How to check dust in lens

[Ramsden]

Hi guys

I'm going to check out a secondhand lens and realise I don't know how to check for dust. Is it microscopic or plain to see. 

Plus I want to keep an eye on my current 3 lenses. 

As ever - your help is appreciated

Ramsden

[shadowsports]

Greetings, 

A small flashlight or bore light would probably work.  The presence of dust can indicate the shooting conditions the lens was used under, and/or how the lens was cared for and stored. 

In most cases, a little dust does not affect image quality. 

[Ramsden]

Thanks - so I guess that if its dusty it'll be plain to see. I'll take a torch and take my time. 

[ebiggs1 · Accepted solution]

"In most cases, a little dust does not affect image quality."

This must be true because a lot of my professional sports events buds never use lens caps on either end of their lenses and never seem to have dust problems. I would look inside and if you can see dust you might want to pass on it just on principle but like Rick says it probably wouldn't hurt IQ. I just wouldn't want to start off with a lens full of dust particles.

[Ramsden]

Thanks - all makes sense. No rush to buy anything that looks 'used'

[Ron888]

The most reliable method is to hold the lens up to a light.Use a strong light if needed.Move the lens around slightly so you can see all internal surfaces.Also operate the zoom as it helps to see some surfaces better. There are actually three things you should check for-

-Dust.Usually you'll see individual specs but sometimes an internal lens surface can be coated in extremely fine dust.That second one is rare though.Several dust specs wont have any optical effect,but they will affect resale value.
-Mold/fungus.These are usually small multi branched filaments and tend to have darker colors.They range in extent from tiny amounts to completely covering a lens surface.
Again,small amounts have no measurable optical effect but do affect resale.
-Lens group separation,also called balsam or glue separation. This is caused by the breakdown of the glue between paired lens elements and looks like small bubbles on a lens surface.It's very rare these days but look for it anyways.Lenses affected this way are typically unusable.They work but give very bad contrast and resolution.

If you're curious why any of these flaws dont show in your photos, the quick answer is they're not in the lens focus plane.A lens can see sharply at the scene it's focused on,but everything else -including stuff inside the lens itself- is a blur. And because these things are blurred ,they tend to affect contrast long before they affect sharpness

[Ramsden]

Hi Ron

Thanks for your detailed explanation. Thats really helpful, and because I spent many years in quality management, its perfect advice for my visit.

Best wishes

Ramsden

[ebiggs1]

"The most reliable method is to hold the lens up to a light.Use a strong light ..."

I would not advise you doing that. The panic that will overcome you and what you will see cannot be unseen. You won't buy any used lens! Some dust can look nasty and it looks bad because we all want perfection, but the reality is that unless it's excessive and closer to the rear element than the front, It's not likely to make much of an impact on IQ or contrast. Inside dust and even dust on the front element is simply out of focus to the lens so it doesn't see it.