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peterkinchington
24-Jul-2018, 15:44
I have an RF5 lomo process lens 450mm f10 that is a symmetrical double gauss design. I was wondering whether I could use the rear element as an ~800mm lens if I stop it all the way down to cover 16x20inch (650mm diagonal) cibachrome that I want to shoot in camera. I am in the process of building a ULF camera and this will help me determine how I construct the camera.
Cheers Kanga

Mark Sawyer
24-Jul-2018, 16:01
The Gauss Lens was originally designed with only two elements (in 1817 as a telescope objective), and later "upgraded" to a Double Gauss with four elements for photography in 1888. (If yours is cemented, that's Zeiss' 1895 refinement.) I believe some Double Gausses were sold as convertibles, so yes, it's possible. That's no guarantee how yours will perform, though.

Mark Sampson
24-Jul-2018, 16:18
IIRC, you lose about two stops of speed when converting a lens. And the lens then requires a longer extension than the nominal converted F.L. I'd say it's worth exposing a few sheets of film to find out what happens... maybe not 16x20 though.... but how else will you find out?

Greg
24-Jul-2018, 16:19
I have tried using only the rear elements from several of my view camera lenses on my 11x14. Trouble is that when you really stop them down, judging sharpness on the GG, I found hard to do. Eventually just sucked up the film costs and went out and shot a negative with each of the rear elements. Some worked, some didn't. Actually liked the Bokeh on one of them that "didn't work out" as far as being evaluated by overall sharpness across the negative.

Mark Sawyer
24-Jul-2018, 16:37
Eventually just sucked up the film costs and went out and shot a negative with each of the rear elements...

That's where the economy and ease-of-processing makes paper negatives a good option.

peterkinchington
24-Jul-2018, 17:09
That's where the economy and ease-of-processing makes paper negatives a good option.

Thankyou all for your comments. Has anybody tried colour with film or paper using the rear lens component?
Cheers Kanga

Dan Fromm
24-Jul-2018, 18:02
Kanga, there's a rule of thumb to the effect that half of a convertible lens covers roughly the same circle as the complete lens. Check carefully before sacrificing a 16x20 Cibachrome sheet. Arne Croell believes that the RF- series of six elements in four groups double Gauss type process lenses cover 45 degrees. If he's right, and he usually is, don't even bother.

I'm sorry, but there just aren't that many lenses that cover 16x20. If the trick you want to try worked well, it would be well-known.

Pere Casals
24-Jul-2018, 19:03
I have an RF5 lomo process lens 450mm f10 that is a symmetrical double gauss design. I was wondering whether I could use the rear element as an ~800mm lens if I stop it all the way down to cover 16x20inch (650mm diagonal) cibachrome that I want to shoot in camera. I am in the process of building a ULF camera and this will help me determine how I construct the camera.
Cheers Kanga


Acording to this info (http://www.arnecroell.com/eastern-bloc-new.pdf) the RF-5 is of APO-Planar type, six elements in four groups.

In general lenses are not convertible to be used in halves. A convertible lens has rear and front cells individually corrected, while in non convertible lenses the aberrations from the front cell are compensated by the ones in the rear cell, being that the common situation.

So you should try it, for sure that the rear element alone will form an image, another thing is if the performace is enough for your application or not, at the end you may need only 6 lp/mm on the print, it is possible that even in the case that the lens is not convertible you obtain acceptable results, as you are not to enlarge later, so you may check it on the GG.



And the lens then requires a longer extension than the nominal converted

Not always, if using the front cell alone then you require less bellows than the nominal, for example with the Cooke Triple Convertible or with the Symmar Convertible, in the Symmar case the aperture scale is not shown for the front cell alone, but it also can be used. The early Symmar (Dagor, IIRC) type has the 3 scales...

Mark Sawyer
24-Jul-2018, 19:21
Kanga, there's a rule of thumb to the effect that half of a convertible lens covers roughly the same circle as the complete lens...

Even though the focal length roughly doubles and the barrel is shorter? How can that be? (Not disagreeing, as Cooke's specifications for the XVA Triple Convertible agree with you.) Is it that the outer image area degrades?

Jim Noel
24-Jul-2018, 19:50
That's where the economy and ease-of-processing makes paper negatives a good option.

How about the lower cost and ease of processing of X-ray film?

peterkinchington
24-Jul-2018, 22:53
Kanga, there's a rule of thumb to the effect that half of a convertible lens covers roughly the same circle as the complete lens. Check carefully before sacrificing a 16x20 Cibachrome sheet. Arne Croell believes that the RF- series of six elements in four groups double Gauss type process lenses cover 45 degrees. If he's right, and he usually is, don't even bother.

