My prized 50/1.4 AIS Nikkor suddenly has a fair bit of oil on its aperture blades. It was fine about six months ago.
So, obviously it'll have to be taken apart and cleaned up.
That lens was bought new in 1988 with a new F3/T. It's been used and taken care of.
I've checked older lenses and they're fine.
What causes this - warm temperatures?
Normally, I leave my lenses on camera bodies, so the blades are usually at full aperture. Would it be better to remove the lenses and have them closed down to minimum aperture?
I suppose that in extremely hot conditions it's possible that some grease types may be encouraged to migrate from focus threads (Eg extended time in a hot car). I avoid this for obvious reasons so cannot offer any direct observations. But generally aperture blades become contaminated by lubricant residue. Greases are typically comprised of solid and liquid components. As they age eventually these components can separate, inducing either oils to flow onto interior lens surfaces such as the aperture blades, or evaporation to occur which results in a greasy or oily residue to form on the blades and usually interior lens surfaces as well for obvious reasons. As lubricant viscosity generally decreases with increasing temperature it should not be surprising that the process might be hastened by warmer temperatures.
You can speculate about the pros and cons of leaving a lens wide open or stopped down but it really is only that. Any benefit of one or the other position might well vary, depending on the lens, and bear in mind that, if it exists at all, it would be marginal, at best, because the blades remain inside the lens at all times, where portions of them will be exposed to the contamination, no matter what f stop they're stored at. Once put back into use, any contamination of almost any part of the blade surface is typically going to spread across the blades entirely, due the way they interact with each other as they're opened and closed. Hence, even if a benefit existed in the first place, (of which, I'm personally very doubtful), it would be forfeited as soon as the lens was subjected to enough usage to spread the contamination across the blades, unless they were first cleaned.
It is also worth noting that contamination can occur in lenses that do not feature a helical type focus arrangement. Eg Rollei twin lens reflexes, as found, often have sticky shutter and aperture blades, even though they use a strut type of focus system driven by eccentric cams (except for some of the earliest models). They can however be prone to contamination as a result of the oils and greases originally applied to the Compur shutters fitted to their taking lenses which, like helical greases, will eventually break down and evaporate some residue onto the shutter and aperture blades.
Cheers
Brett
