TAOP Introduction: Focus at different apertures

17.1.11 Exercise 3: Focus at different apertures (p.18).

The aims of the exercise were to :-

  • Find a subject with depth (e.g a row of things seen from an angle).
  • Stand at an angle to the row, with camera on tripod and focus on an obvious point.
  • Take 3 pictures – widest aperture; mid point and smallest aperture whilst ensuring that  the shutter speed is adjusted to maintain the same exposure.
  • Compare the 3 prints and draw on each one what you see as the limits of sharpness.

Setting up

I had difficulties in choosing what I thought would be a good scene – mostly due to my inhibitions about using my camera in public places with a tripod which I know I will have to get over.  A line of cars would have been ideal but as soon as I reached a row parked on the road or in a car park various scenarios ran through my mind about people rushing along and saying, “What do you think you’re doing taking photos of my car!”  I had similar thoughts about rows of houses.  I think it was the tripod that was causing the problem in my mind because it takes time to set up and I couldn’t just take quick shots. I needed the tripod though to make sure I kept the same focus.

Eventually I decided to go to a nearby village hall which has some nice buttresses on the wall.  It was Sunday morning so it was fairly quiet there.  My settings were lens at 55mm; ISO 100; AV priority; auto white balance; evaluative exposure metering and spot focus.  I stood at 38.4 m away from point of focus which was the third buttress along. I took several shots from f5 to f32 and chose those at f5, f18 and f32 to do the comparisons.

1st photograph

f5  at 1/50

2nd photograph

f18 at ¼

3rd photograph

f32 at 0.8

Viewing on the computer, I was immediately aware that the third photograph was much softer than the other two. Even with a tripod this very small aperture created a larger circle of confusion.  This is caused by an effect called diffraction which occurs even in he plane of focus.  “Diffraction quickly becomes more of a limiting factor than depth of field as the aperture gets smaller” (http://www.cambridgeincolour.com/tutorials/depth-of-field.htm (accessed 17/1/11).

Below are the three photographs marked with what I could see as the areas most in focus (complicated a little I think because the buttresses jutted out from the wall and I was standing slightly at an angle as well.)

1st photograph (f5 1/50)

The first buttress in the bottom right-hand corner is out of focus.  The sharpness begins at the wall besides the second buttress and appears to extend  to the wall behind the fourth buttress.

2nd photograph (f18 ¼)

Sharpness is most evident here from the first to the 5th buttress. Also there is more sharpness in the picture overall than on the first photograph. For example,  the hedge on the middle left show more clearly than it does in the first picture.

3rd photograph (f32 0.8)

There appears to be a larger depth of field than in the first photograph but, due to the very small aperture, this is belied by the softness due to the effect of diffraction.

In terms of an image, the second photograph, at f18, is sharper overall. In fact, I know from my experience that this particular lens and camera work well at f11.I also noted how the shutter speed decreased as the aperture became smaller which would have created problems without a tripod.

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