"Let there be light" is a good title for this heap of Pola-geekery blog fodder, and we're gonna get down and dirty on this one with lots of cross-eyed tech talk, and a smattering of pre-school physics. If that wasn't enough, there's a bunch of obscure pics that will both confuse and astound you. It's the only way I know how to explain it.
Just about every Model 1 SX70 I get in the shop to fix up has some sort of overexposure problem. Images are faded, overexposed, highlights are heavily washed out, there's zero contrast, and some objects are barely visible in bright scenes. Some shots even have a slight blur to them. There's many variables that can affect exposure metering on the SX70 (sticky shutter blades, sluggish S1 solenoid, incorrect faceplate ND filter, LTO - long time out), but the most common is a crunchy-crusty photodiode filter.
The SX-70 was designed to "see" ambient light by means of a photodiode that is built into the shutter's circuit board (ECM - electronic control module) which is mounted right behind the shutter frame. What exactly is this "the photodiode"? Well... simply put, a photodiode is simply an electronic component that takes light and converts it into electricity.
When the shutter button is pressed and the internal mirror flips up, the exposure sequence begins. The shutter blades open to reveal light to the photodiode through secondary aperture holes located right behind the faceplate's "electric eye" (which is nothing more than an ND filter held into place by a pretty chrome ring bezel). Light is converted into electricity that runs into a capacitor on the ECM. When that capacitor is full, it pukes out it's charge in one huge burst which is read by the shutter's IC "brain" and tells the shutter blades that it's time to close the doors, ending the exposure part of the cycle. The remainder of the cycle then mechanically spits out the exposed print.
A good way to understand how the automatic light metering system of the SX70 works is this:
- The less light that hits the photodiode, the longer for the capacitor to charge and the shutter blades remain open for a longer amount of time.
- The more light that hits the photodiode, the faster for the capacitor to charge and the shutter blades remain open for a shorter amount of time.
Keeping the above comparison in mind, because of low levels of ambient light, a lot of indoor shots without a flash or tripod are blurry. The shutter blades remain wide open for a few seconds and any camera movement results in blur. I'm gonna pat myself on the back for that explanation... ready for that second cup of coffee and a valium?
So back to the crusty photodiode filter. The photodiode itself is protected by an opaque housing and only light can get in through a little glass window. This window, filtering out any type of unwanted and interfering light that may cause inaccurate readings, is a tiny green glass gel filter. Without this filter in place, the photodiode lets in tons of nasty light that can't even be seen, throws off the reading, and results in way too fast of a shutter speed. Shots then emerge very underexposed and dark.
As far as what the residue is that grows/leeches out of/accumulates on the filter... I haven't identified that yet. Anyone that has studied optics, chemistry, or even internal medicine, please feel free to jump in on this one. I'm thinking of sending a sample to a lab (a sweet cop-show crime lab) to see what it actually is, but it's most prominent on Model 1s that seem to have been stored in wet conditions. The moldier the camera, the crustier the filter. One of my theories is that it's a reaction between the adhesive used to bond the filter into the photodiode housing and moisture, much like the corrosion on a car battery. Or it could be a reaction to moisture and the glass itself? This corrosion is white, very opaque, and can drastically reduce the amount of light that reaches the photodiode. Remember what I wrote above about the less light that hits the photodiode? Yep... shutter stays open longer. Overexposure.
So there you have it regarding the SX70 photodiode/filter, but I have to end this screed with a little 2nd Shot plug - A primary part of my repair process involves checking/cleaning/rebuilding this filter on every model camera that comes into the shop. This falls within my mission statement that if the camera "kinda" works, it's not working properly. This camera relies on all sequences working 100% for optimal results. I like optimal results.