Project 55: Light Intensity

I have a 1000w studio light which I used for this project. It is a constant tungsten light, and does indeed become very hot. I have had it for about 18 months, and don't use it very often, mainly because I don't really know how to!

Here's the full f-stop scale for reference: f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32. I also had a look back at my notes from T189 and each f-stop approximately doubles the area of the aperture. One stop is an effective doubling (or halving) of the area of the aperture, the shutter speed or the ISO equivalency. One stop represents half as much light. It is possible to derive the f-stop scale by taking the square root of 2 & multiplying it by itself. I've just tried this in Excel & it works, so handy to know if you forget the f-stop scale and happen to have a scientific calculator in your handbag!

The first problem that I had was using the suggested settings from the notes i.e. ISO 100 & a shutter speed of 1/30. The aperture readings for the bright midday sun & cloudy day were f27 & f6.7 respectively. The 1m reading was f11 and the 2m reading was f4 (which was also the widest aperture setting for the lens attached to the camera, so the actual aperture reading may have been below that). Therefore beyond 2m, it was impossible to take any more readings because the aperture had reached its widest value, and the reading at 2m was unreliable because my particular lens was not fast enought. This was a bit of a surprise, because the 1000W lamp seems so blindingly bright and yet is considerably weaker than outdoor light even a short distance directly in front. By 1m distance from the lamp it is 3 stops weaker than midday sun. Beyond 2m it is unusable at ISO 100 and the slowest shutter speed for handheld. 2 readings were clearly insufficient to draw a graph but it was a useful illustration.

So, determined to get my graph, I then changed the ISO setting to 800 (3 stops) in order to get more readings. I am unable to go out to 12 metres through because my house isn't big enough! I then converted the sunny & cloudy readings to approx f76 (using the handy square root of 2 method!) and f19 for comparison at ISO 800. This gave me readings out to 8 metres (the length of my house) which clearly demonstrated firstly the inverse square law (approximately); secondly the weakness of indoor lighting compared to natural light and thirdly that the light meter from the camera is difficult to use for this type of experiment (particularly when you are restricted by the minimum & maximum apertures of a lens)!
















Whilst the results are demonstrated graphically, they can be summarised as follows:

When the distance from the light doubles, the strength of the light falls to one quarter the intensity. As one stop represents half as much light, then it follows that 2 stops represent a quarter as much light.

Therefore as the distance from the light doubles, the intensity of the light falls by 2 stops.

In practical terms, this would mean increasing the ISO from 100 to 400 say, or reducing the shutter speed from 1/100s to 1/25s.

I'm loving the maths, but what does this mean for photography in practical terms? Well, like a lot of things, it comes down to money. For still life photography, it means a tripod is essential to maintain image quality (and good tripods cost money!). For indoors portraiture, it means very powerful mains flash & associated bits & bobs to soften the light (big money). And for indoors sports photography it means very fast lenses and high spec cameras with a huge ISO range to enable fast shutter speeds to capture the action (Very very big money). It also means many digital compacts are not really suited for indoors photography because the high ISO makes many images unusable (see project 39) and they can't achieve the wide apertures required. Image stabilisation has helped to a degree, but there are still issues with image quality.

As always, an interesting project which helped to cement the science in my mind. I am enjoying the artificial light section of course, more so than the natural light section, for the simple reason that being able to control the lighting means you can do the projects at your own convenience. This is the great convenience of indoor lighting! Waiting for the right weather, and then having it coincide with being available to go out shooting, was quite frustrating given my restricted time due to small children & work.