Originally posted by Ian Stuart Forsyth I like to use virtual aperture while other like to use the term (entrance pupil)
The hole point of the exercise is to show that you can have 2 different exposures but when those exposures are across format they can capture the same total light.
Thus demonstrating that the total light in an image is what determines the level of the noise in an image and not the exposure value between formats
An interesting classroom construct of very little use in the real world. I find noise is variable depending on shooting conditions in all ISOs. Without defining what the cut off point is for a given situation, which is the only possible thing it cold be used for, it tells you nothing useful.
Now if you're claiming that by using a terms such as "Virtual aperture" you are defining something useful, fine, tell me how you use it in every day shooting, or what use it is at all? As I said, and you of course ignored me, as is usually done with all useless stuff, just ignore the question...
Where is the setting on my camera for "virtual" aperture? I can set the aperture on my camera. I can set the shutter speed. I can set the ISO. I can't set the "virtual aperture". There's no control for that.
The whole concept of virtual aperture is wrapped up in a few simple statements. Larger sensors collect more light. If noise is an issue, consider trying out to a larger sensor. What noise is acceptable to you personally is a personal preference for the most part, not determined by formula. You can achieve that by taking a few hundred images with your camera. "Virtual aperture" is a totally unnecessary concept. Unless of course someone would like to take the time to test every camera ever made and demonstrate empirically what it's real world results are for each camera compared to some "virtual aperture." That's what amuses me most about this type of concept. People will swear up and down that they are talking about something real, but don't put the time in to prove it.
A proof requires empirical confirmation of the theoretical construct, in every conceivable situation. Exceptions must be documented and noted. Without that documentation, it's not proved.
The only thing you can prove with a "virtual aperture" would be what "virtual noise" you might have. Talking about "virtual aperture" without endless papers in the sensitivity and characteristics of every sensor out there is non-sense. Or should I say "virtual nonsense" ? That's like talking about grain without talking about the size of the silver halide crystals. It's reducing a complex interaction of signal, amplifiers, encoders and processing software into one parameter, the total light on the sensor. Without understanding the affect of total light on the whole process, it means nothing. Without understanding the different sensitivities of different sensors, it means nothing. Without understanding the effect of the signal amplifiers it means nothing, and without understanding the effect of any built in noise reduction software, it means nothing. Without understanding the sensor's response to contrast and dynamic range, it means nothing.
Or in short, it means nothing.