AF assist tool v3.0 – complete
Updated version available here. Version 3.1 is complete.
Version 3.0 of my AF assist tool is complete.
Taking a step back, I should explain why I needed this tool.There are really 2 reasons I built this:
1. When photographing concerts or clubs, or any other venue where light is at a premium, the AF sensor in the camera can only do so much to give accurate focus. I am using off-camera flashes most of the time now so I am not relying on the ambient light to paint my scene; that’s what the flashes are for, but I still need to focus. No remote flash trigger to date has had a AF assist light built into it. I use a Pocket Wizard Mini TT1 trigger and Flex TT5 receivers to trigger my flashes, along with an AC3 zone controller (which allows power control of 3 flash groups). So with a flash trigger in the camera’s hotshoe I loose my AF assist light, which normally I would get from a regular flash.
2. DSLR cameras (that I’ve looked at) don’t do AF assist in AI-Servo mode. They don’t allow you to track a subject in the dark, by continuously adding the light from the AF assist lamp on flashes to help the AF sensor in the camera continuously maintain focus. You get One-Shot focus, and once focus has been acquired, the light goes of. But if I want to track a singer as the move on a stage, like one would track a moving person or a race car during the day, you’re stuck. You can’t. This AF assist solves that problem but allowing full control of the AF assist light to be projected as long as you’re focusing (focusing is done by 1/2 pressing the shutter).
How am I addressing those two shortcomings?
1. Moving the wireless flash trigger away from the hotshoe on top of the camera to the end of a ETTL extension solves that problem. The AF assist light sits on top of the Yongnuo ETTL extension block that sits on the camera but does not electrically connect to anything there. In the next revision I plan on removing the extension cord, and adding hotshoes attachment to the AF Assist box to keep everything mounted on top of the camera, with no ETTL extension cords.
2. The connection is made on the side of the camera on the remote-trigger port. The connection there allows for the AF assist light to be on all the time while you’re focusing but 1/2 shutter press. Note, that this AF assist light does not get turned of from the AF-ON button on the back of the camera; only shutter 1/2 press.
The previous version (version 2.0) worked around a defocused laser pointer beam, and the v3.0 unit works around an af assist light from a Canon 550ex flash. I figured Canon must have spent a bit of research time developing the best system to aid a Canon camera into focusing in the dark, so why not take advantage of that and not re-invent the wheel.
Other changes from the previous v2.0 model are the smaller enclosure, fewer batteries (2x AA in version 3.0 vs 4x AAA in version 2.0) and light is not triggered from the hotshoe anymore (hopefully causing less interference with the Pocket Wizard transmitter. The AF assist is now being triggered by the remote-control terminal (N3) port on the side of the camera. This is where normally a remote switch is hooked up to trigger the camera remotely. There are 3 pins there. One is ground, one will trigger the shutter release a the third pin will engage 1/2 shutter press. Normally (without the shutter button pressed) the second and 3rd pins have 3.3v on them. When the shutter is 1/2 pressed, one pin goes to ground (0 volts) and when the shutter is fully pressed the other pin also goes to ground.
If the camera is configured to engage autofocus when the shutter is 1/2 pressed, then sensing a drop to 0 volts can be used to engage the AF LED simply through a transistor. The AF-ON button on the back of the camera will not turn on the AF assist light.
The schematic used is described in an earlier post, with only a minor change. The potentiometer was not used. The visible light this puts out is not that distracting, and I doubt I will need to dim the light. Besides, a dimmer will waste energy.
I have used an op-amp before the transistor to act as a buffer between the camera and the AF assist v3.0. A diode is used to prevent any current feeding back into the op-amp from the transistor and a 2watt resistor is lined up in series with the LED.
Currently the AF Assist sits on top of an E-TTL extension cable mount. The other end of the extension is attached to a Pocket Wizard Mini TT1 (wireless flash trigger). In subsequent 3.x versions I plan on incorporating a hotshoe pass-through on the case to allow for mounting the Pocket Wizard transmitter on top of the AF assist v3.0.
The AF assist v2.0 works with both Alkaline batteries (1.5v cells) as well as rechargable Ni-MH betteries (1.2v cells). One things I found surprising is that even thought there’s almost no distinguishable difference in teh visible light put out by the LED between 1.2v cells and 1.5v cells, the camera notices a difference. At 40ft on a blank wall the camera (in pitch dark) will acquire a focus lock using the 1.5v cells (alkaline batteries), but will not acquire AF lock using the 1.2v cells (rechagable Ni-MH batteries) . The lens used for both AF acquisition tests was a Canon 50mm f1.4. Will have to test a f2.8 lens (2 stops slower) to see how well this AF test works with a slower lens.
I have attached pictures of the light put out by the 1.5v cells (on the left) and the 1.2v cells (on the right).
I just need to field test the unit, and compare it with my previous v2.0 model. Will post updates in new posts, so make sure you check back ofter or grab the RSS feed for automatic updates!
The parts used: (list of the parts with less then obvious sources)
- The cable with the N3 connector that attaches to the camera’s remote trigger port. This was cannibalized from a cable that came with a Yongnuo RF-602 Wireless Remote/Flash Trigger. That kit came with a cable that allows for the camera to be remotly triggered. I just chopped off the other end of the cable and soldered to my board.
- The hot show was taken from a Yongnuo ETTL extension cable. Upon opening the case (with the male foot that doesn’t have any electrical connections) there are 3 screws that attach the actual hot shoe to the case. Drilled the same pattern in my project box and simply moved the foot over.
- The op-amp I am using is a NJU7016D. It’s a low voltage op-amp that can run on as low as 1V. The LM358 op-amp I had from the previous version of the AF assist needed at least 3v, and since I wanted to be able to run this on a couple of Ni-MH batteries (which would put out about 2.4V) I needed something that operated at a lower voltage.
List of previous posts leading up to this final build:
AF Assist tool v3.0 – schematic
Disecting the 550EX AF Assist module
AF Assist tool v3.0 – laser pointer trials 2
AF Assist tool v3.0 – laser pointer trials
AF Assist tool v3.0 – more info
AF Assist tool v3.0 – line laser
AF Assist tool v3.0 – more testing
AF assist tool v3.0 – testing
AS assist v3 project update: The light source
AF Assist tool v3.0
And for referrence, AF Assist tool v2.0 log
Below is the full photo album on images of this project in it’s v3.0 final form.