Using an External Flash with the Nikon Coolpix 800
The Nikon Coolpix 800 is a fairly inexpensive ($500) 2Mpixel digital camera. Outdoors, this camera does really well, however, it suffers from two problems indoors. The first problem is that it has a fairly slow lens, at full wide-angle, the lens is /f3.5; at full telephoto, the lens is /f4.8. Many other digital cameras boast /f2.8 or even /f2.0 lenses. The other problem is that the built-in flash is quite weak. The effective flash range for this camera using the built in flash is less than 10 feet. There have been reviews of this camera that claim rather extraordinary ranges for the built in flash. Do not believe them!
There are ways to increase the flash range. The shutter can be held open longer using 'slow-sync' mode thus making use of available room light. However, holding the camera still for 1/30sec is impossible. The ISO number of the camera can be increased from 100 to 200, or even 400; however, the noise increases dramatically with the ISO400 setting. With just the built in flash, the best the used can do is to keep the lens near the maximum wide-angle setting, and shoot close up.
There are a few obstacles to using an external flash with the camera. The first is that the camera has no built-in electrical connection for a flash. This requires that any external flash unit be slaved to the built-in flash. This is further complicated by the fact that the built-in flash flashes not once, but twice. The two flashes are 83ms apart. It is speculated that the first flash is used to set the white balance and the exposure, with the second flash being the main flash that is actually used to expose the picture. A standard slave adapter (such as a Wein Hot Shoe Slave) will fires the external flash on the pre flash, and will not help. The other issue with an external flash is that the camera is fully automatic, and it was not known how the camera would react to an external flash.
Special digital slave units are available, but the only ones I have seen are expensive ($200). I was looking for a way to use an inexpensive flash unit with my CP800. I purchased a Vivitar 283 flash and a Wein Hot-Shoe Slave adapter. (Both these products are readily available on eBay for reasonable prices). I was hoping that this configuration might be made to work by simply reducing the sensitivity of the Wein adapter by placing tape over the sensor. The main flash appears to be much more powerful than the pre flash. However, I was not able to get this scheme to work reliably. After making some measurements with an oscilloscope and a voltmeter, I decided to try to build a little interface circuit to suppress the pre-flash from the slave adapter.
Before I present the circuit, I would like to put forth a few words of caution. The Vivitar 283 unit I was using had only 9V between the hot-shoe contacts. I have read account of flashes having up to 200V on these contacts. This circuit is not designed to handle this much voltage, although it probably could with a few modifications. In addition, this is the first version of the circuit I have tested, and I have not had a chance to thoroughly analyze it. I only spent about half and hour on the design and I assembled the circuit out of parts that I had on hand. The good news is that neither the camera, nor the flash, nor the hot shoe slave adapter is in great peril if the circuit should leak some of it magic smoke. And there are less that $5 worth of parts in the adapter circuit.
The schematic is shown below:
Resistors R1 and C1 have a time constant much greater than the time between the two flashes. R3 limits the current into the base of Q1, and ensure that Q1 is turned on long enough to discharge C3 through R4. The time constant of C3 and R4 is long enough to allow the flash to fire. Diode D1 and capacitor C1 form a simple power supply, allowing the unit to be powered from leakage current from the flash. Resistor R5 supplies a bias current to the slave adapter so that it can see the flashes. The pre flash turns on Q1, and Q1 stays on until C2 is drained through R3. The hot shoe adapter employs an SRC in its output, the current supply by R5 must be less than the sustain current of the SCR, but the current supplied by both R2, R3 and R5 should be greater than the sustain voltage. I believe that I may have R3 a bit large. With Q1 on, Q2 cannot turn on. After the pre flash, C2 recharges almost fully before the main flash, however, C1 does not recharge significantly. When the main flash is picked up by the slave unit,.Q1 does not turn on, and therefore Q2 is allowed to turn on and the external flash fires. The resistor and capacitor value are not highly critical, and with some tinkering, this circuit should be adaptable to a large number of flashes and hot shoe adapters. Another good modification would be to replace Q2 with an SCR, and possibly increase C1 so that the unit can suppress red-eye reduction flashes too.
