Exercise – Focus At Different Apertures

Another exercise to help us get to know the camera and another to get us used to the effect that aperture has on depth of field.

I had a walk down to the Great Central Railway and took some cheesy pictures of steam trains. While waiting for the next to arrive, I took these shots of the oil-soaked fence that runs next to the spot where the locomotives stop and then take the strain once more to haul their load towards Leicester.

I didn’t use a tripod and should really have considered having equal range in the foreground and beyond the focus point. Also, because I’m looking down on the subject, rather than (only) along it, finding the ‘slice’ that is in focus is more difficult. I used a 100mm lens and had the camera in aperture priority (Av) mode so that the shutter speed changed automatically to suit the aperture. If I’d been doing this manually, I’d have needed to slow the shutter as the aperture size got smaller (and f-number bigger).

OCA (2)

The focus spot is obviously the centre of the sign. The first shot was at using my largest aperture f2. The in focus area is limited to just a couple of letters in the centre of the sign. While you can see that the sign is attached to a fence, it is difficult to be sure what is beyond the fence and many other details are completely lost. The ground is simply grey, brown and green shapes with our brain filling in the gaps to suggest to us that the shades of grey under the fence are probably stones or gravel.

This view does however, focus the eye onto the sign. There is no doubt about what the picture is of, or which direction the trains are. This very shallow depth of field is an effect that I like and use for many subjects, even though lenses are often not nearly as sharp at these apertures as they may be when stopped down a little.

OCA (1)The second shot is taken midway across the aperture range at f8. Now, much more of the scene is in focus. We can now see a little more of the fence – two of the thinner uprights are in focus, approximately 3 feet apart. We can also see and identify the track and sleepers in the top left corner of the picture and gaps between the slabs on the path on the safer side of the fence. In the top right, a brown shape on the path that because of the others on the floor, can be assumed to be an autumn leaf. The centre of the image is very sharp, even at full size.

OCAThe final shot (I took pictures at more aperture settings than I’ve included here) was shot at the lens’s minimum aperture, f22. Almost all of the image is sharp and every detail can be identified. The slabs can be counted individually, the leaf in top right is clear enough to imagine the crunch if you were to step on it.

The final step in the exercise was to print out the pictures and draw lines at the limits of focus. Here’s where I realised that I’d made things trickier by shooting down upon, as well as along the fence. This meant that I could not draw 2 parallel lines from top to bottom but rather a slice made up of parts of the subject that were the same distance from the camera.

Aperture Exercise Printed

The maximum width of these sections on the printed images were 10mm (3.5% of the overall width), 50mm (18%) and 200mm (71%) respectively.

As I said earlier, I’d usually go for a limited depth of field, unless shooting landscapes or needing to show an entire scene in detail. I’d leave the viewer to do some of the work and think for him or herself what might be contained in the out of focus areas. Also, as demonstrated here, the narrow depth of field concentrates they eye of the viewer onto the area of the picture that I want to show them.

Finally, here’s another picture of the same sign, but this time in the out of focus area of the image. This picture has a slightly smaller aperture (f3.5) and the subject is further away, meaning a much greater area is in focus.To The Trains


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