Point Light Source...The Theory and Application

[Kirk Keyes]

I don't understand why you choose to interject your views on a matter that was not directed to you. Do you often have difficulty in determining who a question was directed toward?

I thought you were directing your comments to the forum members at large. Sorry, I would have thought that if this was a private communication, that you would have use email or pm.

Anyway, I have not used a point source enlarger. Does that really matter? I just believe that information should be presented in an accurate manner, and in this case I believe you are not making a valid comparison. Just stating my opinion.

Sorry to have stepped on your toes...

 

[Kirk Keyes]

Have you printed with a point light source or is your voluminous dissertation one that is based in theory only?

Oh, sorry, I forgot one thing.

AS I mentioned above, I have not used a point source. But I have used both a doubly-diffuse and a doubly-specular densitometer. And you will find an increase in appearant contrast between the two sets of number, with the specular source having the higher numbers. This information is very pertinant to this discussion. That's why you can't use a single negative to make comparisons of light sources.

 

[avandesande]

We do know the callier effect increases contrast, but that doesn't mean that more is not going on. The optical paths are quite different than with a regular condensor. If point source enlarging did not provide more information than other means it wouldn't be used in photo microscopy. Why would they bother?

I agree that a test with a n negative enlarged with point source and a n+1 negative with a regular condensor would be interesting.

 

[Donald Miller]

We do know the callier effect increases contrast, but that doesn't mean that more is not going on. The optical paths are quite different than with a regular condensor. If point source enlarging did not provide more information than other means it wouldn't be used in photo microscopy. Why would they bother?

I agree that a test with a n negative enlarged with point source and a n+1 negative with a regular condensor would be interesting.

It seems to me that there are a couple of issues being raised here. They are being interchanged and intermingled by those who raise them and I am not convinced that they are connected at all. It is a demonstrable fact that a diffusion light source will require a different negative DR then a condensor light source and even different still from a point light source. There are those who seem to believe that this is the cause for an apparent increase in sharpness on the print with a condenser light source. I don't believe that because of the following.

I have a 4X5 negative negative that was developed in ABC Pyro. I can enlarge this negative to an 11X14 print with my Saunders 4550 VCCE XLG and not have any apparent grain in the evenly toned sky. However I can not print an 8X10 print from this same negative with my Durst 138S with the opal lamp without grain becoming apparent. When I go to the point light source it becomes even more noticeable. In each of these cases the lens was my El Nikkor 150 f 5.6 lens. Grain is not a condition of contrast. Noticeable grain is a matter, in this case of the ability to resolve detail.

There is the matter of resolution and also of accutence that is not explained away by higher contrast. I believe and contend that the measure of sharpness of an enlarger, enlarger light source, and the enlarger lens is the ability to collimate and precisely focus the light source at the nodal point of the enlarging lens.

Diffusion scatters light. I think that we can agree on that. Scattered light is neither collimated nor is it precisely focused. That is what I believe because my actual experience indicates this. I can take a 5X enlargement and then view details on the print using a 10X loupe and the resolution and accutance is vastly greater with the condenser point light source. It is greater even then what is obtainable with the condenser opal lamp source.

I am not saying that contrast isn't different. Of course it is. But that is not the same thing that I am saying here. Nor does it explain the results on the print...and that after all is where the rubber meets the road.

 

[Donald Miller]

Oh, sorry, I forgot one thing.

AS I mentioned above, I have not used a point source. But I have used both a doubly-diffuse and a doubly-specular densitometer. And you will find an increase in appearant contrast between the two sets of number, with the specular source having the higher numbers. This information is very pertinant to this discussion. That's why you can't use a single negative to make comparisons of light sources.

You most assuredly can use a single negative to measure resolution and accutence...in fact a common standard would be indicated in order for the results to be something other then someone's subjective analysis.

 

[Kirk Keyes]

Grain is not a condition of contrast.

Why not? But the appearance of the grain will change with the grade of the printing paper.

 

[Kirk Keyes]

If point source enlarging did not provide more information than other means it wouldn't be used in photo microscopy. Why would they bother?

I have not done much photography with a microscope (a friend of mine does microanalysis and I've helped him do both film and digital imaging with his scopes), so I'm sure this will be discounted.

But it seems to me that the images taken with a microscope are of very low contrast. If I remember right, they make color films designed specifically for microphotography that are of very high contrast, at least compared to pictoral photo films. I'm sure the same holds true for B&W micro photo work as well.

So the use of a point source enlarger with microphoto images would allow the microphotographer to overcome some of the lack of subject contrast when making prints. I understand it is equivalent to giving about an N+1 equivalence just by using a point source. And the appearant increase in sharpness/resolution will help with imaging structures that are subject to very low depth of fields and therefore often look (and sometimes are) out of focus.

