606 HANDBOOK OF PHOTOGRAPHY 



to record, bearing in mind the region of the spectrum to which the plate or 

 film responds. 



All filters absorb some radiation to which plates and film respond, so that it is 

 necessary to give longer exposures than would be necessary without the filter. The 

 factor by which the exposure must be multiplied is known as the "filter factor." It 

 has no significance in infrared photography, because infrared plates and films are not 

 used without a filter. It is customary, therefore, merely to denote the time of expo- 

 sure required for a certain subject under definite conditions when used with a particu- 

 lar filter. Since there is usually a gap in the spectral sensitivity of infrared materials 

 from the middle of the green to at least as far as the middle of the red, all filters having 

 their transmission threshold in this region will require the same exposure. On the 

 other hand, if the filter absorbs some of the infrared to which the plate or film responds, 

 extra exposure will be required. For instance, with the Eastman Infrared Sensitive 

 plate, the exposures will be identical through the Wratten filters Nos. 25 and 29, 

 whereas through the No. 87 filter, twice this exposure will be necessary. 



Cameras. — There is no difference in principle between cameras used for normal 

 photography and those for the infrared. There are, however, a few precautions which 

 must be observed. The bellows, the shutter blades, and the dark slides must be 

 opaque to infrared; otherwise fogging of the plate will occur. Bellows used on most 

 modern cameras are quite satisfactory, particularly if they are made of leather or arti- 

 ficial leather containing black-carbon pigment and backed bj'' a sheet of black cloth. 

 Certain kinds of hard rubber and wood are very transparent to the infrared and must 

 be avoided in plateholders and shutter blades. Some manufacturers test the hard 

 rubber of their dark slides for opacity to the infrared. Those made by the Eastman 

 Kodak Co. and the Folmer Graflex Corp. carry five dots embossed on the metal tops 

 if they have been tested for safetj^ in the infrared. Metal is quite safe. 



Photographic lenses made for good quality photography with panchromatic mate- 

 rials are designed so that their focus is the same for light in the violet and yellow^ 

 Other wavelengths are not in the same focus, but achromatizing for these two gives 

 quite satisfactory results for white light and modern plates and films. The position 

 of correct focus changes rapidh^ as the wavelengths increase into the infrared, so that 

 it is sometimes necessary to make a correction of focus if sharp pictures are to be 

 obtained. Lenses differ A^ery markedly in their infrared focus, and it is not possible 

 to give a general rule as to the change in bellows extension which must be made after 

 focusing visuallj' on the ground glass or by scale. In all cases where a correction is 

 required, the lens must be racked out for a distance slightly longer than that necessary 

 for good panchromatic focus. This is equivalent to focusing on a nearer object. One 



recommendation has been to increase the bellows extension by „r>^^, of the focal 



•' 200th 



length of the lens, but this rule is by no means general. For Tessars of //4.5 and 



//3.5 and of moderate focal length, Zeiss recommends an extension of 3 per cent of the 



focal length. Leitz lenses of the Elmar series should be set at a scale reading of 100 ft. 



in order to focus distant objects by infrared. The Hektor series requires somewhat 



less compensation. The Summar lens is provided with a special index mark on the 



mount for focusing liy infrared. The Ross //4 Wide Angle Xpres lens is specially 



corrected for the infrared, and the following Cooke lenses are corrected for coincidence 



of focal planes of 7500 A. and white light: 6 in., //2.5; 919 in.. //2.5; 6M in., //3.5; 



814 in., //4.5; 10^ in., //4.5; 25 in., //4.5; 13 in., //6.3. The Kodak Anastigmats, of 



aperture //4.5 and of focal lengths shorter than 10 in., and the//7.7 Anastigmat are 



generally satisfactory. In the case of lenses of short focal length used at small relative 



aperture, the correction is so small that it can be neglected. Long-focus lenses may 



require a correction. If there is doubt as to the correction to be applied, the lens 



