Theory behind light fall-off

[poco]

File this one under stupid things you think about at 4AM:

I understand that fall-off is a function of change of iris shape as seen from edge of negative as well as the longer distance light must travel to hit that edge. Okay. But what confuses me is that second factor of "longer distance" because that only makes sense if we accept without challenge that the lens, not the photographed object, is the light source. But what makes it the light source? It can't be the shutter since, relative to the speed of light, its open/close function is insignificant. And it can't be the glass, since light doesn't "know" it's gone through it -- right? The only thing that vaguely makes sense to me is that the lens is the source of the FOCUSED light, and that this somehow qualifies it as the light source. But does that mean an unfocused lens has no fall-off (I feel stupid even typing that)? Seriously, what makes the lens the light source?

Huh?
Huh?

Someone please answer quick so I can get some sleep...

[Walter Calahan]

Attempting to think at 4 AM is not advised. Go back to sleep. Grin.

Inverse Square Law

[Struan Gray]

Optical theory was developed to make life easy for optical designers, not for jobbing photographers. In principle you can take the light radiated from your object at various angles, follow its path through the lens system and calculate the irradiance of the focussed spot at the film plane. In fact, that's what lens designers do. But if you do it enough, with enough different lenses, you eventually spot (well, if you're a bright cookie like Abbe) that the lens behaves like a magic black box. Light appears to be collected by the entrance pupil - the hole that you see when you look in the front of the lens - and it appears to be re-radiated from the exit pupil - the hole that you see when you look in the back of the lens. Because the light from the object appears to be spread evenly over the whole exit pupil, you can treat the exit pupil as the source of your imaging light for the purpose of intensity calculations.

This is just re-stating Huygens' model of wave propagation, where every point on the wavefront behaves like a little light emitter, independently of where the wavefront came from. Where the wavefront goes next is then just the sum of all the little light beams sent out by all the little light emiters along the current instantaneous wavefront.

Next up: relativistic length contraction and Schrödinger's cat.

[poco]

So Struan, pretending I understood all that, let me ask, what about pinholes? Do they also function as light sources with fall-off from off-axis travel distance? I forget the name of the guy who pinholed hotel room interiors, but his holes were about 1/4" in diameter which would seem plenty big to let a lot of light rays through without being affected by the hole's edge. At least for those rays, wouldn't the light source be the photographed object and not the hole?

Worst case, I could live with your "magic black box" summation, but I'd still like to understand.

[Ole Tjugen]

Struan's rephrasing of huygens' model of wave propagation explains pinholes too - even easier than it explaing lenses and optics and apertures.

With "affected by the hole's edge" I fear you are mixing diffraction into the question, which opens a whole new can of wormholes.

I claim Heisenberg's: I know where I am, but not where I'm going. Or I know where I'm heading, but not when, why or from where. Where is "here", anyway?

[darr]

I know where I am, but not where I'm going. Or I know where I'm heading, but not when, why or from where. Where is "here", anyway?

At 4AM that would work for me!
:rolleyes:

[Joseph O'Neil]

Next up: relativistic length contraction and Schrödinger's cat.

-snip-

You wanna be careful - most people with zero science background start screaming "animal abuse"when they first hear about Schrödinger's cat. :confused:

A bit more seriously, Edmund Scientific used to publish a book "All about Telescopes" by - I think - Sam Brown. Although the book was written first circa the 1950's as a way to sell surplus optics for the home hobbist to build their own telescope, it is to this day the easiest to read book on basic optical theory that you will ever find.

There are articles on things like grinding your own mirror which will be of little use to people on this list, but even the process of doing so explains a lot how light and optics work.

It is written almost in the style of a black & white comic book, but the illustrations are very easy to follow, and i can even follow the math involved. They do touch briefly on photography in that book too. I highly reccomend it, even reading a copy in a local library if that's all you can find. You can sue the basic concepts in that book to understand on a basic level how just about anything from a pair of binoculars to a view camera to microscope works.

Another really good resourse - again if you can find them - are a series of books - one a year I think - that were published by Popular Mechanics during the 1940's and 50s on photography. Mostly the books were re-prints form articles from the previous year, plus tips written in by readers.

Indirectly from articles such as how to build your own enlarger, you will get an explanation of how some of these optics work. Also there's a pile of good tips on handling view cameras (which was exactly what many people were using then), such as dealing with "bellows flutter vibration" and more. Hard to find but a good read.

joe

[Struan Gray]

So Struan, pretending I understood all that, let me ask, what about pinholes? Do they also function as light sources with fall-off from off-axis travel distance? I forget the name of the guy who pinholed hotel room interiors, but his holes were about 1/4" in diameter which would seem plenty big to let a lot of light rays through without being affected by the hole's edge. At least for those rays, wouldn't the light source be the photographed object and not the hole?

Worst case, I could live with your "magic black box" summation, but I'd still like to understand.

Yup, pinholes behave in exactly the same way. With lenses there are tricks you can pull to reduce the falloff such as 'tilting' exit pupils in wide angles, or the use of asymmetric lens designs to reduce or enlarge the angular spread for a given focal length; but with pinholes you are stuck with cos^4 whatever you do (assuming your film is held flat).

All of this assumes that the light from your object is distributed evenly over the aperture - i.e. that your aperture really is the size and shape you think it is. It also assumes that your light is incoherent. Both assumptions are safe in normal photographic situations.

[Struan Gray]

You wanna be careful - most people with zero science background start screaming "animal abuse"when they first hear about Schrödinger's cat.

A visiting professor recently described his quantum computing project as the search for Schrödinger's dog. The poor beast still has a 50:50 chance of snuffing it, but while it is still alive, it's more likely to do what you say.

