Tuesday, January 27, 2009

More on the lens adapter

In my previous post, Derek's comment regarding the labelling of the focal length of the lenses involved in my test of the new adapter I bought for my Rebel XT, I came to the conclusion that the labelling on my Sigma 18-200mm zoom lens was the "full-frame" equivalent value. Yesterday, I repeated a couple of the flower basket shots, but in addition, I also took some long-distance shots to compare the Bausch and Lomb and Sigma 200mm performance.

Figure 1 shows the flowers taken with the Sigma lens at 200mm again, and figure 2 is an image of the same flowers, taken the Bausch and Lomb 200mm from the same position. The B&L image is obviously larger.

In figure 3 (the Grouse Mountain ski area as seen from our front window), taken with the Bausch and Lomb lens at 200mm, and the same picture (figure 4) taken with the Sigma lens at 200mm, there is no difference in scale. What's the reason?

Well, I neglected to take account of something I routinely do when I use the Rebel XT on one of my telescopes to take pictures. Depending on the accessories used in the process, the effective focal length of the telescope changes considerably, depending on the focussing adjustments required to get a sharp image. The pictures of the flowers were taken at a distance of about 2.5 meters (appr. 8 ft.), which is the closest distance to which the B&L lens can be focussed. The B&L lens focuses by moving the entire set of lens elements forward for close-up work; the Sigma lens adjust focus by changing relative positions of its internal lens elements. You can see the difference in the pictures of the B&L lens in its long distance (infinity) focus position (figure 5), and its close-up focus position (figure 6).

As you can see, the barrel length of the B&L lens in its close-up focus position is appreciably longer than when it is focused at infinity. The effect of the close-up position is that the lens appears to have a focal length of about 230mm, not 200mm. This naturally results in a larger image. The Sigma lens does not physically move lens "outward", and therefore maintains the 200mm distance from the camera's CCD. When both lenses are set to infinity, the scale of the pictures is identical.

This means, contrary to what I said to Derek in my reply comment, that both the Sigma and the B&L lenses are labelled with their correct focal length - there is no full-frame equivalent marking on the Sigma lens. Click on each picture to see it in larger format.

Figure 1
200mm Sigma zoom lens (flowers are 2.5 meters away)

Figure 2
200mm Bausch and Lomb lens from the same position as Sigma lens.



Figure 3
Grouse Mountain taken with the B&L lens at 200mm
(notice the snow-making plume near the top of the "cut")


Figure 4
Grouse mountain taken with the Sigma lens at 200mm
(snow-making plume has changed)



Figure 5
Bausch and Lomb at infinity (figure 3 focus)




      Figure 6
Bausch and Lomb at closest focus (figure 2)

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