 

[Kino]

FWIW, I would tend to agree with Donald Miller..

I wonder if the difference could be attributed to the varying degrees of chaotic/messy circles of confusion at the focal plane of the baseboard due to the "cloud" of randomized light rays that cross the path of the more coherent light at the lens nodal point?

OR is it as simple as randomized the angles of light that hiting the negative and the grain of the image being placed less in bold relief than point source collimated light?

Its no secret that you should use, as a general rule, a highly diffuse light source when copying very grainy material, so it only makes sense...

I know that LED based lamphouses are brutal in their edge enhancing effects and who remembers the laser light prints that were/are too sharp to be true?

Can't get much more coherent than a laser light source...

 

[Allen Friday]

Dear Don,

Why did you jump all over Kirk? Because he had the audacity to disagree with you?

Your first post simply said: "Before I begin, let me say that what I am about to state is based upon my education and my experience. It is not designed to be a point of contention for differing viewpoints. It will be factual and factually supported." Is that a limit on the discussion--you say this is a fact, so we all have to believe it, and no one can dispute it.

You say the discussion will be factually supported. Well, where is the support for your facts? What sources did you use? Where did you get your research? What book can I go look at to verify what you say. Or, did you make it all up? A citation would be nice.

Do you think you can really limit people from analysizing your writing by claiming it to be based on your experience and because you believe it to be fact. You have had your facts wrong in the past. They may be wrong here. I don't know, I haven't looked into it--yet.

Finally, in your attack on Kirk, you say, "This is not intended to be a discussion of various light sources. It is designed as the discussion of one light source and how it differs from the other more commonly available ones."
How can you discuss how this light source differs from the other sources with out discussing the other source?

Finally, I don't know if a point source is sharper or not. I know, however, that Barry Thorton discusses the issue of sharpness in his excellent book "The edge of Darkness." I respect his work and writings. I know he addresses film grain and its effect on sharpness and he discusses condenser and point source enlargers. I think he concludes that the point source is sharper, but it may not be worth it to most photographers because of other issues, such as more difficulty spotting. I may look it up, I may not.

Or Don, you could actually do the research yourself.

Allen

 

[Charles Webb]

I am not aware that Omega or Bessler ever made a point light source. Durst primarily offered the point light source to the medical community for very precise photographs in research from what I understand.

Omega did offer the point source light option for the D2 series back in the fifties. I debated at that time which way I should go, talking with many photographers and lab techs I chose to go the other direction. I have never been sorry. The point is that yes, Omega did offer it a few years back.

Charlie..................

 

[MichaelBriggs]

There is about a century of scientific investigation of this subject. jdef is correct that the Callier effect is a work.

If you are going to visually compare the sharpness or resolultion of prints made by different methods, it is absolutely essential that you match the contrast of the prints. It is well known that the human perception of sharpness is strongly influenced by contrast. Of course, you can visually or artistically prefer one print to the other whether or not it truly has higher resolultion, or only appears to have higher resolution because it has higher contrast.

A brochure for the Durst L1200 states that the point source is for "enlarging low-contrast negatives and electron micrographs".

 

[Struan Gray]

Here's my purely theoretical take. I think you are all right.

The Callier effect is local, while most ways you can change the contrast of a print are global.

That is, the Callier effect reduces the contrast of small details that are close to each other and so can scatter light into each other. It has a lesser effect on large patches of tone, or on details that are widely seperated across the negative.

If you increase the contrast of the print to compensate for the loss of contrast in the details, you will change the contrast of the larger structures too. Thus you unavoidably change the look of the whole image, not just recover detail that was too low contrast to see clearly.

With negative development there is more scope to use things like adjacency effects to compensate for the contrast reduction through the Callier effect. Developers or techniques like stand development which promote micro contrast can therefore be use to make prints from diffusion or condensor enlargers which look just as sharp - have just as much small-scale contrast - as prints from a point source enlarger.

My impression is that for pictorial photographic purposes there is no absolute practical benefit from using a point-source enlarger if you process your negatives and prints with suitable adjustments to global and micro contrast. If you are doomed to low-contrast negs as in some scientific photography, or if you want really tight grain in your prints, or if you simply find point source enlarging the easiest and most direct way to get the look you like, then it is worth putting up with the finicky method and extra spotting.

 

[Bob Carnie]

Over the years I have printed film from various labs in the city and my own processed film(jobo, one shot)
The biggest complaint , from new clients that have film and prints from other labs is :
1. Where is all that crap from on the print.
2. How come I have black scratches running through my images
3. I didn't realize the edges were so sharp in this image.

Any one working in the film/lab envioronment knows the answer.
Diffusion light source hides defects , imperfections , scratches, bad processing.

