Luis: > > No. Karl is right. All changing focal length does is change > > the angle of view. The relationship between still objects > > (and space) in a scene (perspective) remains fixed as long as > > the photographer > > does not move. Greg: > Pardon my interruption but I think what Dave is talking about is compression as opposed to perspective. The vertical and horizontal placement of objects does not change from one focal length to another however the relative size of objects does change with focal length as does the apparent depth of the image. Doesn't it to you? > herin we have a problem, and an answer. Terminology or 'jargon' is a linguistic tool to enable people using the same language with comparable experiences to explain or examine concepts in a minimallistic form of language to prevent the incessent defining of terms in a more long winded way. Compression (apparent) is probably exactly what Dave is writing about (I hope) but there is and always has been confusion when it comes to perspective/spatial relationships and compression or expansion and _ apparent _ compression or expansion. Dave wrote "Changing focal length alone can definitely alter the relationship between objects as seen in your view finder. This will be readily apparent if you spend a moment with a zoom lens" ..which is really not right, but I don't mean to rip into Dave for saying this - the reality IS difficult to comprehend. In fact, the appearance of compression occurs at _distance_ only. the fact that we can easily see this effect with a 1000mm lens is purely because of the angle of view. Let us create a theoretical scene with two closely placed, similarly sized objects located remote from the viewer - they both look far away and they both look of a comparable relative size, if we were to move very close to the first (thus changing our perspective) the closer object would look bigger than the more distant object. In a shot with ANY lens, these items when remote would look like they were the same size (compressed), in a shot with ANY lens taken closer, they would appear to be differently sized (expanded). _Apparent_ compression occurs in the shot taken with the longer lens because the field of view is smaller, the frame tighter. _Apparent_ expansion occurs with the wider lens (taken closer) as the field of view is larger. Let me hark back to the concept of 'normal' focal lengths for a moment and appropriate print sizes for a moment. The theory there is that an image captured with a 'normal' focal length lens, enlarged to 8x10 will produce a print in which there will be no apparent compression or expansion when held at a given viewing distance. If we apply this desire to achieve no 'distortion' to the scene then it follows a shot taken with a wide angle should be made larger if viewed at that same distance and a shot taken with a long lens should be printed smaller. If you actually DO this you'll see that the wide angle encapulates a really big chunk of the scene in front of the camera and the successive lens focal length changes produce smaller and smaller clips of the scene. If anyone wants to try an experiment that really shows how this all works try this one out. Determine the front nodal point of a 50mm lens and a 24mm lens. Fit the 50mm and attach a camera to a ball socket that allows rotation around the center of this nodal point and take a series of shots from left to right, top to bottom of a scene. Now fit the 24mm and frame up to encompass the whole of the area captured in clips by the 50mm and make an exposure. Compiling all those 50mm shots, and allowing for the spherical distortion of the 24mm lens the shot taken with the 24mm will look the same as the montage made with the 50mm. The spatial relationships are unaltered, there is no difference, no compression or expansion - they will look the same. karl
