Approve the Cookies
This website uses cookies to improve your user experience. By using this site, you agree to our use of cookies and our Privacy Policy.
OK
Index  •   • New posts  •   • RTAT  •   • 'Best of'  •   • Gallery  •   • Gear  •   • Reviews
Guest
New posts  •   • RTAT  •   • 'Best of'  •   • Gallery  •   • Gear  •   • Reviews
Register to forums    Log in

 
FORUMS Photography Talk by Genre General Photography Talk 
Thread started 14 Nov 2011 (Monday) 22:50
Search threadPrev/next
sponsored links
(this ad will go away when you log in as a registered member)

Does all light decay with the distance? (Technical question)

 
RafaPolit
Goldmember
Avatar
1,668 posts
Joined Jun 2005
Location: Quito, Ecuador
     
Nov 14, 2011 22:50 |  #1

Friends,

The title may sound absurd, but bare with me for a second ;). I was today at a concert and was wondering about the light, knowing that it decays with distance and thinking about the scene in general, and then... a doubt jumped at me:
- The light coming from a light bulb, a flash, the sun, etc. decays with the distance. We all (or I assume most :) ) know that. But...
- After it hits an object and reflects (which is the light our eyes, or the sensor finally sees) does it still decay? The probably obvious answer is that it does still decay once it is reflected from a surface. Therefore, and this is my question:

If I am on a concert on a row 200 feet away, would I need to expose my frame 2 stops brighter than a fellow that is in a row 100 feet away? And the person 50 feet away could get with 4 stops faster shutter than me. Is this correct? It doesn't seem that way! It would seem logical that everyone needs to expose with the same exposure, yet, the light is still traveling... my sensor can only 'see' what enters the lens and that light is loosing strength with the distance.

So... what am I missing? What am I overlooking here? Does any of this makes sense or does anybody even care about such things or am I, as I have long suspected, a nerd! :) .

Thanks for any input,
Rafa.


Rebel T2i | EF-S 17-55 IS | EF 70-200 f4L | EF-S 10-22 | 430EX II |
Picture Galleries at:
www.rafaelpolit.com (external link)

  
  LOG IN TO REPLY
sponsored links
(this ad will go away when you log in as a registered member)
DC ­ Fan
Cream of the Crop
Avatar
5,881 posts
Gallery: 3 photos
Likes: 50
Joined Oct 2005
     
Nov 14, 2011 23:26 |  #2

RafaPolit wrote in post #13401015 (external link)
So... what am I missing? What am I overlooking here?

The inverse square law. (external link)




  
  LOG IN TO REPLY
RafaPolit
THREAD ­ STARTER
Goldmember
Avatar
1,668 posts
Joined Jun 2005
Location: Quito, Ecuador
     
Nov 14, 2011 23:32 |  #3

DC Fan, that was exactly the point of my question. The inverse square law would dictate that the light reflected from the singer, dancer, scenery, etc., is decaying inversely square to the distance, therefore, someone closer to the subject is actually receiving more light. That was exactly the point of my question... I thank you for the link, but I fail to see how that answers or confirms my question / suspicion.

Rafa.


Rebel T2i | EF-S 17-55 IS | EF 70-200 f4L | EF-S 10-22 | 430EX II |
Picture Galleries at:
www.rafaelpolit.com (external link)

  
  LOG IN TO REPLY
Tiberius
Goldmember
Avatar
2,545 posts
Likes: 8
Joined Apr 2008
     
Nov 15, 2011 00:46 |  #4

RafaPolit wrote in post #13401015 (external link)
Friends,

The title may sound absurd, but bare with me for a second ;). I was today at a concert and was wondering about the light, knowing that it decays with distance and thinking about the scene in general, and then... a doubt jumped at me:
- The light coming from a light bulb, a flash, the sun, etc. decays with the distance. We all (or I assume most :) ) know that. But...
- After it hits an object and reflects (which is the light our eyes, or the sensor finally sees) does it still decay? The probably obvious answer is that it does still decay once it is reflected from a surface. Therefore, and this is my question:

If I am on a concert on a row 200 feet away, would I need to expose my frame 2 stops brighter than a fellow that is in a row 100 feet away? And the person 50 feet away could get with 4 stops faster shutter than me. Is this correct? It doesn't seem that way! It would seem logical that everyone needs to expose with the same exposure, yet, the light is still traveling... my sensor can only 'see' what enters the lens and that light is loosing strength with the distance.

