aerial focusing?

[dh003i]

In Answel Adams' book, The Camera, he says that you can drill small holes in the ground glass, and put a small fine wire across each hole on the ground side. Focusing on that wire with a magnifyer, he says that you can focus on the aerial image with maximum resolution/clarity. Can someone please explain this?

Also, this appears different from a kind of aerial focusing mentioned here, also called parallax focusing (can this be done on LF with non-ground glass?), which I don't quite understand:

The special screens that have a clear center spot surrounded with a ground glass periphery can be used with a technique called "parallax focusing". This procedure takes a little practice. Rough focus is obtained on the ground glass, then on the clear area. Fine focus is obtained by moving one's head in a motion that suggests a a "yes" or "no" type of gesture.

As the eye moves across the viewing aperture the subject will move or "twitch" in relation to the cross hair. When this movement ceases, and the cross and subject remain constant with each other, correct focus has been obtained. This is not subject to eye sight diopter difference. If it appears sharp for me, then it will appear sharp for you if you know the technique.

This is not quite as time consuming as it may sound, and the effort spent mastering this technique will be more than justified In your photographic results. Because of these focusing screen considerations, a camera that can accept interchangeable focusing screens is very desirable if you plan to do much photography with a telescope; even better if the camera can accept and has available an optional high magnification finder.

Ben -- , Jun 11, 2000; 11:57 p.m.

In ground glass focusing, the lens forms an image at or near the focusing screen. The ground glass provides a diffuse but semi-transparent surface, which fixes the location of the image. Because the image location is fixed, when the lens is not in focus, the image formed on the ground glass is not in focus. Essentially, the ground glass restricts your eyes to focusing on whatever is at the ground glass.

An aerial image is an image formed in air, so to speak. If you put in a clear glass screen with some grid lines or whatever, the lens forms an image at or near the screen, and your eye can focus on that image, even if it is ahead or behind of the clear screen. So you see an in-focus image, even if the lens is not focused correctly on the screen. However, because the image is displaced ahead or behind of the grid lines on the screen, if you move your eye from side to side, you will see the image and grid lines move with respect to each other. This is a parallax effect, like looking at a tree behind a picket fence and moving your head from side to side and seeing the tree and fence move with respect to each other.

[Vaughn]

I am not all that familiar with the technical aspects, but I used a camera with clipped corners for many years. I could easily tell what was in the corners by looking at the "aerial image" and seeing what was going to on the film there. The image was quite a bit brighter and more clear than the image in the center of the ground glass.

That's about all I know about it.

Vaughn

[Emmanuel BIGLER]

There is little to add to the explanations given by the photo.net group.

May be we could add that this method was in use in some scientific instruments or on repro benches of the past (the good old days when flatbed scanners were unknown )

Certainly Saint Ansel refers to nothing else but this technique even if he does not mention the use of parallax effects to determine precisely the proper focusing point.

The problem with aerial focusing are the natural fluctations of accommodation of the human eye.
With a ground glass (GG), the grain of the GG is a visible feature on which you eyes "locks-in" and helps to properly focus. The GG also quickly blurs any out of focus image, something that does not occur with aerial focusing, except if the image is observed at a high magnification ratio as explained below. The high price to pay for this blurring effect is a loss of brightness ; you have a trade-off between a good blurring effect and brightness. The compromise has been greatly improved in the last generation of ground glasses, but ideally using a perfectly clear GG, the brightest of all (if we do not tale into account live-video on a computer screen monitoring a silicon sensor !!), brings you back to the problem or aerial image focusing.

If you do not use the parallax method, you have to rely on the very shallow depth of field of a high-power magnifying loupe. If the magnifying loupe is a 2X or a 3X, aerial focusing cannot rely on the relatively broad depth of focus of this loupe.
The parallax method can be used however with a low-power loupe and solves some of the problems associated with images of low ligh level ; it could solve the problem of light fall-of in the corners, but I have no experience.

