I once performed an experiment to see how much the operation of the shutter of an Alpa Reflex vibrated the camera and how much it would interfere with sharpness when used with high magnification optics. I put a Questar 3.5" barrel on a stand on the optical bench. I attached an Alpa body to one of the other fixing screws. So that the camera was beside the telescope, not optically lined up, just sitting there. I focussed the telescope through an open window on a telegraph pole some 200 yards away. While looking through the eyepiece I fired the camera shutter with a cable release. The image jumped about as much as it did when the stand was tapped with a fingertip. I repeated the experiment with other cameras and there was not one in the lab that didn't shake the telescope. We tried taking pictures too, of course, and it was difficult. Long exposures, external shutters, and controlling the exposure with the light are ways of solving this problem. All of them impractical in the field with ordinary subjects.
I can't remember which eyepiece I used for that test, but it was probably the 16 mm one. When used for photography one needs a Barlow lens which effectively lengthens the focal length - making things much worse of course. Even with telephoto lenses of moderate focal length - 500 mm or so - the mirror clacking up and the shutter's sudden acceleration and stop will tend to blur the image to some extent - but of course this only matters if shake is the limiting factor. Considering these things I am amazed at how sharp some of the pictures people take through long lenses at motor races, hockey matches etcetera actually look. D Don Williams http://personal.inet.fi/cool/don.williams Author's Web Site and Photo Gallery Updated: March 30, 2002 ----- Original Message ----- From: "William Robb" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Saturday, December 21, 2002 4:39 PM Subject: Re: Lens sharpness vs. camera shake > > ----- Original Message ----- > From: Dr E D F Williams > Subject: Re: Lens sharpness vs. camera shake > > > > Getting sharp images with long lenses is an exacting task, > much more > > difficult than meets the eye. Forgive the weak pun. I've been > taking shots > > of tree tops recently with a Sigma Apo 400/5.6. They are not > particularly > > sharp although I used the highest shutter speed I could. Its > hard to decide > > if the images are fuzzy because: the lens is not so good wide > open, the > > shutter and mirror vibrated the camera on the tripod, the lens > is not so > > good anyway, or the camera was not properly focussed. The > pictures are on my > > website if anyone wants to look at them and tell me what they > think. They > > are acceptable for the web, but 10 mbyte files on my monitor > seem a little > > soft. I usually focus by the 'rangefinder' method. > > I like your test subjects. My own experience with trying to get > something critically sharp out of a longish lens was that metal > tripods were a bad method of holding a camera. They "ring", and > this vibration will pretty much ruin sharpness. > Also, a fast shutter speed is almost counterproductive, as the > shutter is still only giving you its flash sync speed, in > reality, and this speed is often right in the range to show > vibration. > When I ran my tripod tests with the Super Program, I found that > 1/8 second or longer showed the least vibration, having the > camera vertical showed the most vibration, and that a heavy wood > tripod was better than an equally heavy metal tripod. > I have an untested theory that wrapping a bungee cord around all > three legs of the tripod might settle the ringing down. > > William Robb > >
