dimanche 29 octobre 2006

Horseshoe... Sometimes a tripod doesn't say it all... A shutter speed does This morning the clocks went back, and I remembered for the first time that they had to be turned back and not forward at this time of year... "Spring Forward, Fall Back" being the well-known aide-memoire which I have only recently learnt! Being lighter one hour earlier in the morning suits me... I always rise early and the extra hour gives me more photography opportunities now that I'm more in "digital mode." As I went to feed the hens (not mine, but still to be collected by the previous owner of the house where we're staying) I noticed that the scene from last Friday's weblog looked completely different in the cold, foggy morning light compared to how it had appeared, contre-jour, against the warm setting sun. I made few images with the same Nikon D200 and 300mm Nikkor lens combo used for that previous image, but felt they were not really of any interest or merit... however, I decided to download them onto the Mac to see if there was anything to learn... and so there was! Out of interest, from Peter Marshall's excellent website "About:Photography - Tripods and Camera Supports" one can read... Tripods keep your camera still Tripods are virtually essential to get sharp pictures when you have to use slow shutter speeds. Otherwise your pictures will be less sharp than they should be. What is a slow shutter speed? For a normal telephoto lens, anything under 1/250s should be regarded as slow. If you are using an extreme telephoto, such as a 400mm lens on a 35mm camera, even 1/250s is a slow speed. For standard lenses, anything under 1/60s is a slow speed, although most people can get reasonable results most of the time at 1/30s. With a wide-angle, probably anything less than 1/30s should be regarded as slow. Rule of Thumb for when you need a Tripod If you are using a 35mm camera, or know the equivalent focal length of the lens you are using on 35mm, then you can use the general rule: Slowest safe speed for hand held photography = 1/focal lengths. I should add that this morning I used a tripod because the light was dull and I wanted the same framing as before... the 300mm AF Nikkor was focussed manually, and after taking one shot I checked the focussing accuracy by going into the D200's menu and enlarging the image 10x on the rear LCD screen. All looked fine so I took a few shots at different exposure settings... from using the lens wide-open at f/4 to stopped-down a couple of stops to f/8, letting the camera's "Aperture Priority" program look after the shutter speeds. The two which looked best were taken at 1/125th of a second at f/5 and 1/60th at f/8. Although I thought the shot at f/8 would reveal the best sharpness and contrast, I was surprised to see, at 100% magnification on the screen of the 5Mg file, that there was distinct camera shake... whilst the shot at 1/125th of a second showed none. Although the camera was tripod mounted, on my old but solid Leitz Tiltall, the 1/60th of a second shutter speed didn't produce a sharp image as you should be able to see from the comparison details above. A lesson to learn is that with long lenses even a tripod will not produce exacting results! 3:09:08 PM    comment []