exposure meter

Diego Treves · November 1, 2015

Hello Fellows,

I usually try to get a discrete amount of information before posting any question on the forum, trying to answer myself my own doubts.

In this case I didn't have much time and all I found was inadequate.

So please excuse me if I am writing things already discussed.

please follow my chain of assumptions and feel free to break it (gently and with style please) when I am wrong:

- I am learning exposure on still camera and I've been spending the last few days trying to build a little personal vocabulary of typical exposure situations: with the aid of a surprisingly decent light-meter app installed on my android phone, I've been walking around like a ghost-buster taking notes of the readings I got in different ambients. And I noticed that it was predictably real close to those LV scales you can find out there on the internet. (which, of course, are quite approximate)

I've been even thinking about buying a cheap dedicated luxmeter on amazon (considering that my phone app may not be calibrated) but I stopped. And here is why:

As we all know, exposure of a given picture you are trying to take with your camera, changes accordingly not only to incident light, but also to reflected light.

This means that framing the same subject under the same lighting condition but with a different change in framing may call for an exposure compensation.

Let me explain myself with the aid of a basic visual example, since english is not my first language:

the exposure compensation indicator on this digital camera shows correct exposure at first and then signals slight over or under-exposure (+1/3, -1/3) consequently to small changes in the framing composition.

https://drive.google.com/file/d/0BwUoRCRkAqNHeWF2cjEteVZsSzA/view?usp=sharing

https://drive.google.com/file/d/0BwUoRCRkAqNHMjduaFpXMGdNMmM/view?usp=sharing

https://drive.google.com/file/d/0BwUoRCRkAqNHNzFiZXhyYXE5bWs/view?usp=sharing

We explain this because of course more light "enters" the frame and consiquently we need to compensate for that.

But I don't think a digital camera (or a film camera or whatever) may be able to actually measure the light. My idea is that the camera gives an average reading of the frame according to the "colors" which show in the frame, comparing them to that ideal 18% gray assumed as ideal average exposure. Is that right?

More black is less lighr, more white is more light, more exposure.

Now this seems true (and confirmed) for me also according to expensive exposure-meters.

When you need to measure the reflected light, say, of a distant object like shown in this youtube review of a high end sekonic exposure-meter

https://youtu.be/4C5s8bapVEs?t=10m56s

I don't believe that the instrument actually measures the reflected light (not from that distance of course and not for that price): my idea is that it gives a reading of what it sees based on a color-comparison, as I said before.

Now, if that's true, and this is the point of my topic, it should be quite easy to have a similar application for a smart-phone!!

That would mean you could frame a given suject with you phone and get a reading of that scene (expressed in lux, or whatever kind of value) WITHOUT buying one of those expensive exposure-meters.

you could perform the same exact task of the exposure meter in reflected ambient mode simply with your phone, just as in the YT clip ..

what do you say about that?