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. (You might like to use the specific focal lengths indicated on the lens barrel.)
As you page through the shots on the preview screen it almost feels as though you’re moving through the scene. So the ability to change focal lengths has an obvious use: rather than physically moving towards or away from your subject, the lens can do it for you. The other immediate difference between the shots is the ‘angle of view’, which also depends on the sensor size of your camera. Use the sequence to try to get a feeling for how the angle of view corresponds to the different focal lengths for your particular camera and lens combination. Which shot in the sequence feels closest to the angle of view of your normal vision?
Does zooming in from a fixed viewpoint change the appearance of things? If you enlarge and compare individual elements within the first and last shots, you can see that their ‘perspective geometry’ is exactly the same. To change the way things actually look, a change in focal length needs to be combined with a change in viewpoint.
I took 6 photos in a sequence of an alleyway at the side of a house, these were all shot on my Nikon D750 full frame using a 24-85mm lens. The day was quite overcast so I shot the pictures using an ISO of 400 and a wide appeture between 4-4.5 to allow the light in and create the best exposure. My viewpoint was set in the middle of the doorway at the end of the alley and I was in a standing position at the end of the alleyway to capture the shots.
Photo 1 – 24mm, ISO 400, F4, 1/320 seconds.
Photo 2 – 32mm, ISO 400, F4, 1/320 seconds
Photo 3 – 42mm ISO 400 F4.2 1/320 seconds.
Photo 4 55mm, ISO 400, F4.2, 1/320 seconds
Photo 5 68mm, ISO 400, F4.5, 1/320 seconds
Photo 6 – 85mm, ISO 400, F4.5, 1/320 seconds.
Angle of View
The angle of view is the measure of a scene or the subject that a lens/camera is capable of capturing. It is measured in degrees across the photograph horizontally, vertically and diagonally. The angle of view will change depending on the focal length you use as is displayed in the sequence of photographs I have shot for this exercise. The first one, takes in a lot more of the scene surrounding the viewpoint (the doorway). You can see the entire side of the house with it’s pointed roof, part of the neighbouring roof, the tree beyond the doorway and the plain, overcast sky. If you compare this to photo 6, it captures the viewpoint framed in the doorway, there is less distraction which allows more focus but you see very little of the surrounding scene, the house is now limited to a small portion of the wall, where you notice the window more. The sky and neighbouring house has disappeared from view and the tree beyond the doorway is now cut in half. The depth of field is also more noticeable and the sharper and out of focus aspects to the photo are now more obvious (which would change depending on where you set your viewpoint).
The sensor size of the particular camera I used for this exercise is 35mm (a full frame camera) with a standard lens attached. If I were to use a camera with a smaller sensor, then I would be shortening my focal lengths to achieve the same results as the photos above. This is because a smaller sensor will give a narrower view of the scene using the same lens (effectively increasing the focal length).
The shot in the sequence that I feel represents my normal field of vision would be somewhere between photo 2 (shot at 32mm) and photo 3 (shot at 42mm).