Work on the new AF Assistant has gotten to a point where I thought I would post some progress.
First off, the name is slightly changing. I am calling this one the AF Assistant IR. This will be version 1.x
As the name implies I am working on using an IR lightsource (well, near IR) from a Canon 550ex. I have mounted the LED module in a case which sits under the camera. It mounts to the 1/4-20 threaded hole on the bottom of the camera. The unit is still triggered by the remote shutter release port on the side of the camera just like the AF Assistant v3.1
Fig 1. Pattern at 3 ft.
I have chosen to align the light so that the projected pattern of the module is horizontal (as opposed to the vertical line the laser AF Assistant put out). Due to the fact that the pattern is not just a line, and that the module is closer to the lens axis then a hotshoe mounted device, parallax errors in alignments are quite small. It seems that (in the first alignment of the LED module), the center focus point can pick up on the projected pattern from 3ft to infinity. That’s more then enough for me. I can probably tweak it to get a closer distance, but I’ll play with that later.
Fig 2. Pattern at 30ft.
Several advantages are already apparent by using the LED module.
– The pattern that this puts out is quite wide. So wide in fact that with a 50mm lens on a full frame sensor (of a 5d mk2) the side (left or right) focusing points are able to use the pattern.
– The light the LED module puts out is quite dim. Figure 1 (at 3tf) was taken at ISO6400 , 1/30, f1.4. Figure 2 (at nearly 30ft) was taken at ISO25600, 1/13, f1.4. In a totally dark room, at 30ft, I can just barely see (with the naked eye) the pattern on the wall 30 ft away.
For both the 3ft test, and 30ft test I was focusing on a flat wall with no features (in full light, the camera can not focus on the wall, since there’s nothing there to focus on) and with the AF Assistant IR light, I can focus on the blank wall at 30ft away even with the side focusing points. Quite impressive.
Some internal details
Fig 3. LTC3490 LED driver in action
I have moved away from the 1 battery setup of the previous version. The LED is drawing up about 250mA (which is 10 times what the laser was using) so I figured I’d move up to 2 x AA cells.
I have also moved away from thru hole for most components. Surface mount technology is what I’ll be focusing on from now on.
The LED driver IC is a LTC3490. It can drive up to 350mA though I cannot drive this LED that high. The LED module heats up too much for my comfort. At about 230-250mA it still gets warm but I can keep my finger on it indeterminately. Anything above that current and I can’t keep my finger on it for too long. There is a trimmer pot on the board so I can adjust the current to the LED but I may take it out in the production version.
So far all the tests are looking very positive. The other surface mount IC is a MAX4289 op-amp. It’s able to run on low voltage (down to 0.7V). Most other op-amps need at least 3 volts to run, which I may not have if I’m running two NiMH batteries at 1.2V a piece.
Fig 4. Fully populated board
I’ve only been playing with this setup for a couple of days, and will be testing it at music festival next weekend.
I took some measurements of the wavelength of light from a couple of AF assist lights form Canon flashes. Read about them here.