Reversing a prime lens for macro use limits you to essentially one magnification and working distance. That's because the distance from the reversed lens to the sensor plane is fixed at about the camera's registry distance.
Reversing a zoom lens also fixes the distance from the lens to the sensor, but since zooming changes the focal length both magnification and working distance vary as the lens is zoomed.
Here's an example:
I've tried this with a variety of mild zoom lenses and find it works well. If the zoom lens maximum is around 100mm, far away objects can be imaged at low magnification. I've found a Sigma 28-80mm zoom works well.
Fully automatic lenses don't have an aperture ring so a means to control the aperture must be found. It turns out that a convenient way to control the aperture on a K mount lens is to slip a short piece of 6mm hose over the aperture control lever; friction is sufficient to hold the aperture in any position although I use only full open for focusing and fully closed for exposure.
The basic thin lens theory is simple. Magnification, M, Focal Length, F, and Image distance, I, are related as:
I=F(1+M)
When the lens is reverse mounted the image distance is essentially constant, call it Io. Hence:
M=Io/F - 1
As F decreases, M increases. There is a focal length at which the magnification approaches zero and the object distance approaches infinity; beyond this focal length focus cannot be obtained. I've found the max focal length to be around 100 mm for the lenses I've tried.
Image Quality seems fine and I've observed little distortion.
Dave
PS I purchased a 52mm filter size reverse K mount adapter on ebay for less than $10 delivered.
Last edited by newarts; 01-05-2010 at 08:20 PM.