The main drawback of the method is that the holes and associated wires (= like a cross-hair reticle) are located at some fixed places, may be not on the main subject you'd like to focus on.
Hence, aerial focusing was perfect for a repro bench where you setup the film plane perfectly parallel to the flat object you want to copy (no Scheimpglug there, not because it would be impossible to get something sharp, but because between slanted planes, when perfectly Scehimpflug-ly focused, you get some mandatory trapezoidal distorsion, the absolute evil in the good old analog days of repro benches with no digital post-processing ! and the optical axis perpendicular to both objet and film plane as precisely as possible by a combination of mechanical and optical aids like two flat mirrors face-to-face, one, with a viewing hole drilled at the center, mounted just un front of the camera replacing the lens ; and one layed flat on the object. A similar device was on the Hasselblad catalogue of the past.
Hence in repro benches, any point of the object, for example, the centre of the field, suffice to focus very precisely with the aerial image & parallax methods as soon as you are sure that the objet and image plane are parallel and the optical axis perpendicular to both.
When I mean : precisely, you should keep in mind that some repro lenses of the past (those used to fabricate photomasks in visible light on high-resolution silver-halide plates, a technology still in use in the sixties, eventually repaced by computer-driven pattern generators on chromium photomasks and more recently replaced with no-mask direct laser or e-beam pattern writing) were credited of 300-400 cycles/mm in green light, you cannot achieve this resolution in your image without being extremely cautious with focusing.

My feeling is that the aerial methods might not be very compatible with the total freedom in composition that we demand in LF... but I may be wrong ; although I still do have so hair available on my head that could serve for a home-made cross-hair reticle (the best cross-hair reticles, as the legend says, were made from a thread extracted from spider-web; probably a freshly picked-up one in the morning or during the first crescent of the Moon) I do not really feel comfortable at the idea of drilling holes in my GG..

[dh003i]

Perhaps instead of drilling holes in the GG, one could get a clear glass plate with a grid on the image-facing side to focus on?

[Emmanuel BIGLER]

Perhaps instead of drilling holes in the GG, one could get a clear glass plate with a grid on the image-facing side to focus on?

Yes, definitely but doing so you loose all capability to make at least a quick but coarse focusing to be refined with the aerial/parallax method.

But there comes another major problem, the fact that with a perfectly clear glass, you can't see the whole image, you can only see a tiny portion delimited by the exit pupil of the lens.
Simply take the GG off, and try to see the image.
What you'll see sharp is the rear barrel of the lens ; your eyes locks its focus to this object or the rea l world, not on the optical image floating in air close to your nose ! And you'll only see a tiny a portion of the image coming out of the lens delimited by the cone of rays connecting direcly the exit pupil of the lens (image of the iris as seen through the rear glass elements) with your own pupil.
So with only a clear GG you are unable to frame globally which is a nuisance for composition, and you are not able to do at least a rough focusing.

[OFF-TOPIC]
In the fifities & sixties, some famous German 35mm SLR cameras like the Zeiss Ikon Contaflex and the Voigtländer Bessamatic had a clear glass with a Fresnel.
The reason was to allow the user to get the brightest image as could be delivered by lenses of moderate aperture at the time, I mean : if f/4 can be considered as a moderate aperture !
You could fine-focus only by using a ring of microprism or the central split-image rangefinder.
But there was a Fresnel lens acting as a field lens like in any visual optical instrument !
[/OFF-TOPIC]

Even if the glass is clear, you'll need a Fresnel lens in order to see the whole image at a glance.
This is quite an unusual setup in Large Format, although it was used in the Contaflex and the Bessamatic...

[Nathan Potter]

Emmanuel, I'd like to point out that I still use an old Linhof GG from the sixties that partially solves some of the problems you alluded to with clear glass. The Linhof GG is mostly finely frosted but has a center small clear area with a black cross on the image side. Then there are four clear strips radiating outward to the four corners of the screen. With such an arrangement I can do a critical focus with my 5X loupe on the center black cross while simultaneously focusing the image in that area such that both the cross and image are parfocalized (in exact focus). Such a procedure sets an exact focus at the GG independent of eye focus or eye correction.

Since there are clear strips of glass out to the four corners I can use the same technique to check focus out to the four corners of the image but using the loupe to focus on the edge of the ground part while ensuring the the image is parfocalized at the same time. Additionally since most of the GG is frosted I get a nearly full view of the image for compositional purposes. Essentially I can't live without this piece of ground glass.

Have you ever found any GG similar, because I'd sure like to obtain a newer version in better condition than my old workhorse?

BTW your comments on early lithographic mask making brings me back to similar activities for which, by the way, I used the same type Linhof screen for critical focusing. We cut Rubylith masters which were back illuminated with mercury arc light at 430 nm. the master image was demagnified using an Ultra Micro Nikkor lens and imaged onto a Kodak High Resolution Glass Plate (HRGP). Since the image was small (I think 15 X 15 mm or so) we stepped it across the plate in an array format. IIRC the resolution of that lens was on the order of 800 lp/mm at 430 nm and the HRGPs' were an astounding 2000 lp/mm. We were able to image about 1 um lines quite well using careful development.

