Find a similar subject to the previous exercise such as a row of things seen from an angle. Take three identical pictures. Focus on an obvious point somewhere near the middle of the picture. Take the first picture with the lens at its widest aperture, the second at midway on the lens scale and the third at its smallest aperture, keeping the exposure for all three pictures the same. Print the images and compare them; there should be an obvious difference between at least the first and the third. Draw a band on each picture where you see the limits of sharpness.
Planning – and failure:
I planned to use the same row of motorbikes from the previous exercise (‘Focus with a set aperture’) using a tripod and my 18-200mm lens. Using a focal length of 18mm, I took pictures at f/3.5, f/10 and f/25. However, despite a couple of attempts, I could not get the differences in sharpness to show greatly; a slight difference was apparent between my pictures shot at f/3.5 and f/25 but certainly not as much as I expected and not enough to mark the limits of sharpness on the images. I tried again with a different subject, this time a row of neatly parked ‘Boris Bikes’ (aka the Barclays Cycle Hire scheme in London) with the same result. This baffled me for a while until I remembered that I had encountered a similar problem when doing a basic depth of field exercise in my beginners’ photography course two years ago, albeit with a different camera (Samsung NX5). The solution then was to use a longer focal length when taking the pictures. So it was back to the ‘Boris Bikes’, this time using the full focal length available to me.
What happened next:
I shot all three pictures using a tripod and my 18-200mm lens, using a focal length of 200mm and aperture priority mode. I also used my camera remote release to avoid any camera movement when taking the pictures. I kept the same focal point for all three pictures – the rear brake light of the fourth bicycle from the front, near the centre of the picture. I shot in RAW and processed the images in Lightroom.
With a wide aperture of f/5.6 (the widest available on my 18-200mm lens for the focal length of 200m) the area in focus is very narrow and the only parts of the image that are in sharp focus are the fourth bicycle tyre and brake light, thus showing a very narrow depth of field. The bicycle tyre at the front is very out of of focus and the writing on the sign on the wall at the back of the picture is not readable.
With a mid-point aperture of f/16 the area in focus is much greater (the depth of field has increased). More of the bicycle tyres are in focus (although some of the saddles are not) and the sign at the back, although blurred, is now legible.
With the smallest available aperture of f/36 the picture at first sight seems to be in focus from from to back, showing a much greater depth of field. All the bicycles are now in focus and the tread patterns on the tyres are sharp. The writing on the sign at the back is clear. On closer inspection however, it is noticeable that the stonework and other details at the very back of the picture appear slightly soft and focus seems to have been lost a little.
Key learning point:
As the aperture gets smaller the area of the picture in focus becomes greater and gives a wider depth of field. As the aperture gets bigger the area in focus becomes smaller giving a narrower depth of field.
I have used different aperture sizes in the past to produce different depths of field, but this exercise has helped me better understand the principles and it was interesting to see the different depths of field produced from the use of different apertures. However, I was still unable to produce images showing ‘bands’ of sharpness, so if I was to repeat this exercise I would try a simpler composition with fewer subjects that were further apart to see if that would make the focus bands more obvious.
Having found that shooting with a longer focal length produced far better results for the purpose of the exercise I looked into this further and the basic idea seems to be that the the longer the focal length is, the shallower the depth of field will get. I’m not sure that I understand why, so I will just remember it for future use.
I tend to use a smaller aperture for a greater depth of field when taking pictures of buildings and landscapes which benefit from front to back sharpness. When using a smaller aperture I am aware of the need for a lower shutter speed so I try to use a tripod or other support to avoid camera shake. I am also conscious of the problems of diffraction that can arise from using a very small aperture (see link here for a good explanation of this) so I try not to use an aperture smaller than f/16.
This exercise has reminded me of the value of using depth of field to create interest within a picture so I will try to incorporate this when planning images in the future.