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Archive 2009 · Any EEs out there? DIY focus assist...
  
 
brett maxwell
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p.1 #1 · p.1 #1 · Any EEs out there? DIY focus assist...


With cameras that perform well at ISO 6400 and fast glass it becomes possible to shoot ambient photos in conditions that cause autofocus to suffer greatly. Sure, you can slap a st-e2 on your Canon or 580 with the flash off to get focus assist, but those are expensive and bulky options.

Solution?

A hot-shoe mounted LED light that shuts off during the exposure.

I think the ideal way to do this would be figure out how the Canon hotshoe tells a flash to activate the focus assist and have that activate the LED. My research indicates the function of the additional pins is a secret, and likely this communication is done in a serial manner rather than a simple applied voltage or open/close of circuit.

A good alternative would be an LED that you turn on and remains on, but uses the hotshoe flash signal to turn off during the exposure. The electronics here would have to be quick enough to turn off before impacting the exposure, and be timed to remain off for the length of the exposure, or just a safe number like 1/10 second. Should be just a matter of a few transistors and some capacitor/resistor combo to get the off time right.

I can solder and assemble this easily enough in the housing of an old ebay/cactus transmitter I have, but I don't know how to design the circuit. If someone could provide me with a working circuit design I would be happy to make a second unit for them.

Thoughts? Experience? Feasibility?



Feb 14, 2009 at 08:12 AM
mpmendenhall
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p.1 #2 · p.1 #2 · Any EEs out there? DIY focus assist...


Your second idea would be far easier than reverse-engineering Canon's serial protocol. The basic flash signal is easy to interpret: the resistance between the big circular contact in the center of the shoe and the flash shoe rails drops from high impedance (~10Mohm) when the shutter is closed to ~4ohm for as long as the shutter is open (based on a quick measurement of my 5D). It shouldn't be hard to switch fast enough; based on the flash sync speed, it takes ~5ms (1/200s) for the shutter to fully open, so any shutoff time <~1ms would be more than fast enough --- and LEDs switching times are often measured in nanoseconds. I'll try and think up a simple circuit to do this, and post later (unless someone else beats me to it).


Feb 14, 2009 at 03:58 PM
mpmendenhall
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p.1 #3 · p.1 #3 · Any EEs out there? DIY focus assist...


OK, here's my circuit attempt (I'm not an EE, so don't blame me if it doesn't work!):

http://praetoriusphoto.images.s3.amazonaws.com/fmforums/20090214_flashlight.gif

Requires: 1 op amp, 3 resistors, LED, voltage source
The switch in the circuit is through the flash shoe (between center pin and walls)
V_+ should be ~30% higher than the operating voltage (forward drop) of the LED
Resistor R_LED sets the current through the LED; choose its value so V_+/(5*R_LED) = operating current for the LED.
The opamp power supply lines (not shown) should be connected between V_+ and ground.

Explanation:
In this circuit, the opamp is acting as a noninverting voltage-to-current amplifier. When the shutter is closed, the switch is open, and the noninverting input (+) of the opamp sees voltage V+/5 from the voltage divider formed by the 400K and 100K resistors. This opamp responds by setting the voltage at its output so the voltage at the inverting input (-) is also V+/5, so the current flowing through the resistor R_LED (and thus also through the LED) is V+/(5*R_LED).

When the camera fires, the switch is closed, shorting the (+) input to ground at 0V; thus the opamp works to get the (-) input also at 0, which means no current flowing through the LED and R_LED, so the LED shuts off for the duration of the exposure.



Further improvements: You should also have an on/off switch for the whole unit that disconnects the battery when not in use. Swap out the 100K fixed resistor for a 0-100K variable resistor and you have a dimmer control that you can use to turn the LED brightness up and down.



Feb 14, 2009 at 04:49 PM
mpmendenhall
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p.1 #4 · p.1 #4 · Any EEs out there? DIY focus assist...


Another option, if you want something more like your first choice of only having the light on during focusing, is to forget about using the flash shoe. Instead, get a cheap remote release cable and open it up; wire in an LED circuit to turn on whenever you half-press the shutter button on the remote release --- then it will be on just during focusing. The LED circuit could be the same as above, except move the switch to above the 400K resistor so it will turn on when the switch is closed instead of open.


Feb 14, 2009 at 05:11 PM
mpmendenhall
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p.1 #5 · p.1 #5 · Any EEs out there? DIY focus assist...


Of course, here's the minimalist cheapskate way to do it (with only 1 resistor + LED):

http://praetoriusphoto.images.s3.amazonaws.com/fmforums/20090214_flashlight2.gif

This probably won't hurt the camera. The possible disadvantage to the cheapskate method is the full current normally flowing through the LED gets shunted through the flash shoe. Since this is only a few mA, it is probably fine, but I'd rather be safe than sorry and isolate the flash shoe switch on the high-impedance side of the circuit as in the op-amp circuit, rather than relying on the flash shoe to provide a good solid low resistance path for the main current.



