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)

Some examples of using a lens adapter

On January 16, Derek pointed to some interesting links. One of them relates to lens adapters. As Derek has mentioned in a previous blog, I used to be a studio photographer many years ago, and I have a 35mm film camera among my "collection" of things optical. That camera uses Pentax screw-mount type lenses, so it is about 40 years old. Derek's article prompted me to order a lens adapter for my Canon EOS XT Rebel digital SLR camera. At left, you see the adapter (bottom), and the three screw-mount lenses which I used in the photographic experiment described in more detail below. Not shown is a 2x "teleconverter" - an additional screw-mount lens which can be attached to any of the lenses shown, it "doubles" the focal length of all the lenses. The largest lens is a 200mm, f3.5 Bausch and Lomb lens, the smaller one is a 135mm f2.8 Bausch and Lomb, and the smallest a 55mm f1.8 Asahi Pentax lens.

The pictures below show is the effect of using the "full frame" screw-mount lenses have on the scale of the pictures generated in the Rebel XT, which has a "crop-factor" (APS-C dimensions are 25.1 × 16.7 mm and have an aspect ratio 3:2) type of CCD (it plays the role of the film in film cameras, see Derek's post on Aug 16, 2008). In general, the full-frame lenses act as lenses which appear to have a larger focal lengths, when used with a crop-factor cameras. The effect of a crop-type CCD is equal to "cropping" a 16x25mm section of a full (24x36mm) frame, seemingly "enlarging" a section of the full-frame image. Here are some examples:

This is picture taken with a 18-200 Sigma zoom lens at 200mm and f6.3 - the "upper limit" of what this crop-frame lens is designed for. This lens has optical stabilization features, which makes hand held, longer exposure less like to be blurred by camera motion caused by my hand.

For comparison, this is the image from the full-frame 135 mm Bausch and Lomb lens, also set to f6.3 (nowhere near its "upper limit" of f2.8). The scale of the image is almost identical to that of the 200mm Sigma lens, above (just a tiny amount smaller). This lens has no image stabilization. Exposures are calculated and made just like I used to do in the days of "non-electronic" film cameras. There is no electrical connection to the automatic features of the EOS Digital Rebel. There is no automatic focus - it's done through the viewfinder.

Here we have the image from the 200mm full-frame Bausch and Lomb lens, again at f6.3 (also not the limit of f3.5). You can see the image is larger than the 200mm Sigma image. The "cropping" effect shows - the 200mm Bausch and Lomb lens acts more like a 300mm crop-factor lens..

Here we have a picture taken with the 200mm Bausch and Lomb lens to which the 2x teleconverter has been attached. This makes it 400mm full-frame lens, which acts like 600mm crop-frame lens. Of course, the use of the teleconverter makes the f-stop markings on the lens incorrect. The doubling of the focal length requires a fourfold increase in exposure. In this case, to achieve the approximate equivalent of f6.3, I set the lens itself to f3.5 (its upper limit).

This is a picture taken with the 55mm Asahi lens, again at f6.3 (the upper limit if f1.8). This is a reasonably scaled standard image, with a reasonable angle-of-view. The effect is that of a 80mm crop factor lens. Derek presented me with a 50mm f1.8 Canon EF lens a couple of years ago, which gives a wider view angle, at the same speed (f1.8) as this lens.

The conclusion I draw from all this is that for the price of the lens adapter (about $75.00 after shipping and exchange rates are factored in), I have gained the ability to use my high-quality full frame lenses on my Digital Rebel. The equivalent Canon lenses would cost many hundreds of dollars. It's true that the "old" lenses do not have any of the interactive features of the lenses designed for use with EOS cameras, but this only means that I have to fall back on the techniques which I had to use in the the days of mechanical film cameras (calculating light levels, depth of field, exposure times, etc. in my head), before taking the picture. One advantage of the digital camera is the ability to display the picture just taken on the view screen, in the days of film, you had to wait until the film was developed and printed. In many ways, mentally calculating the required parameters prevents the "mental laziness" to which I succumb too readily when using the automatic features on my digital camera.

The use of the teleconverter effectively doubles the number of focal length capabilites, but there is a fair overlap with my crop-factor lenses, so that some of the combinations are essentially available twice. In any case I now have a fair number of possibilities. Generally, fixed focal length lenses tend to have a better optical quality, and much lower f-ratios - i.e. they are "faster", than the zoom lenses that commonly come with digital cameras of the Digital Rebel level, so if a given picture has to have critical definition, or is taken in low-light conditions, the use of the full-frame lenses may be a better option.

A Planetary Perspective

As I mentioned yesterday, we spent a few days in Mexico (Puerto Vallarta) recently. During that time, and considering my interest in things astronomical, I took a picture of a loose alignment of three planets, and the Moon, visible for a short period after sunset. The planets were Jupiter, Mercury, and Venus (in order of increasing distance from the horizon).

To see the details better, click on each picture.

Jupiter, the largest planet is closest to the horizon, actually seen through a light cloud. Mercury is slightly to the left of, and a little higher than Jupiter. The bright spot to the left of the Moon at the top of the picture, and somewhat lower, is Venus. The leftmost picture shows the actual scene; the right picture is the annotated version.

This is an interesting demonstration in planetary perspective. In this picture, Jupiter, the Solar System's largest planet (it has a mass equal to 318 times Earth's mass), is six times farther away from us than Mercury. Mercury is the planet closest to the Sun, Venus is next closest (our Earth is third) and Jupiter is the fifth of the major planets orbiting the Sun. Jupiter orbits five times farther out from the Sun than Earth. As shown in the picture, even that far away (we're talking about 900 Million km - 560 Million miles), Jupiter shows up brighter than Mercury. Mercury is around 160 Million km - 100 Million miles away. Venus (almost equal in size to Earth) is at this time about 110 Million km - 70 Million miles away. The closest is the Moon, "only" about 380 thousand km - 236 thousand miles away.


Quite apart from these scientific aspects, it was a pretty picture in its own right - certainly quite different from our wintery scene; the temperature at the time I took these pictures was about 22 degrees Celsius (72 F). On the left, you see a wider-angle view. Both Jupiter and Mercury are located to the left of the topmost dark cloud in the lower right (which seems to point straight to them).

We were staying with close friends, one block away from the beach, a couple of minutes' walk to this beautiful scene on Banderas Bay.

^

Away from the snow

I haven't posted anything since Christmas - we were away until yesterday, in Puerto Vallarta, Mexico. To our surprise, on our return, we still found nearly as much snow near our driveway as the day we left. Highly unusual for this city.

Derek had shovelled the snow out of our driveway, so it was easier to get back into the house. He said it was good exercise, and we certainly appreciated his efforts.

Derek had seen no reason for us not to go on this trip - he said that he felt well enough, and also took a little trip to Victoria while we were away. While he didn't enjoy the same kind of weather we had, it is nice to see that he can take some "time off", too.