I'm sorry, but there just aren't that many lenses that cover 16x20. If the trick you want to try worked well, it would be well-known.

Hi Dan,
I tried the rear group of my RF3 300mm f/10 on my 4x5in and it required 575mm of extension. On the ground glass at least there appears to be no aberration at a marked aperture of f/32 (effective f/64). If it does not cover 16x20in then it appears I've got a portable 600mm lens (or 800mm if I use the rear group from the RF5) for my 4x5in or an 8x10in. I will let you know how the actual negative looks after I give it a go on the 4x5in.
Cheers Kanga

peterkinchington
25-Jul-2018, 04:43
Even though the focal length roughly doubles and the barrel is shorter? How can that be? (Not disagreeing, as Cooke's specifications for the XVA Triple Convertible agree with you.) Is it that the outer image area degrades?

That was my thinking Mark- I am not an optical engineer but I thought stopping the lens right down might help. I am aware of diffraction issues but that is always a trade off for me as I generally do macro work and I sacrifice resolution for depth of field.
Cheers Kanga

Thalmees
25-Jul-2018, 08:25
Hope the issue of this thread will also go with Ronars.
Will try my self when I return from my current travel.
Wondering, why most LF photographers do not think the same way?
Testing on paper negative or X-ray will not charge much.
Thanks peterkinchington for the subject.

Dan Fromm
25-Jul-2018, 08:36
Wondering, why most LF photographers do not think the same way?

Separable double anastigmats, symmetrical and not, have a long and glorious history. They were largely abandoned for several reasons. Manufacturing cost, for those with three or more cemented elements. The fact that single cells are, in general, not quite well corrected enough. The fact that lenses not intended to be convertible can have better corrections than lenses intended to be convertible.

Questions about using single cells come up because many of us are hopeful and don't want to spend more money if that can be avoided.

Jac@stafford.net
25-Jul-2018, 14:12
Even though the focal length roughly doubles and the barrel is shorter? How can that be?

That is a question that perhaps Dan Fromm can answer. Where does the nodal point go when the front element is removed. I sure don't know.

Dan Fromm
25-Jul-2018, 14:53
I don't know either.

Amedeus
30-Jul-2018, 16:41
Where does the nodal point go when the front element is removed. I sure don't know.

When the front element is removed you end up with a single doublet or single meniscus. The nodal point is somewhere on the optical axis of the lens in the glass, one and other function of the curvature of the lens.

Cheers,

Dan Fromm
30-Jul-2018, 17:26
Rudi, I have some Boyer Beryls that are convertible. A single cell is a cemented triplet. I've had an f/2.8 6/4 double Gauss type Boyer Saphir that was convertible. A single cell has a singlet outside and a cemented doublet inside. I have some separable plasmat types. A single cell has a cemented doublet outside and a singlet inside. And then there are the old double anastigmats with single cells that have four or five cemented elements. I've never had any of them.

The definition of focal length tells us where the rear node of a lens whose focal length is known is located. When the lens is focused at infinity, the rear node is one focal length from the film plane.

peterkinchington
30-Jul-2018, 17:27
When the front element is removed you end up with a single doublet or single meniscus. The nodal point is somewhere on the optical axis of the lens in the glass, one and other function of the curvature of the lens.

Cheers,

Hi Rudi,
This explains why I needed 575mm of extension for the rear component of the RF3 (300mm lens) the nodal point is probably 25mm in from the surface of the rear element. That is 575+25mm =600mm which is what you would expect the rear components focal length to be in a symmetrical 300mm lens.
Cheers Kanga

Amedeus
31-Jul-2018, 12:21
Rudi, I have some Boyer Beryls that are convertible. A single cell is a cemented triplet. I've had an f/2.8 6/4 double Gauss type Boyer Saphir that was convertible. A single cell has a singlet outside and a cemented doublet inside. I have some separable plasmat types. A single cell has a cemented doublet outside and a singlet inside. And then there are the old double anastigmats with single cells that have four or five cemented elements. I've never had any of them.

The definition of focal length tells us where the rear node of a lens whose focal length is known is located. When the lens is focused at infinity, the rear node is one focal length from the film plane.

Dan, totally agreed on the rear node of the lens as you describe.

My confusion stems from the use of the "nodal point" expression, typically defined as the no-parallax point of a lens system. In compound photographic lens designs, this point lies between front and rear element, in single lens designs, it lies somewhere in the immediate vicinity of the actual lens, depends on the actual shape of the lens. Critical to know in panoramic photography.

Thanks for catching this.