As part of my son's photography class, he wanted to try "painting with light," similar to the shots that Picasso did. So we created a slightly "ghetto" set up: Placed the camera on the tripod, triggered by a remote. While he held the shutter open, I painted using the lights. When I was done he triggered a Nissin flash remotely by hand then closed the shutter. Crude, but interesting.
However, upon closer examination, I noticed a number of red and green pinpoint spots on SOME of the images. While you can see one red one just above my left wrist here, you may have to view at original size to see more of them. It's definitely sensor related, as when they do appear they appear in the same locations. There doesn't seem to be any rhyme or reason (or pattern in the EXIF data) as to why they appear, at least not that I can tell. I don't see anything like this during "normal" use of the camera. I even took a few 20s star exposures the other night and noticed nothing.
So what do you all think? Dirty sensor? Damaged sensor? Limitations of the D90 sensor? A light reflection/refraction issue related to the use and positioning of the flash?
Red and green spots?
(11 posts) (6 voices)-
Posted 5 months ago #
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How long is the shutter open? Does the D90 have LENR? I don't think so, but if so use it. Otherwise you can snap a dark frame with the lens cap on for the same amount of time and subtract it from the photo.
What you are seeing is noise generated by the long exposure. They deal with this all the time in astrophotography.
Posted 5 months ago # -
That's what I thought as well. However, the shutter was only open for 6.7s, and others where the shutter was open for up to 9s didn't display this. The D90 does have LENR and it was on.
Also, I found it interesting that the noise artifacts showed up in the same place each time they did show up.Posted 5 months ago # -
I saw the red dot as I was attempting to clean my monitor...LOL
I mess up all the time in my photos and just use the spot removal tool in Lightroom. It is too much to try and figure out what it is. But, I suppose cleaning the sensor might help. If it is a pixel related issue then it will not go away. And, you may find it in other photos of a uniform background.
Let us know what it is...
Oh, kinda neat image idea...
Posted 5 months ago # -
Dead/stuck pixel probably. Every sensor has them. I think the D90 has the dust mapping feature, if you use that it will usually catch them.
Posted 5 months ago # -
Not sure how hard I'll pursue it. As Moto said, it's pretty easy to edit out. For the record, he was inspired by this:
http://www.thecoolist.com/light-painting-by-pablo-picasso/
Never underestimate what will inspire a 17 y/o.Posted 5 months ago # -
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It may very well be dead pixels or due to higher ISOs. I just did a little analysis of which images had them and which didn't. Oddly, everything with an exposure UNDER 8.0s had them while everything OVER 8.0s didn't. Seems counterintuitive to me. I would think that a longer exposure would lead to MORE noise artifacts (or would be more likely to show dead pixels), not less.
For the record ISO was 400 and aperture was f/7.1.Posted 5 months ago # -
The LENR may kick in at 8sec and map that pixel out for you. Check your owners manual, as there is a cutoff to what is considered Long Exposure vs. not.
Edit: I just checked the manual for you pg 167:
LENR on: Photographs taken at shutter speeds slower than 8 s
are processed to reduce noise.So just keep the exposure above 8s and everything is dandy :-)
Posted 5 months ago # -
Thank you! Amazing.
Posted 5 months ago #
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