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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:
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.
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?
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.
What I see is that in the middle third of the photo, the entire image is duplicated a bit to the left of the main image.
I don't think this can be caused by any sort of lighting issue, because new light sources can't cause something to shift sideways (unless we're talking about two flash bulbs firing while the camera or subject is in motion, and even if that happened, the effect would not be limited to a horizontal band). Similarly any internal reflection inside the lens should exist for the entire duration of the shutter's motion, not just part of it.
If this effect were over the entire image, I would guess that you held the shutter button halfway down a little too long before taking the shot, and then shifted just far enough that the IS mechanism hit the end of its range and recentered itself during the shot.
However, because this occurs only over the middle section of the photo, that suggests that the photo was actually exposed twice (and you moved a bit between those exposures). This suggests to me that the second curtain is opening while the first curtain re-closes, and is briefly getting out ahead of the first curtain, meaning that the first curtain is moving too slowly. This must be a fairly slight mispositioning of the first curtain, as opposed to being truly stuck, or else part of the image would be massively overexposed.
If you are in a mode that shoots repeatedly, and if your camera has multiple speeds, slowing it down might change the behavior. Worth a try. Either way, I'm pretty sure you'll need to send the camera in for repairs. It likely either needs some sort of shutter adjustment/lubrication or a new shutter.
