Plants grow best when they have plenty of light and cameras produce their best photographs under the same sort of conditions. If there is not enough light, plants become yellow and elongated, and cameras produce photos that are fuzzy or too dark.
In gardening there is always a lot of discussion of the business of "enough light" but rarely any clear definition of what constitutes "enough". There are phrases like "full sun" and "partial shade" but these refer to the number of hours in the day during which a given area of the garden will be bathed in the maximum available sunshine. This takes no account of the strength of sunshine available, which may vary significantly depending on the time of year and the location of the garden.
So here's a question: Can we measure the intensity of sunlight using a digital camera? I don't mean with a level of accuracy that runs to the nearest photon. What's needed here is just accuracy sufficient to support meaningful discussion of gardening conditions between people who may live in very different parts of the world. For example, could a gardener in Egypt describe his gardening conditions to a Canadian gardener by using his digital camera to take measurements of light intensity?
I've been experimenting with this a bit and here's one of my first attempts.
This photo was taken at the end of spring (29th March) with the following parameters:
1/80th second exposure
The photograph below was taken with the same camera, lens, and location, but on the first day of December, of the same year.
1.3 second exposure
So can we calculate the difference in light intensity based on these parameters? To make it easier I have taken the second photograph using the manual setting on the camera, and I have set the ISO and aperture to the same levels as in the first photograph.
I had to use a tripod to take the photograph, as the only paramater I wanted to vary was the exposure length. I ended up needing quite a long exposure, which would have introduced a lot of camera shake without a tripod.
The answer is - Yes! It's actually pretty easy to make a direct comparison of light levels using a camera. However, it really helps if the same model of camera and lens is used to measure both samples, and it really very much helps if the camera has a manual mode to enable some parameters to be kept constant between both tests.
The bottom line though, is that if you want to talk gardening with your distant friends, even friends who may currently be in a different season (perhaps Australia versus UK) or in a different latitude (e.g. Northern Scotland versus Botswana) then it's pretty easy to do it.
I don't think we need to worry about expressing light intensity in Lux, or photons or anything tricky like that. As gardeners we could probably stick to expressing it in seconds of camera exposure time, on the understanding that all other parameters are kept the same between our location the location for comparison.
The next question is to ask how this works with different models of camera, but that is a whole other thing. A question for another day I think.
An add-on -
MaryWilliams @PlantTeaching on Twitter has suggested that this work might also be done using a smart phone app. I've had a quick look online and it seems that there are already a number of light meter apps available. If you try this, please do write and let me know how it goes. I will compile the results from different locations.