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Purple Spot - Light Leak ?

Discussion in 'General Technical Discussion' started by antsplan, Sep 22, 2008.

  1. Hi

    I'm hoping someone can help me identify an issue I had whilst out trying to shoot pano's.

    In every shot I noticed a purple spot (when I got home).

    It should hopefully be visible in this image (exact centre of the image in case its faint):

    2878047923_9561d88c4d_b.jpg
    Subscribe to see EXIF info for this image (if available)


    This is a resize of the original but in every image the purple spot was bang right in the centre of the image.

    This shot was approx a 20 sec exposure and I have never noticed anything like this previously whilst doing long exposures.

    My lens was clean as is my recently cleaned sensor.

    I was thinking that perhaps it may be light leaking maybe via the viewfinder ?

    Anyone any ideas on what this is and how to prevent it ?

    Another example:

    2878930114_2928a9400c_b.jpg
    Subscribe to see EXIF info for this image (if available)
     
  2. I can't see it, but to prevent it I believe the D700 and D3 has a vf shutter.

    But on other models just use the VF cover that comes with the camera.
     
  3. Hi Anthony,

    First off, its nice to be on this board with you fine fellow photographers!!

    Honestly, I cant see the purple spot you are talking about, but if its a little tiny spot that under heavy magnification looks like a cross, its a hot pixel.

    I just discovered one on my sensor. Heres a photo of it. If you look at the "1" on the greyhound's blanket go down from there, down the vertical pole on the rail and you will see a shadow under the greyhound. At the tip of the shadow you will see the little red/orange dot. Can you see it? If yours looks like that then you have a "hot pixel" or a "dead pixel".

    Heres the photo:

    2871602653_4e5c692b20_b.jpg
    Subscribe to see EXIF info for this image (if available)


    Sorry to post such a big image, but I wanted you to be able to see it. Is this what yours looks like?

    There are many opinions as to what to do in this situation. Here is one by Ken Rockwell:

    http://www.kenrockwell.com/tech/hot-pixels/index.htm

    Im really on the fence as to what to do. I can clone the pixel out when I get Capture NX2 (right now I have NX 1.3 and it doesnt have a clone tool so I have to use the paint brush which is a hassle), but Id really like to get rid of it. I think what Im going to do is live with the one red hot pixel and hope it goes away, but if I get another one Im going to send the camera in to get re-mapped by Nikon. Maybe my local camera shop can do it. Im going to call them today and see. I just hate sending my D300 anywhere....Im not very trusting sometimes!! Plus from what I hear its just a matter of time before another hot pixel shows up anyways!! With my luck, if they give me a new camera, it will have more hot pixels than the one I have. I think its just part of being in the digital world. Hopefully Capture NX will develop software to eliminate them when the photo is in the RAW converter.

    Let me know what you think!

    Hope this helps.

    my best,

    Rick
     
  4. Phil, apologies its no so easily visible now - not sure why ?

    The spot/blurb is approximately a finger tip size in comparison to this picture exactly right at the centre of the image.

    As in my original post, I thought that due to the positioning of this that it was likely to be a light leak, but I have been told elsewhere that this is not the case - but I'm not so sure how accurate this information is.

    I'll get the VF cap a go, thanks for your help.
     
  5. Rick thanks for your reply.

    I am fairly certain that this is not a hot pixel, its way bigger than that and apologies its so difficult to see.

    Thats a lovely image you posted and I feel sorry that you have this issue and I hope you get it resolved speedily.

    Thank you sincerely for your reply.

    Anthony
     
  6. Hopefully the following link will allow a d/l of the original size image for better viewing.

    http://www.flickr.com/gp/12346061@N02/t49z6F

    After this page has loaded, you can click 'ALL SIZES' and then original size and choose 'Download The Original Size'

    Thanks

    Anthony
     
  7. Slightly worryingly I have just saw a few more examples of this problem with a 50 1.8 on another forum, unfortunately no solution yet however.
     
  8. AllenOwen

    AllenOwen

    20
    Aug 14, 2008
    Houston,TX
    Does it only show up in long exposures? If so, I would second the suggestion of using the VF cover. Maybe do an experiment. Two 20 second exposures at the same spot you shot those. One with the cover and one without.
     
