Question on the inverse square law

R

Revet

Senior Member
I was playing around with my flash, testing the inverse square law and I got results different from the theory.

equipment used: D3100 (D7100 is on my wish list!!!), SB700 Speedlight,

settings used: manual setting, Raw, 1/200 shutter, f/8, Iso 400, manual flash setting. The auto ISO was turned off so the camera couldn't try adjusting ISO.

1st shot taken at 8 feet, flash power 1/16
2nd shot taken at 16 feet, flash power 1/16
3rd shot taken at 16 feet, flash power 1/8 (one stop increase)
4th shot taken at 16 feet, flash power 1/4 (two stop increase)

According to the inverse square law, when I move from 8 feet to 16 feet, I have lost two stops of flash power so the 4th shot should be close to the first. What I am finding is that the 3rd shot (one stop increase to compensate for 2x distance) is much closer to the first shot than the 4th shot (two stop increase in flash power). The 4th shot is quite a bit brighter in both the histogram and visually than the first.

As an added note, the ceiling is 8 feet (white) but I was using direct flash without any diffusers or filters. Also, I did zoom in at the 16 foot range to match the frame of the 8 foot shot for better comparison.

Any ideas as to why I am not proving the Inverse Square Law in this example??
 
RockyNH_RIP

RockyNH_RIP

Senior Member
Revet said:
I was playing around with my flash, testing the inverse square law and I got results different from the theory.

equipment used: D3100 (D7100 is on my wish list!!!), SB700 Speedlight,

settings used: manual setting, Raw, 1/200 shutter, f/8, Iso 400, manual flash setting. The auto ISO was turned off so the camera couldn't try adjusting ISO.

1st shot taken at 8 feet, flash power 1/16
2nd shot taken at 16 feet, flash power 1/16
3rd shot taken at 16 feet, flash power 1/8 (one stop increase)
4th shot taken at 16 feet, flash power 1/4 (two stop increase)

According to the inverse square law, when I move from 8 feet to 16 feet, I have lost two stops of flash power so the 4th shot should be close to the first. What I am finding is that the 3rd shot (one stop increase to compensate for 2x distance) is much closer to the first shot than the 4th shot (two stop increase in flash power). The 4th shot is quite a bit brighter in both the histogram and visually than the first.

As an added note, the ceiling is 8 feet (white) but I was using direct flash without any diffusers or filters. Also, I did zoom in at the 16 foot range to match the frame of the 8 foot shot for better comparison.

Any ideas as to why I am not proving the Inverse Square Law in this example??
Click to expand...

Couple of quick thoughts... Pictures would help us see better...

Are you using on camera flash?? That is your problem (likely) if you are... you are moving flash to subject and subject to lens..

I am not an expert... but I think you want to fix the camera distance for your study and then move flash only..

Pat in NH

(the smarter ones will be along soon!!) :)
 
G

gqtuazon

Gear Head
I am no flash expert but from what I know is the farther you are from the subject the less power you need.



Sent from my iPhone.
 
S

Steve B

Senior Member
The inverse square law is only applicable for the distance between the subject and the flash (light source). If you keep the camera in the same location and double the distance between the flash and the subject the intensity of the light on the subject will be 1/4. Keep in mind that if you meter the incident light on a subject with a hand held meter the settings the meter gives you will be correct for the subject no matter what the distance between the camera and the subject. Wait. Did you have the flash mounted in the hot shoe or off camera? To get an accurate test you need to have the flash off-camera as Pat said.
 
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K

kiwi86

Senior Member
GN=aperture x flash to subject distances
or
aperture=GB / flash to subject distances
or
flash to subject distances=GN / aperture


If your GN is 90 (at 100 ISO) and flash to subject distances si 5 feet this mean, aperture is f/18.
If you use 200 ISO than GB is 90 x 1.4=126
400 ISO means GB x 2
800 ISO meaa GN x2.8


INVERSE SQUARE LAW
intensity=1/flash to subject distances 2


Digital Photography 1 on 1: Episode 59: Inverse Square Law: Adorama Photography TV - YouTube
 
W

WayneF

Senior Member
Revet said:
I was playing around with my flash, testing the inverse square law and I got results different from the theory.

equipment used: D3100 (D7100 is on my wish list!!!), SB700 Speedlight,

settings used: manual setting, Raw, 1/200 shutter, f/8, Iso 400, manual flash setting. The auto ISO was turned off so the camera couldn't try adjusting ISO.

1st shot taken at 8 feet, flash power 1/16
2nd shot taken at 16 feet, flash power 1/16
3rd shot taken at 16 feet, flash power 1/8 (one stop increase)
4th shot taken at 16 feet, flash power 1/4 (two stop increase)

According to the inverse square law, when I move from 8 feet to 16 feet, I have lost two stops of flash power so the 4th shot should be close to the first. What I am finding is that the 3rd shot (one stop increase to compensate for 2x distance) is much closer to the first shot than the 4th shot (two stop increase in flash power). The 4th shot is quite a bit brighter in both the histogram and visually than the first.

