Caltar 210/6.3 II-E plus Nikon 6T ... funny behaviour

[Matus Kalisky]

Hello,

I decided to try some close-up photography with my Tachihara over the last weekend . As I do not have any dedicated macro lens - I decided to try Caltar 210 plus close-up lens from Nikon - 6t which is supposed to have 3 dioptres. I went to the text book of Optics and computed that the focal lengt of this setup should be approximately 120mm. Indeed when I tryed to focus to infinity the lens behaved like that.

Let me just mention that I have very good experience with this nikon close-up lens in small format.

So - I started to focus on a nice sunflower we have at home. For simplicity I kept both standards and the floweritself paralel. Well - I first focused on the central part of the flower (the most distant part - it was close to 1:1) and then I tried to get into focus the yellow leaves of the flower. I could not find the focus so I was focusing closer and closer...and sudenly the central part of the flower came into focus again - now of course somewhat beyond 1:1.

Yes - I checked it twice (I'm kidding - I checked it at least 10 times) - the focus behaviour of this setup was really like that. I've put the flower on the side and took a metalic ruller , focused the lens as far as the bellows was long, then placed the ruller NEARLY parallel to the lens axis (nearly touching the lens surface) - so I could see where the focus is - and started to move the lens back - what in normal way would mean to focus towards the infinity - BUT - the focus started first move in opposite direction as expected - 1 or 2 cm and only then (I kept moveing the lens back) stopped and started to move towards infinity.

Well - I am physicist (though my field is not optics) and this must be a normal behaviour of a compound lens of such a type, but have any of you experienced such a behaviour?

P.S. Concering the pictures - as I was rather confused - I used my fujinon 125/5.6 and made couple of photos at about 1:2 - trying to apply the rule of best f-stop as mentioned in the lasr issue of VC (I did apply the apparent f-stop). Let's see what the result will be....

Matus..

[Michael Gudzinowicz]

It's nothing unusual. The extension required for a given magnification (M) is focal_length x (1 + M), and the lens to subject distance is focal_length x (1 + 1/M). The focus plane to subject distance is the sum of those terms. If you substitute 1/M for M, you'll get the same film to subject distance, however the extension and lens to subject distance are swapped. Since the focal length of your lens is short compared to extension, and the magnification is close to 1, you were able to focus for both magnifications.

[Jon Shiu]

Not sure of your situation, but for close-ups it is necessary to focus with the rear standard as to not change the lens to subject distance. Hope this helps.

Jon