Perceptol & The Microdol Substitute Formula.

[Photo Engineer]

How does fog ever harm an image? It compresses shadow detail (light areas) in the negative and eliminates any detail that was below the level of fog formed. In addition, it is (or can be) colored hence the name "di chrhroic" and this can cause its own distortion of the image under some conditions.

PE

 

[Ray Rogers]

Foggy Bottom

How does fog ever harm an image? It compresses shadow detail (light areas) in the negative and eliminates any detail that was below the level of fog formed. In addition, it is (or can be) colored hence the name "di chrhroic" and this can cause its own distortion of the image under some conditions.

PE

I still haven't figured out what the difference between a neutral general fog and neutral density is. (If there is such an animal)

As I read what you wrote above, it seems you are saying that fog (in general) is damaging even if it is neutral in color... so, what aspect of fog is different from neutral density? Turbidty?
I guess it somehow allows lght to "add" differently across the toe to the shoulder, but I don't see why or how it does so. Is the fact that fog is discontinous a factor?

:confused:

 

[Photo Engineer]

Ray;

Neutral density takes a totally normal image and just moves all data points on the density vs Log E curve upwards by the amount of density in the filter.

Fog changes image areas into density with no discrimination and eliminates differentiation in tone up to the point where the fog meets the density vs Log E curve.

I might add that if you are lucky, fog might not impinge on the image enough to be a pain.

This is one of the basic principles of photography.

PE

 

[archer]

Where can mercaptan be purchased and is it the same as methylsuloxide?
Denise Libby

 

[Ray Rogers]

Ray;

Neutral density takes a totally normal image and just moves all data points on the density vs Log E curve upwards by the amount of density in the filter.

Fog changes image areas into density with no discrimination and eliminates differentiation in tone up to the point where the fog meets the density vs Log E curve.
PE

Is it impossible to create a neutral density filter out of collodial silver?

Ray

 

[Photo Engineer]

Is it impossible to create a neutral density filter out of collodial silver?

Ray

You can create a uniform, non discriminating neutral density filter from colloidal silver, and in various colors. That is what the CLS layer is in color films AAMOF. But, it does not affect image quality while fog does. If you believe that fog and an ND filter are the same, then you will have to rethink your beliefs.

PE

 

[Photo Engineer]

Where can mercaptan be purchased and is it the same as methylsuloxide?
Denise Libby

IDK where to purchase the mentioned mercaptan, as I have never tried. The chemical name you give in the quote is not a valid name, but resembles "methysulfoxide". If this is what you meant, then no, they are not even remotely similar. It might be that you mean the common solvent "di methysulfoxide", and that is not similar either.

PE

 

[Kirk Keyes]

If you believe that fog and an ND filter are the same, then you will have to rethink your beliefs.

I think Ray is trying to get at the idea that if you make an exposure on film, and then give it a fogging exposure, then you have bumped the exposure up the curve, perhaps out of the toe of the film. Similar to what is done when B&W printing and giving a slight exposure to push the image out of the toe, or when shooting and you want to bump the image out of the toe, one can try a slight fogging exposure either before or after the image is made.

The fogging exposure will end up as ND overall in the negative, but the shadow detail in the film exposure or the highlight detail in the print exposure will be enhanced.

Differences in the source of the fog, one is chemical during development, and one is exposure in the film - make the difference I suspect...

 

[Photo Engineer]

Well, actually fog is not like "adding an ND". No matter how you do it, or what the purpose, it does something to the tone scale.

Dichroic fog, produced by solvent action may affect Dmax as well as Dmin, as the Silver fog must come from somewhere. Regular fog by exposure or keeping usually just raises Dmin and does not affect Dmax. An ND affects Dmax and Dmin equally and does not change the tone scale.

When you fog a film as you describe Kirk, you generally soften the toe so that when you bump the image up the curve, the toe can follow and put you right back where you started. However, if the exposure is centered, and you fog the film, then contrast is reduced overall in the normal exposure region.

PE

 

[gainer]

Well, actually fog is not like "adding an ND". No matter how you do it, or what the purpose, it does something to the tone scale.

