Newbie question

jason.moseley

What's up with the little red and blue dots on this picture (it wasn't a keeper anyway)? Dust in the lens? Sensor issue? :scratch

https://www.dropbox.com/s/r0qcokfs3uj2aab/_MG_4285.jpg

Thanks for any assistance!

-Jason

jogle

It's a sensor issue but every sensor has it, and it shows up more when using high ISO and long exposure.

Basically it's a pixel on the sensor that is hit by a little bit of electro magnetic interference and reads out the wrong value. It's an extreme version of sensor noise. Similar to a "hot pixel" but they don't always show up and can move locations.

If youre using your 40D then turn on the "long exposure noise reduction" feature in your custom functions. It takes another photo immediately after the first one but with the shutter closed. It then subtracts this "dark frame" from the photo and those hot pixels go away.

jogle

This rather dated article explains it quite well. http://webpages.charter.net/bbiggers/DCExperiments/html/hot_pixels.html

jason.moseley

Thanks so much for the reply. That explains it quite nicely. I wasn't using a high ISO (~250 I think), but I was doing long exposures. Are there any downsides from turning on the in-camera noise reduction to remedy this?

jogle

The only thing is that it takes longer before you can shoot the next shot because it needs to capture the dark frame. It gives a better result then you can do yourself in post.

You've probably got a couple of modes for it, at least I do on the 5D and I can't remember what one I use off hand, I think there's one that does dark frame subtraction for any shot longer then a second, so check the custom function pages of the manual.