AF Assist tool v3.0 – more info

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.

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)

3 thoughts on “AF Assist tool v3.0 – more info

  1. Pingback: AF Assist tool v3.0 – laser pointer trials | adrian's domain

  2. German

    Hey I was just reading your comment and thought of something. Have you taken into acount that the point of focus of glass lenses shift then dealing with IR light? If the subject is being illuminated with IR light from the AF assist, and the phase detect sensor resolves focus under that exact situation, wouldn’t the picture be off-focus when the flash triggers? Btw, I’m finding your findings quite useful. Best of luck!!

  3. adrian Post author

    I came across the comment that focus could be off, howeverI am using the LED and lens assembly from a Canon flash (550ex flash).
    I figured that since Canon is using this wavelength, their system must be somehow with the near IR light.


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