So... what am I missing? What am I overlooking here? Does any of this makes sense or does anybody even care about such things or am I, as I have long suspected, a nerd! :) .

Thanks for any input,
Rafa.

The Inverse square law says that if you double the distance between the light source and the surface it is reflecting off, the light intensity drops to a quarter. Think of it like this:

If you have a light source that is aimed at the wall, it will light up a certain area of that wall. Let's say it lights up a square one foot high by one foot wide. That's an area of one square foot. If you move the light source away and double the distance between it and the wall, then the light is now shining a square that is two feet high and two feet wide. This is four square feet in area. So each square foot is now receiving a quarter of the light that the original square received.

Once the light reflects off the wall, it is subject to the inverse square law again. The wall reflecting the light is now acting as a light source (as far as the camera or your eyes are concerned) because any photons you see are coming from the wall.

So it's amazing flash photography works at all.

However, since the non-lit part of the wall also has its distance to the camera variable by the same amount (in other words, if you change your distance to the lit part of the wall, you change your distance to the unlit part of the wall as well), the ration of light coming from the lit part to the unlit part remains the same, so you won't see any changes between them. The lit part will always appear twice as bright as the unlit part, for example.

Not quite sure how it applies to the situation you described. I propose that a series of experiments be conducted to find out!


My photography website!PHOCAL PHOTOGRAPHY (external link)

  
  LOG IN TO REPLY
krb
Cream of the Crop
Avatar
8,818 posts
Likes: 7
Joined Jun 2008
Location: Where southern efficiency and northern charm come together
     
Nov 15, 2011 01:00 |  #5

RafaPolit wrote in post #13401149 (external link)
I thank you for the link, but I fail to see how that answers or confirms my question / suspicion.

Perhaps it would help if you phrased the question in a different manner. It appears that the question you want answered is:

"Do the laws of physics still apply to photons after they bounce off an object?"

I'm pretty sure the answer to this one will be unanimous...


-- Ken
Comment and critique is always appreciated!
Flickr (external link)
Gear list

  
  LOG IN TO REPLY
Cyth0n
Senior Member
283 posts
Joined Jan 2007
     
Nov 15, 2011 06:58 |  #6

If I'm understanding the question correctly, RafaPolit makes the point that reflected light falls off with distance according to the inverse squared relationship. The question is, if this is the case, then why do lit objects that are further away not appear to be darker than when they're closer?

It sounds counter-intuitive. I have no idea what the answer is, except to say that I think the inverse squared relationship only applies to point light sources. I'm not sure if a lit object would count as a point light source.


[My Gallery (external link)]

  
  LOG IN TO REPLY
J ­ Michael
Senior Member
970 posts
Gallery: 7 photos
Best ofs: 1
Likes: 51
Joined Feb 2010
Location: Atlanta
     
Nov 15, 2011 07:18 |  #7

All the ISL is describing is the change in intensity of light on the surface of a sphere at varying distances. Although the ISL applies to point sources you can treat a broad source as an array of point sources for computational purposes, but this is only important for relatively short distances since a broad source starts acting like a point source as distance increases. Some sources project a beam and for those you can treat them as having a virtual point source behind the actual source, taking the beam spread angle into account and calculating the change in area with distance.

