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 (http://http://photo.net/bboard/q-and-a-fetch-msg?msg_id=000ukR), 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.

[aduncanson]

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 ?

I was able to make the comparison you suggested since I have a contemporary Canon FTb, also with a 50mm f/1.8 lens. And you are correct that the EX Auto's screen is significantly brighter. In fact, to really compare apples to oranges, it seemed brighter than the rangefinder of my Canonet G-IIIQL 17. I think that I was disappointed with the EX Auto because the microprism focusing aid (all that one has to rely on) was a bit ambiguous and did not inspire confidence.

Drat, I may have to put some film in the camera and actually see how it works. ;)

[rvhalejr]

... 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.

Ive been using developed film grain taped tightly to the clear glass at the ideal Critical Dimension (statistically based on dimensional analysis of a matching set of film holder
septums to within +/- .001" or better).

I first course focus (compose the frame) with the gg, then thats removed and shimmed plenum (clear glass shimmed to the Critical Dimension) is mounted with the piece of developed film taped to the glass.

So I'm using film grain instead of AA's wire (or hair). I focus a 35-40x loupe (its actually a pocket microscope) on the grain in the emulsion. Then I Rack-in the lens to snap in focus in the emulsion plane.

Given this technique, is it any better or worse than the AA wire method ?

I have not used wire with a microscope but have used SEM grids (Calibrated
Scanning Electron Microscope Grids) to quantify film grain sizes.

Any Thoughts ? Thanks beforehand,

R.

[Nathan Potter]

rvhalejr, if I am following you, you first compose the image using regular GG and do a rough focus. You seem to remove the first GG then place a clear glass in its place. The clear glass has a piece of film attached to the image side. Is the emulsion facing the glass? Next you focus again at higher magnification on the silver grains using the higher power loupe. OK you have a parfocalized condition where the film grains and aerial image are both in focus. Now to take a photograph you must have to remove the clear glass and place a film holder in its place. The clear glass and film holder have been custom shimmed to be in the same plane? Seems like a tedious procedure while in the field.

Do I have this procedure correct?

Nate Potter, Austin TX.

[rvhalejr]

Deleted.

[rvhalejr]

rvhalejr, if I am following you, you first compose the image using regular GG and do a rough focus. You seem to remove the first GG then place a clear glass in its place. The clear glass has a piece of film attached to the image side. Is the emulsion facing the glass?

Yes, Absolutely.


rvhalejr
Next you focus again at higher magnification on the silver grains using the higher power loupe. OK you have a parfocalized condition where the film grains and aerial image are both in focus.


Yes, both are in focus in the same .001" plane.

In the example at hand we are using pocket microscopes with fixed magnification. So strictly speaking Parfocal is arguably a term that may not be that accurate describing our procedure.

Not to split hairs here but the definition of Parfocal Lens in Microscopy seem to be that focus stays the same when changing the objective (magnification) of a microscope. Ref: ...en.wikipedia.org/wiki/Parfocal_lens

That can only happen between 10x and 40x, never at 40x to 400x, neither of which is the case here. It might also be used in a context where eye pieces are swapped.

However, if we coin the term "Aerial Parfocal" or "Ansel Adams Aerial Parfocal" (AAAP) then we can say unequivocally that it describes our specific example.



Now to take a photograph you must have to remove the clear glass and place a film holder in its place. The clear glass and film holder have been custom shimmed to be in the same plane?


Absolutely Yes.



Seems like a tedious procedure while in the field.


Thats the down side (AA's technique or not). If I tried to do this in a helicopter I'd probably fall out the door !!!

One does not want to be changing gg or plenum glass in a hurry as dropping one could ruin your day. If one were firmly committed to the technique having a clear spot (or hole) at the center of the gg and the carson loupe (already focused) would be the quickest and least hassle.

If you have the time and preset 35-40x loupes and near-far-mid field positions marked on the plenum it might be setup in a few minutes (or less) with a little practice.

Evaluating diffusion would be the most time intensive under the dark cloth at f32 complicated by the fact that the amount is dependent on the final image size and viewing distance. The calculations for the final image (or print size) would need to be done before hand

Even on clear sunny day it is still very dark and difficult to see at >= f32 and I do not like using laser pointers around people. Indoors photo lights are required >= f32.



Do I have this procedure correct?


Yes, Thanks,

R.

[dh003i]

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.

Do you know where this lens cement you speak of can be purchased? Is it easy to apply to GG to fill in the grain and make it clear? Does it create distortions?

Also, why can't a very fine permanent marker be used to draw a reticule?

As an alternative, couldn't the GG be ground some more to essentially be polished, thus transparent?

