Advanced Technique & Process 2A: Testing Inverse Square Law (pt2)

27th October 2014

Testing the Inverse Square Law of light


^ The set up (here with just the regular head fitting on) ^

Equipment used:
Canon 5D mk iii DSLR camera
Elinchrom Style 400FX, studio light
Soft Box head fitting
Regular head fitting
Connector wires
Camera always set to 1/125 sec and ISO 100 – adjusting aperture only

Soft Box


^ 5ft away from the light source-f11. ^ 10ft(?) away at f11. ^ 10ft(?) away-f5.6

Here we can see that the law hasn’t worked. HOWEVER – there are many factors that affect this and most of them are my fault. I had not had chance to purchase a light meter before this shoot so had to go with the exposure meter in camera to see which f stop appeared correct. However the screen on my camera is more contrasted and slightly darker than when the image is seen on a laptop, so image 1 appeared a correct exposure on my camera screen. So because I didn’t start off with a correct exposure, the other images that followed were inconsistent. I shot first at f11, 5ft away. Then at f11, 10ft away. Then at f5.6, 10ft away. Also at first I forgot about measuring the distance using a tape measure and estimated it. I can see that this also influenced the results. On the right side of the model was also a window and I did this shoot during the day, however used 1/125 sec shutter speed to cancel out as much ambient light as possible.

I wanted to try two different types of light and see if this had any effect, so I switched to the regular head for the light source (as seen in the set up image at the top)…



^ 3ft away from the light source-f16. ^ 6ft away-f16. ^ 6ft away-f8

This time I measured using the tape measure – 3ft to 6ft. It turns out the estimated ’10ft’ from before was actually 6/7ft. However this time round the first exposure was correct opposed to the first exposure with the soft box. I would say the last image exposed is at least a stop too bright. So the Law only works in perfect conditions – with the exact measurements, one exact light source.

I also tried photographing with the light behind me and shining onto the subject, then moving the subject further away from the light to see how this worked – balancing the day light with the flash…


^ The lighting set up (unfortunately my wireless triggers hadn’t arrived yet so I had to use cords).


^ 1/64 sec, f8, ISO 100. ^much further away at the same settings.


^ 1/64 sec, f4, ISO 100. ^ 1/125 sec, f4, ISO 100


^ 1/125 sec, f4, ISO 100. ^1/80 sec, f4, ISO 100.

(Playing around with shutter speed to adjust the ambient light being recorded).


^1/100 sec, f5.6, ISO 100.

I want to keep on testing the Inverse Square Law in different lighting situations to grow in my understanding of it and to see if I can find the ‘perfect lighting scenario’ (if that exists).

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