Will it increase performance of the lens or would it introduce too much distortion?


As you focus a lens, you are changing the distance lens to sensor (or film). The job of the lens is to refract (bend inward) light rays incoming from the subject. This alters their path so that the rays now trace out a shape that resembles a cone. As you focus, you are moving the lens towards or away from the senor or film. Focus is achieved when the apex of the cone just kisses the surface of the sensor or film.

You should know, the focal length engraved on the lens barrel is the distance lens to apex when the lens is imaging a far distant object. Because the lens has limited ability to refract light, the distance lens to apex elongates as the subject distance decreases. When close focusing, the distance lens to apex is long and drawn-out. When life-size, unity, often called 1:1, is achieved; the apex to lens distance is twice the focal length. This distance is now technically not the focal length; it is called the back-focus distance.

As you close focus, you will soon reach the limits of the lens’s ability to travel. If your desire is to image ever closer, you must now cause the lens to move even further away from senor or film, further than the mechanical limits of camera body/lens mount, will allow. Now we employ spacer rings or perhaps a bellows attachment. These are mechanical devices that further expand the lens’s forward movement.

You also need to know that the mechanical forwarded motion of a standard camera lens is deliberately stopped when the subject distance lessens to the 2 to 3 foot range. This is because a standard lens is optimized to image distant subjects and compromised when tasked to image extremely close. Besides a reduction in optical performance, the engraved f-numbers on the standard lens barrel become invalid. This fact can be devastating as extreme under-exposure is likely. This is the main reason why a standard camera lens has a close focus limitation. Macro lens to the rescue: The macro lens is optimized to work in close and slightly compromised when tasked to image distant subjects. Further, the macro lens design preserves the accuracy of the f-numbers.

Your desire is to obtain more magnification with your macro lens. You have reached its mechanical forward movement ability. To defeat this limitation you mount spacer rings. That’s OK however; you now face the problem of the invalidation of the f-numbers. Again, this can bring about under-exposure. The severity of the f-number error is mitigated by the fact that a modern camera features thru-the-lens-metering. This innovative feature reads the exposure and prompts you to correctly set the shutter and / or aperture. You will be opening up the aperture or slowing down the shutter or some combination of both to mitigate what is called “bellows factor”.

You can use a math formula that provides a compensation factor to apply. BF=(m+1) X (m+1). Where m = magnification. Suppose you mount a ring and achieve magnification 2 (twice life size). Using the formula BF = (2+1) X (2+1) = 3 X 3. Thus BF = 9. This tells us to increase the exposure 9X. This is accomplished by lengthening the exposure or opening up the aperture. Suppose the exposure without the ring was 1 second, now it’s 9 seconds.

All this gobbledygook is to tell you that rings likely defeat the camera’s automation and you will be forced to operate in manual mode. Now under-exposure is your nemesis.


It would let you focus closer. If this is the aspect of 'performance' that you need, give it a try. It will not IMPROVE image quality. It may not noticably increase distortion. Extension tubes are cheap. Try!

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