Minolta MC 1,4/50 experiences

Started Mar 10, 2013 | Discussions thread
ProfHankD
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"Glowy" lenses usually caused by haze, scratches, or fungus
In reply to john-photoguy, Mar 10, 2013

john-photoguy wrote:

I bet that there is a haze (or dare I say fungus) on the inside of the lens,

Many older lenses are slightly "glowy" -- which is what you're complaining about. Teleconverters and glass adapters greatly magnify the effect, often producing IQ usable only as a special effect.

The glow can come from many factors, ranging from incorrect reassembly of the lens after cleaning to sensor reflections. Some lenses also have a little glow by nature of their optical design, but generally not as severe as what you're showing. For a sharp image with heavy glow the most likely culprit is some type of diffuser in the lens:

  • Haze is simply the deposition of diffusing material on (inner) lens surfaces over time. Most lenses suck air through as you focus, and humidity, volatile organics, and other materials go along with it, condensing and accumulating on inner surfaces over time. I've also seen a lens where oil had spread out onto an element near the aperture iris. Cleaning internal surfaces is problematic, but the only fix.
  • Contrary to popular belief, scratches don't have much impact on lens sharpness, but they do cause refractive problems like glow. The impact of surface scratches can be reduced by filling them with light-absorbing material, such as black paint, but that is feasible only for very deep scratches. Scratches in the rear element usually impact IQ far more than ones in front.
  • Fungus, with its well-known white branching structures, will reflect a little light. However, it's after a long-term fungus infection that the real glow issue starts -- especially after the fungus is removed. Fungus emit chemicals that can etch lens glass (certain glasses more than others; for example, some older Canon lenses seem to be particularly vulnerable), leaving the equivalent of many-faceted micro scratches after the fungus has been removed. Once etched, no simple repair is possible.

Note that dust usually doesn't cause glow because typical dust particles block light, rather than causing refraction. Close visual inspection of the lens with a penlight should tell you which problem you have....

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