photographic conundrum

[FrankS]

Strickly hypothetical. Just trying to juice up the question/problem.

Specular highlights?

 

[ferider]

There is an advantage of thinking about the light source instead of the observer:

there she is, barely dressed, under soft light smiling at you. Now look at the
reflection of her skin and how these billion little light sources work together
to emit mostly parallel, diffused light 🙂

Cheers,

Roland.

 

[Bryce]

Roland-
That could be distracting...

The star photo / point source thing is a VERY special case and almost never applies.

Frank-
If you view an object from two distances, one exactly twice the other, 1/4 as much light reaches you from the farther object. That is the inverse square law, right? At the same time, the farther object appears half as high and half as wide as the near one, so 1/4 as much area, and that adds up to the same 4:1 ratio as the amount of light reaching you from them.
So the brightness of the object remains the same, just its area and total photon count from your perspective changes.
Make sense?

 

[FrankS]

Roland-
That could be distracting...

The star photo / point source thing is a VERY special case and almost never applies.

Frank-
If you view an object from two distances, one exactly twice the other, 1/4 as much light reaches you from the farther object. That is the inverse square law, right? At the same time, the farther object appears half as high and half as wide as the near one, so 1/4 as much area, and that adds up to the same 4:1 ratio as the amount of light reaching you from them.
So the brightness of the object remains the same, just its area and total photon count from your perspective changes.
Make sense?

Bingo! You and Noel got it. This bugged me for quite a while before I found the answer.

 

[nikon_sam]

Let me throw this into the mix...

From what I have read you are lighting and shooting a subject and then moving back twice the distance and getting the same exposure when you think you should have less light...The subject to light distance is the same just the camera to subject has changed...If you shoot with the same lens the subject will be smaller but still lit the same.
Let's say you move back 100' with the same lens the subject will be lit properly you just might not be able to tell...Now put a tele lens on the camera and bring that sucker in and you're back to square one.
Now if the light source is mounted on the camera this does not apply...

 

[FrankS]

Let me throw this into the mix...

From what I have read you are lighting and shooting a subject and then moving back twice the distance and getting the same exposure when you think you should have less light...The subject to light distance is the same just the camera to subject has changed...If you shoot with the same lens the subject will be smaller but still lit the same.
Let's say you move back 100' with the same lens the subject will be lit properly you just might not be able to tell...Now put a tele lens on the camera and bring that sucker in and you're back to square one.
Now if the light source is mounted on the camera this does not apply...

That's correct.

 

[Bryce]

Frank-
You've caused a great deal of trouble today, I'm sure glad it is over. Like all things that take effort to take in, your post on reciprocity failure caused me to learn a little more about something that has caused me a ton of frustration, the multiple flash underexposure problem.
I can't thank you enough!
So when are you going to come up with another one?

 

[FrankS]

Thanks Bryce!

Okay, 2 trains leave the same station travelling in opposit directions....

 

[Ronald M]

Because the law refers to a point light source or reflected body.

A subject is not a point source.

 

[FrankS]

Anyone have another "photographic problem" to post?