Aperture

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Out of all the photography lessons on this site ,this one is the most important to understand.

The aperture is the opening inside the lens that controls the amount of light traveling through it. It is like the iris in our eye and can be opened and closed, opening the iris will  allow more light through the lens.
The size of the aperture is expressed in f numbers. A 50 mm lens may have the following sequence marked on an aperture adjuster on the lens, each number is called a stop: -

f2.8, f4, f5.6, f8, f11, f16, f22
Increasing from one stop to the next halves the light passing through the lens, decreasing one stop doubles it.
A two-stop increase say from f4 to f8 will allow four times the light through.

Some lenses have half stop markings such as f3.5 and f4.8 for fine adjustment.  The exposure required is obviously related to the aperture.  Reducing one stop allows the exposure to be halved; increasing one stop doubles it.

That’s the boring bit, so now let’s deal with the use of f numbers.  aperture selection is vital to focus and depth of field so if you are not clear about its effect you should read that lesson now.

Canon once made a 50 mm f0.9 aperture lens, which is now very rare, it allowed an enormous amount of light to arrive at the film plane but with little depth of field.  On studio lenses, for plate film work, the aperture can be as low as f 256. I remember often using this stop to increase the depth of field, there was so little light coming through to the film plane that 24 hours exposure was required!!!  The larger the sensor or film the less depth of field, that's why with a 10x8 sheet film camera I needed such extreme f n°s.

Typically a lens will bear the manufacturer’s name next to the focal length followed by 1: and its maximum open aperture.  For example a fixed focal length lens will show: - Canon 200 mm 1:1.8, but a zoom may show: - Canon 18 - 55 mm f3,5-5,6.  In the later case two f numbers are quoted, showing that at 18 mm the maximum aperture is f 3,5 and at 55 mm it is f 5.

Usually the lower the f numbers the better and more costly the lens. In my first example the 200 mm lens has a value of around £5000, whereas some18-55 mm zooms can be in the region of £115.

Better lenses may have a plate mounted on the side of the barrel. In this example of a Canon lens it shows EF, that's the type of lens mount, the focal length is 200 mm, the maximum aperture is f1.8, and Ultrasonic indicates a very fast and silent autofocus, the L, short for Luxury, shows that the lens is very high quality. This lens has travelled around the world with me and I’ve never found better.

 

 

 

A better zoom with a fixed minimum aperture throughout the focal length range.

The specification is usually shown on a rim round the front of the lens.  This zoom lens has a focal length varying from 18 to 50 mm as it zooms. It has only one f stop 2.8 marked showing that this aperture can be used throughout the zoom. The DC mark means that it was designed in the digital age for a sensor smaller than the traditional 35 mm film. Filters of 67 mm diameter will fit the lens.

Always buy the best lens you can afford, the lower the f-number the more latitude to be creative.  In simple terms aperture is the opening that controls the amount of light traveling through a lens.  It is always expressed in f numbers and the difference between f numbers as stops.  The aperture is like the iris in our eye it can be opened and closed, opening the iris, will obviously allow more light to carry through the lens and when fully open will be at its lowest f number.

Tip
Always buy the best lens you can afford, the lower f-number will give you more latitude to be creative.

Focus and auto focus
Most lenses offer the possibility of Manuel or auto focus, although auto focus sounds great it can be a bit of a bore with one or several led’s going on or of inside the viewfinder, it nearly always means focusing on the subject pressing the button half way, recomposing your image and then after all that, taking the picture, if your subject hasn’t moved.  Although I must admit I used it most of the time when shooting Formula1, Those cars can accelerate from 0 mph to 200 mph in a few seconds , my 200 mm lens has a setting that will follow them coming straight towards me, believe me it’s not easy to do manually.   However normally Manual focus will be more accurate.

Tip
With longer lenses use a monopod when photographing moving subjects it gives more freedom than a tripod and it is much easier to focus manually.

Quality
Normally a lens that changes barrel length as the focus is changed would not be as good quality as one that moves the lenses in relation to each other, inside the lens barrel.

Tip
Whatever lens you buy and however much you spend remember this, the longer the focal length the more prone to camera shake we are, so to avoid it use a shutter speed the same or greater than the focal length of the lens, if this is not possible a tripod or monopod will be required.  Example: 200mm lens at least a 200 th of a second shutter speed. And that’s if you haven’t had a drink the night before, in which case go for 500th of a second. If you can't get the shutter speed the either open the aperture or increase the iso (see lesson "understanding iso")

I hope this has helped with your understanding of the importance of aperture in photography.

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