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Ken Lee
31-May-2014, 08:31
We often read that a given lens design is optimized for a particular distance or ratio ... How is that accomplished ?

For what are they optimized: color aberrations, distortion ?

paulr
31-May-2014, 10:01
Only a guess. Since everything in lens design requires compromises, if they do their calculations at a particular magnification, that's where it will be optimized. The real work more likely goes into getting good performance at other magnifications.

8x10 user
31-May-2014, 10:27
I read somewhere that some Apo-Ronars and other process lenses such as the Computar have a thick spacer ring that corrects the lens for 1:1 ratios. It is said that if you remove the ring, the lens will then have an optimization that is closer to infinity.

Arne Croell
31-May-2014, 10:56
We often read that a given lens design is optimized for a particular distance or ratio ... How is that accomplished ?

For what are they optimized: color aberrations, distortion ?

All aberrations are influenced by magnification, since a change in distance changes the angles of every ray hitting each lens surface (except for the central ray). Lens designers will use all parameters (glass types, radii, thicknesses/distances) for optimization. Only one of them is occasionally accessible to us, the cell distance in the case of some process lenses, as in some of the Docter-Apo-Germinar lenses which have a removable washer/ring. Afaik, Apo-Ronars never used such a ring but cell distances were optimized in such a way that the shuttered versions were usually optimized for 1:20 or so, and the ones in barrel for 1:1. Dialytes are also not super sensitive when it comes to cell spacing, that is why even a 1:1 optimized Apo-Ronar works perfectly fine at infinity and f/22. I would venture a guess that the more ray bending occurs in a given lens, the more sensitive it is to cell spacing and magnification changes. This would include all modern wide angles.

Emmanuel BIGLER
31-May-2014, 11:02
Hi Ken !

There is are some very good explations in Rudolph Kingslake's "History of the photographic lens" so what I write here can anly be a poor paraphrase of this most useful book.
At the end of the XIX-st century, optical engineers have recognized that a perfectly symmetrical lens design used at the 1:1 ratio is able to deliver an image 100% free from coma, distorsion, and lateral color.
I remember a classical French textbook discussing about what could be a perfect optical system and the book explained that, yes a lens operating at 1:1 ratio could be nearly perfect, but "will be perfectly useless since the image is equal in size to the object".
People who use xerox copiers and scanners all day long will appreciate this enlighting remark from a wise theoretician of the past ;)

Actually most of the lenses we use in LF, with the exception of tessar types and telephotos as well as some wide aperture planars and xenotars, are refinements of some basic symmetrical designs.
The tessar is as old as 1904 and is a remarkable non-symmetrical lens, hence it has to be optimized for some non symmetrical (object / images) distance configuration. Infinity-focus is in principle the basic starting point of optimization for a tessar. On the other end, the (4/4) dialyte like the apo ronar (the ancestor is the celor patented in 1902) is at the beginning optimized for the 1:1 ratio and is perfectly symmetric.

Hence at the beginning, the degree of symmetry / asymmetry plays a very important role in the best lens-to subject distances.

By playing with the lens prescription, tessars can be optimized to serve as an enlarging lens at their best at 1:5 ratio for example.
And it is rumored that Fuji-C lenses are optimized for large distances although the look very much similar to an apo ronar ..

My understanding is that modern (6/4) standard lenses coverning 70° to 75° are optimized not stricly at infinity but for some distance, about 11 times the focal length (1:10 ratio) but actually those lenses are useable from infinity to 1:2, restricting the use of real / true /symmetric macro lenses to a very limited range of applications.
For example the apo-rodagon D 1:1 evoked here for copying a film to a DSLR sensor is perfectly symmetric and works fine if you do not depart too much from the 1:1 ratio. There is a companion lens named apo rodagon D 2x which is asymmetric and is not at its best at 1:1 but as labeled at 2X or 1:2.

