Trouble with Long Exposures – Part 2 of 2

In the previous article I discussed 4 of the 6 most common problems that occur with long exposures.  Those problems are:

  1. Poor Focus
  2. Dim Stars (low contrast)
  3. Strange Colors
  4. Purple or Pink Glow

In this installment we tackle these two issues

  1. Gaps in Star Trails
  2. Lots of Noise (Colored Speckles)

Gaps in Star Trails

To oversimplify a bit there are four causes for gaps in star trails created from successive exposures:

  1. Camera limitations
  2. Camera or intervalometer misconfiguration
  3. Processing choices
  4. Weather conditions

Camera limitations: I described this issue in my article “How long does a 30 second exposure take?”  All the Canon cameras I own – including the top of the line 5D Mark II require 32.8 seconds to complete a single 30 second exposure. Well there you go: almost 3 seconds of time where there is no exposure. This problem can be compounded by two common misconfiguration blunders:

  • Failing to allow enough time between exposures when using an intervalometer. Or using the wrong drive mode on the camera.
  • Failing to turn off long exposure noise reduction.

To avoid intervalometer misconfiguration I operate in either continuous exposure mode or bulb mode. I use continuous exposure mode when my exposures will be many and a maximum of 30 seconds – e.g. when trying to capture meteors or planning for a time-lapse animation. In continuous exposure mode I set my intervalometer with a start delay and then program an exposure time of several hours… AND I put my camera in Manual, high-speed continuous exposure mode with a typical exposure of 30 seconds. You do not really need an intervalometer for this – a locking cable release is sufficient.

When I operate in bulb mode, I try to get a moderately long exposure. Usually in the 4 to 10 minute range depending on the sky conditions. In this setup it is very important to put the camera in Bulb exposure and program the intervalometer to leave a 3 second gap between one exposure and the next. I have recently discovered, however that the Canon 5D Mark II will work with my intervalometer set to 1 second intervals. That’s goodness. I am still trying to work out whether the problem is due more to the timer or the camera. I do know that in continuous exposure mode all my cameras require 32.8 seconds per each 30 second exposure. Failure to allow a long enough pause between exposures can cause unexpected results.

Photo 1: For the first half of the evening I mistakenly left long exposure noise reduction on. The result was that half of my shots occured at every-other eight minute intervals.

The “dotted lines” in the circle above were caused by leaving on long exposure noise reduction. The result was that the intervalometer timed an 8 minute exposure, waited three seconds and then pressed the shutter for the next 8 minute exposure. However 3 seconds after the exposure completed it was still doing long exposure noise reduction so that cycle was skipped until the intervalometer released the shutter for the next 3 second “off” interval.

I have gotten into the habit of setting my exposure length to 3 seconds less than what I want… e.g. 9:57 for a 10 minute exposure. I then set a 3 second inter-shot interval. I used to set a 10 minute exposure plus a 3 second gap – but the predictability of starting a new exposure every 10 minutes makes it easier to monitor what is going on.

Another cause for gaps: changing the battery. I can offer the following important tidbits when you need to change the battery.

  • Do not wait for your battery to be exhausted. A partial exposure may not stack well or be completely written to your card. Battery exhaustion will likely occur at an inopportune time.
  • Have everything at hand in advance of the change. For example, keep the battery in your front pocket where your body heat will keep it warm.
  • Practice a battery change BEFORE you start your exposures. Only by practice beforehand will you be able to discover that the battery compartment is blocked by your tripod, or impossible to reach, etc.
  • When you DO change batteries beware! Your camera settings may change dramatically!

Processing choices you make when stacking the star trails also affect whether your gaps will be inconspicuous. Do not do any sharpening until you complete your stacking – and even then avoid sharpening the star trails themselves. The method used to stack trails is significant. However, I have observed that people do not notice gaps even in this image of 19 8-minute exposures printed out at 20×30 inches.

Photo 2: Even though it is composed of 19 eight minute exposures the gaps are never noticed even when printed at 20x30.

Weather conditions can also introduce gaps. In a truly dark sky where clouds are not lit by city glow, moonlight or twilight, clouds become “black holes” and block starlight. Low or fast moving clouds can obscure some, most or all of one or more images in the set. This can be perplexing if you happen to be sleeping during exposures which started and ended with clear skies.  Another problem is dew which may form a fog that diminishes or eliminates some or all of the exposures. Vigilance with a rag, the use of a hood or a dew heater are your only weapons against dew.

