> >>> How do you change the sensitivity of the sensor? I'd think it > >>> was fixed. As far as I can tell my camera doesn't allow changing > >>> the sensitivity. > >> That is what happens when you set the ISO on the camera. > > > > I would think that the sensitivity of the sensor would be constant. > > But then I know close to nothing about the sensors. Is there some > > bias Voltage (or something) that changes the sensitivity? > > The saturation sensitivity of the sensor is constant (the White level) > -- a given parameter of the sensor which is why you can't reduce the ISO > setting beyond a certain value (but why don't they make less sensitive > sensors?). However, since there is no Black level (it goes to minus > infinity) you can amplify the output to increase the ISO limited only by > the noise. So you have the base ISO for the chip times the factor used > to get the white in the scene to white in the ADC output and that gives > you the shooting ISO.
Sounds like they aren't really changing the sensitivity of the sensor, just changing which portion of the available range they are recording. Less sensitive sensors? Then you hurt the "available darkness" capabilities. Reducing the light is easy, stop the lens down, use a neutral density filter, ... > Actually, a Kodak app note I read suggested that White should be > considered to be 106% reflectance rather than just 100% (ISO is actually > based on 18% Gray). 106% reflectance? Is this for parabolic mirrors? > >>>> Large format backs have a Peltier cooler to cool the sensor > >>>> chip to reduce noise. > >>> How do they prevent condensation? > >> That is a good question! This is limited to the moisture in the > >> air in the camera or between the protective glass and the sensor. > >> After it gets to 0 C, won't the condensation (now frozen) evaporate > >> (actually sublime)? IAC, if there is a protective glass over the > >> sensor, a desiccant will solve the problem. > > > > According to the fine manual, mine will issue a condensation warning, > > so it must have some sort of condensation sensor. As far as I know > > it doesn't have a cooler. I assume they would brag about it if it > > did. > > You are still going to have issues at over 95% relative humidity. I've never seen the condensation warning, even in the rain. The manual warns that moving the camera from cold to hot might cause it. Back to active cooling: if you don't mind having the light go through more glass, you could perhaps design a vacuume insulated area for the actively cooled sensor? > >>> With an OpenCamera and electronic sensor instead of film you > >>> could program whatever type of metering you like. > >> Yes, a digital camera already has the ultimate auto exposure system > >> in it -- the image sensor. The issue is what to do if the entire > >> brightness range won't fit into the RAW format. > > > > Are you saying that the sensor has a wider brightness range than the > > RAW format? Sounds like the RAW format is defective. Obviously we > > want to be able to store all the data the sensor can provide. > > Actually not. If there were no noise, this would be infinite. But, we > have noise. Figure out how far into the noise we want to go, then design a format that can handle the range. _______________________________________________ Open-hardware mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-hardware
