Archived posting to the Leica Users Group, 2008/12/13[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]
Aram: As best I understand it, the M8 focus issues occur because film has thickness, but the sensor is, for all practical purposes a flat plane. (Yes, I know it has wells and microlenses, but the bottoms of the sensor wells are all in the same plane). When light hits film, as long as it comes to its exact point of focus somewhere within the sensitive emulsion, it's going to zap a group of silver halide molecules, and you will have a sharp image. When light hits the sensor, it must be in focus exactly at the sensor surface, plus or minus what our eyes can detect at a given print or screen viewing size. So let's take a ruler and let it stand for a greatly magnified film emulsion, viewed on edge. Let's say that in this scale of things, the emulsion is two inches thick. The lens is at the end of the ruler, projecting back towards zero. The lens can be adjusted so that our image comes to sharpest focus anywhere between the 0 and 2 inch mark, and all's well. But with the sensor, we have much less tolerance. The light rays must come to focus between 1.75 and 2.25 inches, or it will look out of focus. Now let's say that the lens is adjusted to come to focus at the 0.5 inch mark. It will look fine on film. But it will be out-of-focus on the sensor. NOTE: The diagram below will only look correct with a monospaced font. The distance between the vertical bars on each line must be the same. If not, copy it into a text editor, or use Courier font in a word processor. The x's represent the zone where best focus is possible with each medium, the o's represent where the image will appear out of focus. |xxxxxxxxxxxxxxxxxxxx| (film) |oooooooxxxxxoooooooo| (sensor) | 1 | (point of best focus #1) | 2 | (point of best focus #2) | 3 | (point of best focus #3) If a lens is adjusted so that it comes into sharpest focus at Point of best focus #1 above, it will give in focus images with film, but out-of-focus images with a sensor. A lens adjusted to Point #2 will appear in focus on both film and a sensor. And a lens adjusted to Point #3 will look fine on film, but will have all its depth of field to one side of the focus point. This also shows why focus shift may show up on the M8 when we never saw it with film. Let's say the lens wide open brings the image to perfect focus at Point #1, Point #2 at one stop down, and Point #1 two stops down. --Peter At 11:38 AM 12/13/2008 -0800, Aram wrote: >Question. I am not an M user, but am curious. How can this be? How can a >lens focus properly on a film camera but not on the digital M? If this >happened with every lens, I could understand that - something out of whack >with the camera. But if other lenses that worked on a film camera also work >on the digital M, why would one lens behave this way?