AF Assist V2 (Update 19 May 2011)
Continuation of AF Assist v2 post.
The change o the op-amp did not do the trick as expected. In fact even though the NE5532 has a much quicker slew time, I didn’t make a difference. In fact I think it made things worse.
Two things I noticed about it’s function:
When the AF Assist tool is supposed to be off (ie. when on the camera but not activated by the camera) the laser LED is slightly on. Just barely, but enough to be noticeable on. There’s some leakage somewhere… and probably just drains the battery.
The other more pronounced effect the change in op-amp had was that the NE5532 had a brighter recorded image of the AF pattern (there really should be none) then the LM358. I took some photos with both op-amps at 1/10, 1/20, 1/40 and 1/80 to show the difference.
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| NE5532 – 1/10 | NE5532 – 1/20 | NE5532 – 1/40 | NE5532 – 1/80 |
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| LM358 – 1/10 | LM358 – 1/20 | LM358 – 1/40 | LM358 – 1/80 |
So there you have the comparison…
Maybe consider a MOSFET like i thought about before.
UPDATE:
I think I may now know why the LED light is visible in the frame, after watching this explanation on how camera shutter sync works. The dot would likely all but dispensary around the camera’s highest flash sync speeds. What is happening (I think) is that the laser only turns off for the duration of the flash pulse (which is very brief). When the laser comes back online, the shutter is still open, and will record the light of the laser. I think what I need is something to delay the laser from back on for a little while.
any thoughts on using a remote shutter release cable and a microcontroller (arduino, etc..)? a quick google search on “arduino remote shutter release” should pull up a few good examples.
A couple thoughts…
A mosfet will probably switch quicker and be easier to tune than an op amp
You could shunt the power to turn off the LED – basically when the signal goes low, not only does it turn off the op amp – it activates a second circuit which is in parallel with the laser LED that is basically just a short circuit (important that it has an equivalent resistance much lower than the LED) – then the excess power gets dumped into that circuit instead of slowly draining from the LED…
The assist light will actually hit the sensor between the time when the first shutter curtain passes the assist light’s location, and the sync pulse. This is why you’re getting the light in your pictures. You need to turn the light off on the ETTL preflash.
Thanks for the info Shizuka.
Turning the light off at the ETTL preflash is not so simple. I have been considering using a micro-controller in this project, so I may be able to achieve this after all.
Why not just use an optical trigger to catch the preflash?
Because there is not flash on the camera (only the laser pointer) and a wireless flash trigger.
The flash trigger does not use ETTL. I manually set the power on the flashes.
So I have nothing putting out the pre-flash, hence the need to read the pre-flash from the hotshoe.
Have you considered using a Canon 160E or 200E as an af assist light? You could also remove the flash guts to make it smaller too.
I just picked up a non-functioning 380ex flash for a couple of bucks. Hoping to steal the AF lens and LED out of it.
You may have moved on I don’t know but I thought I would offer my 2 cents.
I found this on hack a day btw.
I went out and got my own 2 dollar pointer and began tinkering, I didn’t take it out of it’s case just yet but did ad a piece of diffuser from a apollo gel sample pack. this has taken the laser dot and diffused it to about a 4 inch spot at 12 feet. Couple this with your defocus trick and I would think you would have a pretty easy on the eyes spot.