes. Aperture and shutter speed is controlled electronically from mid-seventies Canon AE1 camera or maybe even before that. Now days you have face recognition, smile shutter, all kinds of scene modes that help consumers, amateurs (not that I underestimate amateur photography and “casual” photographers) and people who know nothing about photography except phrase “smile” or “cheese” and then they press shutter button in one move, all way down... There are all sorts of these so called scene modes; from helpful like portrait, landscape and action, to downright bizarre ones like candlelight, sunset, food, party, or even pet scene modes. And then are so-called effects, for people who are not familiar with post production, like B&W and sepia, or effects that simulate some legacy film emulsions, or even pin-hole effect, and so on... Better I don’t write about live view and video in modern cameras. Sure I missed plenty of them.

 

Electronics, firmware and hardware are developing in very high pace. So every year we have new “useful” features. Some are turning out useful and most of them really are not. Some of this year’s “new photographic” features are: Wi Fi incorporated in camera, so you can control camera by your phone, and wirelessly transfer images, camera equipped with phone android operating system, so you could benefit with all sorts of application, useful or not, for your camera. And also you can share freshly taken photos on your favourite social network... But feature that stroke me most is that on one of new camera that was presented from giant in consumer electronics at Photokina last week. It is called Auto Portrait framing function. When it’s enabled the camera use face detection to locate your subject, crops the image based on a rule-of-thirds, and resample the picture back up to the same resolution as is the original shot. Effectively camera decides about framing instead of you!!! Where this is going I think don’t need to tell.

So whatever these are useful, helpful and needed photographic tools, I prefer a purist way of taking photographs. With all manual and mechanical way of controlling my camera. So when I’m taking pictures and they didn’t turn out in the way I wanted to, it’s only my fault. I prefer working with my light meter, manually turning knob to specific shutter value, turning the aperture ring on selected f stop, zone focusing and manually rewind the film... And then, when I press the shutter button, it’s a pure mechanical joy!

Cottage Tip: Building a small exposure meter-Part I

It has been said that necessity is mother of all invention, but I disagree. Other »mothers of invention«, in my view, are also vanity, frustration and other »virtues«, but they are often confused with the first mother, necessity. Anyway, there too many things to list humans invented just for the sake of their own satisfaction, not really necessity, and some of them are even nice gadgets.

I always »needed«-actually just wanted to have-a small, pocket-size exposure meter, but never wanted to spend a small fortune for one. I have my trusty, almost 20-year-old Minolta Auto Meter III which never let me down. But it is a bulky meter. I just wanted to have a small meter when I get out with a 35 mm camera, with no bags or whatever. Thus, the meter should fit in a small pocket. You can even get an old used one for about 20€, but they tend to be unreliable and in many cases, not working. You can get the small and sweet Sekonic Twinmate, but I just never wanted to spend some 100€ or so for one. Nowadays even less so, since for the same money you can buy a film SLR in very good condition....

So, the plan is to build one simple but precise (enough) incident light meter for about 20€ or so in materials and components. The exp.meter must be:

-small

-lightweight

-simple (both in operation and circuitry)

-precise within ±0.3 EV

-usable at least from 15 EV (the »sunny 16« conditions) down to about 4 EV (exposure value, ISO 100)-this is also the range of many commercial meters

I already checked the availability/pricing of components, but they still need to be delivered...in the mean time, I'll lay down the theoretical aspects of this building issue.

The sensor

The easiest approach (and maybe even the most effective) is to use a CdS (cadmium sulfide) photoresistor (light dependent resistor –LDR). It has  the nice property of a logarithmic response (of its resistance) to lighting conditions, but inversely (the resistance decreases with increasing illumination).  The EVs we deal with, are also a logarithmic arrangement, since by every EV step the lighting conditions change by a factor of 2 (1 stop). We can thus get (with a bit of gimmicks) the electrical output (current or voltage) directly related to the EV value. But we also need to make first an appropriate light diffusor (and attenuator) for the sensor (since they are quite sensitive).

