Bokeh defects?

[oleleclos]

Do you know if this is this something that gets worse in certain conditions etc?

I won't pretend to be an authority on this, but I don't believe delamination is caused by a breakdown of the cement as mentioned in that piece, because then it would be much more commom. I think it's more likely to be caused by the handling of the lens, say if it is knocked or dropped or subjected to rapid temperature or pressure changes as can for instance happen in an unheated and/or unpressurised aricraft hold. But I admit these are guesses, not knowledge.

[oleleclos]

Could be either the glue of a doublet lens element (red arrow, my guess too) or a coating scratch. In any case it is a lens material failure.

One problem with my theory is that according to this lens diagram none of the three lens elements in front of the diaphragm is cemented into a group, only the two elements just behind it... Back to the drawing board

[arith]

I've never seen anything like this but for what it is worth not all the discs are the same. I think it is a dirty sensor; light reflects off it then off the back of the lens, apart from dust which doesn't reflect light. Apart from that I would just buy a new lens.

[ajohnw]

I won't pretend to be an authority on this, but I don't believe delamination is caused by a breakdown of the cement as mentioned in that piece, because then it would be much more commom. I think it's more likely to be caused by the handling of the lens, say if it is knocked or dropped or subjected to rapid temperature or pressure changes as can for instance happen in an unheated and/or unpressurised aricraft hold. But I admit these are guesses, not knowledge.

It is largely caused by breakdown of the cement but this area has improved significantly over the years. There are 2 main reasons. Canadian balsam used to be used. This is a resin and always contains a certain amount of solvent which can continue to evaporate over the years. The remainder shrinks as a results and the stress causes the lens to delaminate. Helped by the fact that the cement gets more brittle. Some older ziess and other camera lenses are well known for having this problem. This lens probably used an epoxy cement. That sets up a certain amount of stress as it sets. I have seen comments about certain lenses always having delamination problems from that era. It's usually down to the characteristics of the 2 glasses that are joined or the steepness of the curves or both. The glasses expand and contract with temperature changes at different rates and this stress alone can delaminate a lens. More recently UV curing cements are used and these may be more reliable. Dropping a lens might encourage delamination but I would say it's also very likely to crack the glass at the same time. Elements in camera lenses are usually held in place fairly firmly and securely - unlike precision optics. Out of interest in relationship to precision optics the largest lens that can be cemented is about 75mm dia. Any more than that and the stress caused as the cement sets distorts the optic and it wont be precision any more and will in any case delaminate at some point. The thickness of the cement is measured in nano meters and ideally less than the wave length of light.

:-) All comes from the rather frustrating experience of buying used zeiss microscope objectives. Some other makes never ever have that problem even in their exotic forms. Most amateur microscopists get to know a lot about delam and coating defects fairly quickly.

If this lens is delaminated I suspect the mark is only showing part of it and the area is actually a lot larger. One other thing it could be is a defect that has been there since day one. Getting all of the air out of a cemented joint for instance takes some care and attention as does deducing it's thickness to suitable levels. The usual effect of these sort of problems is more scattered light and lower contrast. The whole area of the lens makes up the image so small defects often don't have much effect at all. In simple terms this one might make up a couple of percent of the entire used area of the lens which is hardly significant. Large delaminated areas also may not have all that much effect but if one of the lenses has become displaced in relationship to the other it can have a dramatic affect. Black dots though - difficult to see how. The usual effect is loss of contrast and more scattered light. Both will reduce resolution and general sharpness of the image.

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[milleniummuppet]

That's very interesting.

Could be either the glue of a doublet lens element (red arrow, my guess too) or a coating scratch. In any case it is a lens material failure.

Was the lens exposed to heat (e.g. sun or next to a heating device) or shock (e.g. dropped) ?

Not too sure, I purchased it about 5 months ago, I suspect the seller didn't even know it was there (or maybe they did). Kind of a shame though.

[milleniummuppet]

If this lens is delaminated I suspect the mark is only showing part of it and the area is actually a lot larger. One other thing it could be is a defect that has been there since day one. Getting all of the air out of a cemented joint for instance takes some care and attention as does deducing it's thickness to suitable levels. The usual effect of these sort of problems is more scattered light and lower contrast. The whole area of the lens makes up the image so small defects often don't have much effect at all. In simple terms this one might make up a couple of percent of the entire used area of the lens which is hardly significant. Large delaminated areas also may not have all that much effect but if one of the lenses has become displaced in relationship to the other it can have a dramatic affect. Black dots though - difficult to see how. The usual effect is loss of contrast and more scattered light. Both will reduce resolution and general sharpness of the image.

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Thanks for the detailed response. The images I get from the lens are fantastic, but I do distinctly remember thinking when I got it that it wasn't quite as sharp or contrasty as I might like (though only fractionally), compared to other fast primes I've used. So it seems to behave exactly the way you describe obviously aside from this newly found issue. I have to say though its a rare case, because I don't expect there are many people taking out of focus images at 1.8 in the dark, pointing into a light to see what the patterns look like, so perhaps its an occurrence that simply hasn't been widely observed? Not sure. I know though that the bokeh in a standard scenario, under normal lighting conditions at 1.8 shows no such noticeable defects.

[AB26]

Hi Matt,
The only solution would be to have the lens checked out and serviced/repaired by a Nikon service centre.