Twilight & Low-light Photography
Wikipedia defines Twilight as "... light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this illumination occurs." To make things more confusing dusk (the time between sunset and night) is split into three twilights, civil, nautical and astronomical. Each relates to how far below the horizon the centre of the sun is, civil twilight is between 0 and 6 degrees below the horizon, nautical twilight is from 6-12 degrees and astronomical twilight is from 12-18 degrees. When the centre of the sun is more than 18 degrees below the horizon then dusk ends and night begins.
On the 5th of June the following data was collected. These measurements were made using a Sekonic 608 Light meter, measuring the brightness of the same spot of cloudless clear sky, 20 degrees above the horizon, looking in the direction of the sun, using a 1° field of view. Sunset was at 21:12 (t+0) and measurements were made relative to that time.
| Low Light | t-30 | t+0 | t+10 | t+15 | t+20 | t+25 | t+30 | t+35 | t+40 | t+45p | t+50 | t+55 | t+60 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Lux | 2600 | 560 | 180 | 110 | 65 | 40 | 21 | 12 | 6.6 | 3.5 | 2.3 | 1.3 | 0.7 |
| Shutter | 1/1000 | 1/560 | 1/180 | 1/45 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 | 0.7 | 1.5 | 1/2 | 1 |
| F-stop | f8 | f8 | f8 | f8 | f8 | f8 | f8 | f8 | f8 | f8 | f8 | f8 | f8 |
| Iso | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 400 | 400 |
| EV vs t-30 | 0 | -0.8 | -2.5 | -4.5 | -5.1 | -6.1 | -7.0 | -8.0 | -9.0 | -9.5 | -10.5 | -11.0 | -12.0 |
| Sun elevation | 3.51° | 0.00° | -1.54° | -2.18° | -2.79° | -3.38° | -3.96° | -4.52° | -5.07° | -5.61° | -6.14° | -6.66° | -7.16° |
The first measurement taken half an hour before sunset shows the normal brightness towards the end of the day, the sky was bright and clear at t-30. As the sun reaches the horizon at t+0 the direct lighting stops and the only light is that reflected from the sky and artificial light. The sky luminance decreases as the sun moves further below the horizon. The difference between the amount of light from a clear sky and a cloudy sky for twilight photography would be significant.
The Exposure Value (EV) is calculated relative to the t-30 setting, it compares the camera view of twilight relative to the correct exposure at the end of the day. This brings up one of the problems with using cameras to replicate human vision. As twilight proceeds human eyes become dark adapted and make use of their higher dynamic range. Most cameras can capture 8-12 stops of exposure in a single image. Human eyes can adapt to the dark (given time) and in comparison have a dynamic range of around 46.5 stops between bright daylight and moonless starlight. Even under normal daylight conditions human eyesight provides 24 stops of exposure. Each "stop" lets in half or twice as much light as the stop to either side, so f4 would let in twice as much light as f5.6.
We can see from the chart that at t+30 there are 7 stops less light than at t-30. This also explains why you will only get one viewpoint setup per night as you will not have time to setup a second location and have enough twilight to work with. Obviously we can take photographs all the way till it is fully dark but would need to increase the ISO, have longer shutter speeds or decrease the f-stop (all of which have other photographic implications). The problem with this approach is that the general scene is getting dimmer as night approaches and you are trying to maintain detail in the landscape by letting in more light. However all of the artificial illuminations are fixed points that are not changing their brightness. Letting in more light in general means that the artificial illumination will be over-represented relative to the t+30 Scottish benchmark.
Looking at the Landscape Institute Visual Representation of Development Proposals, TGN 06/19, Appendix 5 Night-time Photography refers us to the forthcoming publication "Landscape and Visual Assessment: Artificial Light and Lighting" which unfortunately still has not seen the light of day. They also refer us to "SNH 2017, paras 174-177", which is SNH Visual Representation of Wind Farms Guidance, Version 2.2, February 2017 which is mostly about lighting turbines as a warning to civil or military aviation.
Para 176 of the above says "We have found that approximately 30 minutes after sunset provides a reasonable balance between visibility of the landform and the apparent brightness of artificial lights, as both should be visible in the image. It is important that the photographs represent the levels of darkness as seen by the naked eye at the time and the camera exposure does not make the image appear artificially brighter than it is in reality. It can also be helpful to note the intensity of other lights in the area to enable comparison (e.g. television transmitters) as this can aid the assessment process. SNH may prepare further guidance on assessment of lighting in due course."
As these guidelines were based on images produced in Scotland we need to be careful about using a set time after sunset. The different latitudes for locations in Scotland means that the sun is closer to the horizon half an hour after local sunset than it is in London half an hour after the sun sets there.
| Nelson's Column in London | Fort William in the Highlands | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Solstices & Equinoxes | Local Sunset Time | Sun -6° | Sun -12° | Sun -18° | Time between 0° - 6° | Sun elevation t+30 | Local Sunset Time | Sun -6° | Sun -12° | Sun -18° | Time between 0° - 6° | Sun elevation t+30 |
| 20th March | 18:12 | 18:48 | 19:27 | 20:09 | 36 minutes | -5.055° | 18:33 | 19:14 | 19:59 | 20:47 | 41 minutes | -4.545° |
| 20th June | 21:19 | 22:09 | 23:24 | n/a | 50 minutes | -3.836° | 22:14 | 23:26 | n/a | n/a | 72 minutes | -3.000° |
| 22nd Sept | 18:56 | 19:32 | 20:11 | 20:51 | 36 minutes | -5.137° | 19:16 | 19:57 | 20:42 | 21:30 | 41 minutes | -4.542° |
| 21st Dec | 15:51 | 16:34 | 17:17 | 17:58 | 43 minutes | -4.206° | 15:38 | 16:31 | 17:21 | 18:08 | 53 minutes | -3.456° |
Note the n/a in the above table are because the sun doesn't get that far below the horizon at that time of year for that location.
As can be seen from the above table the local sunset times are significantly different throughout the year between Scotland and England. In Scotland it takes longer for the sun to get between 0° and -6° below the horizon than it does in London. Also on average the sun is 0.67° further below the horizon at t+30 in London vs Fort William. The "30 minutes after sunset" guideline from SNH is closer to the lighting conditions experienced at t+25 for London. If we were to stick to t+30 for southern England then images will be a whole f-stop darker than those taken in Scotland.
Interested in learning more?
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