The display

The nicest way would be to use an analog milliammeter or millivoltmeter, but unfortunately such small meters are very hard to find, plus they are substantially more expensive than their digital conterparts-and then you can be quickly out of budget just for the meter! Digital millivoltmeters (we need them in the range of ±200 mV) are quite expensive (10-15€), more than half of the budget, but they include all the necessary circuitry for a precise metering. They usually run on 9 volts. You just provide a battery source and the measuring leads for your signal, and of course, some room in the housing of your meterJ The use of a 7-segment or LCD display (while cheaper) would dictate the use of the necessary A/D converter and drivers chips, which would substantially complicate the circuitry and eventually, you could spend (more or less) the same amount of money.

The battery

An ordinary 9 volt battery is just fine, but not of my taste-too heavy and bulky. Instead, you can get a 9V battery of the 23A-type(used in remote controls), but very rare. Or you can use its 12v counterpart (also used in remotes), since its easier to get and cheaper. Smaller than a AAA battery, and you can even adapt a AAA battery holder for it.

The housing

Any plastic case of suitable dimensions can do the job-you can even recycle some old stuff. Or you buy one for a few € or $. Just be sure it can fit the voltmeter, the battery and the circuit. Along with the housing, we also need a switch to turn on the circuit for metering.

The circuit

I really like simple things, so I did not include any integrated circuits, only discrete components, and the least amount of active (semiconductor) components as well. In the hand-drawn circuit draft shown below there are no actual values shown, since I don't know yet how the LDR will behave. Only after that I can calculate the actual values to fit my needs. But I do know that I want the output to be 10 mV/ EV (for increased accuracy); at 15 EV (»sunny 16«) I want to get as close as possible to 150 mV, while at the low end I want about 40 mV for 4 EV.

 

The resistors Rz1 and Rz2 are just regulating the currents for the voltmeter and the sensor circuit, respectively. The first Zener diode ZD1 serves to supply the 9V needed for the voltmeter, while the ZD2 will be set later, when the LDR behavior will be known, as also the values of other resistors R1, R2 and R3. But we do know that the output will be according to this formula:

U_OUT= (U_Z2 * R1)/(Rx+R1)  * R3/(R2+R3)

So we have 4 parameters to play with (U_Z2 and R1-3) to adjust them to our needs. Maybe we can later add a couple of diodes to adjust the input voltage level, but we'll see. Anyway, I like this simplicity and I just can't wait to do the actual test, next time.

Portfolio of the Week: Gaston Torres

Dear Readers,

This week we move to Buenos Aires, Argentina, to meet another one-of-a-kind photographer, Gaston Torres.  First, I must state something about his person: as I found out during our correspondence, he is an exceptionally cooperative and kind person (well, all photographers so far have been so, not to be mistaken), in his very own way. He is primarily a 35 mm fashion photographer, using different cameras and mostly color neg film, even for many of his commercial assignments and editorials. His talent is instantly obvious, once you see his work, as he is capable of handling both very complex and also very simple photo setups. By simple I don't think it as a diminutive, on the contrary. And especially in those »simple« (film) photo shoots I think he really excels in. He did a very good job on the  recognizability of his style, although I am having a hard time how to clearly define his style.....since his photos emanate quite some ambiguity. After a long thought, however, I think the best way to describe Gaston's style is a »an elegant mix of Terry Richardson and Nobuyoshi Araki, with a pinch of Helmut Newton«. I openly invite you to visit Gaston's work on his website and on his photostream on Flickr.

Mitja

 All photos copyright: Gaston Torres

"Film Lives: The Enduring Allure of Analog Photography"

I thought it would be worth to post the link to this contribution appeared in Popular Photo last week.There are a few featured photographers in this article, among them Michael Massaia, Dina Kantor and others. While from the technical point of view there are no new statements made to us, analog photographers, it's just nice that some things get reminded once in a while; even for contemporary professionals, film has some distinct advantages. The biggest one today is surely standing out of the crowd of the digital majority, because film has its own specific look. Take it to your advantage!