Condensor light source is unforgiving and will show all the films defects if put in a glass carrier and use a good apo lens.

Point light source will be less forgiving than the two above light sources.

I have made a career from making good contrast/tonality prints. I would still not be in buisness if these prints did not reflect what the buyer is looking for. *I use condensor for visually apparent sharper Prints*

Mini Labs use diffusion light to soften , wrinkles, defects and film scratches.

So is this*sharpness* *acutence* or just my imagination.

 

[Claire Senft]

There is a point light source for a Beseler 4x5 enlarger for sale on Ebay right now. Check it out if you are interested.

 

[bill schwab]

There is a point light source for a Beseler 4x5 enlarger for sale on Ebay right now. Check it out if you are interested.

This looks like a simple condensor head to me.

Bill

 

[jon koss]

Hi Donald - Great post. Love the sun analogy. It does bring a question to mind though. Why are reflectors needed at all in your design? The sun has none that I have noticed. And any reflector will have thousands of times as much area as a point bulb filament. If we are to develop the sun analogy, the point bulb should hang in front of a black velvet background rather than a reflector. What do you think?

j

... we need look no further then the effects of the sun on a clear cloudless day....
... The other problem is that the light must be even across the projected area...

Reflectors are used in the design that I developed and installed. The reflector design must be clearly defined. It's dimensions must be carefully calculated and adhered to. The fact that light does have defined and predictable characteristics does help in this regard. In building a point light source as in all aspects of lighting the law that the angle of reflection equals the angle of incidence holds true.

I hope that this has answered those who have had questions.

 

[MichaelBriggs]

......Love the sun analogy. It does bring a question to mind though. Why are reflectors needed at all in your design? The sun has none that I have noticed. And any reflector will have thousands of times as much area as a point bulb filament. If we are to develop the sun analogy, the point bulb should hang in front of a black velvet background rather than a reflector. What do you think?

j

A condensor optical system focus the point or near point light source onto the entrance pupil of the enlarger or projector lens. This design principle has been known for many decades. For a simple and clear discussion with diagrams and photos, see Rudolf Kingslake's 1951 "Lenses in Photography". A reflector is frequectly added to capture some of the light rays that would otherwise head away from the consensor and therefore be wasted. A reflector is used to focus the rays back onto the filament of the lamp, maintaining the point nature of the source.

The known contrast effect is the Callier effect. The sun analogy ignores the role of the enlarger lens, which redirects rays originating from the same location on the negative but at different angles to the same spot on the paper.

 

[Kirk Keyes]

"Not only are these light rays in these types of light sources diffuse (scattered) but they are also not collimated (directed along a defined axis). Thus we experience a loss of sharpness between different density regions of the camera negative, we also experience a lack of information on the print."

"However because the light is not collimated and diffuse we have effectively "washed" away the defect by the scattering of light. We must recognize, it would seem, that we can not "wash" away the effects of dust and defects without suffering an accompanying loss of local contrast and sharp detail."

I think the original discussion falls apart in the first paragraph here. A jump is made correlating the colimation of light with the amount of sharpness or resolution. I disagree. An increase in light colimation will increase the contrast of the projected image, but it will not affect sharpeness, or even resolution.

As others have pointed out, the Callier Effect will lead to an increase in contrast when using a point source. This is an increase in overall contrast (and yes, even including microcontrast). It does not cause an increase in resolution. If negatives are not correctly matched for the type of printing light source, there will be an increase in appearant contrast, which the viewer of the subsequent prints may mistake for an increase in sharpeness or resolution.

Resolution will be limited by the enlarging lens, and not by the type of light source.

As to the second paragraph about dust, I covered the explaination of that in a previous post. It has nothing to do with a lack of resolution or sharpness. It has to do with the way the diffuse light is able to get around and under the dust.

You have to remember that light travels in a straight line, unless it is reflected off something (ignoring refraction here, of course). You also have to remember that the grains of silver in the negative are 3 dimensional, and irregularly shaped. That means that there are lots of little random surfaces on the developed grains of silver that can catch light from all angles, and reflect some of that light directly into the lens. So any dust that is between the diffuse light source and the film will be minimized.

It's kind of neat to realize that light from one side of the diffusion light head can fly all the way across the enlarger head until it goes into the neg, and gets bounced at an oblique angle by a speck of silver grain, and then go flying into the lens and onto the paper. That beam of light does a complete end run around our speck of dust sitting way up above the surface of the negative with the diffusion enlarger. The point source has no option but to run straight into the speck of dust and make a shadow on our print.

Notice that nowhere does the resolution of the enlarging system come into play here in minimizing the dust with a diffusion enlarger. Local contrast is affected when compared to a point source due to the Callier Effect, but not resolution.

