frog

In the previous article (https://passionford.com/forum/viewtopic.php?t=262702), we looked at how a camera is made and touched on a few important subjects that are discussed in more detail here.

Aperture, film (or CCD), over and under-exposure. So, how does this all work then ?

First of all, we must add to the above and talk about shutter speed.

Shutter Speed
The shutter in a camera is essentially like a curtain which protects the film or CCD from exposure to the light coming through the lens.
When you press the shutter release button to take a picture, the shutter opens for a period of time, this time is dictated by the shutter speed selected by you, or, the camera.

Going back to the eyes analogy, the shutter is like your eye lids, when they’re shut, your retina (the film) receives no light, when they’re open, the retina is exposed to the light coming through your pupil (the lens).

Depending on the selected shutter speed, we can therefore conclude that the film or CCD will be exposed to light for a given period of time, and from there realise that the shorter the time, the less light the film will receive, the longer the time, the more light it will receive.

You may indeed be able to look at bright subjects by blinking rapidly without being blinded, however, in the dark, you need to keep your eyes wide open to expose your pupil to as much of the available light as possible.

A camera works just the same, open the shutter for a long period of time and low levels of light can be captured on film, open it for a short period of time, and only bright light will be recorded.

Typically, shutter speeds range from 1s to 1/8000s in modern cameras, an additional speed known as ‘B’ allows the shutter to remain open for as long as you press the button, enabling longer exposures (say 30s if you are taking pictures of a starry sky).

The range of shutter speeds is also further divided, generally (although not always) in half steps (or next nearest), so, starting from 1s, we have…
½ s, ¼ s, 1/8s, 1/15s, 1/30s, 1/60s, 1/125s, 1/250s, 1/500s, 1/1000s, 1/2000s, 1/4000s, 1/8000/s.

So, for a given amount of light coming through the lens (more on that later), we can see that if the shutter is open for 1/250s, twice as much light will be able to hit the film or CCD than if the shutter was open for 1/500s.

We have just encountered another key aspect of photography, most things, when exposure is concerned are worked in halves or doubles. Certainly in the early days of photography. Cameras these days have a wider range of speeds than simple double of half increments, for example, I can select 1/750s or 1/1500s on my camera. We’ll see why later.

So, correct exposure, or, the recording of just the right amount of light on film or CCD for an image to be correctly exposed (not over, not under) can be controlled by shutter speed.

The lighter the scene, the shorter a shutter speed you need to select.

The second parameter in exposure is Aperture.

Note: Another name for aperture is F-Stop or F-Number.

As we saw in the previous article, aperture represents the amount of light that is allowed through a lens, the smaller the aperture number (e.g. F2.8), the greater amount of light passes through the lens.

Inside your lenses, is a mechanical device called an iris or diaphragm which is a set of concentric blades that can be opened or closed to a smaller or greater extent. The more the iris is closed, the less light is allowed through the lens.
In your eyes, the iris opens and closes just the same, depending on light conditions.

How much the iris is closed by is expressed by the aperture you choose when taking a picture. A wide (or large) aperture (F2.8) will leave the iris wide open, allowing lots of light through, a narrow (or small) aperture (F22) will close the iris such that very little light is allowed through.

If your camera has a depth of field (DOF) preview facility, you can see this very easily by putting your camera in manual (M) or Aperture Priority (A) mode and choosing an aperture such as F22 to start with, then push the DOF Preview button.
Looking through the end of the lens at the same time, you will see the iris close, and open again when you release the button.
Looking through the viewfinder, you will notice how dark the image appears.

If you don’t have a DOF preview facility, choose a slow speed such as 1s, and take a picture while looking at the end of the lens, you will see the iris close then open again.

Changing the aperture from F22 to F16, F11, F8, etc… and repeating the experiment, you will see that the iris closes less and less as a result.

Aperture numbers usually range between F2.8 and F32. And follow this range of intermediate numbers; f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, etc.

Again, as per shutter speeds, each increase or decrease represents a halving or doubling of the amount of light being passed through the lens (remember F numbers going up means less light coming through).
However, modern cameras and lenses offer intermediate aperture numbers which correspond to less than half or double, examples of such apertures are f/6.3, f/7.1 which can appear between f/5.6, f/8, again, we’ll see why later.

