This is why...
It is a simple, science-based, pocket calculator - and its results are always correct.
As a rule of thumb, it has the benefit of being easily remembered. However, it only leads to an approximate exposure for unshaded, nearby scenes during 12 of the 77 UK's different natural lighting conditions throughout the year. Moreover, more light enters the lens the further the distance between the camera and the subject - so distant scenes would be 1EV over-exposed, and cloud shots by 2EV. Unshaded close-ups would be under-exposed by 1/2EV - and shaded close-ups by between 2EV and 3EV. Add in the 2.5EV drop in natural light (and its associated ground temperature) that occurs from its brightest annual level over the remainder of the UK year as the Earth moves further from the Sun (and often during daylight hours as that heavenly body rises and sets) and you will soon realise just how inaccurate that easily memorable rule of thumb actually is. It does provide you with an approximate key value - but you need to adjust it for the scene you are shooting, the month, and the time-of-day. (I say 'approximate' because an accurate key is 1/125th second at 160 ISO/ASA).
Had I used that basic rule of thumb to capture the above "Evening Tide" shot, the image would have been -3.5EV under-exposed, and resembled something like this...
If your subject does not reflect light, it does not matter how fast your film or chip is - you will not be able to capture its image. Similarly, if the light is too cold, greens and reds will be muted. In order to capture the perfect image, we need the perfect light - conducted by the Sun around which the Earth revolves in its wobbly orbit. A single, heavenly spotlight around which our rickety studio revolves - sometimes near, sometimes far - and always at an angle.
Fortunately, the Earth's orbit is entirely predictable over the course of a year - as is the distance from the Earth to the Sun (and consequently its light's intensity). High temperatures give rise to low air pressure that conduct surface water molecules upwards to form clouds that obstruct the light and cool the planet - and, as the Earth revolves, wavelike wind actions are born between areas of high pressure where the air is cool, forming strong anti-clockwise spirals in our Northern Hemisphere as they break. Those strong winds then scatter and bear the clouds away, the Sun retreats, and the air cools to become more dense creating high air pressure that reduces the strength of those winds.
As the air cools, its molecules become compressed, it becomes more dense and sinks to the surface - pushing outwards and seeking-out areas of low pressure - whilst, above in the clouds, warm air is pushed over cold air to condense further moisture that combines and adds weight to the frozen crystals that have formed at the higher altitude. Those droplets then become bigger and bigger - until their weight overcomes the forces of the warm air drafts - and fall back to the earth as snow or rain. Then the skies clear; and the Sun returns to continue the process all over again.
The pattern is entirely predictable, and never changes; it is the local terrain and its retention of the Sun's heat that creates local changes to ancient weather patterns. Diverting rivers, reservoir creation, heavy industry, airports - they each create new cool-spots and hot-spots that the global process absorbs into its annual rhythm of N/S winds created by high equatorial temperatures seeking out the high pressure of our planet's frozen poles, which are then deflected by the E/W Coriolis Effect of the Earth's rotation. It is the interaction of those two processes, which lead to warm air clashing with cold air, that forms the warm and cold fronts beloved of meteorologists - and the same fronts that were easily analysed by those old farmers who held a moist finger in the air to judge the wind's temperature and direction whilst noting the clouds, their direction and speed, and the course and strength of the sun as they approached by observing their own shadows.
Cold finger? Cold front. Warm air is being forced upwards, condensing more water in the clouds. Are they light or dark? How heavy are they? How long will it take them to arrive? What's the time?
How dark is my shadow? Is the sun rising or setting? Will its course provide the warmth needed to combat the incoming cold air? Is the wind's temperature and direction about to change? What do my exposed cheeks and moistened finger tell me?
Despite what others would have us believe to sell their fashionable Armageddon theories, sunlight (and consequently nature's local ground temperatures) does not have a seasonal pattern. The Earth's Sun orbit is elliptical - not circular; and the distant body does not bathe Earth in a constant heat that the Global Warming Activists assume. In the 45-55 degree Northern Latitude of the UK: its midday intensity from March through September is 8 x greater than its weakest moments that occur during the hours of 5am and 7pm GMT during May and July. (During those hours, the Sun actually provides less heat as it does during its first and last daily hour in December).
It is simply about distance and angle - and you cannot base a global warming theory upon ground temperature readings that are taken in different latitudes, under different light values, during different weather conditions on different local terrains.
All weather is local - and the only reason seasons differ is because they are exposed to different quantities of sunlight. In towns and cities, winters are now warmer - because of the heat generated by their ever increasing populations and the energy they consume during hours of darkness. They are increasingly active hot-spots whose low air pressure is increased when the sun rises to suck precious water from the soil and local reservoirs to be carried away in the daylight clouds upon those man made spirals of warm air clashing with the cold fronts of farms and villages (either bringing them rain or casting it across the oceans - depending upon those fronts' directions).
Winters are not warmer in the countryside - but they are wetter because of the increased population in our towns and cities. (It is nature's way of ensuring that our crops and livestock might be bulked-up to compensate).
It is not about farting and its methane, which quickly rises into the atmosphere under its own impetus and is destroyed when exposed to the heat of the rising sun...
Here are the light values from that exposure calculator again - although, this time, I've compiled them to show how sunlight and its associated ground temperature is distributed throughout the year by adopting a rainbow of warm and cold colours - to highlight how those warm, and cold, foreign fronts are assisted or opposed.
(Please point-out its dark hours and time-scales to the next idiot you find proposing that we should adopt a different time-zone to increase the amount of light we enjoy each year.).
|Annual UK Light Values (0.5EV steps)|
Of course, these days it is much easier to use a hand-held or the TTL light meter of the camera we are using at the time - and the science upon which the exposure calculator is based is unlikely to produce the same results as it does with traditional stock. That is because individual sensors - even between identical camera models - are not manufactured to the same exacting standards as film. Different sensor batches, like different film brands, are quality controlled to produce consistent results - but the common ISO scale by which they are judged obscure their actual differences. Generally speaking, you will always gain a full EV from doubling the ISO - on any digital camera - but you are unlikely to find that, on another, the subsequent shutter speed and aperture settings produce identical results for the same shot. Neither are you likely to find that the same exposure combination produces precisely the same result on different brands of film.
In order to produce consistent results, you need to calibrate your equipment to mid-tone grey (for each batch of film, or each ISO setting provided by your digital camera). Then everything snaps into place.
That's also when things become interesting, because you can then check the accuracy of that simple Johnson calculator and the science from which it was derived. (All its dialled exposures resolve to the chosen scene's mid-tone grey). Moreover, you are then in a position to use those daily light values to determine when to take that magnificent landscape. If you want to incorporate some glorious clouds - or use them as a heavenly reflector to lighten the scene's dark shadows - then just ignore the Met Office and stick a moistened finger in the air...
|The canny concession owner knew that it would pass over; but the locals read the weather forecast and decided to stay at home|
And never forget nature's own fill-light (or the power of Photoshop)...