INTRODUCTION – FOCUS

EXERCISE 1  –  FOCUS WITH A SET APERTURE.
Well…. I thought it was going to be easy!  I was confident that I would have no difficulty with this exercise as I knew about the relationship between aperture and depth of field and I had every confidence that there would be no problem.  Wrong!  The purpose of the exercise was to demonstrate that the lowest f stop setting (widest aperture) would give the smallest depth of field thereby producing images with limited areas in sharp focus.
I set up my tripod mounted Canon 7D camera with a Canon EFS 17 – 85mm lens alongside the village street at Trefin and pointed it towards a row of colour washed terraced cottages that I thought would be ideal.  The weather was overcast with rain imminent so I set the White Balance (WB) to ‘cloudy’. I set the camera mode to Aperture Priority (AV) so that I could select the lowest f setting (f5.0) and focussed on the first of the cottages before changing the angle to take in the whole row.  I repeated this twice more changing the point of focus to the middle of the row and then the far end.  On checking the images on the camera’s viewing screen all looked OK.  I then moved on to Exercise 2 Focus at Different Apertures using the cottages as the subjects again of which more below.  On returning home as the rain started I loaded the images onto my computer full of confidence but disaster!  There was no appreciable difference between the images with none of the expected effects of narrow depth of field.  I was not happy especially as I did not understand what had gone wrong.  The images are shown below.

—–   o0o   —–
Determined to crack this first exercise I set out again to the cottages but this time I used a line of railings as my subject.  Before taking the shots I checked that the focus setting was on AI Focus to make sure that the focus point selected would remain the same even if I moved the camera to re-frame the shot.  I took 3 shots again with the focus on the far end of the line of railings, the middle and the near end.  On returning to the computer, I found that these images were better and did show the effects of the limited depth of field (see images below).  However, I am still not clear why this should be as, as far as I am aware, the settings were the same.
1.  Focus at near end of railings.  Aperture set at widest (f5) producing narrow depth of field covering just a few railings at near end
2.  Focus at centre of railings.  Aperture set at widest (f5) producing narrow depth of field covering just a few railings in the centre
3.  Focus at far end of railings.  Aperture set at widest (f5) producing narrow depth of field covering just a few railings at far end
As there was some doubt in my mind as to why the results were different between the cottages and the railings, I carried out a final test on a row of plant pots and the results are shown below.
1.  Focus towards near end of row.   Aperture set at widest (f5) producing narrow depth of field sharp only on pot No. 3
2.  Focus towards middle of row.   Aperture set at widest (f5) producing narrow depth of field sharp only on pots in centre
3.  Focus towards far end of row.   Aperture set at widest (f5) producing narrow depth of field sharp only on the pots towards the far end
My preference is marginally towards the 1st image with the sharp area towards the nearer pots as I find that the eye is drawn naturally from the front backwards towards the further, less focussed pots.  Interestingly, there is little difference between image 2 and image 3, possibly because the camera is pointing slightly further down the row in image 3.  This highlights the need for me to be precise in checking the image in the viewfinder to ensure that it frames exactly what is required.
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