[dh003i]

Posting from an email between myself and Steve Hopf, photofixation on eBay, a maker of excellent ground glass (http://myworld.ebay.com/photofixation), here's what he had to say:


Dear Steve,

I bought a piece of 4x5 ground glass from you around February '09. I remember asking you if you could drill fine holes in your excellent GG, but you said that you weren't setup for precision drilling.

I have another question regarding this. I know that GG is "ground" and displays an image because it is rough. If it is ground further, it becomes polished and transparent, right? Hence, I wonder if small "circles" (2mm diameter or so) could be polished at selected points on the GG for fine aerial focusing with a wire (or even line drawn across on the GG side).

I remember you said you sell ultra-fine glass using a special ultra grinding compound. As you said, any finer and it would polish the glass. Is that something you could do in small-diameter circles for the purpose above to make the GG transparent there for aerial viewing? If not, is there a tool I could buy to do this myself?

Best,
David



Hello David,
...
I thought about --and even experimented with different masks, but the grinding process is much too rough and nothing I have found can really withstand.

One option might be acid etching. If you are willing to go with acid
etched glass instead of ground glass, masks can be made in any shape with a computer --and DIY kits can be bought online. You could make your own gg and mask it precisely. I have no experience with this, but I have read about it online -- it seems very possible.

Other than that, the only idea I have is possibly a to find someone
with a CNC mill set up for etching glass --that might be able to make
polished 2mm circles into a gg...not sure though.

Hope this helps! Thanks ... --Steve

PS: I asked Steve for permission to post his comments, and he granted it, also asking me to give his regards to the LF forum :-) He also referred to glass etching kits (http://www.glass-etching-kits.com/).

[Nathan Potter]

dh003i and Steve. Masking against abraisive grit in a water solution can be tricky especially if the area to be masked is small but it can be done. I've used platers tape which is waterproof for the purpose. Also I've used Apiezon black wax applied with modest heat from a hotplate (probably around 50 to 60 degrees C).

If that fails for very small areas then the abraision can be done by sand or bead blasting - a most elegant technique that can be done very quickly. Use 600 to say 300 grit size for the sand blaster.

Holes in glass are best accomplished by ultrasonic grinding or diamond drilling and is straight forward in shops so equipped.

Nate Potter, Austin TX.

[Bob Salomon]

Linhof offered ground glasses with a clear center spot for years. It proved to not be a very successful or popular item. We may even still have some in stock that we haven't thrown out yet.

[dh003i]

dh003i and Steve. Masking against abraisive grit in a water solution can be tricky especially if the area to be masked is small but it can be done. I've used platers tape which is waterproof for the purpose. Also I've used Apiezon black wax applied with modest heat from a hotplate (probably around 50 to 60 degrees C).

If that fails for very small areas then the abraision can be done by sand or bead blasting - a most elegant technique that can be done very quickly. Use 600 to say 300 grit size for the sand blaster.

Holes in glass are best accomplished by ultrasonic grinding or diamond drilling and is straight forward in shops so equipped.

Nate Potter, Austin TX.

Nate,

Thanks for your response. I have a piece of hand-ground glass (Steve Hopf's ground glass). So what you're saying is that part of it it can be sand blasted with 600 grit size sand-paper to make a small strip or small circles clear?

[ic-racer]

Funny I saw this thread. Last night I was doing aerial viewing on my 8x10. I'm nearsighted, so I was getting a great aerial image in the clipped corners. I took the whole GG off and had a good look around. Main problem is you can only see a small part of the scene at a time.

[Nathan Potter]

Nate,

Thanks for your response. I have a piece of hand-ground glass (Steve Hopf's ground glass). So what you're saying is that part of it it can be sand blasted with 600 grit size sand-paper to make a small strip or small circles clear?

dh003i. I think I confused you. The masking against abraision needs to be done on the clear glass prior to grinding. Thus the masked area stays clear while the unmasked area is frosted. What's a bit tricky is to keep the mask (wax or tape) from abraiding away during the grinding operation since it is constantly attacked by the grit along with the glass. Sand blasting is more successful since the impinging particles hit the mask orthogonally so tend not to apply a shear force against the mask.

Since your glass is already frosted you need to defrost (so to speck) a small area to make it clear. This can be done and I think was mentioned above. You need to apply a small drop of clear material that preferably has the same index of refraction as your glass and that will spread out to the diameter desired. It will fill in the fractured frosted area and render it clear in appearance. You'll want the material fairly thin and flat so as to not distort the aerial image. What's the material? I don't know. But I might look at some clear wax; fingernail polish?; crazy glue? Maybe even vinyl alcohol. Possibly someone else here has done this and will offer a suggestion based on real experience.

Nate Potter, Austin TX.