Find a scene that has depth. From a fixed position, take a sequence of five or six shots at different focal lengths without changing your viewpoint.
I took these at Lacock Abbey. The sequence I had in mind was in the cloisters, however I took another sequence in an upstairs corridor that I found more interesting, you can see how the corridor seems to shorten and the stairs move closer as the focal length increases. There is camera shake visible on the last two images, I’d like to re-take this sequence with either a tripod or a higher ISO (I used ISO800 for these).
On looking at the exif data I realised that even though I’d been shooting in AV mode I didn’t have the same aperture for each image. I tend to work with prime lenses and had forgotten that my zoom lens doesn’t have a full aperture range available from one end of the zoom to the other. Using the zoom lens, as I increase the focal length the “widest” aperture available to me decreases rather than the selected aperture remaining constant throughout. Hence my realisation that neither of these series were at a constant aperture, despite my setting the camera mode to AV. Both environments were not well lit so I had selected the widest possible aperture, which then become unavailable to me as I increased the focal length of the zoom. So I’d met the requirement to work in AV mode but it hadn’t been implemented by my lens in the way that I’d expected. That will teach me not to set and forget. For the sake of completion I will take another set at a constant aperture (ie one at f5.6 or narrower) and add that series here. The sets with varying apertures are not affected in terms of composition or showing the effects of changing the focal length, but they will show larger variations in the histogram and depths of field.