Another interesting lens optimization is the 5.6 / 135 mm Zeiss makro planar, a fixed-element lens (7/5) design, a compromise for a lens able to shoot from 1:1 to infinity. This lens now belongs to history, at the first glance, it is very similar to one of our LF lenses but bears more assymetry, though.
There is an interesting article on the Zeiss Camera Lens blog explaining how the use of floating elements yields higher performances for a lens operating from 1:1 to infinity. They do not say that the 135 Makro Planar is obsolete, but you can read it between the lines ;)

Lenses departing from the symmetry principle are in fact the majority of lenses for 35 mm, with the exception of macro lenses.
The reason I see for this is that 35 mm photographic lenses developed mainly around the idea of reportage and hand-held photography, for subjects located at large distances and with wide apertures. If you need both a wide aperture and optimization at infinity, a symmetric lens design is certainly not the good choice.
Need a wide-angle lens on a reflex camera body ? You can't use LF WA lens symmetrical design, you have to use a retrofocus, which is higly asymmetric. Don't want a lens-to film distance as big as the focal length ? You have to use a telephoto design, again something asymmetric that will not be good at close distances
Apo-ronars were unknown in 35 mm photography, at least after WW-II !

If we look at recent view camera lens designs for digital photogrfaphy with a small view camera, they are definitely asymmetric but bear some similarity with our classical LF lenses.

Hence for LF lenses a short summary to the original question is that the optimization starts from some symmetrical design and departs from symmetric by changing lens surface curvatures and spacings, and changing the actual location of the iris.
But those degrees of freedom are somewhat limited and many modern lenses for small formats use floating elements to achieve the best image quality in a large range of lens to subject distances.

On the other hand, highly specialized lenses like those used for photo-lithography work only a one wavelength and one magnification with a fixed f-stop e.g. f/4. They are definitely asymmetric and lens designers cannot compromise anything outside the highly specialized technical goal they have to achieve, designing the the best of the best lenses for one single task, one single wavelength, one object field and one single magnification (1:5 for current wafer-steppers).

In our LF lenses optimization at one distance e.g. 1:1 or 1:10 is quite loose since those lenses are general-purpose lenses.
But we could dream of floating-element LF lenses that would outperform our classical "fixed" designs.
Suffice to look at the last generations Zeiss MF distagons with their floating element control ring, manually operated. This could perfectly be applied to our manually-operated LF lenses.
But since we operate on film on a non-reflex camera we do not need any retrofocus lens. The good old symmetric lenses are still so good !
Only the constraints of silicon sensors with micro-lenses unable to capture slanted rays pushes lens designers to refine some special retrofocus designs like the very last Zeiss 55 mm "Otus" design ... but this is another story 100% off-topic here ;) (https://www.flickr.com/photos/carlzeisslenses/sets/72157635236491881/)

hoffner
31-May-2014, 11:33
For what are they optimized: color aberrations, distortion ?

Lenses are optimized for their optical performance which in its turn is demanded by the optical requirements. The optical requirements are dictated by the intended use of the lens. As the use differs so do the optical requirements and the necessary optimization.
Each optical designer can decide what optical defect optimization is preferred at the expense of other defects.
Therefore your broad question has no single answer.

ic-racer
31-May-2014, 13:04
We often read that a given lens design is optimized for a particular distance or ratio ... How is that accomplished ?

For what are they optimized: color aberrations, distortion ?

In terms of Large Format Photography and Photographic Enlarging, I'm familiar with fine-tuning flatness of field with front-rear cell spacing. I suspect there are also many considerations that would concern the lens designer.

Dan Fromm
31-May-2014, 14:58
Interesting points about the effects of changing cell spacing. One of the things it does is change focal length. That's what puzzles me about the fact (which I believe) that Rodenstock used a spacer to convert an Apo-Ronar optimized for 1:1 into an Apo-Ronar optimized for 1:20.

8x10 user
31-May-2014, 18:43
Afaik, Apo-Ronars never used such a ring but cell distances were optimized in such a way that the shuttered versions were usually optimized for 1:20 or so, and the ones in barrel for 1:1.

I have a Sinar branded version with a small normal shim and a thick removable shim which I believe to affect optimization. I have only seen this with the Sinar version... Also it is multicoated and a "CL" version of the lens... Very rare.

8x10 user
31-May-2014, 18:48
I believe the Red Dot Artar was a dialyte that was optimized for ~1:10-1:20 (depending on FL) while the Apo Artar was for 1:1.

