On Apr 27, 2005, at 7:48 AM, mike wilson wrote:
Isn't a macro lens designed for flat field work? If so, wouldn't a regular lens
used with an extender, a bellows, or helicoil focusing device, be a better
choice for 3D objects, such as close-ups of flowers or small objects? Or maybe
close-ups of not so small objects?
Yes, most dedicated macro lenses are flat field corrected. If the primary point
of interest is in the centre of the frame you might get away with a non-macro
lens pretty easily but the point is that the generally distorted plane of focus
on the subject side of most non-macro lenses can become a problem at macro
distances. It certainly would be no better for 3D objects, a flat field lens
will generally record any object in close focus with less optical distortion.
In addition, I suspect the curve of the plane of focus in the non-FF lens will be the "wrong way". In other words, if focussed correctly in the centre, the curve of plane of focus will move back towards the lens towards the edges of the frame. In most macro pictures, one would want the plane of focus to move away from the lens as it moves towards the sides. I think...
Yes. I recall Minolta once sold a special lens for three dimensional close up work, with variable/adjustable field curvature that would allow that kind of "negative" field curvature when needed. Their typical example was a photo of a rack of billiard balls.
Most macro lenses are corrected for flat-field work because, in the greater scheme of things, corner to corner sharpness and correction for copy work and other small, flat-art objects is their priority.
For three dimensional objects, flat-field correction is less important ... more focal length is more important so that you have working distance for proper lighting and better perspective. A 200mm lens is brilliant for 35mm format cameras, a 90-135mm lens is equally great for 16x24mm DSLRs.
Godfrey
