The Art of projection and complete magic lantern ... - Yesterday Image

136The Lens.—This should be of the very best quality only ; aninferior lens is of little service. A portrait lens that will cover the platewell from corner to corner is without a doubt the best combination,passing more light than any other ; it is somewhat surprising to find thatany amount of stopping down, with a good lens has little or no effect onsharpening the picture, an open aperture gives as good a result as a lensstopped, if properly corrected.Rectilinear lenses give a very sharp definition, only that the loss oflight on passing through such a small aperture is very great, quite 75 percent, is cut off. For registering the light and focussing, a specially sharpnegative should be kept. The further the lens is racked out the closerthe illuminant to the condenser and vice versa.A wide angle lens is unsuitable, it produces an unequal illuinination.A cap of ruby glass fitting over the lens is a great convenience, allowingthe picture to be centred on the bromide paper to a nicety, if the lenshas a slot in front or as it is called by the optician a diaphragm mount,the non-actinic glass may be slipped into this. An ordinary cap with thebottom knocked out to allow a piece of orange or ruby glass co be boundon by the leather is the best form.Exposure.—The length of time required varies considerably and nofixed rule can be given, a thin negative with a good oil illuminantrequiring from one to two minutes, while a dense negative may requirefrom 20 to 30 minutes exposure, the colour and density being thegovernors, a dense negative with a poor illuminant will not give the sameresult, as when a more brilliant light is used, no matter how long theexposure ; at the same time, if a very thin negative is used a less brilliantilluminant yields the best results. Perfect negatives alone should beussd, every defect being multiplied and enlarged. Supposing thenegative to be dense in one portion and very thin in the other, this maybe made quite as effective as an equally dense negative by shading thethin portion with a piece of cardboard, holding it some distance away fromthe paper taking care it does not cast a hard line which is avoided bykeeping it continually moving. Exfa exposure to certain portions of thepicture greatly enhance the result, extreme white drapery or a white-washedcottage require more exposure than the distant objects.After giving thenormal exposure, cap the lens, cut a hole about the size of the objectthat requires the extra exposure out of a sheet of paper or card andallow the light to pass through this, thus shading all except the extremewhite object, keep the card moving and away from the enlarging paper.