Ken Lee
1-Jun-2014, 03:43
Thank you all for your very helpful and informative replies. The expertise on this forum is... remarkable !

ruilourosa
1-Jun-2014, 05:00
Hello

In my lenses it does not change the focal distance (at least with "minimal" increases of up to 10 inches), i have tried in plasmats, tessar (not symmetrical), celor, double anastigmats, reversed double anastigmats, super angulons (and similars), type lenses, and even in a rapid rectilinear that i built out of achromatic doublets.

each lens element (or group of lenses aka cell) has itīs own focal length and it does not change regarding the spacing, in zoom lenses this (in theory) is made with a moveable negative element(s) that moves between two positive elements in a afocal system plus a positive focusing element.


i would love that changing spacing would change focal distance :)

i think optimization could exist in some lenses but in general itīs a built system that is made towards a particular end, but i would really love to see if someone could check possible otimization in ronars, g-clarons and other process lenses,

cheers

Rui Lourosa

Dan Fromm
1-Jun-2014, 06:16
Rui, you're mistaken. I have a couple of 6x9 folders with tessar type lenses that have front cell focusing. They're not unit focusing, i.e., they're not focused by changing the entire lens/shutter assembly's distance from the film plane. Their distance from the film plane is fixed (when, of course, the camera is erected). They focus by unscrewing the front cell. This wouldn't work if it didn't change focal length.

ruilourosa
1-Jun-2014, 06:55
Iīm not mistaken... iīm generalizing, (for large format) you are talking in a particular design and in minimal change in focal distance, i also have a minox that works that way! and itīs not the best way to focus a tessar... but itīs simpler and easyer to make... large format tessarīs do not use this focusing mecanism, and i never even seen a floating elements system nor zoom type for large format.

my ilex caltar 375mm is a tessar and i have experimented in changing the distance from the back element to the front element (and aperture) and no change in focal distance is observable,

my experience is empirical not achieved through equations and i am only talking about the designs i mentioned, residual changes in focal distance might be verified but i have no means to see them.

the mentioned dialite (apo ronar) does not change the focal distance of the sistem (at least in eye verified experiments) although it changes the amount of aberrations

i am interested in some testing about otimizations of (specially g-claron plasmats)

Cheers

rui

Dan Fromm
1-Jun-2014, 07:28
Rui, you are still mistaken. Here's the test:

Set up your camera with any lens you like on it. Focus on a subject. Unscrew the front cell two turns. If you've lost focus, unscrewing the front cell has changed the lens' focal length.

ruilourosa
1-Jun-2014, 07:52
just the optical center is displaced, and since you are also moving the element forward you might need some correction of the plane of focus. not the focal distance. If you have a +4 diopter (250mm sistem) system you will not change the total sum focal distance if you change the distance between the elements, however the optical center will change so you will lose focus (regarding your experience in focusing a subject)

maybe in some optical systems focal distance will change, but for the case that is being discussed here they are of no importance

and maybe minor changes also occur but not a zoom effect.

cheers

Rui Lourosa

Emmanuel BIGLER
1-Jun-2014, 07:55
If you've lost focus, unscrewing the front cell has changed the lens' focal length.

.. or the focal length did not change at all, but the facetious principal plane H' has moved just enough to bring contradiction to Dan's explanations ;)

To the best of my knowledge, front-cell focusing works only with classical Triplets and Tessar designs (may be Heliars as well) but does not work at all with, say, Sonnar designs.

With a 6x9 folding camera of the thirties, a 100mm lens has to move as a whole block by about 10 mm in order to bring focus from infinity down to one meter.
With front cell focusing, the same effect is obtained by un-screwing the front lens element by less than 1 mm.
The pitch of the focusing thread is very fine in those cameras and can be easily damaged if you unscrew completely and re-screw it wrongly.

Emmanuel BIGLER
1-Jun-2014, 07:57
just the optical center is displaced,

Could you please define what is the "optical center" of a compound lens?