Feb 14, 2009 at 07:10 PM
brett maxwell
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p.1 #6 · p.1 #6 · Any EEs out there? DIY focus assist...


Great stuff, thanks! I think your "Further improvements:" are great ideas, especially the dimmer! I was thinking the power to the unit would be a 3 way switch with off, always on, and blink off during exposure.


Feb 14, 2009 at 07:15 PM
Timm
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p.1 #7 · p.1 #7 · Any EEs out there? DIY focus assist...


Very nice work.

I'm replying so I can follow this thread....



Feb 15, 2009 at 12:19 AM
mpmendenhall
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p.1 #8 · p.1 #8 · Any EEs out there? DIY focus assist...


A little further poking at my 5D's flash shows that there may be an easy way to tell when the camera is ready for a focus assist light. The upper left of the four small dots on the flash shoe goes to ~+4V when the shutter button is half-pressed, and stays there until ~7s after the button is released, or 3s if the shutter is fired. You could use this signal (again buffered by an opamp) to switch on/off the V+ supply to the above circuit, so the lamp would automatically come on when you half-press the shutter button to start focusing, and turn off a few seconds after the shot is done (while still blanking out during the exposure).

Circuit addition:
http://praetoriusphoto.images.s3.amazonaws.com/fmforums/20090214_flashlight3.gif

Here we are using an opamp as a voltage amplifier with a gain of 4, so an ~4V signal from flash pin 1 will peg the output at the full supplied rail voltage (which you can use as the V+ input voltage reference in the preceding circuits).

This pin on the flash also flickers on other camera operations (hitting just about any other button). You might want to add a little capacitance to the circuit to keep down flickering on every little piece of digital chatter --- for example, split the 400K resistor in the earlier diagram into 2x200K in series, and put an ~.1-1microfarad capacitor from between the two resistors to ground.



Feb 15, 2009 at 01:12 AM
Badtz
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p.1 #9 · p.1 #9 · Any EEs out there? DIY focus assist...


Someone out there sells something along these lines, if only I could find it again... I remember thinking it was much too expensive for what it was...


Feb 15, 2009 at 04:44 AM
brett maxwell
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p.1 #10 · p.1 #10 · Any EEs out there? DIY focus assist...


awesome work! I love the idea of it only coming on with a half-press!

I'm sourcing some parts and will probably spend a few hours Monday seeing if I can get this working. Of course needing one of the additional pins will mean I need to cannibalize an ETTL cord, but I'm okay with that. I'm thinking of using this "super ultra-bright" red LED, or maybe this infrared LED so you wouldn't even see it going. (do you think the AF sensors can see IR?)



Feb 15, 2009 at 06:19 AM
 

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mpmendenhall
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p.1 #11 · p.1 #11 · Any EEs out there? DIY focus assist...


The AF sensors are definitely quite sensitive to deep red (as used by Canon's flashes), and I would guess also sensitive to IR (I doubt Canon would have any reason to put an IR-blocking filter over the AF sensors, and solid-state light sensors tend to be very IR sensitive on their own).


Feb 15, 2009 at 03:49 PM
andc
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p.1 #12 · p.1 #12 · Any EEs out there? DIY focus assist...


If you're going to have the LED on for focus assist aren't you going to get an exposure based on that light level (and not the actual light level when the shot is actually taken with the LED off)? Maybe using IR as you mention would get around that.


Feb 15, 2009 at 05:32 PM
brett maxwell
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p.1 #13 · p.1 #13 · Any EEs out there? DIY focus assist...


that's a good point, but I use manual focus in those extreme low-light situations and base my exposure off chimping the histogram. still, I like the IR idea.


Feb 16, 2009 at 01:30 AM
invalid2
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p.1 #14 · p.1 #14 · Any EEs out there? DIY focus assist...


brett maxwell wrote:
With cameras that perform well at ISO 6400 and fast glass it becomes possible to shoot ambient photos in conditions that cause autofocus to suffer greatly. Sure, you can slap a st-e2 on your Canon or 580 with the flash off to get focus assist, but those are expensive and bulky options.

Solution?

A hot-shoe mounted LED light that shuts off during the exposure.

I think the ideal way to do this would be figure out how the Canon hotshoe tells a flash to activate the focus assist and have that activate the LED. My research indicates the function of the
...Show more

I think the easiest would be an st-e2 (probably still cost competitive with diy). If you really want to make your own thing, I think it would be easier to use a microcontroller with software for the logic, rather than hardware. Easier to fine tune and I think less worry for process variation.



