Since my last post about what extension tubes and a teleconverter behind a 50mm macro can accomplish was so popular, here's another test I've done on the opposite end of the spectrum: telescope photography.
I recently joined a group on Facebook regarding building your own telescope eye pieces from broken or obsolete binoculars. I got a brain wave and though to myself "Why can't I build one from pentax lenses in such a way that I can still use the lens?"
So I set off and built myself a 2" eyepiece adapter that can turn any pentax lens (as long as it has a filter ring to go from x filter size to 52mm) into a 2" very bright, sharp and great eye relief eye piece. So far my favourite is the old pentax 50mm f2 (or f1.7 is even better) lens. Not much to offer int he way of magnification, it probably has the same field of view as my camera attached to the telescope directly, but it's SO VERY BRIGHT. I can't wait to try it on some bigger targets like orion and andromeda. My other favourite is the Pentax 18-55mm @18mm simply because it offers really nice magnification.
When Igot it built, I had the brilliant idea to try attaching my camera to the lens that was inserted into the telescope as an eye piece. I tried the 18-55 @18mm and while I got it to focus, the image circle was extremely tiny; I had the lens focused to infinity for that test.
Then, being on 50mg of prednisone, I couldn't sleep last night and was running a bunch of thought experiments in bed. I thought "well what if I focused the lens to it's nearest point? that should technically enlarge the circle?" While thinking about it my thought went to extension tubes and how they work. Technically at 18mm you could only get away with 12mm of extension tubes because under normal photography your subject would be near touching the lens surface for focus to be achieved and any more than 18mm, your focus point is actually behind the front element inside the lens. But, we're not dealing with normal photography where the subject is a solid object. In this case, the subject is a beam of light focused by the telescope and, by extension, could very well indeed become focused inside the 18-55mm lens. So that should, in effect, push my focal point even closer and increase that circumference even more. And not to be outdone there, I thought about my macro lens and how they behave differently than normal lenses and wondered what the results wold be if i attached my camera to my 50mm macro with and without extension tubes and took a shot.
I got up early this morning and completed the tests. here are the results. the images are all labelled as to the combination.
For reference, when I say "max mag 1:1 life-size without telescope" I am referring to the max mag I would get with the macro lens in that configuration without the telescope attached. I have no idea how to actually calculate what kinds of magnifications I am getting with it all assembled in the telescope without actually shooting something of a known scale.
I am very curious now what the results would be with a wider focal length macro lens. The wider you go in telescope eye pieces, the stronger the magnifying power. Hence why 50mm offers little magnification and 18mm offers the most.
Please, unless I know you personally and know that you're a great physicist, don't try and explain the physics of this to me. This is a fun post, and I don't really want an explanation that simply doesn't line up with the physical tests I've done.
Originally posted by aitrus3 
Post #1 by aitrus3
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