> Bolo wrote: > > > [my opinion] > > 2a) It seems a ball-head does better than a 3d (xyz) head since it has > > a larger lever-arm (ball radius or diameter) to resist the camera forces > > than the small diameter XYZ spindle locks on a 3d head.
Paal wrote: > It has nothing to do with lever arms but mass coupling. All tripod > heads I'm aware of locks completely. With a ball head the camera is > bolted to the tripod via a thin stem. Most pan-tilt head offer two > point support and much beter mass coupling than any ball head. Here > in Europe, the general wisdom is that ball heads can only hold lenses > up to 300mm (for 35mm). I'm not disagreeing with you, rather bringing a few more points forward on this topic. Thanks for starting some discussion! Above 300mm it seems that people should be using gimbal heads instead of 3d heads or ball heads; they balance the weight of the camera+lens combination and eliminate most of the mechanical lever-arm problems. You still have inertia to contend with, but many other problems are eliminated or minimized. Speaking of the 67, the gimbal head doesn't help an, as the camera is still bucking around, but now on the end of the lens. Perhaps a gunstock style support for camera + lens is really needed? Mass coupling is a good point, and mechanically I can't disagree with you. However, note that a ballhead is quite compact compared to a 3d head. This gives the ball head a mechanical advantage over the 3d head, or to be more precise, gives camera-induced motion a greater mechanical advantage on the 3d tripod head. With the 3d head the camera has a greater lever arm on the tripod head, and thus a given force can exert a larger torque on the joints in the head. You mention the "thin stem" on the ball head. Again, as above, a great mechanical point. However that same thin stem, at least on the ballheads I've looked at or on the one I own, is rather short and stout. For example, my 308RC ball head has a stem of 3/4" long and 5/8" diameter. The length could be reduced to 5/8" without affecting the operation of the head. The lever arm from the base of the camera mount to the center of the ball is approx 1-1/4". To contrast, the lever arms on my 3030 3d head are 1-1/2" to the first (roll) joint, 2-3/4" at the next (pitch) joint. Hmmmm, I'll leave the above, but I think I have a better way of examining this: A ballhead is compact compared to a 3d head. It is also massive in the same scale. The compact size reduces the lever arm and torque of camera induced forces. The mass concentration allows a great deal of strength in that same compact volume. The other advantage is leverage on the tripod itself. The compact ball head makes the lever arm to the top of the tripod shorter; the camera is closer to the top of the pyramid formed by the legs and can direct the force directly to that platform, rather than to a lever arm to that platform. I think another important thing to emphasize is the QR plate mentioned. Although I don't own any, the arca-swiss dovetail QR plate and holder are more massive than the bogen systems I have. Instead of a 3 point grip with a reversible wedging action, there is a full dovetail which is clamped solidly. There is more surface area in the QR system to hold the load and to transmit forces effectively. The other important part is the RRS plate, or some other plate which holds the camera well and transmits *torque* effectively between the camera and the tripod/head system. The RRS plates do this by "bedding" the body in a cradle. Pentax's own plate system does this with the anti-rotation hole in the base. The bogen plates (unless modified as some people in the group have mentioned) don't have any such provisions. Bolo -- Josef T. Burger - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