  9. Anthony, I can see it but it's the first time I see such an effect. It looks as it is always in the center?
    It looks round and if it's always there and in the center my guess would be that it's not VF light but actually coming through the lens. Does the size of the spot vary with your aperture setting?
    I could imagine that it's some light reflected back from the sensor into the lens (or some reflection inside the lens).
    Did you use any filter? (Doesn't look like a reflection from a filter but worth a check anyway.)
    Does it also happen with a different lens?
     
  10. ChrisA

    ChrisA Guest

    Some raw conversion programs allow you to specify a "defective pixel file" where you list the pixels that don't work (hot or dead) and then the interpolation algorithm can work around those pixels. This results in a better image then if you simply cloned them out in Photoshop. It's also easier because you don't have to do the work. One of the best raw converters out there is "dcraw" and it has this feature. Also many othe raw converters are based on dcraw ad so they also have this feature.

    Astonomers have lived with this problem for decades now. They calibrate there sensors and then routinly apply a calibration to all there images. I think Nikon cameras can do something like this for long exposures. Does the feature work for short exposures? It's called "dark frame subtraction" and would have the effect of removing hot pixels.

    You could also do a dark frame subtraction by hand in Photoshop. (Shoot a frame with the lens cap on at the same EVas the real image then subtract it in PS.) but the clone tool is quicker.
     
  11. JohnK

    JohnK

    540
    Aug 6, 2006
    Pacific NW
    I can see it, it's very faint - absolutely not a hot pixel, it's a circle about 70 pixels across, if you crank up the blue channel it becomes very easy to see. It actually looks rather like a flare/glare like you might get shooting towards the sun. Maybe it's a reflection from one of those lights you're shooting towards?
     
  12. nostaws

    nostaws

    6
    Aug 29, 2008
    az
    I noticed right away (if I didn't know it was there maybe I wouldn't have noticed).

    Have you tried the same shot with other lenses? I lean toward a problem with the lens (if you haven't already ruled it out).

    To me it looks like more of a problem with the optics, not the sensor/camera.
     
  13. Craig/Daniel/John/Nostaws (I hope I didnt miss anyone)

    Thanks for your input. I returned to the same location last night and shot pretty much in the same conditions with a 24-70mm and not once did I get the purple spot ?

    Reading elsewhere it seems as though I'm not the only one with this problem with the 50mm f1.8.

    I shall return again with the 50 this time and try some of your suggestions and see if I can nail what is causing the issue.

    Thanks again gents.

    Anthony
     
  14. Then my guess is that it's a lens internal reflection back from a lens (group) in front of the aperture blades.
     
  15. Thanks Daniel, much appreciated.

    Can i do anything about this, or do I simply live with it ?
     
  16. I haven't experienced that myself, so that is really just a guess made out of your pictures and description. If my guess is right and the reflection is caused by direct light (not strong light coming from the frame edge or near, where lens hoods help), I don't have an idea right now how you could do anything about it. Sorry.
     
  17. Lurker

    Lurker

    Jul 21, 2007
    NJ
    :actions1: Why can't our cameras do this automatic? It's not that hard (Minolta had automatic dead-pixel mapping almost 5 years ago in the 7D. How difficult is it, Nikon?). Apparently Canon isn't making life hard enough for our guys. [/RANT]
     
  18. I think Daniel has the answer. I've seen this happen with certain lens/camera combinations and it's apparently due to internal reflections and I think the body is involved some how.. The worst was an 85 f/1.8D on a Kodak Pro14n. This lens never did it on any other camera but the Kodak and not just for long exposures. We never found a cure. I must try my 50 f/1.8 and see if I get the same.

    Ronnie
     
  19. daveg

    daveg

    Jul 14, 2008
    UK
    Just a thought:

    Does it show up in "Live View" under any circumstances?

    If it does then covering the viewfinder with the palm of your hand should prove or disprove the viewfinder leak theory.

    The blob is similar in colour to the light on the bridge. Does the colour change if the brightest light in the image changes?

    If you were shooting in bright sunshine with the sun in or near the field of view you would get a similar effect but not necessarily dead centre as yours appears to be.

    DaveG
     
  20. mvnsnd

    mvnsnd

    626
    Jul 12, 2008
    Western NY
    I don't think you have a hot pixel either. Other than an internal lens reflection, my only other suggestion is that it could be some contamination on one of the lens surfaces. Have you looked at both the front and rear lens elements to make sure they are both really clean? A dried spot or dust playing with the light passing through the lens may cause that type of region also.
     
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