Dust specks in 80mm lens.

Discussion in 'Medium Format' started by andrew_gardiner, May 18, 2011.

  1. Hi there everyone. I have just bought a used 80mm lens for my Mamyia 7II over the internet from a very respected dealer. It looks good in all respects ( some minor scuffs on the lens hood is its only sign of use).
    However it does have some specks of dust inside the lens, a couple of tiny ones when looking at the front element, but a slightly more significant one when looking at the back element of the lens ( which annoyingly is right in the exact middle ).
    Is it possible that these flecks can in any way affect my images. I guess I need to shoot a reel and carefully examine the negs,but I need to do all this fairly quickly as I only have a short period of grace if I want to send it back. It would be a shame because I think I got quite a good deal on this lens ( £ 300 +vat ). But ultimately I bought this camera to make really lovely images and theres only so far I want to compromise that.
    Any prompt advice/experience on this would be hugely appreciated.
    Yours Andrew
     
  2. "Is it possible that these flecks can in any way affect my images."
    My magic 8-ball (which is never wrong) says, "Not likely".
     
  3. There's no way you'll see anything on the negatives. And if you trade it in for another, you'll probably get dust on that one, too.....
     
  4. ALL lenses are infested with specks of dust. In theory they will scatter a little light due to diffraction, but in practice you won't see ANYTHING until you start throwing handfuls of dirt on the glass....
     
  5. Depends on where the specks are and how big they are.<br>When on or near the rear element(s), the light they scatter/block no longer arrives at it's intended spot on the film behind the lens. The result will be darker spots with undefined edges.<br>There's only one way to know for sure: test. Shoot a preferably evenly lit, featureless, bright, but not too bright thing (the sky is good, but make sure you do not overexpose), and inspect the slides or negatives.
     
  6. There was a large spec of dust inside one of my lenses last week. A few judicious taps on the table and it relocated somewhere else, and it's no longer on a glass element.
     
  7. Very unlikely unless there are a lot of them. 4-5 unlikely to make any difference
     
  8. Even on the back element a dust speck would have to be more like a rice grain in size before it became visible on the negative. Think about this: Dust right on the surface of a digital sensor doesn't cast that much of a shadow; and does the image develop a dark outer ring when the aperture iris of a lens is closed down? - No it doesn't, it just gets evenly darker. Well, that's about all that dust inside a lens will do. It will simply reduce the intensity of the image by an indiscernible amount.
     
  9. Put that iris diaphragm in another place, Rodeo, such as the just in front of the front element or just behind the rear element, and the effect will be quite different.<br>So with dust also: it depends. Size matters (as always), yes. And location too.<br>Easy test: put something not too small on the rear lens, and look through the viewfinder.
     
  10. I'm not sure that dust on the front or rear element is really relevent here, since it could be easily cleaned away. However, I thought I'd experiment to see exactly how much of a problem dust behind the lens might be.
    I wasn't going to risk attaching anything directly to the rear element of any of my lenses, so I chose a lens with a bit of distance between the mount and the rear glass. This happened to be a 105mm f/1.8 Nikkor. I suspended a sizeable blob of Blu-tak on a human hair across the rear of the lens - see picture. The blob was about 10mm rearward of the actual lens element.
    The picture shows: Top left the blob attached to lens mount, top right the image at f/1.8, bottom left at f/8 and bottom right at f/22. Camera was focused at infinity and pointed at a white wall.
    As can be seen the lens vignetting was more obvious than anything else at f/1.8, by f/8 the blob became fairly visible, and at f/22 became extremely visible. What was more surprising was that the hair became visible at f/22 as well!
    I guess the lesson is to either stick to apertures of f/8 or above, or to keep the rear element clean of Blu-tack and human hair.
     
  11. Ended up with a double post - sorry!
    This has happened a couple of times since IE "upgraded" itself to version 9. Anyone know how to get back to version 8?
    00YllW-361275584.JPG
     
  12. Good test. Thanks!<br>The effect will be a bit different, but still there, when the dust is on the other side of the rear element or group, where you can't easily brush it away.
     

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