Dichroic fog, produced by solvent action may affect Dmax as well as Dmin, as the Silver fog must come from somewhere. Regular fog by exposure or keeping usually just raises Dmin and does not affect Dmax. An ND affects Dmax and Dmin equally and does not change the tone scale.

When you fog a film as you describe Kirk, you generally soften the toe so that when you bump the image up the curve, the toe can follow and put you right back where you started. However, if the exposure is centered, and you fog the film, then contrast is reduced overall in the normal exposure region.

PE

We can see mathematically that adding a uniform exposure to an exposed negative will change the shape of the characteristic curve, as PE has said. The characteristic curve is plotted as density vs log(exposure). Adding the same increment of fogging exposure to a small image exposure has a greater effect on the log of the total at that point than does adding the same fog increment to the other end of the brightness range. It is not a matter of guesswork, but can be planned. It does change the shape of the curve. A film that showed a straight line without the fogging would show a curve.

Some old uncoated lenses are revered for the overall fog they bestow.

 

[Ray Rogers]

I think Ray is trying to get at the idea that...

First, Kirk, let me say thank you.

Having spent more of my life trying to learn, rather than actually learning,
I have observed that questions about the unknown* are frequently malformed.

I appreciate your clarification of what I was trying to say...
I am not totally sure if that was actually it, but I appreciate your help.
Certainly, you were able to open the conversational door, and for that I am thankful!

It is not my desire to divert this thread any further so I will hold my comments/questions untill a better time. I do think the comments Gainer has made are important to questions I have concerning fog, and density manipulaton with intensifiers and reducers etc., but as I said before, perhaps we can return to that at another time in a new thread.

In the meantime, I will just follow along quietly.

(After one or two quick questions for Gadget that is)

Ray

*(bzw. poorly understood or conceptualized)

 

[Ray Rogers]

Warning! Questions are probably malformed...

We can see mathematically that adding a uniform exposure to an exposed negative will change the shape of the characteristic curve... A film that showed a straight line without the fogging would show a curve.

Some old uncoated lenses are revered for the overall fog they bestow.

Thanks for your comments.

I wonder then, what do you see mathematically by SUBTRACTING a uniform exposure...
and How does this not equate to the use of an ND filter?
If adding exposure changes the curve, doesn't removing it do the same?

What are some of thoes old lenses?

Ray

 

[Photo Engineer]

Patrick;

How does a lens cause fog? That is new to me. It can have flare, but fog?

PE

 

[gainer]

Patrick;

How does a lens cause fog? That is new to me. It can have flare, but fog?

PE

Flare light is not image light. It softens an image by the same mechanism, though in a somewhat more complicated manner, taking light from brighter parts of the image-bearing rays and diffusing it into the more or less nearby image areas.

 

[gainer]

Thanks for your comments.

I wonder then, what do you see mathematically by SUBTRACTING a uniform exposure...
and How does this not equate to the use of an ND filter?
If adding exposure changes the curve, doesn't removing it do the same?

What are some of thoes old lenses?

Ray

A neutral density or in fact any color filter does not subtract, it divides by a Filter Factor. Thusly, the film sees Illumination/FF, not Illumination - FF. If the filter factor is 2, interposing it between the scene and the lens is equivalent to stopping the lens down by 1 stop.

Have you devised a way to subtract a uniform exposure?

 

[Kirk Keyes]

Have you devised a way to subtract a uniform exposure?

Yes, by using log(exposure) or log(optical density).

It makes the math easy with using the log. My 1-stop ND filter subracts a log(filter density) of 0.3 from my exposures.

 

[Keith Tapscott.]

Some old uncoated lenses are revered for the overall fog they bestow.

Some of the older single-coated lenses are also highly revered.
James Ravilious used Leica range-finder cameras and preferred to use the older Leitz lenses for his B&W photography.

 

[gainer]

Yes, by using log(exposure) or log(optical density).

It makes the math easy with using the log. My 1-stop ND filter subracts a log(filter density) of 0.3 from my exposures.

