Possible Purchase of Used Lens

Derrel · Mar 22, 2014

Stuff on the FRONT element or in front of the front element at close range does not affect image quality much, except at times on bright sources of light. So, yeah, like Gavjenks showed, even a pair of pliers held in FRONT of a lens is not a big issue. But...things at the REAR of a lens are more of an issue...the light that's headed OUT of a lens and destined for the image plane CAN EASILY be affected by spots or fungus patches that are far back in the element groupings. If that spot is close to the rear-most part of the light path, you might have some issues with it, especially on things like "bokeh balls", where the rounded, out of focus point light source "bokeh balls" might show a dark spot in some of them. You'd need to carefully clone that out on those types of photos, but on normal "everyday" scenes I don;t think this would cause a lot of easily-seen issues.

Here are three examples of what crap inside a lens near the rear element looks like:

https://www.flickr.com/photos/40243617@N08/11878275216/in/pool-nightstreets

https://www.flickr.com/photos/mishifuelgato/9437062623/in/pool-nightstreets

https://www.flickr.com/photos/smellyfreckles/8262715398/in/pool-nightstreets/

Now, keep in mind, this only shows up in this exact type of shooting!

Gavjenks · Mar 22, 2014

Even stuff on the rear lens depends. It's more about how concentrated the light is / how big the glass is at that point. Usually rear elements are smaller and the light is more condensed, at least in modern lenses, in the rear. Thus a piece of dust is larger compared to the whole stream and blocks/interferes with a larger proportion of the stream.

However, dust or little scratches on the back still don't matter hugely much. If a pair of 3" pliers on a 4" front element is okay, then a similar 1.5" obstruction would usually be equally okay on a 2" back element, proportionally. Note that the pliers probably did hugely affect contrast, but that's obviously an extreme example.

Also, older or simpler lenses often are more symmetrical and have nearly equal sized elements on both sides of the aperture, making the distinction almost moot. I have an old large format lens with only a few elements in it of roughly equal size, and I dropped it in some quartz gravel and then accidentally stepped on it (yeah...) on the rear element. Has like 4 big rice grain sized gouges in the glass, and I was all depressed that I just ruined $250, but then shot a bunch of things with it and saw nothing. I tried pushing it to extremes to intentionally see something, and couldn't (bokeh balls, out of focus stuff in general, on the near side and the far side, in focus stuff near and far, nothin'. Contrast is probably lower, who knows, but I still get all I need). I will post a picture of the gouges when I'm back home if I remember.

And the shape of bokeh balls is affected by the entire light path, at any point. The actual point of the optical aperture (rarely in front or the rear) will have the most affect on the shape of the EDGES of the bokeh ball, because that's the bottleneck. But anything on the glass at any point can affect the middle, proportional to the size of the light stream. For example, the obstruction on the front and middle of a mirror lens (but not the back and not necessarily the aperture) creates donut bokeh.