If you have a +4 diopter (250mm sistem) system you will not change the total sum focal distance if you change the distance between the elements, however the optical center will change so you will lose focus (regarding your experience in focusing a subject)

This is not correct, sorry, the focal length changes a bit.
The triplet and tessar are excellent examples. In a triplet or tessar, the focal length of the front cell is much shorter than the focal length of the whole lens. Computing the change in focal length induced by the change in spacing between the first lens element and the rear group is not very difficult to compute, and the focal length does change.
The formula is elementary: (known as Gullstrand's formula))
1/f = 1/f1 + 1/f2 - e/(f1.f2)
where f is the focal length of the whole lens, f1 and f2 the focal lengths of the 2 separate groups. "e" is the distance H'1 H2 between principal planes of both groups.
Unscrewing the front cell changes "e" without changing anything else. Hence font cell focusing or any change in group spacing DOES change the focal length. By a tiny amount in several cases, certainly.
And the principal planes move as well.

The advantage with front-cell focusing of triplets and tessars, at least from the point of view of an amateur photographer of the 1930's to 1950's, is that you do not degrade too much image quality by moving the front lens element by 1/100-th of the focal length.
This does not work with many other lenses.
(4/4) dialytes are weakly sensitive to group spacings, but sensitive enough so that manufacturers adapt group spacing to the actual use of the lens!

Dan Fromm
1-Jun-2014, 08:12
since you are also moving the element forward you might need some correction of the plane of focus. not the focal distance.

You won't admit it, but we agree. If the plane of best focus has moved and must be corrected (after the front cell is unscrewed) then lens' focal length has to have changed.

Emmanuel, my friend, Rui doesn't want equations. More's the pity.

ruilourosa
1-Jun-2014, 09:00
if you move the front element forward you are altering the light path and bringing the optical center of your sistem forward, if you rectify the focus pulling your front standard back towards the film plane you are just putting you optical center where it was...

I have no problems to admit in whatever i am wrong but i think in this case you are wrong, not me, and i am not afraid of equations, as they are quite simple. if you add diopters you will get less focal lenght, and if you add distance between two elements you will change the optical path, but the strength of the elements remain the same.

maybe aberrations will come into place, and with complex sistems maybe this will change, but with a two cells plasmat itīs verifiable

If you are into testing: prepare a still life and point your camera to it, measure an element in your ground glass, take notes, change the spacing, focus, do another measurement, check for coincidence :) you got it

changing cell spacing by it self cannot be (in the normal large format or process lenses) a way to change focal distance, or if i am completely wrong, zoom lenses would be really easy to make

cheers! no pity :)

rui

ruilourosa
1-Jun-2014, 09:10
what i intended to say: cell spacing is not a zoom mechanism.

if it changes for 3mm in a 300 is certainly of no practicable value and should be disregaded, i am a photographer not an optician and i have mainly an empirical aproach to my photography (not only) and this has the same photographic value as the substitution of 1+24 to 1+25 in rodinal dilution.

thanks for sharing you knowledge

any otimization conclusions?

rui lourosa

Dan Fromm
1-Jun-2014, 09:14
Rui, the equation that Emmanuel posted is from a textbook.

To expand on his post, the effective focal length of a thick lens can be written as f = fa * fb/(fa +fb -d) where fa is the front cell's focal length, fb is the rear cell's focal length and d is the distance between the cells. This is Emmanuel's equation rewritten.


The position of nodal points N1 and N2 relative to the first vertex (A) and last vertex (B) of the system are given by

AN1 = df/(fa + fb -d)

BN2 = -df/(fa + fb -d)

I took the quoted text from S. F. Ray's book Applied Photographic Optics, 3d edition. See chapter 8. By the way, changing cell spacing changes focal length and moves the nodes. The various effects' magnitudes depend on the cells' focal lengths and cell spacing, don't have to be large but are never zero.

Dan -- police up the battlefield and dispatch the wounded -- Fromm

ruilourosa
1-Jun-2014, 10:09
Thanks for your knowledge, proving i am wrong, and proving that i am right.

From my intended use of photo optics point of view (and yours too, certainly) i am right

from a optic physics point of view you are absolutely right, although not very useful here

i think you understand me as i understand you, what i really do not understand is your petulance, not needed...