Lastly, film (and CCD’s) play a role in this whole exposure malarkey.

ISO
ISO (or ASA in some countries) for film or CCD represents how sensitive film or CCD is to light.
The lower the ISO number, the less sensitive, for example ISO25 film is not very sensitive to light, ISO1600 on the other hand is very sensitive.

So, applying the rule of halves and doubles, we have ISO 25, 50, 100, 200, 400, 800, 1600, 3200, etc… each step going up being twice as sensitive as the previous one.

We’ll go into more detail about this in a subsequent article, let’s assume that for now, all film and CCDs are ISO100.

Finally, we get to exposure…

So, we have two (three if you add film ISO) parameters which govern exposure in a camera, shutter speed and aperture.

A combination of shutter speed and aperture will therefore allow a quantity of light to hit the film or CCD, if this quantity of light is sufficient, the image will be correctly exposed, if it is insufficient, it will be under-exposed and you can guess the opposite.

What this quantity is, is measured by the camera. Inside the camera is a light meter which knows how sensitive the film or CCD is. Depending on the light coming into the camera through the lens, and, the selected speed and aperture, it will be able to tell if too much or too little light will hit the film or CCD when the photo is taken.

If the light meter says too little light, we have to either increase aperture (changing from F8 to F5.6 for example), or, decrease shutter speed (from 1/250s to 1/125s for example).

If the light meter says too much light, we’ll have to do exactly the opposite.

So, once we have chosen a combination of aperture and speed which satisfies the light meter’s requirements, we can increase aperture and decrease shutter speed together without affecting the end result.
For example, if F/8 and 1/125s is the correct combination, F5.6 and 1/250s, or, F11 and 1/60s will work just as well, and result in a correctly exposed photograph.

There should be a little scale inside your viewfinder which shows the effect of changing speed and/or aperture on exposure

Easy eh !

Well, not quite, remember I said that everything is a compromise in photography ??? Well, aperture affects something called Depth of Field, and speed is very much dependent on what you are photographing, typically fast objects require fast shutter speeds to “freeze” them, slow or static objects on the other hand may be photographed at considerably slower speeds.

I’ve talked too much already in this article to go into detail about Depth of Field, this will be done later, however, let’s look at what other factors control the speed you should choose when taking a photograph.

First, let’s assume you don’t have a tripod, and are taking you photos without any support.

The first consideration is the lens you are using, and the focal length you have chosen. Typically, the speed you choose must be at least 1/focal length. With a 200mm lens you should choose a minimum speed of 1/250s, 1/500s being better and so on.
Why ? Because you’ve got a great big lens in front of you which weighs a ton, and you are focussing on something that’s 100’s of yards away, chances are you will be shaking a little, and a slower speed (1/60s for example) will most probably result in a blurred image because you can’t keep steady long enough

If you have a tripod, monopod or some other form of support, forget the above and choose whatever speed you want, bearing in mind that the slower the speed, the more likely it is that the image will be blurred.

The second consideration is the subject, a portrait photo can be done at 1/60s without any problems, a racing horse however will be completely blurred if it comes at full speed.
If you want to freeze fast motion, you have no choice but to choose a fast shutter speed.

Lastly, shooting hand held, anything slower than 1/30s is also likely to result in blurring.

Note: Modern cameras with anti-shake, VR, etc… technology allow these simple rules to be broken somewhat, but don’t expect that you will be able to use a 500mm lens at 1/30s, it won’t happen. At best, Anti Shake, is good enough for one or two slower speeds (e.g. 1/60 or 1/125 for a 200mm lens).

So here you go, speed and aperture determine exposure, compromises have to be made when choosing both in order to obtain correct exposure.

Do you understand now why fast subjects in dark environments will always be difficult to photograph (or should we say expose) properly ?

Next time we will cover Depth of Field and the effect of Aperture on this very important aspect of photography.

Lesson Three here: https://passionford.com/forum/viewtopic.php?p=3813539