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01-05-2010, 11:44 AM   #1
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Macros with a reversed zoom lens

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.
01-05-2010, 12:56 PM   #2
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Thanks Dave for this great report. I have often wondered about reversing zoom lenses, but never got around to seeing how well it might work.

Steve
01-05-2010, 01:01 PM   #3
mel
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I've done this several times with the same lens, using a finger on the aperture thingy (technical term) but I never did any kind of mathematical analysis of it. I generally operate by the "ooooo pretty" principle. If the shot makes me say "ooo pretty" it works. Yeah I'm a girl. Lol.
01-05-2010, 08:26 PM   #4
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QuoteOriginally posted by mel Quote
I've done this several times with the same lens, using a finger on the aperture thingy (technical term) but I never did any kind of mathematical analysis of it. I generally operate by the "ooooo pretty" principle. If the shot makes me say "ooo pretty" it works. Yeah I'm a girl. Lol.
Mel,

The key trick is a short piece of rubber or PVC flexible tube or rubber bands to control the aperture thingee.

Then vibrations can be low enough to actually take a photo.

Dave

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