Minimal focusing distance of Caltar-II 90mm/6.8 (Rodenstock Grandagon-N 90mm/6.8)

Alexz · Feb 21, 2006

Wasn't able to find such info yet.
Perhaps you have their spec. where minimal focusing distance is mentioned ?
I'm trying to get it in practice driven by a smiple lens formula (1/L +1/l = 1/f) which hints that given maximum bellows extention of about 30cm I can focus as close as about 14-15cm. But in practice, having the subject about 30 cm away I can't get it in focus even rouhly.
What mistake am I messing around ?

Thanks, Alex

Nick Zentena · Feb 21, 2006

Front or rear focussing? If you have enough bellows you should be able to keep going. But front focussing changes the distance between the lens and the object.

I think that's right-) I'll let those who do lots of macro to explain.

Dan Fromm · Feb 21, 2006

Alexz said:
Wasn't able to find such info yet.
Perhaps you have their spec. where minimal focusing distance is mentioned ?
I'm trying to get it in practice driven by a smiple lens formula (1/L +1/l = 1/f) which hints that given maximum bellows extention of about 30cm I can focus as close as about 14-15cm. But in practice, having the subject about 30 cm away I can't get it in focus even rouhly.
What mistake am I messing around ?

Thanks, Alex

Alex, the lens' minimum front node to subject distance is 90 mm; at that distance magnification will be infinite and the film plane will have to be infinitely far from the lens' rear node. It won't focus closer than that no matter what you do.

With extension 300 mm = 3.33*f, magnification will be 2.33:1 and front node to subject distance will be 129 mm. All this subject to the assumption that the lens' rear node is coincident with the lens board's plane.

You're probably looking in the wrong place.

Nick, don't you go all mystical too!

Alexz · Feb 22, 2006

Yes, I was doing front focusing, as usual.
Dan, you obviously meant front and real principal planes, right ? (not a nodal points).
If we assign P to be the distance between len's rear principal plane and film plane and L to be the distance between len's front principal pane and subject, I simmulated L as a function of P in Matlab using just a plain formula I mentioned in my original posting above.
I obtained the plot of this dependency which certainly isn't linear and at about 300mm bellows extention (L ~= 300mm) it showed P ~= 130-140mm which is in line with what you said.
However all this was good for theory, once I was trying to get in focus with this lens, I wasn't able to do even at the distance of about 30-40cm to the subject (and at this distance bellows shouldn't extend as long as 300mm in theory).
Weird. Perhaps I still did something stupid ...
Will make another endeavor today...

Dan Fromm · Feb 22, 2006

Alexz wrote "Dan, you obviously meant front and real principal planes, right ? (not a nodal points)."

Alex, the nodal points are also called principal points and lie in the principal planes. You've made a distinction without a difference.

The lens you're using isn't optimized for closeup work, so you may have trouble discerning when it is in focus. I've had this difficulty with the 105/2.5 Nikkor I bought new in 1970; I couldn't tell when it was in focus wide open around 1:1.

Alexz · Feb 22, 2006

Thanks Dan, however, according to what I was taught, pricipal planes and nodla points are different things definition-wise. They may coincide with each other for patciular optical design, but this isn't thesame thing in general. Never mind, this isn't oru point.

Can you please elaborate what does mean optimized or not for close-up work ? I thought such kind of optimization is just better optical performance at clsoe distances, but from what you're saying this may also be expressed in inability to bring focus in in the case of close subjects ?

Helen B · Feb 22, 2006

"Thanks Dan, however, according to what I was taught, pricipal planes and nodla points are different things definition-wise. They may coincide with each other for patciular optical design, but this isn't thesame thing in general."

It's not the optical design that is important, it is the refractive indices of the media on the two sides of the lens. If they are the same, then the nodal planes coincide with the principal planes.

As Nick has already mentioned, have you tried fixing the lens extension and moving the subject or the camera, or using back focus?

Best,
Helen

Dan Fromm · Feb 22, 2006

Alexz said:
Thanks Dan, however, according to what I was taught, pricipal planes and nodla points are different things definition-wise. They may coincide with each other for patciular optical design, but this isn't thesame thing in general. Never mind, this isn't oru point.

Can you please elaborate what does mean optimized or not for close-up work ? I thought such kind of optimization is just better optical performance at clsoe distances, but from what you're saying this may also be expressed in inability to bring focus in in the case of close subjects ?

Aberrations can be minimized for only one magnification. At other magnifications, image quality will be worse. Some designs, e.g., dialyts (Artars, Apo Ronars, Repro Clarons, some Apo Nikkors, WF Ektars, ... ), hold their corrections well at all distances. Others, like my 1970 105/2.5 Nikkor (I think its the early Sonnar type, could be mistaken), don't. That lens could be focused wide open at 1:1, but image quality in the plane of best focus was so poor I couldn't recognize when it was in focus. Is that clearer?

Your ApoGrandagon wasn't made to be used closeup, let alone at magnifications > 1:1. If you insist on abusing it try reversing it. If it is in a #0, just swap the cells. If in #1, you're out of luck.

Alexz · Feb 22, 2006

Well, just tried the setup once again in controlled environment (in the lab), wasn't able to bring 90mm in focus with subjects closer then at least several meters away.
No matter how long the bellows were extended - the subject just becomes more and more diffused (albeit enlarged on GG). It seems 90mm isn't an option for such kind of work.
However, my 150mm/5.6 pleased me being able to snap in focus as close as 25-30 cm from the subject at fill bellows extention. The maginifcation was almost 1:1 (according to the subject's image size on GG comparative to the real one).
I guess the 150mm will be come my work horse for this job..

Minimal focusing distance of Caltar-II 90mm/6.8 (Rodenstock Grandagon-N 90mm/6.8)

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