Dust question Canon EF 100-400mm f/4.5-5.6 L IS II USM

[Redhead-Husker]

I'm looking to buy a used EF 100-400mm lens. The one I found at a local camera store is in like new condition with one exception. It has quite a few dust spots behind the front lens and on the second lens from the front.  I've been told this won't effect the images, but before I invest $1600 I'd love to know what others might think. The dust spots are very small,  but there are quite a few. Maybe 20 or more. 

Also,  any particular methods to test if they will affect the images?

[Waddizzle]

I believe the OP said it is a Mark II.  Reports and complaints about this model being prone to collecting dust are few.  It uses internal focusing and not the pump action for zooming. 

So a copy that exhibits such issues is a major red flag in my book. 

[Ron888]

That many dust specs cant affect the optical performance much,if at all.If compared to a perfect lens i'd be surprised if it can be measured (see below).
However it will affect resale value so make sure the price is good! (i'm unfamiliar with prices for that model)

DPreview did a video where they tested for dust and also damage to the front element.It's shocking how much damage can be done without it showing in the images! Contrast is first thing to be affected,then resolution.
DPREVIEW damage testing

 

[FloridaDrafter · Accepted solution]

I bought mine (EF 100-400mm f/4.5-5.6L IS USM II) new shortly after its release in 2014. It was heavily used until about 2.5 years ago when I started using the R system and switched to RF glass. I never thought about it [dust] so I just took mine out of its case and looked at both ends and only saw a couple of small specks inside, and I had to look hard to find that.

IIRC, this was one of several L series lenses that Canon recommended using a "protector" filter on to enhance its weather/dust sealing. If the lens never had the filter, it could be why it's so dusty inside. Another possibility is that the seals inside the lens are worn.

Judging from the way mine looks inside after years of heavy use, in some pretty dusty conditions at times, I would be cautious. Without seeing it myself, it's hard to say if the amount of dust in it will effect image quality, but if indeed there is a lot of dust, it may indicate an underlying condition that is permitting dust to get by the seals.

Newton

[normadel]

The question was about images. Sure, water inside a lens is a problem in other ways.