Nate Potter, Austin TX.

[aduncanson]

My father-in-law designed optical instruments for years and has recommended creating a clear spot for parallax focusing on your ground glass using a microscope cover glass, Canadian balsam and hair for a reticule. At least I think it was hair, I tend to tune him out when he gets on the subject of parallax focusing.

[QUOTE=Emmanuel BIGLER;433697]
[OFF-TOPIC]
In the fifities & sixties, some famous German 35mm SLR cameras like the Zeiss Ikon Contaflex and the Voigtländer Bessamatic had a clear glass with a Fresnel.
The reason was to allow the user to get the brightest image as could be delivered by lenses of moderate aperture at the time, I mean : if f/4 can be considered as a moderate aperture !
You could fine-focus only by using a ring of microprism or the central split-image rangefinder.
But there was a Fresnel lens acting as a field lens like in any visual optical instrument !
[/OFF-TOPIC]

A friend had a Bessamatic and I never realized that it had this feature. I believe that an early Leicaflex did also, but that they used a conventional solid (continuous) condenser rather than a fresnel. Last year I bought a Canon EX Auto only because it had this set up and I wanted to see how well it works in various lighting conditions. I have to say that I was not real impressed with the increased brightness.

[Emmanuel BIGLER]

From Nathan P. :
The Linhof GG is mostly finely frosted but has a center small clear area with a black cross on the image side. Then there are four clear strips radiating outward to the four corners of the screen.
Thanks Nathan for the information !
I have never seen such a ground glass, the best of both worlds, in a sense ! Too bad ! I would be happy to find one as well !
What about suggesting the idea or partially clera GG like the old Linhof to independant ground glass manufacturers that serve our community today ? A question to ask Geert in Belgium for example !
I can understand that you do not want to change it.
But you were trained to critical "scientific" focusing before
BTW, never I've seen an utra-micro Nikkor lens... the plates I used were Kodak HR plates type 1A, 2"-1/2 square. probably the same as yours. the lens I used was a second-hand Cerco bought as a lot with the copying bench discarded by a French lab much richer than the lab where I was a student. This Cerco lens covered probably 5x5 cm 2x2" but the performance was far from the ultra micro nikkor and it required only regular visible light. So your equiment was more performant although the covered field was smaller.
At the time used those special lense, at the beginning of the eighties, rotating-slit pattern generators were gradually killing those photomask benches.

However, coming back to your parly forsted Linhof GG, you probably do not have any additional Fresnel lens ?
In the corners with wide-angle lenses opening at f/8 like older super-angulon lenses, can you use the clear portion of the glass and check for parallax or any other clear-glass focusing technique ?

From Aducanson :

At least I think it was hair, I tend to tune him out when he gets on the subject of parallax focusing.
And ask him about the use of spider webs for ultra-strong cross-hair reticles !

I wanted to see how well it works in various lighting conditions. I have to say that I was not real impressed with the increased brightness.
This is not fair You should have compared with a regular ground glass of the same vintage
Not kidding, I agree, I have compared my Bessamatic with my wife's recent 35mm Canon SLR ground glass, and my 2.8 skopar with the clear GG is not brighter than the f/2 Canon lens with the modern frosted GG But is the comparison really reasonable ?

[Ernest Purdum]

Focusing with a clear spot and lines has been used quite a bit in scientific work - astrophotography, photomicrography, etc. The Polaroid MP-4 system had a focusing panel rather similar to the Linhof item. It's fairly easy to make one. Rather than bothering spiders, somebody like Surplus Shed usually has some reticles for sale that can be glued onto a groundglass. The lens cement fills in the grain of the groundlgass making it essentially clear.

[Nathan Potter]

One of the labs I consulted with was doing some Nomarski interference photomicrography using a Nikon metallurgical microscope. The technician had ground his own GG leaving a clear spot in the center of the 4X5 frame for aerial focusing accuracy. He had cleverly used IC chip wire bonding material for a focusing reticle. The material was gold wire 1 mil (25um) in diameter and was stretched taught and held in place using cyanoacrilate crazy glue. The stuff is also available in aluminum in diameters of 12 um, 25 um, 50um and 75 um. Seems ideal for the purpose.

Nate Potter, Austin TX.