Lens element spacing and its effects

[razzledog]

As it is one of my favourite wide angle lenses for 4x5, we all know the tiny 90mm Schneider Angulon is a brilliant performer. I would love to increase its image circle, or improve its area of resolution. Has anyone tried variations in spacing of the front and rear elements to improve its performance? I noticed some time ago that I could improve the overall sharpness at the edges of a 90mm Rodenstock Grandagon by decreasing the distance between the front and rear cells. What is the optimum distance required to achieve the largest and sharpest image circle of the 90mm Angulon and how is it calculated? Checking a Linhof select example in a Linhof (Synchro-Compur) shutter, I measured 25.5mm at the outermost dimension. :confused:

 

[ic-racer]

http://www.largeformatphotography.info/forum/showthread.php?t=41218

May be of interest.

 

[DaveOttawa]

As it is one of my favourite wide angle lenses for 4x5, we all know the tiny 90mm Schneider Angulon is a brilliant performer. I would love to increase its image circle, or improve its area of resolution. Has anyone tried variations in spacing of the front and rear elements to improve its performance? I noticed some time ago that I could improve the overall sharpness at the edges of a 90mm Rodenstock Grandagon by decreasing the distance between the front and rear cells. What is the optimum distance required to achieve the largest and sharpest image circle of the 90mm Angulon and how is it calculated? Checking a Linhof select example in a Linhof (Synchro-Compur) shutter, I measured 25.5mm at the outermost dimension. :confused:

I would suggest that you would only need to adjust the spacing IF the lens had been disassembled and then re-assembled incorrectly at some point. The spacing between surfaces is a fundamental part of the lens design that should be entirely controlled by the original lens construction. It is also one of a number of degrees of freedom the lens designer will have used to optimise the design, to me it is unlikely that the final design could be significantly improved by changing one degree of freedom like that in isolation. Having said that if you want to play your best bet (in the absence of professional lens design knowledge and the appropriate software tools such as Zemax) is to carefully experiment making sure you can aways go back from any dis-assembly step you take.

 

[razzledog]

I have found that fitting these elements into shutters of different manufacture, the distance between front and rear cells varies considerably as does the resolution. Assuming that the lens/shutters we purchase S/H have been factory fitted is foolhardy...I was not referring to alteration of the relationship of individual elements, only the spacing between front and rear cells. I have found many lenses to have rather poor performance but by shimming/machining have found vast improvements can be made. I have re-cemented many defective lenses with excellent results, as it is not difficult providing one has the necessary tools and equipment for correct alignment.
I was hoping someone had data available on correct spacing of the 90mm Angulon cells as some sort of reference point which to compare.

 

[razzledog]

Exactly!

http://www.largeformatphotography.info/forum/showthread.php?t=41218

May be of interest.

You are right...exactly what I mean, many thanks for the link. It's great to see someone else checking their lenses to make sure all is at a premium, the same can be said for the ground glass and its relationship to the film plane. Many cameras are way off.

 

[razzledog]

Well the test results are in.... After machining the shutter's rear thread thereby reducing the distance between front and rear elements of this little Angulon, I arrived at the optimum distance. The images are somewhat amazing. Now the lens resolution has improved beyond recognition and I have excellent edge sharpness. I set the hyperfocal distance at 12 feet using the brilliant DOF Master calculator and finished up with a 'no need for focusing' 4x5 that is sharp at just about all distances when shut down to f22. Just goes to show that your not so sharp lens might not be what it could be if the distance between its elements is incorrect.
Just for your reference, my Angulon now measures 25.70mm externally.

 

[Chazzy]

So your conclusion is that Schneider erred in the placement of the rear group on this particular lens at the factory? Or are you suggesting that the Schneider design itself was not properly optimized?

 

[Ian Grant]

Or is it variations in the Compur shutters. I've used 2 90mm Angulons neither were stunning performers adequate is about all you can say.

I'd be interested in testing my current Angulon to see if it's possible to eek out that extra performance. Somewhere there are some resolution tests on the 90mm Angulon and the variations between individual lenses is astonishing perhaps this could be one reason why.

Ian

 

[Chazzy]

I have a 90mm Angulon myself, so I'm very interested in all this.

 

[Ian Grant]

Just for your reference, my Angulon now measures 25.70mm externally.

Well Schneider themselves publish the external length as 25mm. So it would be useful to accurately measure the lengths to see how much they vary.

Ian

 

[razzledog]

Angulon element spacing

So your conclusion is that Schneider erred in the placement of the rear group on this particular lens at the factory? Or are you suggesting that the Schneider design itself was not properly optimized?

I think the problem arises when shutters are switched. As the Angulon is getting on in years, it's possible that their shutters have been replaced several times. Some shutters have a step inside the rear thread which the Angulon rear cell requires in order to seat properly. It would be great if those who have these lenses could measure them?

 

[edtbjon]

The Angulons have been around for a long time and they are somewhat infamous for their varying performance. (Look at Chris Perez's tests.) As we rolled into the '60ies qualitycontrol at Schneider got much better, so the last of the Angulons have a much better reputation. It seems like razzledog have found part of the answer to how to get the best out of these small pearls. I know that Ole T. have pointed out that the innermost 2 elements are held in place only by the optical cement (Canadian balsam) and that he recommends them to always be stored lying down, else due to gravity the inner elements could float out of their optimal position. In both the adjustment that razzledog is doing and the issue which Ole brings up it's a matter of 1/10'ths of millimeters.
A different lens in two versions comes to mind. The famous Heliar and the very rare Universal Heliar. On the latter there is a possibility to move the central lens element a little bit backwards to control softness. I recon most of us have only heard of this feature, but anyone with an ordinary Heliar can try this out (at least in part), as the inner element is threaded into the front barrel, so that it can be moved backwards a millimeter or two, before falling out. (I read about this in some thread but havn't tried it yet.) As most people who are using Heliars nowadays probably wants that special Heliar highlight softness at f/4.5 before overall sharpness at f/16-22, this is a special case, but still...

//Björn

 

[Ian Grant]

It may well be that the Angulon is very sensitive to it's cell spacing, more than Schneider realised.. My first one was Linhof selected but it wasn't good, none of the images I shot with it were fit to exhibit, and I was never happy with it.

You have to remember what it's main European competitors were in the 50's / 60's and in the UK the Wray 89mm was a real dog of a lens. Taylor, Taylor, Hobson (Cooke) under Rank's ownership were beginning to concentrate on Cine camera lenses, so few of their WA lenses were available. So back when they were made Angulons were accepted for however they performed, it's only in hindsight and comparing to later designs we've become more discerning.

My current Angulon is definitely in it's original shutter, but there's no step in the rear thread to seat the cells, I'll get it's overall length measured accurately in the next few days.

Ian

 

[paul ewins]

Well, as measured with my highly accurate plastic micrometer on the metric scale, my angulon is 25.4mm or exactly 1" thick. That is serial number 71543xx (1961 according to http://www.schneideroptics.com/info/age_of_lenses/index.htm) and it currently resides in a Synchro-Compur-P. Dean, if you want to play with another sample for comparison I'd be happy to lend you mine for a few weeks. I live in Donvale near the freeway tunnel so I'm probably not far away.