Archived posting to the Leica Users Group, 2003/01/15
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Last week, before I had read all this, I tested a Summicron 50mm that I had bought used. I have a test chart and set it up and put the camera at 40 times the focal length, like I do for all tests. Usually I only test developer and film combinations and therefore I normally place the test chart/camera so that the relevant portion of the chart appears in the middle of the negative. For the lens test I also wanted to see how the lens performed at the edges. It was clear to me that the camera to test chart distance was different if the chart was placed at the center of the negative or at the corner, but I did not know what distance to set the lens at for the pictures with the chart in the corner of the negative. So i tried it two ways. For testing the lens´s corner performance, I first left the test chart where it was and tuned the camera (without moving it) so that the chart would be in the corner of the negative and exposed a few negatives. Then I turned the camera back so the chart was in the middle of the viewfinder but then I moved the chart so that it appeared in the corner of the viewfinder and took some more pictures. The distance setting was kept absolutely unchanged all the time, that is as I had focused on the test chart when it was in the middle of the viewfinder. I shot at 2.0 and 2.8 and 4.0 and 5.6 apertures The results: Resolution at the edge was more or less the same in the two cases. I have to say more or less because the resolution chart bar lines is in steps as follows: 40 - 50 - 63 -79 and 100 and I can not determine any middle values with any accuracy. The values as I read them were 79 lines in all cases, except for the shots at f/2.8 Resolution at the edges at 2.8 was much lower than at all other apertures including 2.0 - I have 16x enlargements from four different 2.8 negatives (two different films) and they are all dizzy. I could recognize 50 lines only. Resolution at 2.0 may be good very good, but the improved contrast at smaller apertures is what enhances the print quality Resolution at the centre was never better but some times worse than the resolution at the edge, and this was true for all apertures and for negatives from two different films. YES, I am a bit confused by the results. BTW and FWIW, a dozen new photos will be up on my home page very soon. To increase the number of viewers, the will be a nude picture and you will have a chance to win an original print absolutely free. Watch this space for opening details! Chris > >Technique has nothing to do with it. > >What Martin Tai's page is about is the fact that if a lens has a >perfectly flat field, ie, it images a flat plane in front of the >camera onto a flat piece of film, then the parts of the image that >are at the edges of the frame are further from the camera than the >part of the image that is at the center of the frame. > >It's like standing on a sidewalk, looking at the opposite side of >the street exactly perpendicular to the row of buildings along the >other side (we're assuming a straight street with all buildings >starting the same distance from the sidewalk). > >We are, say, 25m from the building (building A) directly opposite. >If we use a lens that has a 90 degree angle of view horizontally, >the edges of the frame will image buildings that are still the same >distance from the sidewalk we're standing on, but from our camera >position the distance is longer than 25m; it is (25m x the square >root of 2), or 35.4m away. However, our camera is focussed at 25m, >and this is correct. If we now want building A opposite us to be at >the edge of the frame, we have to turn our camera 45 degrees and are >now centered on a building that used to be at the edge. Now, to get >the camera to image Building A sharply, we have to set focus at a >distance of (25m/square root of 2) or 17.7m. As before, we are >focussed at 17.7m (which happens to be in the middle of the street, >but the edges (also perfectly in focus) are a lot further away; 25m >in this case. > >In large format work this discrepancy is more important due to the >longer focal lengths involved, but less important due to the small >apertures used. So it is generally ignored as well. > >-- > * Henning J. Wulff - -- Christer Almqvist D 20255 Hamburg and / or F 50590 Regnéville sur Mer please look at my b+w pictures at: http://www.almqvist.net/chris/new - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html