Many of the shops use gelatine filters for studio work, which are placed in a slot in between the front and rear combination of the lens. When they are so placed, it is easy to change the filters on a run of plates and does away with changing the focus after the filter is on, which is quite a good point and they are somewhat cheaper than filters put in optically plane glass, but the filter used on the front of the lens seems to be the most popular.
If you are going to photograph all-corners, you will need the complete set of commercial filters, but if you are just doing a certain line of outside work, you will only need the correction filters—the yellow series.
Of the Wratten series of filters, the Kl will be used mostly fOr a very light correction inside and fast exposures outside and is a good filter to use with a yellow sensitive film or plate.
The K2, which is somewhat deeper, and with mazda or nitrogen lamps on copies, will give practically a true correction, is possibly the filter more used than any other of the Wratten series.
The K3 is said to give an absolutely true rendering of colors in mono chrome, but, for outside work, it is a little too deep.
The "G" filter is a contrast filter and should never be used on correction work, as it is too contrasty and therefore almost kills the yellow—makes the yellow photograph white—and, with a fairly light yellow, kills it completely.
The "F" or red filter is the other contrast filter which absolutely kills most red—makes it photograph white.
The "A," "B" and "C" filters are what are known as tri-colors. The aver age commercial photographer has but very little use for the "B" and "C" filters, as they are used practically only in separation work, that is, "B" kills the green and "C" kills the blue.
The "A" is used a great deal in red separation and correction work.
Many operators hesitate to use panchromatic plates on account of the trouble in developing them. In shops where they have automatic timers it is fairly smooth sailing, but otherwise it is a bit more unhandy, as the plates are sensitive to all colors and the only really safe light is no light at all. The \Vrat ten (E. K. Co.) and Cramer people both furnish a fairly safe light which is of a deep green, and while, when first turned on you think there is no light at all, owing to the faintness of the light, after you are in the dark-room a little you begin to wonder if someone did not leave the door open.
For myself, I develop all panchromatic plates in absolute darkness, depend ing upon time and temperature alone and which I, of course, think is the best way. However, there are many different contrivances to use in this connection. About as good a way as any is to have a double tray or a tight fitting cover for the tray. After the plate is immersed in the developer, put the cover on, turn on the ordinary dark-room red light, take note of the time, turn out the light and proceed with what you think is about the proper time. Then, turn on the light, check up the time, and at the end of the proper period, stop the development.
In the large photo-engraving color plants, where they use panchromatic plates all the time, they have timers which are nothing more or less than alarm clocks which can be set for three or four minutes and are very handy.
The time to develop panchromatic plates also changes with different emul sions, but I find that most operators settle on a certain time ascertained by experience, and which will fit their formula. I develop them from four to four and one-half minutes, using pyro developer with a little more water than the \Vratten formula calls for—otherwise exactly like the formula.
The Wratten plate is a backed plate and consequently will have to be rocked all the time in the developer or the backing will settle on the plate and cause streaks, and the backing should be washed off before the plate is put into the fixing bath or it will soon get the fixing bath too dirty for regular use.
Panchromatic plates all have more or less of a black edge around them which, under ordinary circumstances, would be taken for old plates, so there is no need to worry if you see a black edge around them, but you have to allow fpr that and cannot figure on using up to the limit of the size of the plate. These plates also come wrapped in black paper and, in the case of backed pan chromatics, come packed face to face. In opening panchromatic plates and taking the black paper off of a pair of plates, that black paper should be thrown away and not wrapped around the remaining plate or you will have a streak across your negative, which will mean a make-over.