The word photography is derived from the Greek φῶς (pronounced.. phóss) or in its genitive form.. φωτός (pronounced.. photóss) meaning "light" and... γραφή (pronounced.. graphí) meaning "drawing", which when taken together is literally... "drawing with light".
Amazingly enough, since its beginnings over 175 years ago, the principles of photography have remained unchanged.. namely the capture of light onto a light sensitive material... and in particular the 3 controlling parameters available to achieve that capture have also remained the same... those being, the level of sensitivity of the light sensitive material, the aperture of the lens through which that light is both directed and focussed and the speed of the shutter which opens to expose that light onto the light sensitive material.
So, what is a correct exposure? As most photographers and teachers of photography will tell you... the correct exposure is the one that achieves the image the photographer had intended... a little bit of a "cop-out" answer but also true nonetheless.
Technically however there is another answer... both photographic film and digital sensors are physically restricted in their ability to record light. This spectrum of recordable light is known as the dynamic range of the film/sensor and therefore any light values which fall out of that range are recorded as "black" (under-exposed) or "white" (over-exposed)... meaning that strictly speaking a correct exposure is one where the balance of light between the lightest and darkest parts of an image is even.
In reality however, there are many light conditions where the difference between highlight and shadow light values may be larger than the ratio between the maximum and minimum values of the film's/sensor's dynamic range... In this case, adjusting the camera's exposure settings only allows a choice between underexposed shadows or overexposed highlights as it cannot bring both into the dynamic range at the same time... so what to do and how do we as photographers get around this.?
Some examples of options available to us are.. using flash or reflectors to provide "fill" light to increase illumination to shadow areas or using filters which can reduce the amount of light emanating from highlight areas; we can also employ HDR (High Dynamic Range) techniques whereby we "bracket" exposures (ie. take separate pictures exposing in each in turn for highlights. mid-tones and shadows) and then combine the images in post production to create perfectly evenly exposed images... but more on these and other techniques later.
To achieve all of this however, we first have to get our heads around the 3 elements at our disposal to create an exposure... ISO, aperture and shutter-speed, what aspect of the image they control or affect and how they relate to each other...
ISO stands for International Standards Organization and is an internationally agreed upon scale of measurement of the sensitivity to light of film or nowadays, digital sensors. In film, the light sensitive material is SIlver Halide which is in the form of crystals embedded in a gelatin emulsion. The size of the crystals embedded within the emulsion governs the sensitivity to light, so that tiny fine crystals with a small surface area consequentially have low sensitivity to light (low ISO) but can create finely detailed images and large crystals with a bigger surface area are more sensitive to light (high ISO) but consequentially cannot create as finely detailed images.
The same principle is true for modern digital sensors whereby a low ISO rating will be a lot less sensitive to light but create beautifully detailed images and a high ISO rating will be more sensitive to light but create less detailed "noisy" images.
The dilemma for us as photographers is balancing our need for quality/detail i.e.. keeping as low as possible an ISO rating, which will allow us to shoot at the shutter speed or aperture we need to in order to photograph whatever it is we're shooting... in the light conditions that are prevailing at that moment.
The aperture or "f-stop" is probably the greatest source of confusion amongst photographers.. both beginner and professional..!
The aperture is basically the size of the hole in the lens through which light is directed and focussed on to the film/sensor to make a photograph. It is a diaphragm made of very thin interlocking blades which can be closed down to make a tiny hole or opened up to make a large hole. The size of the hole controls the amount of light that is allowed through the lens to hit the film/sensor... and most importantly it governs what we call the "depth of field". Depth of Field is basically how much of an image is sharp from the foreground to the horizon.
A large aperture (big hole) such as f/2.8 has a very shallow depth of field.. meaning that only a small part of the image will be in focus;
A small aperture (small hole) such as f/22 has a very deep depth of field... meaning that a large part of the image will be in focus;
HOWEVER..! It is not just the aperture that has an affect on the depth of field... a very important consideration is the Focal Length of the lens;
i.e.. its angle of view.. ranging from super wide fish-eye lenses at 8mm to very narrow super telephoto lenses at 600mm and even greater;
The human eye is considered to have a 50mm angle of view and in photography a lens of 50mm is considered to be "standard".. anything smaller is considered "wide" and anything longer "telephoto".. simple enough... BUT... the longer a lens gets both in its focal length and physical build... the longer it takes for light to travel through it at any given aperture.. logical enough.. that being so, f/2.8 will not be the same on a 14mm wide angle lens (which is 9cm long) as on a long 400mm telephoto lens (which is 35cm long)... in terms of depth of field this has the effect of shortening the area of focus the longer a lens gets.. so again using the f/2.8 example.. f/2.8 on a 14mm lens could give a depth of field from 3 feet in front of you to the horizon whereas f/2.8 on a 400mm lens could give you a depth of field of only 2cm forward and back of your point of focus..!
As mentioned above, aperture is measured in "f-stops" which typically number as follows... F/2.8 F/4 F/5.6 F/8 F/11 F/16 F/22
Each one of these increments is called a "whole" stop in exposure... in most modern DSLRs we can however use apertures in-between these whole stops.. usually numbered in thirds or half stop increments.
I think the reason most people become confused with apertures and f-stops is because the size of the aperture is not reflected by its number..
a large aperture (big hole) has a small F number.. e.g.. F/2.8
a small aperture (small hole) has a large F number e.g.. F/32
It is useful to note that lenses are usually referred to by their widest possible aperture.. for example.. 24mm F/2.8 or 400mm F/4
You might also hear photographers talking about "fast" lenses... these are lenses which have a very wide maximum aperture for example f/1.4 or f/2.8... they are referred to as fast because the wider an aperture a lens can open up to, then the "faster" the light can pass through it to hit the sensor.
Fast lenses are hugely more expensive but have many advantages.. the first and foremost being that you can shoot in lower light conditions and maintain a lower/better ISO to shoot at and/or maintain a higher shutter speed; secondly, the optics are larger and usually of better quality, and thirdly your view finder will appear much brighter making it easier to shoot and focus in low-light conditions.
In order to "expose" the camera's film/sensor to the light we have a mechanical shutter that opens and closes to let the light in. The shutter speed is very simply the speed at which this shutter opens and closes again.
If the shutter opens for too long... too much light is let through and the image will be over-exposed (too bright)..
If the shutter is not open for long enough... not enough light is let through and the image will be under-exposed (too dark)..
The other main effect of shutter speed has to do with movement.. both of your subject, and of your hands holding the camera.
It is important to note that Aperture and Shutter Speed have a reciprocal relationship.. so that at any given ISO, changing one will affect the other.
Where the most important effect of aperture is in controlling how much of an image is in focus... the most important effect controlled by shutter speed is movement.. with fast shutter speeds freezing movement and slower ones blurring it.
From this relationship you can see that depending on what it is you will be shooting you have to prioritize one or the other... for a portrait you want to throw the back-ground out of focus to bring all the viewers attention to the subject, so your priority is to have a wide/large aperture to limit the depth of field.. if you wish to photograph a fast moving bird in flight you have to freeze the action and so your priority is to set a fast enough shutter speed to achieve this.
So there we have it... the 3 main parameters controlling our exposure... each of which has a direct and reciprocal relationship with the other, and each of which can be used as a creative tool to artistic effect in making our photos.
So now that we know what each of them is/does.. how do we know at what to set them in any given situation..? that... will be the subject of my next post in a few days...
Hope of some help..