As an added note, the ceiling is 8 feet (white) but I was using direct flash without any diffusers or filters. Also, I did zoom in at the 16 foot range to match the frame of the 8 foot shot for better comparison.

Any ideas as to why I am not proving the Inverse Square Law in this example??
Click to expand...



Zooming the camera is not a factor, except if the flash was on camea, the flash zooming in is a factor, the point of which is to concentrate the flash power into a small area, which is brighter. Generally in many speedlights, zooming from minimum to maximum zoom is near double GN, which is a couple of stops brighter.

If you change absolutely nothing except monitoring distance, then double distance (of direct bare unmodified flash) should be two stops weaker (1/4). Zooming the flash however is modification.

In a small constricted area, it might appear not quite so weak, due to reflections from side surfaces redirected back in, adding to combine.
 
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R

Revet

Senior Member
I had the flash on manual and I did not touch the zoom button so I don't think the zoom changed (but I can check that). I did use the flash on hot shoe (again in manual mode); does it zoom in manual mode without pressing the zoom button??? How about the camera and flash moving from 8 feet to 16 feet together. Is that a factor or should I repeat the experiment with the camera on a tripod, moving only the flash??

If that is the case, this will be interesting because I need to use SU-4 mode (no cable or radio-transmitter yet). I could try to reflect the pop-up backwards to trigger the SB-700. This is like chem lab again!! Love it
 
R

Revet

Senior Member
gqtuazon said:
I am no flash expert but from what I know is the farther you are from the subject the less power you need.

Sent from my iPhone.
Click to expand...

From what I understand, the further you are away, the less drop off of light over a distance (like the sun, because it is so far away, drop off of light intensity is insignificant over any distance on earth). The inverse square law still holds with the sun but it just doesn't matter to us (insignificant). You would need more flash power; however, the further you are away. I am a newbie to flash so if any of you experts out there want to make sure my reasoning is correct, feel free to chime in.
 
S

Steve B

Senior Member
Revet said:
From what I understand, the further you are away, the less drop off of light over a distance (like the sun, because it is so far away, drop off of light intensity is insignificant over any distance on earth). The inverse square law still holds with the sun but it just doesn't matter to us (insignificant). You would need more flash power; however, the further you are away. I am a newbie to flash so if any of you experts out there want to make sure my reasoning is correct, feel free to chime in.
Click to expand...
Yes and no. The light drop off is dependent on the starting light source to subject distance and the amount of change. Think about in a studio environment. Say you had your strobes 20ft away from the subject (big studio :) ) if you move the subject two inches away from the light source the intensity will decrease by a small amount. But if you move the subject twice as far (40 ft) the intensity will be 1/4. It doesn't matter how far the camera is from the subject it only matters how far the light source is from the subject. In your test environment I am sure that the reflective white ceiling is having an effect on your results. Paint your ceiling, walls, and floor a flat black and try it again. :)
 
W

WayneF

Senior Member
Revet said:
I had the flash on manual and I did not touch the zoom button so I don't think the zoom changed (but I can check that). I did use the flash on hot shoe (again in manual mode); does it zoom in manual mode without pressing the zoom button??? How about the camera and flash moving from 8 feet to 16 feet together. Is that a factor or should I repeat the experiment with the camera on a tripod, moving only the flash??

If that is the case, this will be interesting because I need to use SU-4 mode (no cable or radio-transmitter yet). I could try to reflect the pop-up backwards to trigger the SB-700. This is like chem lab again!! Love it
Click to expand...


Yes, the flash still zooms in camera M mode (assuming it all understands the Nikon CLS hot shoe communication system).

You can set the flash zoom to manual, by zooming it manually (to be a mismatch), and the M appears on the LCD, and it won't zoom automatically then. Reset it (Nikon flashes) by by manually zooming it back to match the lens zoom, and the M disappears, and it zooms again.

If not accounting for zoom, I think that was a factor. Zooming the lens to same view at 2x distance is 2x focal length (for same magnification). Zooming the flash is very much cruder and rougher, not the same concepts, and 2x mm is probably only about one stop, but ought to be 2 stops (but isn't). That part is about designing the reflector, not about the power.

Otherwise (assuming manual camera and manual flash and manual zoom), sure, moving the camera and flash (and leaving all else alone) would be same as observing from a different distance. Either way, all else the same, 2x distance should be two stops down. 1/4 power at 2x distance should match 1/16 power at 1/4 the distance. There could be different room reflections occurring then.
 
R

Revet

Senior Member
WayneF said:
Yes, the flash still zooms in camera M mode (assuming it all understands the Nikon CLS hot shoe communication system).

You can set the flash zoom to manual, by zooming it manually (to be a mismatch), and the M appears on the LCD, and it won't zoom automatically then. Reset it (Nikon flashes) by by manually zooming it back to match the lens zoom, and the M disappears, and it zooms again.

