I took a few days out to shoot the migration in the Masai Mara. This event happens at various times in the year as two vast herds of zebra and wildebeest swing both clockwise and counter-clockwise following the rains as they sweep across the Mara and the Serengeti bringing with them new grass. It's a time for new life but also for death as the herds make the perilous crossing of crocodile infested rivers and run the gauntlet of the great cats - lions and leopards. For a wild-life photographer it has its challenges as dozens of safari trucks converge on the major crossing points. They say all is fair in love and war - and, I would add, positioning your safari truck to maximum advantage should also be counted.

It is hard to describe the dramas that you confront in wildlife photography. Having been to the Masai Mara three times now, I am struck by the raw elemental power of the place - the scenery is vast, the sky endless and the wild animals and birds ever present and huge in their diversity. The emotional impact of seeing a zebra taken down by a crocodile or a hyena cub being nursed by its mother is immense and it is that feeling I try to convey in my photography.

Photographing wildlife does demand a long lens and a steady hand. I always, if I can, shoot hand held looking for sharpness and clarity through high shutter speed and low apertures. Going into the field I set my camera at ISO400, f5.6 and Aperture Priority. I find it just possible to hand-hold a 300mm prime with VR and freeze motion at 1/2000. I don't like tripods, not keen on monopods and only use a bean-bag if I have to. There is something very intimate holding your camera to your eye, controlling your nerves, absorbing and connecting with the scene and then, taking the shot. What I do find useful is to fold a tea-towel (one of the chunky variety) into four lengthwise and wrap it around the lens body. This, I have found eliminates the tiny movements in the hand which can undo a shot. Try it, you should find you can get good sharp images at considerable lower speeds than you would otherwise.

General tips for great wildlife shooting: always look for the closest eye as the prime focus point. Also, look for that elusive catch-light in the eye which adds vitality to any image and for reasons I will write about later in this blog chimp-a-lot. I avoid continuous high (CH) shooting as far as possible - it does not guarantee good image capture - even at 1/2000 and 10fps the shutter is only open for 1/200th of every second. For 199/200's it's shut so your chance of getting that perfect moment are still remote. Be Hawkeye rather than Rambo and anticipate the moment and fire on single shot or continuous low. It takes practice but, guess what, it maximises your chance of capturing that elusive moment.

I will post discussion pieces on the art and practice of photography as the days and weeks go by. But here, I hope, for your enjoyment are a small selection of images from the Great Migration.

## Thursday, 27 August 2015

## Sunday, 23 August 2015

### The Mathematics of F-stops

I am occasionally asked the question: what is an f-stop. Here is the answer.

The aperture is the hole in the camera where the light gets in when you press the shutter. The

The aperture is the hole in the camera where the light gets in when you press the shutter. The

*f*number which is used to measure aperture is formally expressed in dimensionless numbers from the ratio of the lens (F) to the diameter (D) of the ‘pupil’ of the camera. The pupil of the camera is the ‘hole’ into which the lens fits and is designed to equal the width of the sensor or unexposed film from corner to corner. So, for a given combination of lens of given focal length and camera the ratio of F/D is defined.
The diameter D can be found by rearranging the formula for the area of a circle (Area = Pi x Radius squared). The radius is, you may remember from school days, half the diameter. So the

*f*number is given by the Focal length of the lens divided by the diameter as defined by the Pi and the area of the hole in the front of the camera the light goes through. A swift bit of algebra shows that the*f*number is given as the reciprocal of 2 x square root of the fraction of the hole through which light passes to hit the sensor times (still with me?) a constant value determined by the camera lens combination. If 100% of the hole in front of the sensor is open to light then the*f*number is 1/(2*sqrt(1)) = 0.5. Halve the exposed area and the*f*number is 1/(2*sqrt(0.5)) = 0.707. Halve it again and*f*= 1/(2*sqrt(0.25)) = 1 and then it goes 1.4, 2, 2.8, 4, 5.6 and so on as we reduce the area through which light passes by a half each time. Well, if you are up to the maths you will notice that it's actually 2.828 and 5.657. By convention the number is rounded.
An

*f-stop*is, therefore, half the area and hence half the amount of light passing through the next highest aperture. A reduction of three stops would reduce the area by 1/2 x 1/2 x 1/2 = 1/8th of its previous area. So the take home messages are:- the f number is the reciprocal of the increase or decrease in the area through which light passes into the camera body.
- The smaller the f number the larger the 'hole' or aperture.
- The more expensive lenses have wider apertures for a given focal length. A 50mm lens may open to 1.4, a professional 300mm lens probably two stops lower at 2.8 and a 'prosumer' lens maybe to only 5.6.

A f-stop represents the change in the amount of light that hits the sensor. As far as aperture is concerned it represents either a doubling or a halving of the area of the hole through which light passes on its way to the sensor. The same logic applies with shutter speed and ISO - an f-stop is a half or a doubling of the previous number. So, if I am shooting at 1/500th of a second, I reduce the light by a stop if I increase my shutter speed to 1/1000th, I increase the light by a stop if I slow the shutter speed to 1/250th and so on. With ISO 100 to 200 is a stop, 200 to 400 is a stop but this time what we are changing is the sensitivity of the sensor to light - in each case doubling it or reducing it.

I know all this looks horribly technical but, in terms of technicality, that's all there is to it. From now on, the trick is to understand this and use it intuitively to estimate the impact of a change of aperture, shutter speed or ISO on the job of creating an image.

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