Showing posts with label electronics. Show all posts
Showing posts with label electronics. Show all posts


When digital comes to help film shooters

We certainly live in a weird era of humankind...while many of us, analog photographers mourn the "old good days" when you could get all the photo materials one could ask (including the revered Kodachrome), there are some good things in being dived into the digital age. Even for "pure" analog lovers. Sometimes, digital gadgets just come in handy for the analog world as well. Oriol Garcia kindly asked me for the opinion about his new iPhone application (PhotoExif) he wrote. The application is intended to store EXIF and GPS data  of analog shots which can then be useful later when these shots are digitized. While not being an iPhone user myself, I can hardly comment the whole thing  per se, but I like the idea...


Cottage Tip: Building a small exposure meter-Part III

When I finally got a working panel voltmeter (measuring range 200 mV) I got an unpleasant surprise; it needs a separate supply, other than the circuit, but it wasn't specified in the catalog I bought from. There are also voltmeters able to share the same power supply with the rest of the circuit (the so-called common-ground type), but unluckily not this one. So I needed first to make somehow a split power supply with the same battery for both the sensor circuit and the voltmeter, with the available resources. So I redesigned the circuit a bit. The aim is to keep both voltages (for the meter and the sensor) as »far away« as possible from each other, preventing to get faulty readings. Below, see the circuit schematics.

 After that, I needed to do a quick test to such a circuit on the breadboard. I also needed to do a short-circuit on one contact pair on the voltmeter, in orderto get into the 200 mV range (following the instructions). Oddly, there was already one short-circuit present (factory-made). These »shorts« also set the decimal point. So two decimal points are now present. Nothing dramatic, only funny somehow (note in the photo).
As the weather wasn't really appropriate (for several days) to fully test the meter outside and in sunny conditions, I relied on my instinct so I soldered the final circuit on the prototype board.

I then made the apertures on the enclosure for the display, the sensor and the activating switch. The battery holder is just an ordinary AAA battery holder shortened by about 15 mm and glued together (the batteries A23 and AAA are of the same width).

the battery holder
Finally, I put everything inside the enclosure: switch, panel meter, sensor, circuit and battery-using solder and glue. As it seems, it works nicely! Now, a necessary addition should be to mount (below the panel meter) also an exposure calculator wheel to be able to set the exposure settings. But I first need to buy some plastic printable film to do that.
The inside of the exposure meter
The finished "product"
 Should the precision of the meter be a bit off, I will still need to change the Zener diode in charge to power the panel meter, i.e. using one with a bit lower voltage-just enough to ensure the meter to work, by changing the existing diode in the circuit with another one.


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:
UOUT1= (UZ2 * R1)/(Rx+R1)  
Where UZ2 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

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.


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:
-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 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:

UOUT= (UZ2 * R1)/(Rx+R1)  * R3/(R2+R3)

So we have 4 parameters to play with (UZ2 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.