When working with focal lengths, we are not talking about the size of the lens in the physical sense of the word, we are talking about the distance between the image sensor and the lens itself, (the glass). Lens’ have the focal length stamped on them. The two I have are, wide angle (18mm to 55mm) and the tele lens is 75mm to 300mm. As can be seen, this distance is measured in millimetres (mm). By zooming in and out we digitally change the image/scene we can see through the viewfinder.
In this exercise, we are working with a longer focal length. This compresses the space, resulting in a shallow depth of field (DoF). The background is very blurry. The focal length, together with the wider aperture setting and the distance from the subject, will give varying DoF. Different combinations of these elements will determine, how blurred or sharp the image will be.
f/5.6 1/500 ISO200 55mm f/5.6 1/1000 ISO200 55mm
The two images above were taken on a sunny day. The sun was behind me and I found it hard to get the settings right, but eventually it worked. I used a wide angle lens to take the photos of my brother in law, at the longest focal setting, which gave me the blurred background I wanted. I found an excellent explanation of focal length, that made really interesting reading on understanding camera lens at this link (click here)
The link goes onto say “optical aberrations occur when points in the image do not translate back onto single points after passing through the lens — causing image blurring, reduced contrast or misalignment of colours (chromatic aberration). Lenses may also suffer from uneven, radially decreasing image brightness (vignetting) or distortion” The website, Cambridge in Colour, had a very good interactive image, that allows you to experience the different problems that can occur.
f/5.6 1/160 Iso100 155mm f/5.6 1/160 Iso100 75mm
—————————————f/5.6 1/125 Iso100 75mm —————————————
Above are photographs of my husband. In this case I used a tele lens 75mm to 300mm, to experience the difference it would make to the shot. I needed to be further away from my husband, and I had to ignore all I know about composition in order to get enough of the back ground in to see the difference. I tried 3 different backgrounds, our hedge, the wall and the window.