[poco]

Think I'll google what Schrodinger did with his cat and try to replicate it this afternoon. Ten times a day mine demands attention to her entry pupil and what comes out the exit pupil I can't even begin to describe. Her meowing is what woke me up.

[keith english]

I have also wondered about light fall off from subject to camera distance vs. flash to subject distance. If light from a flash falls off so fast, why doesn't light reflected from the subject back to the camera fall off just as fast. We don't make calculations based on subject to camera distance. I asked a professor once and he basically said "that's one of those things it's best not to think about". Photography professor of course, not physics.

[CG]

Illuminate two target surfaces, say for example that each are two foot by two foot white cards, to the same brightness - one ten feet away and one twenty feet away from you - and they'll look the same brightness. Note that the more distant one is also perceived as half the perceived height - or a quarter the perceived area.

All the light coming your way from the more distant target is coming from a narrower angle, and the reduction in area "concentrates" the light, which exactly offsets the effect of distance in terms of the brightness of the surface.

So, a spot meter pointed at the targets would show the same reading from both.

Put down the spot meter. Grab an incedent meter. Stay at your station ten feet from one card and twenty from the other. Now, turn one light off, then the other. Use the incedent meter to determine how much light is hitting YOU from each card. The total light you are receiving twenty feet away from the distant surface is one quarter of that which you are receiving from the nearer ten feet from you.

So the fall off is in total light received - but not in apparent brightness when equally bright targets are at different distances.

Clearer? Or clear as mud?

C

[poco]

"All the light coming your way from the more distant target is coming from a narrower angle, and the reduction in area "concentrates" the light,"

Something doesn't sound right. Wouldn't the same "concentrated" light hit the dome of the incident meter?

Edit -- never mind, I think I get what you're saying.

[Rakesh Malik]

I have also wondered about light fall off from subject to camera distance vs. flash to subject distance. If light from a flash falls off so fast, why doesn't light reflected from the subject back to the camera fall off just as fast. We don't make calculations based on subject to camera distance. I asked a professor once and he basically said "that's one of those things it's best not to think about". Photography professor of course, not physics.


It DOES fall off just as quickly, but since you're measuring the light from the subject AT the camera, you're measuring the falloff as well. Hence you don't have to care about falloff; your meter sees what it sees.

The reason that you have to take falloff into account for a flash is that you're illuminating a subject, and you need to give it more light if it's farther away. However, the TTL meter is still measuring the light at the camera, so it's seeing the reduced intensity that comes from falloff, so in the end, you still don't need to care except when you're setting up the lights.

[keith english]

the TTL meter is still measuring the light at the camera, so it's seeing the reduced intensity that comes from falloff, so in the end, you still don't need to care except when you're setting up the lights.

But if you meter at the subject and back up 100 feet, you don't open up to compensate do you? Always has been a mystery to me. CG's answer about subject size make sense though.

[Rakesh Malik]

But if you meter at the subject and back up 100 feet, you don't open up to compensate do you? Always has been a mystery to me. CG's answer about subject size make sense though.

I don't do that, I meter from where I shoot using a spot meter, so I hadn't thought about it reading this thread :)

[walter23]

So Struan, pretending I understood all that, let me ask, what about pinholes? Do they also function as light sources with fall-off from off-axis travel distance?

If you think about how a pinhole works (just lets "rays" moving basically directly from a point in the world to a point on your film), you'll notice that not only are the edges of the film further from the pinhole than the center, but also the real world objects that illuminate the edges of the film are also further from the pinhole in front of the camera because they are off axis (at least in a two dimensional case). This difference is probably minimally important in reality, and who photographs 2D scenes anyway.

[Helen Bach]

Thank goodness that poco didn't ask 'which theta?'

[walter23]

You wanna be careful - most people with zero science background start screaming "animal abuse"when they first hear about Schrödinger's cat. :confused:


It's not animal abuse until you open the box, and then it's only abuse if the cat is dead. Who can say whether he's suffering when he's both alive and dead though. Actually I don't know much about quantum mechanics (though maybe a little more than the layperson having taken a QM course as an undergrad) but I suspect the Schroedinger's cat illustration is just an issue of confusing the mathematical map with the territory. Yeah, QM is based on statistics and probabilities... doesn't mean both states exist, just that since you aren't observing them you can only talk about their probabilities, in the case of the cat 50:50 that he's alive or dead.

Also the concept of "observation" blurs things a bit. As organic machines made out of quantum mechanically based matter, are we really any different from a geiger counter that also observes the quantum state of the particle? I mean you've got a geiger counter observer to induce "wavefunction collapse" or whatever the hell, so the dual state cat doesn't really exist except in the mathematical model. These common interpretations of quantum mechanics as this big mystery kind of neglects this I think... but I really don't know much about it and I might be missing the point (which actually seems to be in accord with Schroedinger's point - a strict interpretation of the descriptive mathematics as reality fails, the map is not the territory, or a blurry photograph from a shaken camera is not a cloud bank).

The cat thought-experiment:
http://en.wikipedia.org/wiki/Schr%C3%B6dinger's_cat

[Marko]

You wanna be careful - most people with zero science background start screaming "animal abuse"when they first hear about Schrödinger's cat. :confused:

You can always direct the confused souls to some lighter reading such as The Cat Who Walks Through Walls (http://en.wikipedia.org/wiki/The_Cat_Who_Walks_Through_Walls).

But then you'd have to be careful with apug members, since that particular cat goes by the name of Pixel...

I suppose that Garfield would be entirely appropriate cat story for those who fall into the intersection of these two groups.

:D

P.S.

But now, what about Schrödingbug?