Do extension tubes decrease image quality?

Started Dec 12, 2008 | Discussions thread
Leonard Migliore
Leonard Migliore Forum Pro • Posts: 18,722
Conjugate ratio
2

Lenses are designed for a specific working distance. The term "conjugate ratio" refers to the value of the object distance over the image distance. So a lens designed for an infinite conjugate ratio is meant to focus at infinity, while a lens designed for a conjugate ratio of 1 is meant to produce an image the same size as the object.

Most lenses work well over a conjugate ratio of infinity to about 10. Then aberrations come in because the light is entering at angles other than what was intended. Lenses vary in their sensitivity to this. Very fast or very sharp lenses will exhibit more change than slow ordinary lenses.

When you put an extension tube on a lens, you are using it in a regime other than that for which it was designed. You will generally see some additional spherical aberration and other problems. I would guess (but I haven't checked) that zoom lenses lose more when you put them on tubes than do prime lenses.

I know that when I used my 105 f2.5 Nikkor (considered one of the sharpest lenses ever made) with an extension tube, the sharpness was quite disappointing.

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Leonard Migliore

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