Quote:
Originally Posted by Peter Waites
This is absolutely fascinating stuff Don. Please keep the instalments coming when you find time.
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Another to keep you happy Peter.
Aquiring the subject.
This was a bit of a problem when I came up with the MkI. As magnification increases above 1:1 this becomes an ever greater problem due to the ever lessening depth of field.
Option one was to put a spike on a mount to bayonet on the front of the bellows, but that idea was discarded as unless sthe subject distance was within the coarse focus movement of the bellows sliding plate it could still take time to aquire the subject.
Next up I found what looked like a good optical solution. Back in the 1980's I bought a 'lens scope converter' from Jessops. Quite a handy bit of kit before the age of decent small bins. Simply attatch to a lens and you can use it as a monocular. So in use, a 50mm camera lens becomes a x5 monocular, 100mm lens x10 and so on. Sounds impressive, but in reality the exit pupil is only about 2.5mm so the image is quite dim unless used with lenses with an aperture greater than f2.8 or in bright conditions. Image quality also does not compare with any half decent bins or scope. But anyway I dug mine out and even at the least bellows extension magnification was too high. However cranking the bellows out and using good light on the subject, magnification was really impressive.
Mmmmmmmmmmmmmmmmm ...... Probably quite obvious to some, but it took me a day to think about it before I had my eureka moment.
How about using a spotting scope optic. OK it would not be an erect image ( upside down ) but never the less worth a go. Quick toilet roll and black tape job and it worked. The image was far brighter than the lenscope. So next up I made a proper bayonet mount for the bellows and it worked brilliantly. For general use we had a 20x-45x zoom that Stevie got as a freebe when she got her scope. So I nicked that. With that mounted and set to 20x and the bellows at minimum extension I found the subject would be dead on when the bellows was at full back extension using a 75mm lens. As the reversed 50, 28 and 24 have only a few mm between them in terms of lens to subject distance, problem well and truely solved. Use that with a reversed lens and you have a seriously good bench magnifier. All very good and a major advance on the dim lenscope, but using a 20x widefield scope eyepiece, or for more magnification, a 38x widefield, gives a huge exit pupil and so a really bright image.
If Dave Smith was about I am sure he could try out an astro telescope eyepiece with a diagonal, on a long lens, as that would give an erect image.
NO Andy, I am not going down that route for this before you suggest it as a further spend.
Some pics attatched.
I am not a photoshop buff so I was unable to give a circular aperture to the frame edges to give a view similar to that you would actually see looking through it, as all birders would recognise, but the magnifications are accurate. Oh, and to be true to form I should have turned all pics through 180 degrees so please excuse that.
Don