On Jan 13, 2007, at 4:37 PM, John Francis wrote: > The MTF is a measure of the resolving power (or sharpness) of the > lens, > and is dependent on aperture.
Not really. MTF stands for Modulation Transfer Function. This is an expression of how accurately a lens transmits modulation, which is not resolving power at all. Think of it this way. A lens is presented with a target consisting of alternating black and white bars of equal width. This target is often on a drum which can be revolved. At the image plane a photo sensor of some sort receives the image of the target as it turns. A lens with perfect MTF would produce a square wave output from such a target. But in reality the "corners" of the square wave are rounded off, tending toward a sine wave. The less rounding, the higher the MTF point on the graph. Generally a number of points on the image plane, from center to corner, are tested and the points connected to make a curve. That's the MTF curve at that aperture. The higher up on the graph, the better. A perfect MTF, which is impossible for many reasons, would be a horizontal line across the top of the MTF chart. MTF tests are usually done at several apertures and may be done at a variety of spatial frequencies. The above explanation is slightly simplified. Some lens designers heavily favor MTF results when designing lenses. Others don't pay as much attention to them and consider resolution and contrast independently. MTF actually combines both into one measurement. Bob -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net
