lundi 17 janvier 2005

Less is More Use a lens Wide-Open Yesterday's weblog image was not too easy to choose because I could tell from viewing the 6 x 7 cm contacts with a loupe that not much was in focus. I'm not saying that the 180mm Sekor lens on the Mamiya RZ67 was soft - Mamiya in fact make a superb 180mm "soft focus" lens for portrait and wedding photographers - on the contrary, it's simply that working close-up to a subject the depth-of-field becomes very narrow. Many times I see captions in photo magazines stating that the photographer "stopped the lens down to f/22 or f/32 for maximum sharpness." Unfortunately all lenses are at their worst stopped down to minimum apertures... and sharpest at perhaps two or three stops down from maximum aperture. Coupled with any decision to stop the lens aperture down to f/22 or f/32 is the necessity to set a shutter speed (if shooting in "auto" mode it will set itself) at a progressively slower setting until the photographer finds himself reaching for a tripod. Now, whilst a tripod isn't a bad thing... indeed I always remember Bert Eifer saying in his excellent book Developing the Creative Edge in Photography that "...the tripod is the sharpest lens you own," most photographers don't use them despite owning them. However, that argument is for another day. Keeping this theme within hand-held photography there are many instances, perhaps most, where light levels are good enough to stop down the lens aperture more than the optimum setting. The old adage "f/8 and be there" is a good rule of thumb but its surprising how much depth-of-field you can achieve at f/8, especially with standard lens or wider. For the scene above, of Handforth Hall in Cheshire, using a standard 50mm lens at f/8 from the viewing distance of 10m would have given a depth-of-field, where everything was acceptably sharp, of between 5m and infinity. OK... that would have been fine for an overall sharp shot... but that wasn't what I wanted. Also, the 50mm Nikkor lens would have included much more of the tree's branches, distracting trunk and the half-timbered Elizabethan building would have been too small in the background. Here I only wanted a suggestion of the main subject in the background because I was attracted by the starkness of the scene... bare branches, snow on the ground and the black-and-white "skeletal" appearance of the building. And, I was restricted as to my viewpoint because of boundary fences and other obstructions at the location. To throw as much as was required out of focus I selected a longer 135mm Nikkor lens, focused on the nearest branches, stopped the lens' aperture down and carefully studied the effect in the viewfinder. Stopping-down and viewing the effect in the viewfinder used to be much easier in the old days (or is still easy if you use older SLR cameras as I do) because the granularity of the older screens made it easier to see the effects of stopping down. With more recent cameras I've looked trough the viewing screens are so bright and grainless that I find manual focusing is much more difficult and, with my aging eyes, probably inaccurate. The 135mm Nikkor at the same f/8 aperture would have given me a depth-of-field at that same viewing distance of just under 9m to around 11.5m... which on stopping the lens down actually snapped too many of the fine branches into sharp focus, so becoming distracting against the half-timbering. So I opened the lens aperture progressively to f/5.6, f/4 and then f/2.8 checking the stopped-down view each time... at f/2.8 the D-o-F was approximately 9.5 to 10.5 meters, call it 3 feet in old money, which was just right to isolate some branches in sharp focus but let all the others blur out of focus. And there you have it... don't automatically stop down a lens for maximum depth-of-field and sharpness. The effect of shallow D-o-F is often to show greater sharpness because the lens is being used at it's optimum aperture... is allowing the use of a faster shutter speed (doing that usually gives better sharpness)... and the visual separation or isolation of the foreground and background elements gives the impression of greater sharpness. As an aside... I didn't know about bokeh in those days (the smooth or wiry rendering of out of focus parts of the image) but I'm pleasantly surprised at the good bokeh of the old 135mm f/2.8 Nikkor-Q lens. I can see no wiry "doubling" of the out-of-focus twigs on the original print... something I've seen a number of times on other lenses... well, on Nikon and Canon lenses, yes... but never on those I've used for the Leica-M system. Ouch... I can hear the flak already! 9:42:19 PM    comment []