The presence or absence of truth in photography is, at very least, significant. At one level, the absence of truth does not matter in any way. We see what is printed on a piece of paper and have complete freedom to appreciate the aesthetic value of the image or dismiss it as unworthy. The subject matter may be completely irrelevant provided the viewer can take from the presence of the image some inspiration or pleasure. A particular print may prove calming or recall some past experience. In these circumstances is surely does not matter how the image was produced or whether it was assembled from a dozen other images. However, the situation may be quite the opposite when the use to which an image is put, or indeed the subject matter, is significantly different. If we see an image of a leopard with unusual spots, apparently taken in some remote African location, it may be vital to know whether it is an accurate representation of what the photographer encountered. Anyone with a copy of Photoshop can rearrange a leopard's spots, but the significance of the resulting image depends almost entirely upon whether the image is "genuine". Otherwise the viewer can never know whether the image represents entertainment or the discovery of a new species of leopard. The decision certainly matters - at least to some people.
Truth in photography, at least at a basic level, is about whether a viewer can believe that an image represents what a photographer saw and experienced. In the case of a leopard with unusual spots it is vital to know whether we are being deceived by clever manipulation, or whether this new animal is really out there somewhere. Why send an expedition to gather details of a newly-discovered species if the unusual spots were devised in Photoshop?
When we dig a little deeper, truth is not so easy to define. A close-up of the unusual leopard might exclude environmental information and make it almost impossible to assess the size of the animal. A wider shot that included recognizable environmental details might render an assessment of the animal's size quite accurate. The best photographic societies, such as the UK's Royal Photographic Society, are generally very good at distinguishing "genuine" shots from those, for instance, taken in a zoo. Their experts also have very sharp eyes for details which may betray any subterfuge on the part of a photographer.
Simple techniques for emphasizing particular aspects of a subject are of course used all the time by photographers. A wedding photographer may routinely photograph a larger person from an angle that makes them appear slimmer, and an older woman may be photographed in softer light with relatively little contrast to reduce the prominence of wrinkles. A landscape photographer photographing a building high up on a cliff may deliberately exclude the level ground at the bottom to leave the viewer with the impression that the precipice continued below the bottom of the image. Such techniques all represent a degree of deception and have been used for as long as photography has existed.
So what is truth in photography? Unfortunately it does not exist in the absolute sense but that does not mean that honest attempts to approach it have no purpose. It is probably true to say there is no 100% pure-white photographic truth, but there are countless very pale shades of grey on the scale between black and white. Photographers must ultimately decide for themselves whether their work is appropriate for its purpose, and whether it is also sufficiently genuine to justify openly any techniques or manipulation used in its creation.