" FIXED FOCUS " lenses are so often mentioned in catalogues and advertisements that the impression has gained ground that this is some distind kind of lens, with the extraordinary power of focusing all objects near and far in one plane. There is no inherent quality in any lens that makes it " fixed focus " ; it is such when it is immovable and that is all. Any lens can, therefore, be made " fixed focus ", but the extent to which it will focus all objeds in one plane depends upon its length of focus and size of stop or diaphragm used, and upon that only.
The reason for this is that the rays of light from near and far objeds do not focus at the same point. For instance, in the exaggerated diagram, page 6, we will assume that B is the point at which obje&s too feet distant will focus and that A is where objeIs to feet distant will focus.
Now the distance between A and B will vary in ratio to the focal length of the lens. In a lens of 3 inches focus it is ascertained mathematically to be of an inch and in one of 12 inches focus, inches. Hence if using a
3 inch focus lens the sensitive surface is placed between A and B, the objed at 100 feet and beyond (all objeCts beyond ioo feet come practically within one focus) and objeels io feet distant will none of them be more than 43,,- out of focus, which, with the size of stop ordinarily used for snap work, creates so slight a blurring of the image that it cannot be deteded. Experience has shown that the limit of focus for a lens which is to be employed for snap-shot work with the focus fixed is under 5 inches ; in other words that a 3y2 x 3% plate is about the largest that can be used to advantage under such circumstances. It is probably true that for cameras of this size and under, little can be gained by having the focus adjustable and that, on the other band, better average results will be obtained with a fixed focus, owing to the faa that there is no adjustment that will allow the lens to be put out of focus by a mistake in measuring distances.