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Cement separation in older Pentax lenses (quick check when buying lenses)

HarisF1
Posted 03/04/2018 - 19:56 Link
Hi guys, just wanted to mention something to look out for when buying older Pentax lenses, or even when inspecting your own lenses.

Many lenses have cemented element groups for various reasons, and sometimes under tough conditions (temperature changes mainly) this cement can weaken or break. This starts a process of separation between the elements and can be seen as a strange shimmery pattern around the edges of elements. It can be detected by looking at the periphery of internal lens elements while holding the lens up against a window or light source. Rotate the lens to make sure you don't miss a spot.

In the past few months I've bought a number of lenses where this problem has been evident. In most cases, the lenses work perfectly fine with only the slightest loss of contrast (and a bit of glow around objects) wide open. In one case, the lens (Pentax K 15mm f/3.5) had separation right down the middle of an element group. This caused terrible softness and loss of IQ in the centre of the image, while the edges were perfect.

I've seen the problem a lot in the 50mm lenses (f/1.2, f/1.4 and f/1.7 versions of the K, A, M series) and also the K 15mm f/3.5. There will be others too but these are the main culprits, being very common lenses.

Repair for balsam separation can be quite expensive, with one quote of $100 not including delivery to the US.
All the gear with no idea
danofmk
Posted 04/04/2018 - 11:30 Link
I have this on a couple of my old MF lenses and had been wondering what it was. Now I know!

Cheers

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