SPOT METER

FOR

EXPOSURE

By Rita & Major Pearle.

The highlight method of exposure for Cibachrome C. relies on the fact that you give sufficient light exposure to the paper from the larger to, in the case of a white object in the transparency such as white cloud for instance, sufficient light is put onto the paper to completely bleach or destroy all the coloured layers in that area so that when the developing process is completed you are left with a white paper and the parts of the picture that are going to be blank are very dark, there the light doesn`t shine on it at all anyway from the transparency and those colours are left in position so the three of them together of course, make black.

I suggest you start off with a box about 3" wide and 4" in hight by 1¼" thick, the back coming off so you can get to the bits. You cut a hole in the top of the box, up at one end, to carry the meter and of course, size depends on the meter that you get, and below that you want a hole drilled to carry the spindlefor the Potentiometer which is the method used to balance the needle. They are all put into the box along with the push-button switch and an on-off switch too and they are all wired up.

I think anyone would be able to follow the theoretical diagram as regards the wiring. The light sensitive resistor of course is taken out of the box by a pair of leads (thin playable leads if available) and the CDS cell itself is mounted onto a 1/8th inch square and you put the cell, held on with a piece of sticky tape, underneath and you can put a little coil of solder on the bottom (I did) of the tin to give it weight so when you lay it down somewhere it doesn`t incline to slide all over the place. The cable of course, is brought out of a small hole in the side of the tin.

The theory of the circuit is as follows:- A current runs through the meter and it is controlled on the right hand side of the circuit by the potentiometer, variable resistance, that`s the one with the knob on, and a numbered scale, anything from one to a hundred will do, it is only a means of re-setting if you ever have to alter it for any reason. Tha`s one side of the circuit.

On the same portion on the left hand side of the meter circuit is the light sensitive cell, and as you may know, these cells vary their resistance according to the amount of light which is falling upon them. So if you put the cell on the easel and switch on the enlarger, close the lens down to about F8 or whatever, and set the head of the enlarger to give an 8" x 10" print size, you find that when you switch on the current of the meter the needle will swing to one side or the other then zero and be either increasing or decreasing the light coming through the iris of the lens in the enlarger, or by putting the variable scale up or down, you re-balance the needle to zero, and when you have done that the resistance underneath the knob you are controlling equals exactly the resistance of the CDS cell that has been activated by the light from the enlarger. So the principle of the whole method is that you can balance the light from the enlarger iris of the enlarger, you don`t alter the setting of the variable resistor exept in the initial setting up. Once it is set up to balance, you leave it where it is.

Now the method of setting up the balance is as follows:- All your processing incidentally, from now on, will be done at a constant time. All the exposures will be given the same time. In our case it worked out at 20 seconds. Once the 20 seconds has been verified and arrived at, it then remains the same, the only alterations from now on is to open and close the iris of the enlarger to balance the needle in each case. You set about it as follows:-

Set your iris at F8 and the size of the picture on the easel to 10" x 8", put the necessary filters in for correction as for paper in use, that`s on the back of the packet, along with the infrared filter as well, for the heat, and the U.V. filter. The pack goes in automatically and is there all the time.

You now proceed to make a series of test exposures at varying times. At this stage you leave the iris set at F8. You cover up a strip of paper in the normal way and you uncover it at an inch at a time, going up in steps, of say, 2 seconds, and when you have a strip of paper about 10" long with the varying exposures along it`s length, say from 5 seconds at one end, to say, 20-25 seconds at the other, when you have done that you then process it and you, remember, make a little note of the times each of the steps are and you then process it in the normal way and hopefully you will find that this strip of paper will be grey in tone and at one end it will be white, the first 2-3 squares that have the most exposure will quite probably be white. Shortly after that there will be very very slightly grey and the grey will go on progressively down till hopefully, at one end of the scale the paper will be black. That will be the ideal strip.

Now you look along this line of squares and you find the one which is just off white, not the one that gives the whitest white, but the one that gives one down, You now look at what time that square had, just imagine that that was the square that had 20 seconds, if you have an auto timer, you also switch on your meter and you balance the needle to zero at F8, the same stop you made your test strip from. Put your cell on the white part of the picture, the whitest white there is, go down the next step to one that is preceptably off-white, put your meter spot in that particular square and balance the needle at the point to zero using the variable potentiometer underneath the meter dial and that will give you a balance to white of 20 seconds. Now, if for any reason you have to alter anything at all, you keep the time the same, any alteration is done purely by means of the iris. For instance, say you wish to do a picture a lot larger, you put the head of the enlarger up, you focus as before, you find a white part of the picture like a cloud or whatever it is, you put your cell in this particular white part of the picture and because you put the enlarger head up, the needle will have gone down on the meter because there is now less light. So you bring back to zero by opening the iris until it balances again, that exposure will give a white that is just off. The point of having this white is that things like clouds and dresses and so forth have a certain amount of detail in the whiteness and you don`t want to burn them out completely to white paper, hence you take one step down from white.

CHEMICAL PRICE CHANGES

TIPS

To clear the sludge when mixing EDTA NaFe type Bleach, balance the pH with 30% Ammonia. Ron`s mix needs about 5ml.

To avoid the possibility of a green cast keep the RINSE stage between Reversal and Colour Developer brief as possible, slight carry-over is essential
(Likewise with the RINSE after the Conditioner).

E6/E7

Editorial

CRCMain