Project 38: Measuring the intensity of light

This is a project I was very keen to do, as I am interested in the "science" of photography (although I still find it hard to grasp). I plan to do this in 2 attempts. The first is an attempt at the end of February and I plan to repeat the exercise in the summer.

My first attempt did not go as well as I hoped, mainly because of logistical difficulties and fluctuating weather and the impact of scattered clouds. The problem I experienced was finding a day in Winter which had sunshine throughout, which also happened to be a day when I wasn't at work or otherwise engaged. This meant it took 3 attempts to finally produce some meaningful data which took me to 15th March. It also gave me an understanding of how much light is reduced indoors. I am fortunate in living in a "light" house (rather than a dark Victorian terrace house) and it receives a lot of sunlight indoors. Even so, I was surprised how much this was reduced, even in the Conservatory, where I thought the intensity would be similar to outside. I found the light indoors in direct sunlight to be around a stop less than direct sunlight outside. What was also interesting was how quickly the light fell away in late afternoon. Using shutter priority in February, it was not possible to take a picture at 1/125s by 4.30 as the aperture couldn't get wide enough to let in enough light (and this was with a fast lens that opened to f2.8).

The notes require the exercise to be done in shutter priority, using a shutter speed that enables handheld photography. As I tend to operate mostly in aperture priority, I was interested to view the results for an aperture of f11 which tends to give the sharpest images. This would give me a feel for when that sort of aperture would be feasible. All images were of a piece of cork board (to ensure no reflected light, and a neutral colour) taken in direct sunlight. I used an average ISO of 200.

For my mid February results, using an aperture of f11, sharp handheld photography would only be possible in direct sunlight between 11 and 2 o clock. Outside of those times, a wider aperture could be used, with a resultant loss of sharpness, or a tripod, or a higher iso with a resultant loss in quality. My conclusion is therefore that a tripod is essential for professional sharpness outside of midday sun in Winter. By 15 March this had extended to most of the day, provided the subject was in the sun, right up to 5 o clock. Obviously using a higher ISO would extend that range further. The impact of Spring arriving is therefore quite significant on the light intensity, and this is a phenomenon that is also experienced in our conservatory. During the months of winter (from November through to the beginning of March) a sunny day has little effect on the temperature in our conservatory. In March, the sun reaches a height and intensity that is sufficient to heat the room (and our house) even if it is cold outside.

Going forward, I plan to repeat this exercise in June (unless we have yet another soaking wet summer). I expect that the graph will have an elongated shape due to the longer days, with much greater intensity at the middle of the day (hence sunburn is common in Summer but rare in March). Had I completed this project in January, the graph would be more of a spike due to the shorter days, and a much lower peak intensity would be recorded as the winter sun does not get high in the sky. During my 3 attempts at the project, I experienced the effect of cloud which reduced the intensity of the light. I imagine the thicker and blacker the cloud, the greater the impact on the light intensity.

UPDATE 23 JUNE

Well I completed a whole day of readings as we're having glorious sunny weather at the moment. I thought it would be interesting to upload my results from summer solstice - 21 June 2010. I chose to fix the settings at f11 & ISO 100, which are generally the gold standard for image quality. Note that I used ISO 200 for the winter readings. The key findings were:

1. When light intensity is at its peak during the day, there is no difference in intensity between early spring & summer.
2. The camera meter is fallible, in that I got different readings when shooting vertical as opposed to horizontal? If I placed the cork board on the ground, I got a lower reading than if I shot it vertical, even if it was fully sunlit in both instances. This one is a mystery to me.
3. On the longest day, provided it is sunny, it is possible to shoot handheld at the gold standard settings of f11 & ISO 100 for most of the day from 7 am onwards (although an extreme telephoto lens may require additional image stabilisation or a higher ISO setting) right through to 8pm.

So here's the graph of my results.