Feb 16, 2009 at 04:10 AM
mpmendenhall
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p.1 #15 · p.1 #15 · Any EEs out there? DIY focus assist...


invalid2 wrote:
I think it would be easier to use a microcontroller with software for the logic, rather than hardware. Easier to fine tune and I think less worry for process variation.

Given that the design as it stands consists of a grand total of 2 opamps and 6 resistors (with no "fine tuning" needed and tolerant of a wide range of "process variation"), I doubt that a programmable microcontroller would make life much easier. There are projects where a microcontroller makes a lot of sense (yay for ATmega168's!), but this is not one of them.



Feb 16, 2009 at 04:46 AM
invalid2
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p.1 #16 · p.1 #16 · Any EEs out there? DIY focus assist...


mpmendenhall wrote:
invalid2 wrote:

Given that the design as it stands consists of a grand total of 2 opamps and 6 resistors (with no "fine tuning" needed and tolerant of a wide range of "process variation"), I doubt that a programmable microcontroller would make life much easier. There are projects where a microcontroller makes a lot of sense (yay for ATmega168's!), but this is not one of them.


If it is trying to time the shutter or something like that, I think a uC is still the way to go. If it can have the camera ask for the light (as you posted a little above this), sure just an amplifier would work.



Feb 16, 2009 at 04:57 AM
mpmendenhall
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p.1 #17 · p.1 #17 · Any EEs out there? DIY focus assist...


If it is trying to time the shutter or something like that, I think a uC is still the way to go.
Fortunately, the "timing the shutter" is being done by the camera, which provides a very clear signal (connecting the center pin of the flash shoe to ground) for the entire duration of the shutter opening. The circuit doesn't need to know anything about the shutter timing at all, it just needs to check whether a switch is open or closed.



Feb 16, 2009 at 05:06 AM
400d
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p.1 #18 · p.1 #18 · Any EEs out there? DIY focus assist...


Finally something interesting.. I am not an Electical/Electronics Eng, but here's my .02.
I own the ST-E2 and 580EX. ST-E2 AF beam sucks big time comparing to 580EX's, I did a shoot on a beach in the evening once, the ST-E2's beam won't go farther than 15m (if I remember correctly, the manual of ST-E2 may tell you the range of the AF assist light). Another thing about ST-E2 is the battery-2CR5..it doesn't last long, since every time you fire it, it fires the flash too (yes, ST-E2 has a small flash tube inside). 580EX is better, but it's not like we want to have the bulky hotshoe flash on our camera all the time..
Now, there are two ways to do it:
1) Get an IR flashlight, and hook it up to a pressure switch, turn on the IR when you are focusing. I need to do more test before I can confirm Canon's AF works with the IR LED I have..
2) Figuring out the pinout of the Canon hotshoe. In other words, once you identify the + and - for controlling the AF assist light, you can simply hook it up to a device with external power and a powerful LED (say, a red CREE LED @ 700mA + aspherical lens will blow the 580EX's AF LED away). Using an op amp like mpmendenhall said, will be the easiest way to implement it.

So, for 2) we have to identify the pins..here's a site which may be useful: http://81.216.246.116/e/fo/



Feb 16, 2009 at 08:50 AM
invalid2
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p.1 #19 · p.1 #19 · Any EEs out there? DIY focus assist...


If it is trying to time the shutter or something like that, I think a uC is still the way to go.

mpmendenhall wrote:
Fortunately, the "timing the shutter" is being done by the camera, which provides a very clear signal (connecting the center pin of the flash shoe to ground) for the entire duration of the shutter opening. The circuit doesn't need to know anything about the shutter timing at all, it just needs to check whether a switch is open or closed.


Quite - I was thinking of the first idea of trying to have it always on and then switch it off for a some fraction of a second based on some other signal. I think snagging the signal off that pin is good.



Feb 16, 2009 at 09:21 AM
invalid2
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p.1 #20 · p.1 #20 · Any EEs out there? DIY focus assist...


400d wrote:
<cut>
Now, there are two ways to do it:
1) Get an IR flashlight, and hook it up to a pressure switch, turn on the IR when you are focusing. I need to do more test before I can confirm Canon's AF works with the IR LED I have..
2) Figuring out the pinout of the Canon hotshoe. In other words, once you identify the + and - for controlling the AF assist light, you can simply hook it up to a device with external power and a powerful LED (say, a red CREE LED @ 700mA + aspherical lens will blow the
...Show more

Though having a nice bright light (maybe a flashlight head with all the stuff preassembled) could be fun - it might also be interesting to have a small focused light just covering one focus point. Of course this could be difficult with a really small light beam as the subject distance changes, but it might be moderately unobtrusive.



Feb 16, 2009 at 09:24 AM
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