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Thursday, 1 April 2010

Focal Lengths – for cameras with variable focal lengths

This exercise builds on the earlier exercise  Focal length and angle of view. This is to appreciate on the effect of changing lenses from one focal length to another and how this effects the angle of view.


I went to Greenwich Park for this exercise as I knew there was a good vantage point that had an open view but with detail in the distance. It was a cold afternoon with a little wind (but later turned into rain) and there was a lot of cloud in the sky. The light was overcast and quite ‘flat’. I used a tripod for the exercise as recommended in the course text. I used a combination of 2 lenses through the exercise a 18-55mm and a 55-250mm lens. As I was using a tripod so I wasn’t concerned about longer exposures so I left the ISO at the lowest setting of 100.

I had planned to use the camera in an Aperture priority mode, controlling setting the f-stop manually then letting the camera set the shutter speed for exposure, with the intention of setting a small aperture, however in my eagerness to get started I didn’t do this in the first few shots and left it in program mode by mistake; The camera automatically sets the exposure for both aperture and shutter speed but allows manual ‘stop’ changes to the exposure (pretty much an auto mode!). When I realised this I changed to the aperture mode and set the aperture small, but rather than re-shoot the images again, I just checked them and felt that for the purpose of the exercise there was no need, as you will see below.

Secondly I’m a strong believer in recording all aspects of the exercise, even the bits that don’t always go to plan; I find it normally sinks in more that way.


18mm f/5.60 1/90 sec ISO 100 Image taken on the 18-55mm lens at the widest angle (shortest focal length) setting.


29mm f/6.70 1/60 sec ISO 100

Still using the 18-55mm lens another shot, but with increased focal length (less angle of view).

After this shot I realised that I was shooting in the wrong setting; I had intended to use a smaller aperture to maximise the depth of field, but the first few shots were with larger apertures. Thinking that the depth would be out I was surprised to find that even though taken with larger aperture the depth of view was still deep. I was a little confused at first, however concluded that as I was using the lens’s wider angles and was some distance from the subject, there was more than enough depth being created to be noticeable by using wider apertures. However as a rule the smaller the aperture the more depth would be in focus.


55mm f/22 1/4 sec ISO 100

This image was still with the 18-55mm lens but at it’s maximum focal length (shortest angle of view). Note that even less of the foreground has been included as the angle reduces.


55mm f/22 0.30 sec ISO 100

At this point I changed the lens from the 18-55mm to the 55-250mm lens. I took this shot at the shortest focal length of the larger lens to show that the angle was view was the same as using the 55mm end of the smaller lens. (albeit a slightly different view of the subject)




1/4 sec

ISO 100 This was centred back to the main subject on the 55-250mm lens at it’s maximum focal length, the angle of view is has all but removed the foreground and background from view, and the view is greatly magnified in the frame. (not the same as moving closer – see next exercise)




0.30 sec

ISO 100 Just a final image of the building at the 250mm focal length.

I found this exercise similar to the Focal length and angle of view exercise from earlier in the course, however it was good to reaffirm what was learnt and experiment further with focal lengths, this time changing lenses to much longer focal length and how this effects the angle of view and size of the subject (magnified) in the frame.

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