There is an f-stop distance trick that's useful for using a flash over a long distance. Let's say you know your flash gives a good exposure for f8 at 8 feet. Then for every increase in distance in f-stop feet, open a stop. So, for 11 feet f5.6, 16 feet f4, 22 feet 2.8, etc.




  
  LOG IN TO REPLY
arkphotos
Senior Member
455 posts
Likes: 1
Joined Mar 2008
Location: Plano, Texas
     
Nov 15, 2011 07:26 |  #8

~warning~ not really scientific & I may have some facts wrong ...~

I think it was already stated, but the practical application relates to the distance between the light source and the subject - not the light once its reflected off the subject toward your camera.

As distance between the light source and subject grows, 'less light' is hitting the subject. This defines the brightness of an object.
As distance between the subject and camera grows, less light is reaching your camera, but its proportional to the 'size' of the subject so ths subject still has the same brightness.


1.6 crop & some lenses

  
  LOG IN TO REPLY
digital ­ paradise
How do I change this?
Avatar
14,736 posts
Gallery: 1 photo
Likes: 8034
Joined Oct 2009
     
Nov 15, 2011 07:30 |  #9

A single photon which is energy does decay eventually but after it has travelled a very, very long way. So the reason that you require more flash when the subject is further away from the light source is due to fall off/inverse square law and not due the decay of the actual photon itself.


Image Editing OK

Website (external link) ~ Buy/Sell Feedback

  
  LOG IN TO REPLY
Martin ­ Dixon
Goldmember
Avatar
1,864 posts
Gallery: 59 photos
Likes: 274
Joined Sep 2009
Location: Ealing
     
Nov 15, 2011 07:36 |  #10

Perhaps this helps:
A lit infinite wall will be the same brightness however far away you get (forgetting dust, vapour etc. in the atmosphere) - alternatively a camera will only get 1/(d*d) of the light from a particular square inch but there will be d*d x as many square inches in the viewfinder so cancels out.

If you had a (theoretical!) simple lens with variable focal length and exactly framed the same brick, increasing distance would then reduce the brightness (f-stop would be increasing as focal length changed). In practice clever zoom optics can allow say 70-200 f2.8 to have constant aperture over the zoom range which is amazing if somewhat confusing.

Flash on the other hand is a near point source and the inverse square law applies to illumination of the subject (assuming it is moving with the camera and not fixed relative to subject)


flickr (external link) Editing OK (external link)

  
  LOG IN TO REPLY
jra
Cream of the Crop
Avatar
6,565 posts
Likes: 35
Joined Oct 2005
Location: Ohio
     
Nov 15, 2011 08:09 |  #11

The law does apply, but, instead of the object appearing dimmer, it appears smaller on the sensor the further away you get. Think of it this way, if you take a photo of a 2'x'3' wall and completely filled the frame from 5' away, if you backed up to 10' and took the same photo, your wall would now only occupy 1/4 of the frame, your camera is now only capturing 1/4 of the light that it was when you were at 5' although your exposure settings didn't change.
To keep the object framed the same as you move away, you would need to use a longer lens and when you use a longer lens, you will be using a physically larger aperture to keep the same exposure (considering that the aperture setting is a fraction of the focal length).
The fact that the light falls off as you move away is the reason that a lenses aperture is directly related to its focal length.




  
  LOG IN TO REPLY
digital ­ paradise
How do I change this?
Avatar
14,736 posts
Gallery: 1 photo
Likes: 8034
Joined Oct 2009
     
Nov 15, 2011 08:17 |  #12

This is pretty good.

http://www.youtube.com​/watch?v=f5BIvSBjvLg (external link)


Image Editing OK

Website (external link) ~ Buy/Sell Feedback

  
  LOG IN TO REPLY
tzalman
Fatal attraction.
Avatar
13,475 posts
Likes: 196
Joined Apr 2005
Location: Gesher Haziv, Israel
     
Nov 15, 2011 08:17 |  #13

Simple common sense tells me that if I stand 8 feet from an evenly lit wall (sunlight) and it meters at f/4, when I move to 4 feet away it will not suddenly become 4 times brighter and meter at f/8 or need f/16 at 2 feet or f/32 at 1 foot. If that were to become true, macro shooters would celebrate.


