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Oil immersion as others have said would be a disaster for any DSLR.
The sensor in my Canon 5DH inside the fridge is 17C above the fridge temperature when exposing continuously. As long as the camera is turned on before cooling the fridge and dummy exposures are taken when the fridge gets near its set temperature there is no condensation as the sensor is the hottest thing in the fridge. I keep the lens at 20C with thermostatically controlled Kendrick dew heaters. Focus does not then change with ambient temperature.
In winter I typically run the fridge peltiers at -10.0C and the fridge with the cameras heat load is at -4.8C. The real advantage of the fridge is that the temperature is constant to + or - 0.1C. This makes dark correction very accurate. The thermal noise is lowered but not totally absent. Using air to cool the camera (there is a fan inside the fridge) means that temperature changes and gradients are never likely to damage the camera. It takes forty minutes to reach equilibrium after first turning on the fridge.
I also always leave the camera on until the fridge reaches ambient after switching it off.
Any water vapour present inside the fridge condenses and freezes on the Peltier heat exchangers.
Cooling the sensor directly would make it the coldest object and then extreme measures need to be taken to keep the sensor and shutter dry.
It would be interesting though to try oil immersion with a much cheaper camera. What is a battered 300D worth now? I am sure it would end up a one shot camera.
Bert
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