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What I've learned over the years with digital cameras -- you can't trust the preview image. The histogram tells much more of the story (if not the whole truth)

What I've learned over the years with digital cameras -- you can't trust the preview image. The histogram tells much more of the story (if not the whole truth). My photographs have become much better (in a technical sense) after I learned to expose almost to the point of blowing highlights, making sure there is little blank space at the right (where the highlights are displayed) of the histogram as possible (hence the term Expose To The Right). As someone downthread stated, this improves the signal-to-noise ratio of the captured data. There's an unwritten assumption that the camera's built-in auto-exposure capability isn't sophisticated enough to expose the scene properly, and the photographer has to take control. This is often the case. And speaking of SNR, there are a few tricks astro-photographers have been using for years that occasionally find their way into mainstream photography. One is image stacking, where you combine multiple photographs of the same scene (useful for landscapes if a little blur isn't a problem for you) into a single image. This pushes the shot noise (the random distribution of noise) down even further. I've gotten clean images out of cheap cameras, and even from cheap cell phone cameras (given enough images). The other technique is dark frame subtraction, where you take an photo with the lens covered at the same ISO/shutter/aperture as the rest of the set. Digital sensors have readout noise and heat noise which tends to be very predicable. Subtracting this noise gives you a cleaner shot as well. On a side note, I do wish when talking about ETTR and even digital photography in general that the term "overexpose" wasn't used when explaining the technique of overriding the cameras default metering with exposure compensation. Overexposure would be blowing the highlights, or at least the highlights of details that the photographer considers important (e.g. blowing the highlights of street lights is usually okay). The "over" portion makes it sound like the photographer is doing too much of something, when in reality ETTR is a technique of giving enough exposure, but not too much.

vanderZwan 9y ago
> One is image stacking Since you seem to have experimented with this, I'd like to ask: don't you need a certain "minimum" time to capture the faintest of stars, because their signal otherwise wouldn't rise above the noise floor? I've been curious about this with stacking in atrophotography for a while. > The other technique is dark frame subtraction, where you take an photo with the lens covered at the same ISO/shutter/aperture as the rest of the set. Digital sensors have readout noise and heat noise which tends to be very predicable. Subtracting this noise gives you a cleaner shot as well. For long exposures this heat noise becomes more problematic: long exposures heat up the sensor quite a bit. Have you tried building your own customised electric coolbox to keep temperatures down? It can be a fun project! I actually used this in a different context: during long Skype sessions at home with my people abroad, my phone heats up a lot and the video would start stutter. Putting it on a cooling block actually fixes that. This had more to do with the system not having to scale back CPU performance to let the phone cool down, but a side-effect according to people on the other end of the line, was that the image quality improved too
- jxcl 9y ago
  > don't you need a certain "minimum" time to capture the faintest of stars, because their signal otherwise wouldn't rise above the noise floor? I'm not an expert or even very knowledgeable on image stacking, but I'll share my experience. I've taken a stacked picture of the Orion constellation. 15 exposures at 5 seconds each, and combined them to get this photo: https://www.flickr.com/photos/abliskovsky/31756623694/ https://www.flickr.com/photos/abliskovsky/31756623694/ In none of the individual exposures is the Orion Nebula visible, but in this photo it's pretty clear. So in the end I'm fairly sure that your statement is true, but the signal required is very, very faint.
  - craftyguy 9y ago
    Do you know of any good resources for "getting into" image stacking for a complete newbie? 2 years ago I tried stacking images I took of Jupiter in my 16" dob (taken with a Canon DSLR), but it was terrible and I'm not confident I used the app I found (I think it was AstroStack..) for image stacking correctly.
    - jxcl 9y ago
      I don't have any particular advice. I searched the web for astrophotography stacking and used some software I don't remember the name of to do it. Sorry I can't be more helpful!
    - jpk 9y ago
      RegiStax is popular for solar and planetary imaging. I've used it several times on H-alpha solar photography. It has a fairly well defined workflow and there are lots of video tutorials on how to use it.
      - craftyguy 9y ago
        Awesome, I will check it out. Thanks! Also looks like it runs under Wine on Linux, which is good news for me!
  - 9y ago
jxcl 9y ago
> The histogram tells much more of the story (if not the whole truth). I feel that this is worth expanding on. Because a raw file isn't actually image data and instead just straight readouts from the sensor, it doesn't make sense to show the histogram of raw data; the data has to be manipulated into a jpeg first. The histogram in the camera will show the lightness values for what the in-camera image processing computer does to convert the raw file into a jpeg, but the in-camera image processor is usually not as advanced as a desktop image processor. It's probably running older image processing algorithms and has to worry about battery life among other things. So generally what this means is that if your camera is telling you that parts of your image are over or under exposed and therefore clipped, it might still be possible to recover those parts of the image in a more powerful image processor. It's also worthwhile, if you've got really old raw files, to go back and reprocess them with modern software. Raw file processing has come quite a long way in the past decade!