If not accounting for zoom, I think that was a factor. Zooming the lens to same view at 2x distance is 2x focal length (for same magnification). Zooming the flash is very much cruder and rougher, not the same concepts, and 2x mm is probably only about one stop, but ought to be 2 stops (but isn't). That part is about designing the reflector, not about the power.

Otherwise (assuming manual camera and manual flash and manual zoom), sure, moving the camera and flash (and leaving all else alone) would be same as observing from a different distance. Either way, all else the same, 2x distance should be two stops down. 1/4 power at 2x distance should match 1/16 power at 1/4 the distance. There could be different room reflections occurring then.
Click to expand...

The flash was in manual (with a big M in the LCD). I have a feeling it was the ceiling causing a problem, I was also next to a light wall on my left. Plus, i was under the interface of a drop ceiling (a little over 6 feet) and the 8 foot ceiling. Maybe I should try it outside at night (when its not snowing). You do raise something that I'm just not grasping. Other people had mentioned that moving the camera and flash together may have been my problem. I thought that made sense at the time. Isn't the reflected light off the subject returning to the lens subject to the Inverse square law also??
 
R

Revet

Senior Member
Steve B said:
Yes and no. The light drop off is dependent on the starting light source to subject distance and the amount of change. Think about in a studio environment. Say you had your strobes 20ft away from the subject (big studio :) ) if you move the subject two inches away from the light source the intensity will decrease by a small amount. But if you move the subject twice as far (40 ft) the intensity will be 1/4. It doesn't matter how far the camera is from the subject it only matters how far the light source is from the subject. In your test environment I am sure that the reflective white ceiling is having an effect on your results. Paint your ceiling, walls, and floor a flat black and try it again. :)
Click to expand...

Makes perfect sense, I don't think I worded my question quite right but thats ok, I get it. I'm doing this in my Man Cave, not a photo studio so forget the black paint!!!! I could do it outside where it is dark though (after it stops snowing)
 
W

WayneF

Senior Member
Revet said:
The flash was in manual (with a big M in the LCD). I have a feeling it was the ceiling causing a problem, I was also next to a light wall on my left. Plus, i was under the interface of a drop ceiling (a little over 6 feet) and the 8 foot ceiling. Maybe I should try it outside at night (when its not snowing). You do raise something that I'm just not grasping. Other people had mentioned that moving the camera and flash together may have been my problem. I thought that made sense at the time. Isn't the reflected light off the subject returning to the lens subject to the Inverse square law also??
Click to expand...

No, exposure does not matter where the camera is (OK, technically if avoiding macro distances, other factors kick in there).

It is one of the hardest subjects to explain, so beginners sources never do. It is enough to know that the camera distance does not affect exposure.

Yes, inverse square law always exists, but our use of it is about light sources, traveling from there to us.

The camera view is a different situation, seeing an illuminated surface out there, and the area it sees is much reduced when it is close, by exactly the same inverse square law relationship, which exactly cancels out, so that camera distance is not a factor of exposure. Yes, twice closer is 4x brighter, but the camera then sees 1/2 size area (1/4 area), so the surface brightness per unit area says constant. See Camera distance does not affect exposure if you dare. :)

Same reason that f/4 is the same f/4, regardless of the focal length or actual corresponding aperture diameter.
So same reason that zooming in tight with a long telephoto lens does not change the brightness of the exposure.

Regarding your result, try again paying close attention to the details. Move the subject or the camera, it only matters how far the flash has to travel. But all else must be the same.




But for example, the sun gives us Sunny 16 exposures of the mountain, no matter how far the camera is from it.
 
R

Revet

Senior Member
gqtuazon said:
What was I thinking?... Never mind.



Sent from my iPhone.
Click to expand...

Not enough coffee?? Too much wine or beer?? I think everyone who has seen the quality of photos you post on this site knew that if you moved back from a subject, you would not have turned your flash power down!! LOL
 
R

Revet

Senior Member
Ok, I repeated the experiment outside in the dark, shooting at my off-white garage door. This time the results were much closer to the expected result. When I was twice the distance, I had to increase flash power by two stops to get a similar exposure. It was slightly more exposed but I attribute that to the snow on the driveway acting as a bounce.

I think this demonstrates nicely that a close by white ceiling and wall will definitely affect your exposure even if using direct flash. I think it also shows that bouncing a flash puts more even light into a room; ie. in the first trial indoors I only needed to increase the power by one stop when I doubled the distance because of the close proximity of the flash to the ceiling and the wall. I assume the bounced light off of these reflective surfaces influenced the exposure enough that I only needed one stop increase in flash power to get an equal exposure at half the distance. And yes, the distance of the camera to the subject was not a factor so the inverse square law holds if the flash is on the camera, moving with it, or off the camera, moving independent of a fixed camera position to the subject.

Thanks for all the comments!!
 
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