Bernice Loui
1-Jun-2014, 10:28
APO Artar / Red Dot Artar are basically the same except for date of manufacture, coating and barrel -vs- shutter mounted. They are both Dialyte designs.

According to the Goerz catalog and literature, the ones in shutter tend to be optimized for 1:20_ish and the barrel version are often optimized for 1:1. With all that published and discussed, over many years of testing and images made close up and at infinity with both shutter and barrel versions, the differences are nil or not worth getting worked up about on film in real world image making.

There is an image posted of the SF bay bridge, it was made using a 19" RDA in barrel. Along with that posted image is a cropped section scan of the speed limit sign from that negative which is clearly legible.

On table top images using 8x10, the 14" RDA proved better image quality wise (resolution, contrast and etc) compared to a 360mm Rodenstock Sironar N @f22 / f32 based on my own experience..


This swayed me towards using barrel versions with a Sinar shutter far more than the shutter versions. As the barrel versions have a nice round iris and the ones in shutter can appear with shutters with less than round iris (Copal re-mounts and etc). IMO, the best shutter versions are in the older Compur shutter with the round muti blade iris. This is the same shutter that can give a lot of problems due to it's complexity, congealed lubricants and dirt.

As for differences between APO Artar -vs- Red Dot Artar, they are mostly the same. I have more than a few coated APO Artars with serial numbers under 76xxxx, that are coated. Generally Red Dot Artars (serial numbers 78xxxx and above) are known to be coated. Performance wise, it depends more on the individual lens than just it's markings.

The Red Dot thing was more marketing by Goerz much like Gold rim / Gold Dot Dagor. Exception to this would be those made by Kern.


These remain one of my all time fave view camera lenses.



Bernice




I believe the Red Dot Artar was a dialyte that was optimized for ~1:10-1:20 (depending on FL) while the Apo Artar was for 1:1.

Dan Fromm
1-Jun-2014, 11:08
Bernice, do you remember the old Rodenstock propaganda that insisted that an Apo-Ronar (f/9 and slower dialytes much like Artars of all vintages and symmetrical Apo-Nikkors) was better than the equivalent telephoto lens?

Bernice Loui
1-Jun-2014, 19:43
Don't remember the Rodenstock promo about APO Ronars being better than telephoto lens designs for LF. How they are used and for specific imaging needs are likely more relevant.

Problem with using this family of APO lenses for longer than normal focal length lenses is bellows draw which related to possible focal lengths possible for a given camera. The longer the bellows draw, the bulkier the camera which directly affects camera stability affecting image quality.

While this family of APO lenses can produce great resolution and image quality, it is subject to numerous other factors that can negate their optical performance over telephoto design lenses for LF.

Both have their place, both have their advantages and negatives. It is much about knowing when to choose a given lens for a particular image making need.

Some where in the pile, there lives a Zeiss S-Tessar with front focus. It was designed for a projection system where rotating the front element to achieve focus works nicely.


:)
Bernice




Bernice, do you remember the old Rodenstock propaganda that insisted that an Apo-Ronar (f/9 and slower dialytes much like Artars of all vintages and symmetrical Apo-Nikkors) was better than the equivalent telephoto lens?

Emmanuel BIGLER
2-Jun-2014, 04:45
While this family of APO lenses can produce great resolution and image quality, it is subject to numerous other factors that can negate their optical performance over telephoto design lenses for LF.

I cannot but agree with this wise statement.
This is the reason why I own a 360 Tele-Arton AND a 360 Apo Ronar.
(collectors are always hungry for new excuses justifying the accumulation of equipement ;) )

In the seventies of the last century in France, at a time when professional photographers still used LF cameras, a famous book "Moyens & Grands format" by the late René Bouillot (Paul Montel editions, a famous publishing house dedicated to photography) stated a very negative appreciation of telephotos (poor sharpness, horrible distorsion and minuscule image circle) and, on the contrary, nothing was better than a dialyte.
I remember reading this book at a time when I never thought I would ever use a LF camera myself
I eventually succumbed two decades later for a 360 Tele Arton. I do not regret the purchase, but some day after reading various forums, I knew I HAD to acquire a 360 Apo Ronar as well ;)