But that is not the math that is the subject here. The filter does not change the shape of the characteristic curve, only its intercept on the log(exposure) axis. OTOH, an amount of light you add by applying a number of non-image-forming meter-candle-seconds to the entire negative before development has a much different effect, not attainable by ordinary filtering. If you could subtract light in the same fashion, you would take away more information in the toe than in the shoulder, reducing film speed while increasing contrast. You could achieve this effect by uniformly bleaching the latent image, but who woud want to do that?

 

[Photo Engineer]

Yes, by using log(exposure) or log(optical density).

It makes the math easy with using the log. My 1-stop ND filter subracts a log(filter density) of 0.3 from my exposures.

But you cannot go below Dmin so the effect is not pervasive throughout the D Log E curve. That is a point to consider, because it changes the shape of the curve, a point made in different words by Patrick.

All of these are non-linear but those of an ND, and they stop working at Dmin but work beyond Dmax. The others stop working beyond Dmax but distort the curve up to Dmin.

PE

 

[Photo Engineer]

I would like to add that my experiments with Rodinal and other substitutes indicate that there are additional ingredients that act more effectively than Sulfite at this high pH value. Any comments from anyone? Kirk? What is more effective at scavenging oxygen at high pH than Sulfite? What is the pH profile of the redox potential of Sulfite as a function of pH?

PE

 

[Kirk Keyes]

What is more effective at scavenging oxygen at high pH than Sulfite?

Well, since you asked -- For use as an oxygen scavenger in boilers/steam generators, hydrazine, sodium sulfite, or saccharide, such as glucose, has been widely employed.

My 32th Ed. CRC Handbook lists the use of pyrogallol as an oxygen scavenger for removing oxygen from atmospheric air to produce oxygen-free nitrogen. Somthing like this was used as an oxygen absorber:

15 g. pyrogallol to 100 cc. 20% NaOH or KOH could be used.

Mind you, this solution is quite effective - much faster than sulfite is at reacting with oxygen.

Perhaps it's a touch of pyrogallol in there that's turning the Rodinal black with age?

 

[Ian Grant]

The problem is a touch of Pyrogallol would materially change the developer characteristics particularly when fresh with no oxidation. If it was Pyrogallol going black in Rodinal it would do theis very quickly, days not many months once the bottle was opened and partially full.

But any developing agent that oxidises easily could be used as an oxygen scavenger, although perhaps not so efficiently. Hydroquinone has been used for boilers too.

More importantly is the combined effects of p-Aminophenol with Pyrogallol and the super-additivity. While I've never seen a formula using this particular combination Ilford, Agfa and Kodak have all used P-Amininopheol with Hydroquinone. p-Aminophenol can replace Metol, so could easily be used in a formula like PMK.

So if Pyrogallol was used as an oxygen scavenger in Rodinal the fresh developer would have an increased activity, and as the Pyrogallol oxidised during storage the activity would drop quickly, it would oxidise first. On the contrary Rodinal's activity only changes marginally as it goes darker.

Ian

 

[Photo Engineer]

Ian;

I agree, but do remember that Agfa kept Rodinal for a few weeks before it went on sale, so that it could "age to perfection". Could that be something decaying to zero that was used as an oxygen scavenger? Could it be something that had to vanish before use?

IDK.

I am merely posing questions based on what I have seen regarding Rodinal.

PE

 

[Ian Grant]

I don't think so as the ageing and bottling of Agfa Rodinal (as opposed to Calbe R09) is done under nitrogen.

It's much more likely that the ageing has to do with the reaction of the p-Aminophenol with the hydroxide, that needs time to stabilise then the pH is adjusted prior to bottling.

In the "Original" formula the hydroxide had to completely react and the formula then balanced so that there's no free hydroxide. Th more recent formula uses an excess of hydroxide but still need the time for the same reaction to take place.

Ian

 

[Photo Engineer]

That is what I cannot understand. Acid base reactions take place very rapidly, even this one. So the reaction is over almost the instant full mixing is achieved. So, something else is going on.

PE