 

[Struan Gray]

Resolution is about the preservation of contrast at high spatial frequencies. Not only does the Callier effect enhance that contrast, it does so preferentially at low negative densities where the paper shoulder can make it harder to preserve shadow detail in the print. There are good reasons for using a point source enlarger if you really need that contrast.

I agree that most pictorial photographers can find that contrast in other less fernickety ways, but that's another issue.

 

[Claire Senft]

Point light sources have found much favor with very high quality printers such as Robert Pace. The had made a good deal of its use in making very high qualty separation negatives and prints. Normally it would be used with an oil carrier.

Do you think there is a reason that they used a point light source or was it they did not read the right books?

 

[Donald Miller]

I think the original discussion falls apart in the first paragraph here. A jump is made correlating the colimation of light with the amount of sharpness or resolution. I disagree. An increase in light colimation will increase the contrast of the projected image, but it will not affect sharpeness, or even resolution.

As others have pointed out, the Callier Effect will lead to an increase in contrast when using a point source. This is an increase in overall contrast (and yes, even including microcontrast). It does not cause an increase in resolution. If negatives are not correctly matched for the type of printing light source, there will be an increase in appearant contrast, which the viewer of the subsequent prints may mistake for an increase in sharpeness or resolution.

Resolution will be limited by the enlarging lens, and not by the type of light source.

As to the second paragraph about dust, I covered the explaination of that in a previous post. It has nothing to do with a lack of resolution or sharpness. It has to do with the way the diffuse light is able to get around and under the dust.

You have to remember that light travels in a straight line, unless it is reflected off something (ignoring refraction here, of course). You also have to remember that the grains of silver in the negative are 3 dimensional, and irregularly shaped. That means that there are lots of little random surfaces on the developed grains of silver that can catch light from all angles, and reflect some of that light directly into the lens. So any dust that is between the diffuse light source and the film will be minimized.

It's kind of neat to realize that light from one side of the diffusion light head can fly all the way across the enlarger head until it goes into the neg, and gets bounced at an oblique angle by a speck of silver grain, and then go flying into the lens and onto the paper. That beam of light does a complete end run around our speck of dust sitting way up above the surface of the negative with the diffusion enlarger. The point source has no option but to run straight into the speck of dust and make a shadow on our print.

Notice that nowhere does the resolution of the enlarging system come into play here in minimizing the dust with a diffusion enlarger. Local contrast is affected when compared to a point source due to the Callier Effect, but not resolution.

Kirk,

You are wonderful on theory but somewhat short on practical experience. I am longer on practical experience. My practical experience says that your theory is faulty...based upon my prints printed in my darkroom. How about your prints printed on a point light source in your darkroom?

 

[phfitz]

Hi there,

Not to start a war but I think Donald is right and Kirk is wrong. Pure theory.

"It's kind of neat to realize that light from one side of the diffusion light head can fly all the way across the enlarger head until it goes into the neg, and gets bounced at an oblique angle by a speck of silver grain, and then go flying into the lens and onto the paper. That beam of light does a complete end run around our speck of dust sitting way up above the surface of the negative with the diffusion enlarger. The point source has no option but to run straight into the speck of dust and make a shadow on our print."

It also does an end-run around the fine detail in the negative. A diffuse light source comes thru the negative from 360* AND about a 90* angle thru the film. This hides the dust but also hides the detail because the light paths cross.

"Resolution will be limited by the enlarging lens, and not by the type of light source."

Any decent lens will out-resolve film and film out-resolves paper, so the limiter here would be the paper and presentation lighting.

There should be an easy way to end the resolution contest by using a vapor plated glass resolution slide enlarged onto ultra-fine duplicating film. I think the point-light will win out for resolution but lose badly for convenience in use.

It might be fun to install a 'MagLight' bulb into a 23C to play around with. Just a thought.

 

[avandesande]

I have heard all the dissenting opinions on here and PN but it doesn't appear that any of them actually have done it.

 

[Papa Tango]

I am not aware that Omega or Bessler ever made a point light source.

Yes, Beseler made one, and occasionally they pop up in eBay for an exhorbitant sum. The primary difference from the standard condenser assembly, beside the light source itself, are that the internal elements are movable.

I have a cobbled together point source that was built on this paradigm, and used for medical photography. At some point I hope to acquire the Beseler original, albeit a non-eBay price.

 

[Billy Bob]

Donald how do you get even illumination w/ your point source? I have 2 point sources and was considering using one to make a huge enlarger for 12x16 plates. I guess you are using the ps with the condensors in the 138s? I wonder about making a simple diffusion type enlarger with a point source sitting at the top of some reflector. Below the reflector I assume I would have to diffuse the light to get it even with ground glass or some other opal glass or white plexi. thoughts?