Light source

I just received an AF assist module out of a Canon 550EX. Bought it here on Ebay from seller procamerarepair. The ebay item title is ‘CANON SPEEDLITE 550EX AF LED ASS’Y NEW REPAIR PART OEM’

It’s the same part as the AF assist lights in my Canon 420EX flash

So I measured both the voltage and current at the LED in my 420EX flash. The voltage across the LED is 2V. The power drawn is 400mA (yes, 0.4Amps; pretty crazy, I know). The Supply voltage (with the LED disconnected) is 5.5V. That means the resistor that’s in line with the LED must be 8.5Ω.

I powered the ebay LED with 3V and with a 3.6Ω and it was drawing 0.25A and it seemed just as bright as the flash AF assist LED. I had them both going at the same time.

http://www.everlight.com/datasheets/Shuen1W_datasheet.pdf

Because of the high current draw, I am betting these are IR emitters rather then visible light LEDs. I have seen what a 30mA red LED looks like, and it’s just as bright as these LEDs. So for the extra 10x the power draw, I am betting on them being IR emitters.

I found a similar power IR emitter at Digikey. 400mA, voltage drop of 2.2v. 730nm, which should be a deep red, considering 660nm is the other LEDs I bought, and they’re pretty bright red. Will look for more, but will order this one next time I put an order in. They’re not very cheap at $4.16 a pop.

I don’t really need to buy an LED for this. It’s more for me to figure out roughly what wavelength these AF assist LEDs are in commercial flashes.

Remore trigger port characteristics

I measured the 3 pin connector on the side of the camera where the remote trigger goes. One pin is ground, one pin corresponds to 1/2 shutter press, and one pin corresponds to full shutter press. I don’t know if the full shutter press also engages the 1/2 shutter pin.

What happens on those pins? When the shutter button is not pushed, there’s 3.3v between both pins and ground. When the shutter button is pushed 1/2 or full, the respective pins go to 0V (ground). So if I’m going to use an op-amp (which I am) to pick up and use the the signal, it will have to be an inverting comparator. When there’s a voltage on the op-amp input I need 0V to be seen at the output and when there’s 0V at the op-amp input I need to see full voltage to drive the LED.

Speaking of op-amp, I will need to source an op-amp that can handle 1/2 Amp. The LM358 I have I think can handle 30mA, which is fine for the laser pointer.

Going forward

I think I will pursue both the laser pointer (line laser) as well as the AF assist emitter from ebay (the 550ex emitter)

From my last post(AF Assist tool v3.0 – more testing)I discovered that the LEDs I have found are just not cutting the mustard. Today it just happens that I receive a red cross hair laser I ordered from Deal Extreme a few weeks back. I had all but forgotten about it.

So I stuck 3 volts onto it, and put it through the paces like I did the LEDs in the last post. The laser is bright. At 40ft (in pitch black), it’s very visible. Too bright in fact. So I stuck some resistors in series with the 3v battery to see if I can dim it. At 60Ω, the laser is dim. So dim in fact that in a brightly lit room it’s hard to see at a few feet away on a white wall. Yet when I turn the light off, and point at the wall 40ft away, the camera can still focus (with the center focus point) on the flat featureless wall. (I will post some pictures soon)
Perfect!

Not having had much success so far with LEDs and this laser module arriving in the mail I think I will rebuild my tool using this cross hair laser. Really I just need one line. The perpendicular line is of not much use. I really want this line laser from DealExtreme. I just ordered a few.

Ideally it would be nice to have several lines (close together) to allow for a little misalignment of the laser line and the focusing points be projected out of the same laser, but I haven’t yet found a diffraction lens that will do that.

Since contract is really what makes the difference in helping the AF sensor focus (as opposed to more light), the laser line is perfect since the light it produces remains sharp (distinct edges) up to a very long distance. And the line gives the enter focus point something to focus on regardless of the subject distance, whereas with projecting a spot, the will be some distances where the center focus point will not line up with spot of light.

As a start, I will add a potentiometer in series with the battery to adjust the brightness (a 100Ω pot should do it). Eventually I want to go with full PWM dimming control, so I don’t needlessly drain the battery.

Now if I can only find a laser of a higher wavelength, so that the light is nearer IR. The current laser is 650nm. Something in the 750nm range would be interesting to experiment with. If anyone knows of a source of inexpensive, low power (under 5mW) near IR laser modules (700nm – 800 nm), please let me know.

UPDATE:

So you can get any pattern projected… Frankfurt Laser does a variety of patterns that may work very well. The Concentric circles I think would be awesome since it could cover most/all AF points very nicely