D90-Red dot on pictures?

lilmomma · May 25, 2009

I noticed the other day on some of my pictures there is a red dot right when I open my photos in lightroon then it disappears. Today I noticed it's there when I take the photo and look at it on camera. It's very small, and when I zoom in it's just one pixel, and one on each side and up and down. It's hard to explain...but does anyone know what it is?

swintonphoto · May 25, 2009

It's a hot pixel. It is a very common thing. Lots of imaging software is programmed to find them and get rid of them - hence the reason it disappears for you in lightroom. Many DSLR's have a hot pixel finding function that finds it and eliminates it with the software in the camera. Does yours have such a function?

lilmomma · May 25, 2009

I don't know? I just got the camera last week.. How would I find out?

swintonphoto · May 25, 2009

Check your manual for a function called "pixel mapping". At least that's what it's called on my Olympus DSLRs.

Tim Kamppinen · May 25, 2009

I have a couple of hot pixels on my D90 that only show up at high ISOs. Just when I was about to get all pissed off about it, I realized that apparently Camera Raw was removing them for me automatically! They showed up in bridge but when I opened the file in CR they were gone... I had never heard about this feature but I'm very glad that adobe came up with it. Also, I never had to do anything "extra" with camera raw, it just did this automatically. I think it's version 5.3 or 5.4 (whichever one is the second-newest release).

lilmomma · May 25, 2009

Well my question is, why is it there? I mean I have only had it for less than a week. I don't have camera raw, but swintonphoto says lightroom gets rid of them which is good, it's just annoying to see it!

Tim Kamppinen · May 25, 2009

lilmomma wrote:

Well my question is, why is it there? I mean I have only had it for less than a week. I don't have camera raw, but swintonphoto says lightroom gets rid of them which is good, it's just annoying to see it!

It's a defect in the sensor. There's something wrong with that pixel (I don't know the proper explanation for the problem) that makes it show up as red all the time regardless of what you're photographing. There's 12 million of them on there, so the chances that you'll have problems with at least a couple of them is pretty good. Like I said though, on mine I only notice the hot pixels at high ISOs. Do yours show up at 200 as well?

lilmomma · May 26, 2009

Yep, and it's there at even lower than that. Wierd, but I guess if it's no big deal I don't need to worry about it. Thanks for the info!

HenryPeach · May 26, 2009

I've had DSLRs that came with hot pixels and DSLRs that developed them over time. The default chroma noise reduction in Adobe Camera Raw and Lightroom always gets rid of them, so I've never even bothered with the pixel mapping.

ziggy53 · May 26, 2009

Folks, the photosites in a camera's sensor are just transistor (or diode) junctions. The first imager I ever saw was just 2000 pixels, 2000 photosites and 2000 transistor junctions and/or diode junctions. (Let's stick with diode junctions just for the sake of discussion.)

When these electronic junctions are allowed to be exposed to photons (light) they are affected by the photons and that affectation can be measured. That measuement is translated into data that forms the digital image. (This is a gross oversimplification of the process.)

When a photosite is defective it is either dead or stuck "on" (hot). Every manufacturer of digital imagers has criteria and tolerances for a "pass/fail" measure for defective photosites. With the large numbers of photosites on a modern imager the vast majority of imagers have some defective photosites when they leave the factory.

"Mapping" the defective photosites simply measures the output from each junction and, if it is dead or hot, replaces the data from that photosite with either a "nearest neighbor" or an average of surrounding sites. The effect is that you will never see the affected defective photosites in general photography.

Whether the mapping occurs in camera or in computer software is immaterial as far as the resulting images.

In other words, if you don't see a problem with either prints or "distribution" images, typically either JPG or TIF, you should not worry about it. The "system" you are using is managing the defect mapping and all is well.

DaC · May 26, 2009

Thanks, this is a very timely thread for me. (d-300)

D90-Red dot on pictures?

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