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?

[normadel · Accepted solution]

Dust and other artifacts (fingerprints, smudges, water spots, even scratches)  on the front, rear or inside of a lens do not show up in images. They are so far from being focused-on that they are invisible. An EXTREME amount can cause loss of contrast, but it'd take a lot. You don't see them in the viewfinder, do you?

You test a lens for problems in this regard by taking picture and looking at them.

[normadel · Accepted solution]

Dust and other artifacts (fingerprints, smudges, water spots, even scratches)  on the front, rear or inside of a lens do not show up in images. They are so far from being focused-on that they are invisible. An EXTREME amount can cause loss of contrast, but it'd take a lot. You don't see them in the viewfinder, do you?

You test a lens for problems in this regard by taking picture and looking at them.

[Waddizzle]

“ Dust and other artifacts (fingerprints, smudges, water spots, even scratches)  on the front, rear or inside of a lens do not show up in images. “

True! 🙂 But, any condensation is more than a little problem.  Internal moisture can lead to electrical issues or even lead to mold and mildew. 

[normadel]

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

[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