2015 09 - Understanding Apertures

25th September 2015
Retford & District Photographic Society - Press release - Joy Allison
Images by Alan Townsend

Understanding Apertures

Members welcomed professional sports photographer Gary Bailey to demystify the subject of apertures – the confusing and hard to remember numbers referred to as ‘f-stops’ which appear in our camera settings. He recognised that there would be a range of understanding and experience in the room and adopted a helpful tutorial format, illustrating his points with everyday subjects. The key point, we were told, is that the smaller the number the less the light reaching the camera sensor. Later the implications of this were explained.

In Gary's view we photographers have four obligations to our subject, namely to be creative or true to it, to get it in focus, to avoid camera shake, producing a sharp image and to expose it correctly. Aperture has a role in achieving all of these.

An aperture is a hole in a flat surface. In our case that hole is the one allowing light to enter the camera. Its size is adjusted by the camera as the shot is taken dependent on the setting the photographer has chosen. As the aperture becomes smaller the image is not cropped, but rather the whole image has to pass through a smaller hole, meaning less light reaches the camera sensor. The photographer’s goal is to select settings which deliver the maximum light to the sensor without delivering too much, creating burnout, or too little leaving no detail in dark areas. These are points our competition judges regularly point out, so everyone was keen to learn more.

Other settings such as shutter speed and ISO also come into play, but for tonight, we assumed these were not changed.

Gary brought an impressive selection of lenses to demonstrate the choices we make when we purchase and the implications of those choices. He used the idea of a tunnel to describe the amount of light reaching the sensor when either the length or the diameter of the lens changes. Less light enters a small tunnel entrance than a large one. So for any lens the lower the f-stop quoted for it, the more light will reach the sensor and the bigger its diameter will be. This explains why, sadly, the lower the f-stop, the greater the price as it is technically harder to make high quality larger lenses.

Gary had some good news for those without huge budgets, suggesting they could consider teleconverters, extension tubes or small fixed focal length, prime, lenses at affordable prices. Better still lenses and accessories can be hired so potential buyers before they buy.

Later Gary looked at the practicalities of getting the settings right on a shoot. Much of his work is capturing fast motion in sharp shots, requiring fast shutter speeds. We were reminded of the inverse rule – when shooting with a 200mm lens, have a shutter speed no slower than 1/200th second and similarly for longer focal lengths. We were also advised to bear in mind that all lenses give the sharpest images at an aperture of around f8.

Correct use of aperture helps to achieve the required depth of field. Gary demonstrated the effects with photos of the same scene at different apertures, showing how to ensure the key features are sharp. Rendering the background out of focus will draw the eye to the sharp area, potentially changing the story the shot tells.

Other speakers advised considering the viewpoint we to shoot from and Gary demonstrated this very clearly. Something as simple as opting to shoot from a lower position can make a big difference. By getting close to the subject and selecting an angle where the background will be distant, we can achieve a sharply defined subject with a pleasantly diffuse background.

Returning to the context of sports, Gary said that he is sometimes forced to choose a high f-stop because he needs his subject sharp but cannot predict exactly where it will be on a track or in the air as it comes into the viewfinder. He has to set up in advance to cover the possible options and this means accepting a sharper background. Alan Townsend is one of our most experienced photographers and had succeeded in capturing enviably sharp sporting images.

With a restricted viewpoint it may be better to take two shots of the scene at different apertures to get both parts sharp and put them together in the computer to create an image you could not easily take. It is best to take both shots close together in time in case the subjects move or the light changes.

Gary concluded by saying that as long as we select the right part of the range for our purpose the precise aperture we select will not usually be critical. He suggested we set up our cameras and take test shots across the range in good and in low light to see how they performs to inform our future work. He left us with the suggestion that we could remember the f-stop we need by thinking that to get one book in a row sharp looking from the side, we need a low setting while to get 22 sharp we need a higher aperture.

The meeting on 5th October will feature a talk from Dave Butcher, called ‘Lake Light’ and on 12th we will be able to see what members have been doing over the summer with our first competition of the season.

President Alan Burkwood told members that the Society seems to be much in demand lately, as we have been invited to set up a display at the Idle Valley Nature Reserve event at the end of October as well as the display at Lincoln soon afterwards. If your event would benefit from a display of our photographs we would like to help if we can. Please contact us in good time.