Advantages of 35mm Camera Lenses

Fitting The Lenses To The Camera

The only snag with this idea is that, as far as I'm aware, there are no adapters for owners of bayonet-fit lenses to enable them to be fitted to the 416XT CCD camera. You might be able to fit an old M42 screw-fit lens using an adapter, but (in my case) I had a fair selection of Minolta MD fitting lenses which I wanted to use and there appeared to be no off-the-shelf solution. I looked at the problem of machining a female Minolta MD bayonet mount but, whilst it was possible, the precision work involved was substantial. Browsing around a camera store the other day I dropped upon a used but perfectly serviceable 2x teleconvertor in MD fitting for just 9 UKP, I certainly couldn't make something like that for that kind of money. So I bought it, removed the lens elements (they were in a screwed cell so easy to take out) and then threaded the convertor's body internally to accept standard 1-1/4" filters. This latter modification was a good move because it's essential to use an infra-red (IR) rejection filter to get sharp star images. Interestingly, you might notice that on some camera lens bodies an additional fiducial line is marked, usually red, which indicates the focus position for infra-red. You'll also notice it's some way from the visual focus! You could actually leave the teleconvertor's optics in place, but I would advise against it - short focal lengths are what you're after.

The only other modification is to remove the male MD bayonet ring from the convertor body, this is (usually) held in by 3 small srews engaging a groove in the ring. Once taken out it can be replaced by a custom threaded ring to fit the CCD camera. On my example I simply used the bayonet ring as the basis for machining a simple adapter - the same groove on one end, and a threaded section on the other which screws into the 416XT's front plate.

	The front of the Teleconvertor showing the female Minolta MD bayonet fitting. IR rejection filter is fitted inside the teleconvertor's body.
	The rear showing the new adaptor ring with thread to fit the 416XT front plate.

Of course, there is the other minor detail of having the camera lens at the correct distance from the CCD chip to focus correctly. It's not essential that this distance be correct as minor discrepancies can be accounted for during focussing. However, if the distance can be set correctly, then lenses of all focal lengths can be interchanged without the need for focussing at all. For most 35mm lenses the distance should be 55mm from the lens seating to the CCD chip surface. While experimenting I made an adjustable adaptor so that the distance between lens and chip face could be adjusted. I placed a test image some 10 feet way, adjusted the lens focus ring to read 10 Feet, then turned the adjustable mounting until the image was in focus. This gave me the dimension for the thickness of the mounting ring. I then machined another non-adjustable fitting with this dimension.

Curiously, each lens was slightly - and one not so slightly - different from each other when reading from the focussing scale on the lens barrel. An object brought to focus will give a slightly different reading on the distance scale. I later repeated this process using actual star images and setting the lens for infinity (and machining yet another fixed length ring). This was more successful and all lenses were then correctly focussed when attached and set at infinity.

Another curious thing - when the lens is stopped down to say, f/16, the indicated depth of field is quite large (10 ft to infinity for example). Yet in practice to achieve precise focus required very exact postioning of the focus ring - as though there was very little depth of field. An object some 50 feet away should have been in focus (according to the scale) with the lens set at infinity and f/16 - but no way, it requires fine tweaking to get optimum focus for close objects.

Mounting The 416XT Camera

	416XT camera mounting ring. It uses the 2 threaded holes meant for mounting the 616 colour filter wheel (which I don't have), with 2 thumb screws passed through the two curved slots.
	Using the 416XT in horizontal piggy-back mode. The mounting block is the same as all my other dovetail fittings and thus will fit either the upper or lower bar.
	Using the 416XT in vertical piggy-back mode.
	Using the 416XT on a camera tripod. A short section of dovetail fitted to the tripod head enables the same mounting block to be used.
	The camera mount also fits on the lower dovetail bar...
	...and it clears the forks too!!

The Tamron 80-210 zoom lens (shown above) produces very sharp images, but I did have to tighten the one-touch focussing barrel so it was much stiffer to move - and thus less likely to slip during a long exposure. The Tamron zoom has 3 small screws at the lower end of it's barrel which adjust the friction. In addition to this lens I have the standard 50mm Minolta lens (not to be overlooked - the standard 50mm lens is usually excellent), a 28-80 Tamron zoom, a 17-28 zoom (which is rubbish), 28 and 24 fixed f/l lenses, and another 2x convertor. All of these lenses can be attached in a second.

Listed below are a few pictures taken with the 416XT and various 35mm camera lenses (on 8th August 1998 - full moon, high thin cloud). I didn't need to focus any of these shots - just stuck the lenses onto the front of the 416 with the focus set at infinity. Conditions were far from ideal, there was a bright full moon and pesky high stratus cloud which seemed to be invariably attracted to whatever it was I was trying to image.

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