I built the circuit out of surface mount parts, but this is not required. The competed circuit was about the size of a nickel and I potted it with hot glue. I built it into the PC cable that was supplied with the Vivitar flash. I mount the slave adapter on the hot shoe of the flash for convenience, but a piece of electrical tape prevents that hot shoe from making contact. The configuration is shown below:
Does it work? Try taking a picture:
Using the External Flash
For close work, I have been using the Vivitar 283 as a bounce flash. Compare the following three pictures. The first is taken with nothing but the built-in flash, the second with the built-in flash covered with aluminum foil to direct the built-in away from the subject and the Vivitar 283 as a bounce flash, and the third was taken with both the built-in flash and the Vivitar as a bounce flash. All pictures were taken at a range of about 4 feet. The camera was on fully automatic mode, and there is a 100W incandescent light behind me. The pictures have not been retouched, but the size has been greatly reduced by Corel Photo Draw so they will load in a reasonable time.
/f3.8 1/60sec 8.8mm with built-in speed light. This picture was taken at close range (5 ft) and represents about the best that the built-in flash can do. Note sharp contrast between the foreground and the background, the shadow behind the blue chair, the reflection off the window glass, the dark tabletop, and the dark background.
f/3,8 1/60 sec 8.8mm with Vivitar 283 bounce. It is not possible to turn off the built-in speed light and still trigger the external unit, but the internal flash can be redirected by holding a piece of foil in front of it. Note the addition color in the rug, the lack of glare from the glass, and the different flesh tone, and the slight yellow cast to the picture.
Finally, both the bounce flash and the built-in speed light are combined. The picture is a bit overexposed, but offers significantly better exposure than was obtained in the previous photos. Note the lack of shadows on faces. I am not completely pleased with the exposure, I think the picture is a bit flat, but I am still learning. This picture included more telephoto, which usually hurts the exposure with just the built in speed light. I could probably compensate for the over exposure with a little bit less EV.
f/4.3 1/60sec 11.3mm built-in and bounce flash. Finally, both the bounce flash and the built-in speed light are combined. The picture is a bit overexposed, but offers significantly better exposure than was obtained in the previous photos. Note the lack of shadows on faces. I am not completely pleased with the exposure, I think the picture is a bit flat, but I am still learning. This picture included more zoom, which usually hurts the exposure with just the built in speed light. I could probably compensate for the over exposure by adjusting the EV of the camera.
The next set of picture were taken under circumstances where the built in flash was inadequate. The room is dark wood paneling, and light from overhead with 2 40W floresent lights. I am shooting about 8 feet from Foard (in the Jar-Jar pajamas.)
f/4.3 1/60sec 11.3mm built in only center weighted average. The spotlight and coal elevator in the foreground are bright, and it could be argued that they are throwing off the exposure, however, I think that the built-in flash just doe not have enough pop to light the room. I have other picture without the bright objects in the foreground, and they have about the exposure setting. I have shot in this room before by setting the camera to ISO200 with adequate result as far as exposure is concerned, however, the increased noise was readily visible. The bounce flash also solves the problem with objects in the foreground reflecting too much light.
Here are a similar pictures taken with the bounce flash in addition to the built-in:
f/4.3 1/60sec 11.3mm built in flash and Vivitar Bounce flash. Center weighted average. The additional flash really lighted up the room. I do not really like the composition, but it does show what the flash can do, and that is what I was after. These pictures also show another problem with my set up, I am holding the flash in one hand and the camera in the other and I am having a very hard time keeping the camera level. I really want to shoot this again with a tripod and a camera with manual controls. I would like a longer shutter to get the lights from the layout, and maybe some motion blur from the moving train, with a little flash head-on to bring out the specular reflections to give the trains the look of metal…Maybe I should go ahead and pre-order a Nikon CP990…Oh yeah, I need to convice my dad to have this room painted, that paneling was out when it was installed in 1971… Anyway, I am having fun!
I am a better electrical engineer that I am a photographer, so I will gladly trade advise on building the slave unit for advice on taking better pictures of kids! Hope you enjoyed this as much as I enjoyed making it!
Charles Naumann