Mirrored Light Sources?

Discussion in 'Beyond the Basics' started by E-Van, Mar 30, 2007.

  1. E-Van

    E-Van TPF Noob!

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    I've been noticing in a lot of my night pictures, bright light sources will be mirrored and show up in other parts of the picture. What causes this?
    I'm shooting with a Rebel XTi with the 50mm f1.8 lens.
     
  2. MACollum

    MACollum TPF Noob!

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    I assume that with practice we train our eyes to be on the lookout for distractions, such as reflective surfaces that might destroy our picture. I'm not at that stage yet :) I always find something wrong with my pictures, whether it's a flash burst on glass, trash in the scene, etc. I'm too impatient, just want to take the picture, not clean up a scene. ;)
     
  3. E-Van

    E-Van TPF Noob!

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    No, this isn't reflective surfaces. The light sources appear mirrored in other parts of the picture such as the sky or other random places.
     
  4. The_Traveler

    The_Traveler Completely Counter-dependent Supporting Member

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    If there is a very bright source of light off-axis to the lens, you might be getting images refracting off the surfaces of the glass lens elements.
     
  5. E-Van

    E-Van TPF Noob!

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    here's one example.

    [​IMG]

    You can see the top light is mirrored down by the no-parking sign, and the other two are mirrored as well.
     
  6. Hertz van Rental

    Hertz van Rental TPF Noob!

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    Looks like the image is bouncing off the sensor, reflecting onto the back of the lens and then being picked up by the sensor again.
    There is also a second ghost at the bottom of frame which looks like a reflection of a reflection.
    Digital cameras seem prone to this problem but usually the reflections are just of speculars.
    Could be enhanced by the lens. Cheaper lenses usually have less coatings and this can exacerbate the problem. Until they produce a matt surfaced sensor we are just going to have to live with it - or go back to film, which doesn't suffer from this problem.
     
  7. fmw

    fmw No longer a newbie, moving up!

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    It is called ghosting. Light goes through the elements of the lens and is bent (refracted) as it travels through the lens. If you have a point light source entering the lens straight one, some light will refect as opposed to refract from the areas where the lens elements meet. The reflected light creates a ghost image of the point light source. The more complex the lens (more elements) the more prone to ghosting it is. In other words, if you had been using a zoom lens you likely have more ghosts in the image.
     
  8. Garbz

    Garbz No longer a newbie, moving up!

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    A good qulality lens reduces this issue. This is ofcourse spoilt if you have a filter on the front as the glass in filters is rarely as good as in the lens untill you get in the +$100 mark. Try screwing the filters off if you use them and if the problem is still there then it's internal lens reflections.
     
  9. Hertz van Rental

    Hertz van Rental TPF Noob!

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    It is a form of ghosting, but the one described is associated with lens flare and the image formed is not focussed. But it does generally pick up the shape of the lens aperture. You see that type of ghosting when you get the sun in shot and you see a row of shapes running through the lens axis.
    On the example given the ghost images only appear for the two side lights and not the central top one. They are also quite well defined and in focus.
    It is far more probable that they are reflections of the lights bouncing off the sensor and reflecting back from the rear surface of the lens. Which is what I originally said.
    This is a well known problem in Astronomical optics and you get this kind of ghost particularly with spectroscopes using CCD sensors.
    A multicoated lens might reduce the ghosting but the only cure is non-reflective sensors.
     

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