I think you’re looking for a lux meter. Precision is far rougher than single photons, but good enough for any human-scale use. It’s what you’d use for things like calibrating indoor lighting, quantifying the difference between a sunny day and moonlit night, or testing flashlights. Last I checked a good basic one was under $40.
I suppose there would be a rough conversion between that value and photon count (at some wavelength range). I just imagine the error bars to be very wide.
Not really, it depends from the application.
Even something like 20 years ago the CCDs available at the time were capable of astrometric measure of star magnitudes with 1/100 of magnitude precision using some control stars.
Everything in our universe is made up of waves. Does it really matter whether it's an oscillation in one field or another? Photons are discrete systems, can you clearly differentiate why anything else in the universe is "physical"?
How so? My understand is that a photon is a discrete packet of energy. Why can't I count those? If there is an excitation in the electromagnetic field, and then there is not, and then there is one - why are those not separate, countable photons?