Lens Separation

[Bill_1856]

Just how serious is the separation so common on some lenses? (Yes, I know the answer is "it depends," but is it possible to generalize?) For example, I've never seen a Turner-Reich which doesn't have a silver ring around the edge, and both accessory Carl Zeiss lenses for my Contaflex have a silver reflection over virtually the entire lens. Does it affect the resolution? Or the contrast due to the additional internal reflections? If the separation is only at the periphery does stopping down eliminate those rays? My inquiring mind (d'oh) wants to know.

[Emmanuel BIGLER]

Wilhelm,
My feeling with lens separation is that contrast is affected by flare first before you could see something wrong with resolution. You need only a fraction of a micron of separation to start seing fringes and get some parasitic reflections. The defect like classical Newton's fringes is much more visible by reflection than by transmission, which is the normal route for useful photographic rays.

Well, only a lens designer could tell us whether a cemented doublet, separated by one micron is really affected or not, but let us try some simple arguments at risk of being too optimistic.
My feeling is that it does not really affect the MTF. If 1 micron could affect the MTF significantly, it means that the positioning of lens elements in the barrel is critical within one micron, which I do not believe.

MTF charts, to the best of my knowledge are re-normalized with respect to the contrast of plain subjects with no fine details like blue sky or a white surface without texture.
So in terms of MTF, some stray light originated from internal reflections does not count for resolution, it only affects the zero spatial frequency contrast with respect to which the results are re-normalized.

However, the effect of stray light and flare can of course be visible on the greyish aspects of deep shadows and de-saturation of color images. This can be visible easily on film without need of a high magnification.

[John Kasaian]

I'll take a philosophial approach---the failure of balsam at the edges reminds us that life is fleeting, be it the lens or the bloke behind the gg ;-)

This problem won't heal itself, but if it dosen't degrade the image that you can notice, work around it until the condition worsens ---then you can sell it on eBay to a Sally Mann wannabe (or put it up on the mantle to remind you of your own mortality)