vanderZwan 9y ago

> One is image stacking Since you seem to have experimented with this, I'd like to ask: don't you need a certain "minimum" time to capture the faintest of stars, because their signal otherwise wouldn't rise above the noise floor? I've been curious about this with stacking in atrophotography for a while. > The other technique is dark frame subtraction, where you take an photo with the lens covered at the same ISO/shutter/aperture as the rest of the set. Digital sensors have readout noise and heat noise which tends to be very predicable. Subtracting this noise gives you a cleaner shot as well. For long exposures this heat noise becomes more problematic: long exposures heat up the sensor quite a bit. Have you tried building your own customised electric coolbox to keep temperatures down? It can be a fun project! I actually used this in a different context: during long Skype sessions at home with my people abroad, my phone heats up a lot and the video would start stutter. Putting it on a cooling block actually fixes that. This had more to do with the system not having to scale back CPU performance to let the phone cool down, but a side-effect according to people on the other end of the line, was that the image quality improved too

jxcl 9y ago
> don't you need a certain "minimum" time to capture the faintest of stars, because their signal otherwise wouldn't rise above the noise floor? I'm not an expert or even very knowledgeable on image stacking, but I'll share my experience. I've taken a stacked picture of the Orion constellation. 15 exposures at 5 seconds each, and combined them to get this photo: https://www.flickr.com/photos/abliskovsky/31756623694/ https://www.flickr.com/photos/abliskovsky/31756623694/ In none of the individual exposures is the Orion Nebula visible, but in this photo it's pretty clear. So in the end I'm fairly sure that your statement is true, but the signal required is very, very faint.
- craftyguy 9y ago
  Do you know of any good resources for "getting into" image stacking for a complete newbie? 2 years ago I tried stacking images I took of Jupiter in my 16" dob (taken with a Canon DSLR), but it was terrible and I'm not confident I used the app I found (I think it was AstroStack..) for image stacking correctly.
  - jxcl 9y ago
    I don't have any particular advice. I searched the web for astrophotography stacking and used some software I don't remember the name of to do it. Sorry I can't be more helpful!
  - jpk 9y ago
    RegiStax is popular for solar and planetary imaging. I've used it several times on H-alpha solar photography. It has a fairly well defined workflow and there are lots of video tutorials on how to use it.
    - craftyguy 9y ago
      Awesome, I will check it out. Thanks! Also looks like it runs under Wine on Linux, which is good news for me!
- 9y ago

jxcl 9y ago

> The histogram tells much more of the story (if not the whole truth). I feel that this is worth expanding on. Because a raw file isn't actually image data and instead just straight readouts from the sensor, it doesn't make sense to show the histogram of raw data; the data has to be manipulated into a jpeg first. The histogram in the camera will show the lightness values for what the in-camera image processing computer does to convert the raw file into a jpeg, but the in-camera image processor is usually not as advanced as a desktop image processor. It's probably running older image processing algorithms and has to worry about battery life among other things. So generally what this means is that if your camera is telling you that parts of your image are over or under exposed and therefore clipped, it might still be possible to recover those parts of the image in a more powerful image processor. It's also worthwhile, if you've got really old raw files, to go back and reprocess them with modern software. Raw file processing has come quite a long way in the past decade!