Elie / אלי

  
  LOG IN TO REPLY
SkipD
Cream of the Crop
Avatar
20,476 posts
Likes: 158
Joined Dec 2002
Location: Southeastern WI, USA
     
Nov 15, 2011 08:22 |  #14

RafaPolit wrote in post #13401015 (external link)
If I am on a concert on a row 200 feet away, would I need to expose my frame 2 stops brighter than a fellow that is in a row 100 feet away? And the person 50 feet away could get with 4 stops faster shutter than me. Is this correct? It doesn't seem that way! It would seem logical that everyone needs to expose with the same exposure, yet, the light is still traveling... my sensor can only 'see' what enters the lens and that light is loosing strength with the distance.

IF all of the light illuminating the stage is coming from the same place for all of the photographers (such as continuous stage lighting, not flash units at the photographer's locations), there will be NO DIFFERENCE in the exposure settings for the photographers at various distances from the stage.

If the stage is being lit by flash sources on each photographer's camera, then the more distant photographers will either have to have more powerful lights or wider aperture settings.


Skip Douglas
A few cameras and over 50 years behind them .....
..... but still learning all the time.

  
  LOG IN TO REPLY
PhotosGuy
Moderator
Avatar
75,866 posts
Gallery: 8 photos
Likes: 2516
Joined Feb 2004
Location: Middle of Michigan
     
Nov 15, 2011 08:38 |  #15

tzalman wrote in post #13402206 (external link)
Simple common sense tells me that if I stand 8 feet from an evenly lit wall (sunlight) and it meters at f/4, when I move to 4 feet away it will not suddenly become 4 times brighter and meter at f/8 or need f/16 at 2 feet or f/32 at 1 foot. If that were to become true, macro shooters would celebrate.

And astronomers would have nothing to look at! ;)


FrankC - 20D, RAW, Manual everything...
Classic Carz, Racing, Air Show, Flowers.
Find the light... A few Car Lighting Tips, and MOVE YOUR FEET!
Have you thought about making your own book? // Need an exposure crutch?
New Image Size Limits: Image must not exceed 1600 pixels on any side.

  
  LOG IN TO REPLY
sponsored links
(this ad will go away when you log in as a registered member)

5,093 views & 0 likes for this thread
Does all light decay with the distance? (Technical question)
FORUMS Photography Talk by Genre General Photography Talk 
AAA
x 1600
y 1600

Jump to forum...   •  Rules   •  Index   •  New posts   •  RTAT   •  'Best of'   •  Gallery   •  Gear   •  Reviews   •  Member list   •  Polls   •  Image rules   •  Search   •  Password reset

Not a member yet?
Register to forums
Registered members may log in to forums and access all the features: full search, image upload, follow forums, own gear list and ratings, likes, more forums, private messaging, thread follow, notifications, own gallery, all settings, view hosted photos, own reviews, see more and do more... and all is free. Don't be a stranger - register now and start posting!


COOKIES DISCLAIMER: This website uses cookies to improve your user experience. By using this site, you agree to our use of cookies and to our privacy policy.
Privacy policy and cookie usage info.


POWERED BY AMASS forum software 2.1forum software
version 2.1 /
code and design
by Pekka Saarinen ©
for photography-on-the.net

Latest registered member is xemlicham
1073 guests, 244 members online
Simultaneous users record so far is 15144, that happened on Nov 22, 2018

Photography-on-the.net Digital Photography Forums is the website for photographers and all who love great photos, camera and post processing techniques, gear talk, discussion and sharing. Professionals, hobbyists, newbies and those who don't even own a camera -- all are welcome regardless of skill, favourite brand, gear, gender or age. Registering and usage is free.