What makes hot pixels during long exposure?

I thought maybe my sensor was dirty. I took a look tonight, and it doesn't appear to be dirty, even with magnification. Lenses are clean. I blew the sensor out with the rocket.

This is typical with different lenses. I don't see this under normal exposures. Well maybe a dot or two, but not this bad. This is just a crop of a larger image. It's pretty bad across the entire image. This was probably a couple of minutes LE.

I bought a cleaning kit, but I read another thread here where someone made a smudgy mess. I'm leaving for a photo trip soon, and I don't want to make it worse.

** Just took a picture of a piece of paper, and not a spot to be found. Is this just a matter of using the in camera LE noise reduction that I don't use? I don't do long exposures really, but could have some opportunities on my trip.

Yes hot pixels from long exposure, they are much more noticeable when the rest of the image is very dark like here, and yes you need to be using LENR. Only thing about LENR is that you have to wait for the same length of time as the original exposure before you can use the camera again. You don't need to keep the camera pointing at the subject, since it is making the dark frame exposure without opening the shutter. If you are going to be making a series of exposures at the same shutter duration then you might be better off shooting all the images, then doing a single dark frame, and doing the subtraction in post. If that means little to you then you are probably much better off doing the in camera LENR. LENR is one of the few in camera processes that can actually be applied to RAW data, and is actually probably best applied to the RAW data if it is actually needed.

Because of the way that the image is converted from the RAW data to an RGB image, what starts out in life as a single hot sensel becomes a small dot of light spread over a few pixels. You will not see sensor dirt manifest itself as a really small spot like this, and sensor spots will always be darker than the surrounding area. Sensor dirt is also ƒ/number dependent, because of the way that light strikes the sensor at wide apertures a speck of dust or pollen on the sensor will create a shadow that is many tens of pixels in diameter, it may even break in to the hundreds. It is only as you stop down past about ƒ/11 will the shadows of the spots become sharper and more defined, but they will always be shadows. So the best way to see if you have dust issues on the sensor is to stop the lens down as far as it will go, this will make the shadows as small and pronounced as possible. Ideally you will set the ISO to minimum and white balance to AWB. I then like to take a shot of the sky that will have no detail, as this will give the most even illumination. This will make the sensor dust glaringly obvious. If you need to do this indoors then shoot a section of plain painted wall. The exposure will be a number of seconds, and while the exposure is underway move the camera around slightly. This again ensures that you only see the sensor dust and not any detail on the surface you are shooting.

You will have to try very hard to see any marks on the lens. On some lenses you might see some very large spots that are on the rearmost element, but generally they would be so large you would obviously notice them as you mount the lens. As for the front surface, well you can put large bits of paper even over the front element, and you still won't see them in the image at "normal" aperture values. You can make them up to about half the diameter of the lens without "seeing" them. What they do is affect the shape of out of focus highlights, and reduce overall contrast.

Alan

Thank you Alan! This is an area I haven't played much with. I appreciate the in depth response.

BigAl007 wrote in post #18717781
Yes hot pixels from long exposure, they are much more noticeable when the rest of the image is very dark like here, and yes you need to be using LENR. Only thing about LENR is that you have to wait for the same length of time as the original exposure before you can use the camera again. You don't need to keep the camera pointing at the subject, since it is making the dark frame exposure without opening the shutter. If you are going to be making a series of exposures at the same shutter duration then you might be better off shooting all the images, then doing a single dark frame, and doing the subtraction in post. If that means little to you then you are probably much better off doing the in camera LENR. LENR is one of the few in camera processes that can actually be applied to RAW data, and is actually probably best applied to the RAW data if it is actually needed.

No, that all makes sense. So these hot pixels will always be the same hot pixels, or the same for a given subject/setup? It sounds like the LENR is just heating up the sensor again with the mirror down. So the pixels that got hot once already, will likely heat up easier during the LENR to create the perfect dark image?

I'll do that again tonight with moving the camera around, to be sure.

When you make the dark frame for any set of conditions the dark frame noise for each sensel should be the same. The reason you need dark frames for each exposure time, ISO value, and temperature conditions is that those variables can change the exact level of noise generated by each sensel. All of the sensels produce this type of noise, but of course with some they will always end up saturated, which as you noticed show up really well when the image is generally dark. The great thing is that other than the time required, even in camera LENR is very effective, and doesn't really even degrade the image quality.

If you do a check for sensor dust, it can be a really good idea to shoot with around +1 EC applied. This will ensure that the resulting exposure is a little brighter than a mid tone. You might even find +2 is even better. Again it's usually easier to see the dust spots when the image tone is on the lighter side. Although I would always do my checks at the smallest aperture possible, IIRC the kit 18-55 is good here, IIRC mine goes down to ƒ/45 at 55mm. Still once you have done the check, and cleaned the sensor if necessary, I would do another check at ƒ/8, if you did a reasonable job of cleaning the sensor this shot should be pretty damn near perfectly clear of spots. I often find that I end up needing to shoot at ƒ/16 to ƒ/22 at ISO 100, to get the shutter speed at 1/160 or slower, and finding reasonable quality 95mm round ND filters is not easy, nor cheap. The only ones I found available here in the UK are £199 each!

Alan