Change of focus and focal length

I performed the exercises on the frontage of Boots shop in Guildford, Swan Lane, and on a row of plants in the street market.

I expected that the depth of field would be narrowest using the 70-210 mm lens on maximum zoom. At full zoom, the maximum aperture is f/5.6. However, whilst the change of the focussing point from the nearest to the farthest point on the shopfront was discernable when the image was magnified, this did not really transform the image at normal magnification.

For the row of plants, the zoom was set at close to 100 mm where the maximum aperture is f/4.2. In this case, the change of focussing point had an obvious impact, and the plants outside the focussing area were significantly blurred.

Taking account of the weak depth of field effect at f/5.6 on maximum zoom, changing to an aperture of f/25 had little impact on the image of the Boots frontage. The effect of changing to an aperture of f/25 brought the whole row of plants into focus. (However, the image was more grainy as I needed to increase the ISO to 1250 due to low light levels.).

My preferred image of the row of plants was the one focussed on the nearest plant with a shallow depth of field. This is probably because this blurred the background of the market stall which was otherwise distracting.

I also tried the same exercise with the 18-70 mm lens where the blurring effect was hardly noticeable, as might be expected with its maximum aperture of f/4.5.

More surprising to me was absence of blurring effect with the Canon Powershot. This has a maximum aperture of f/2.7 which led me to expect that it would have a narrower depth of field. However, there was virtually no blurring effect noticeable at 100 mm f/2.7 for either the Boots frontage, or a row of plants.

The "succesful" images of the row of plants demonstrating the depth of field effect are included above in this post:-

No 1 - focus on near plant - aperture f/4.2 (maximum)

No 2 - focus on middle plant - aperture f/4.2 (maximum)

No 3 - focus on far plant - aperture f/4.2 (maximum)

No 4 - focus just behind near plant - aperture f/32.