so, i did the experiment !
i sat at home all day on the 17th November taking photos every few minutes. the photos i took throughout the day are arranged on the sheet below. The most recent picture is at the top left. The shutter speed and aperture are shown under each picture together with the time stamp.
A few points are worth noting – the easiest setting to change (for my camera) is the ISO setting which i had to set to 1600 at the beginning of test in the morning at 7.27am. The weather forecast suggested an overcast day so i expected to have to use relatively long shutter speeds at a low ISO.
To begin with you can see i experimented by changing the ISO value from 1600 to 200 which allowed me to set the aperture at f/6.3 (although you can see this gave a slightly darker exposure). I then settled on ISO 200 for the rest of the experiment, although over time i had to vary this to give readings at 1/50 (rather than 1/10).
One interesting point to note was the effect of the weather ! it began to rain at 9.38 which initially made the scene darker – i assume this was both because of the increased cloud cover, but also because the rain drops made the concrete slab darker.
Then, after a few minutes, the rain stopped, and the rainy surface actually reflected more sunlight back this increasing the brightness.
Images 28 and 30 show the light levels inside the house at the same time as images 29 and 31 respectively). The location of these photos was around half a metre from the leaf pictures, but was not in direct sunlight. The difference between these photo is startling.
The lighting level started to stabilise between about 1pm and 2pm. Again i moved to a different ISO to see the effect – noting what the equivalent values would be for the aperture at the different ISO, which would give an indication of the same EV (exposure value).
This is interesting as EV is a way of showing equivalent settings for aperture/shutter speed/ISO that give the same exposure – ie increasing the aperture one stop requires a reduction in shutter speed by one stop to get the same exposure. Therefore these three variables can be changed to produce the same exposure, but with different characteristics – ie a higher ISO might allow a faster shutter speed at low light levels, but it also increases the graininess/noise in the picture. Or a high quality photo with a large depth of field would require a low ISO and a small aperture, and would therefore require a long shutter speed – possibly requiring a tripod and likely to blur any moving objects in the picture.
The other key variable in the experiment was the weather which changed the light source !
In terms of what to expect from sunlight, when the sun is above 40 degrees above the horizon . However, the sun doesn’t reach this height in the UK for some of the year (according to this calculator). Various charts show how the sun moves across the sky at different times of year (eg here). ‘Capping’ these charts at the 40 degrees elevation would show how the light levels change throughout the day and seasons
Here are some useful pictures from CLEAR
The height of the sun not only impacts the light level, it also changes the angle of the light – for example, in winter (as now) the sun does not lift high above the horizon which creates dramatic highlights and shadows which reveal the textures of objects such as ploughed fields and walls. This can be compared to the more direct ‘flat’ light when the sun is more overhead in summer.