


Originally Posted by Q.G.
There's no real difference indeed, whether point or line, except that you don't calculate the effect of a change in distance from subject to one point light source, but to many. Each behaving in the 'inverse square way'.
All I can say it's much easier to think of a line source as a line mathematically in this case and not as a collection of points. One can do that problem, but it's a heck of a lot easier to look for the symmetry in the problem.

Originally Posted by Tim Gray
All I can say it's much easier to think of a line source as a line mathematically in this case and not as a collection of points. One can do that problem, but it's a heck of a lot easier to look for the symmetry in the problem.
It hides the fact that this inverse square thing is at work then too.
And i'm not going to do the math.

Alright you win. Look at a big line source and think, "That's a collection of points, all behaving the inverse square law." What does that tell you about how the light falls off with respect to distance? I'd rather look at it and say it's a line source and it falls off as 1/r.
It's geometry. That's it. Inverse square law is for a point source (hint  it's not just for light). A collection of points is a line. It doesn't hide anything about that the 'inverse square thing is at work'. It's not a point source. A collection of points is not a point.
Again, the math is pretty dang easy. If you can write a fraction, you've done the math. I'm out. Hope I didn't annoy anyone being too pedantic.

You're not one for representing things the way that makes most sense in a given context then?

Originally Posted by Q.G.
There's no real difference indeed, whether point or line, except that you don't calculate the effect of a change in distance from subject to one point light source, but to many. Each behaving in the 'inverse square way'.
Not in any radiometric sense. As Tim stated, a point source, a line source, and an extended source are all different. The inverse square law is for point sources only. An area is not a collection of point sources not does it behave the same. This is well recognized in astrophotography where extended objects and stars are effected differently based on fnumber and entrance pupil.

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As stated before and from the Focal Encyclopedia of Photography:
Point source E = I/d squared
Line source E = I/d
Extended source E is invariant with d

Originally Posted by Hikari
Not in any radiometric sense. As Tim stated, a point source, a line source, and an extended source are all different.
Of course they are.
Where did you get the idea that only Tim thinks or says so?
Originally Posted by Hikari
The inverse square law is for point sources only. An area is not a collection of point sources not does it behave the same.
Why does this happen all the time..?!
Yes, it is, and it does.
I'm not going to tackle the, judging by how hard it has turned out to be to explain something very simple, no doubt unsurmountable problems that would be encountered explaining that to you.
So read a good book about photometry.

Originally Posted by Q.G.
Of course they are.
Where did you get the idea that only Tim thinks or says so?
Why does this happen all the time..?!
Yes, it is, and it does.
I'm not going to tackle the, judging by how hard it has turned out to be to explain something very simple, no doubt unsurmountable problems that would be encountered explaining that to you.
So read a good book about photometry.
Thank you. I have radiometry and photometry books. And I have read them. You do not need to explain anything to me because I understand how light works and behaves in this context. It is well documented.

Originally Posted by Hikari
Thank you. I have radiometry and photometry books. And I have read them. You do not need to explain anything to me because I understand how light works and behaves in this context. It is well documented.
Good.

My earlier comments applied to the great outdoors.
On the other hand, if using incident light metering indoors (e.g., light source is a window, lamp or flashgun) then distance from light source does matter and the inverse square law (subject to light source, but not subject to camera) does apply and readings should be taken as close as possible to the subject, meter pointing towards the camera.
Folks, incident light metering (like the tides) does work in a predictable fashion. If you want to explain just how it is that it works, there may be a PhD. thesis in it for you.
Incidentally, (pun intended) for those who agonize about metering a 3dimensional subject half in the light and half in its own shade (say, a subject's face), that is exactly why the meter has a dome (some domes being more sophisticated than others) which, if the meter is pointed correctly, results in an inmeter averaging of the required exposure with a single reading (the dome being a model for the face).
In fact, this is what makes incident light metering so convenient: this relatively simple device with a single reading results in the same exposure outcomes as is generated by the averaging of a number of spot metered readings, shade and light averagings taken by reflected light metering, and any number of "you beaut" "matrix" and "honeycomb" metering contraptions.
Last edited by Galah; 02092011 at 06:11 PM. Click to view previous post history.

