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I'm having a problem with my lens (Sigma 70-300mm f/4-5.6 APO) and this is what is happening sometimes:

What's those ghosts?

You can see the ghosts happen only by those red lines I've drawn, for example that metal structure is ghosting just under that area.

Can someone explain me what is this? I didn't even know what to google for!

My camera is a Nikon D3100.

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Did this happen to more than one photo? – Jim Sep 21 '13 at 5:17
What was the shutter setting? Do you have other pictures made during that session at the same shutter speed with the same problem—maybe in a different place in the frame? Do you know if the shutter travels horizontally or vertically in your camera model? – Stan Sep 21 '13 at 6:13
Can yo provide a link to a version of the photo that doesn't strip out the EXIF info? – Michael Clark Sep 21 '13 at 7:39

From what you show, there could be a few assignable causes and a couple of contributing factors.

It does not look like a lens-based artifact due to the horizontal zone where the ghost appears. It appears to have a rather definite, but not sharp, cut-off. The areas affected are detailed which rules out haze or flare. They are not reversed so they are not reflections, etc.

I do not know the camera model but I think the shutter must travel vertically, given the limited vertical extent of the affected area.

It does not appear to be shutter bounce which appears in older cameras with heavier shutter curtains near the edges of the image.

That points to shutter speed timing but not a malfunction, per se.

Lighting: Outdoor gas discharge lights (Argon, Sodium, Neon, others?) are not continuous and pulse in sync with alternating current. In North America it's 60 CPS and in Europe 50 CPS. The ghost could be the overlapping of a second source while the shutter slit was traversing the sensor. Given the difference between the primary and ghost, a second source is a plausible explanation.

To begin to nail-down the answer, We'd need to know if supplementary lighting was used. What kind. Where located. We'd need to know if other shots immediately before or after were similarly affected. You mentioned it happening sometimes. Do you have a second shot?

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Thank you, I'm in a rush now, but the camera is a Nikon D3100, it was sunny at that time, the image is underexposed, there were no flashes too. I have other shots with this problem! I'll add them later, thank you by now! – Aerohix Sep 21 '13 at 22:37

This is a wild guess, but without any additional information regarding the shooting conditions, equipment used, and camera settings there isn't a lot here to go on.

It appears you had something cause ghosting only during the specific time the slit between the first and second shutter curtains was over the part of your camera's sensor that recorded the affected area in you photo. When you shoot at shutter speeds faster than your camera's flash sync speed, then entire sensor is not uncovered at the same instant. Rather the edge of the first curtain moves across the sensor to expose it to light and the edge of the second curtain follows it to re-cover the sensor. The shutter speed is determined by the distance between the two curtains as they move across the sensor at a constant speed regardless of the selected shutter speed value. If it takes, for example, 1/250 second for each shutter curtain to cross the sensor then a shutter speed of 1/500 second would mean the second curtain starts to move when the first curtain is halfway across. A 1/1000 second shutter speed would mean a 1/4 sensor height offset, a 1/2000 second shutter speed would mean the slit between the two curtains is only 1/8 the height of the sensor as they moved across it.

If there was a flat filter mounted on your lens, then a reflection off another element in your camera's lens may have bounced off the back of the filter. What might have caused the ghosting during the precise time the gap in the shutter curtains was over the specific area of the camera's sensor is a fairly weak flash from the camera of another photographer taking a photo of the same scene or some other source of off axis light that caused the lens/filter to create additional reflection for a precise instant.

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