Chris_Malcolm
500mm reflex with long lens hood and red dot sight
500mm lenses are difficult things to aim, especially when the thing you're trying to find is against a featureless background, and almost impossible if the thing is moving fast, such as a bird in flight. Like a big telescope they need some kind of sighting aid. A red dot gun sight with no magnification is very good, because it allows you to look with both eyes open and see a red dot projected on your target (an optical illusion, not a laser beam :-).
They're designed to clamp onto a gun sight wedge mount, so some kind of adapter is required. I played with the hot shoe mount, but it was too flexible -- the sight needed re-zeroing at every mount, and was easily knocked out of calibration. The degree of precision required to aim the central focus sensor at the target via the dot also made parallax error a problem on the hot shoe. So I decided to mount it directly on the lens. Least parallax error, plus the geometry of the lens barrel and the sight mount naturally lines it up with the lens. To protect the lens barrel I glued the sight clamp to a cardboard tube slightly too small, slit open to provide a sprung grab on the lens body. The slit also handily accommodates the focus hold button on the lens barrel.
It works amazingly well! It's now trivially easy to aim the lens at anything very quickly indeed, including birds in flight. Getting the lens to lock focus on a bird is a bit trickier. Without bothering to zero the sight carefully I found that If the flying bird occupies at least 1/10th of the image the AF manages to lock focus quite soon. But tiny birds, or distant birds, often fail to lock just using the red dot sight, and if you drop to viewfinder or live view the view becomes so narrow the bird is easily lost. So experiments with careful calibration and technique are required.
But initial results are very encouraging! This has suddenly become a VERY much more useful and easily used lens! Although having a gun sight on top of it does reduce one of the advantages of a 500mm reflex lens -- it doesn't look like a long lens, so people are less nervous of it than a foot long lens with a dinner plate objective.
Of course if you feel a little inferior at having a much smaller 500mm lens than the big boys you can always add a long lens hood to it. This one is threaded at both ends so allows you to remount the original short rubberised hood on its end. Lengthening the hood is supposed to improve the contrast of reflex lenses, which are inherently flatter and greyer than refractors. in part due to the increased diffraction edge length created by the hole in the middle. So far I've failed to verify this effect, but I haven't yet tested it in provocatively difficult conditions.
DSC05981X
500mm reflex with long lens hood and red dot sight
500mm lenses are difficult things to aim, especially when the thing you're trying to find is against a featureless background, and almost impossible if the thing is moving fast, such as a bird in flight. Like a big telescope they need some kind of sighting aid. A red dot gun sight with no magnification is very good, because it allows you to look with both eyes open and see a red dot projected on your target (an optical illusion, not a laser beam :-).
They're designed to clamp onto a gun sight wedge mount, so some kind of adapter is required. I played with the hot shoe mount, but it was too flexible -- the sight needed re-zeroing at every mount, and was easily knocked out of calibration. The degree of precision required to aim the central focus sensor at the target via the dot also made parallax error a problem on the hot shoe. So I decided to mount it directly on the lens. Least parallax error, plus the geometry of the lens barrel and the sight mount naturally lines it up with the lens. To protect the lens barrel I glued the sight clamp to a cardboard tube slightly too small, slit open to provide a sprung grab on the lens body. The slit also handily accommodates the focus hold button on the lens barrel.
It works amazingly well! It's now trivially easy to aim the lens at anything very quickly indeed, including birds in flight. Getting the lens to lock focus on a bird is a bit trickier. Without bothering to zero the sight carefully I found that If the flying bird occupies at least 1/10th of the image the AF manages to lock focus quite soon. But tiny birds, or distant birds, often fail to lock just using the red dot sight, and if you drop to viewfinder or live view the view becomes so narrow the bird is easily lost. So experiments with careful calibration and technique are required.
But initial results are very encouraging! This has suddenly become a VERY much more useful and easily used lens! Although having a gun sight on top of it does reduce one of the advantages of a 500mm reflex lens -- it doesn't look like a long lens, so people are less nervous of it than a foot long lens with a dinner plate objective.
Of course if you feel a little inferior at having a much smaller 500mm lens than the big boys you can always add a long lens hood to it. This one is threaded at both ends so allows you to remount the original short rubberised hood on its end. Lengthening the hood is supposed to improve the contrast of reflex lenses, which are inherently flatter and greyer than refractors. in part due to the increased diffraction edge length created by the hole in the middle. So far I've failed to verify this effect, but I haven't yet tested it in provocatively difficult conditions.
DSC05981X