Lots of Noise (Speckled Colors)

I purposefully left the noise in Photo 1. It’s quite noticeable in the rock silhouette at the lower right and appears mostly as red specs. Annoying? Well, yes, but it is not the end of the world.  In order of effectiveness here are your best approaches to keep the noise manageable:

  1. Shoot at a lower ISO (100 or 200)
  2. Shoot and stack shorter exposures – longer exposures generate more noise.
  3. Capture the foreground and the star trails separately. A better lit foreground will exhibit less noise.
  4. Shoot during colder seasons – lower temperatures result in lower noise.
  5. Control stray light with a lens hood – and close or cover your viewfinder while exposing.
  6. Use high ISO noise reduction
  7. Use noise reduction post processing tools. Chrominance noise is usually most in need of correction.
  8. Use long exposure noise reduction.

Hopefully you noticed that long exposure noise reduction (LENR) is last on the list. If you are trying to stack star trails it is impossible to get continuous trails with LENR on. It is also the least effective unless you are only going to shoot one shot.

Before we go much further, it is worthwhile to note that there are 4 causes of “noise” and each has a different source. The random speckles are usually what is meant by noise. Those random speckles are created by heat, limitations in the electronics, and things as bizarre as electromagnetic phenomenon like sunspots. No kidding. True noise is by nature random and LENR can not do a thing to combat random noise except to diminish it by reducing the luminance of the offending pixels – which also reduces the sharpness of your image. But there are 3 other kinds of noise that are not random though often lumped into the same general category: hot/stuck or degraded pixels, local heat noise (sometimes called amp glow), and high ISO noise. LENR is effective for these because they are not random.

Hot or stuck pixels usually appear as bright pink, red, blue, green, white or purple spots. They are caused by either electronic problems on the sensor chip or by the dyes used to detect the color.  A pixel detects the intensity of the color red by use of a red dye (inkjet droplet) over a sensor site. If that red dye is insufficiently thick, or missing altogether then that pixel location will always read hot if there is any light falling on it – and if the problem is electronic it may read hot even if no light is striking it. Dead or degraded pixels are just the opposite. Too much dye or dead electronics at a pixel site. Degraded pixels are stuck black or darker than the surrounding pixels and are seldom if ever noticed in night photography.

Locally caused heat noise is noticeable in some cameras and is due to the heat of electronics in proximity to the sensor. In my opinion this problem is a design flaw in the camera. However this kind of noise is repeatable so LENR can help correct it. The “Pink or Purple Glow” that results from this flaw was discussed in Part 1.

High ISO noise has an understandable parallel in the world of audio. Take nearly any cheap radio. Turn it up. At some point the sound will become distorted and harsh. This harshness is because there are limitations in the signal, the amplifier circuitry and the speaker used to produce the sound.  Increasing the ISO in your camera is the photographic equivalent of the audio scenario.  At some point amplifying the light measurements made at each pixel makes the noise more obvious.

14 thoughts on “Trouble with Long Exposures – Part 2 of 2

  1. Pingback: Exposing for Stars | Star Circle Academy

  2. Steven Christenson

    Film solves a lot of problems, of course, but also creates difficulties… including a steeper learning curve (no results until the film is developed), and won’t solve the problem of getting very long exposures when light pollution or moonlight are present – in those cases long exposures must by necessity either over expose the sky, or use an f-stop so small that no stars will be present.

    Stacking shorter digital exposures solves the long exposure problem quite elegantly. One bright light accidentally aimed into a long film exposure destroys hours of work. Going the digital route with stacking, you may lose one frame.

  3. Paul Caldwell

    It’s actually more of a question. I live in Arkansas, with very high heat indexes and 100% humidity during the late spring and all summer. By far here the best time to shoot is winter, early spring and fall.

    You mention shooting several 8 minute segments, which you then stack. When you are shooting this long a segment, are you using LENR? or in certain temps can you get a clean enough 8 min exposure? Or do you take a manual dark frame at the end of the series and then subtract it. I have been working with night photography now for a little over a year. Most of the time I prefer to shoot a longer single segment as a raw file as it seems to me that the overall image quality is much better (using a 5D MKII or 1D MKIV). I recently started working with stacks with my 5D, shooting 25 to 30 minutes of 30″ exposures. I set the quality to Ljpg instead of raw and left LENR off. The amount of “stuck” pixels was amazing and really ruined the shot, I also did not take a manual dark frame. I have been able to clean them up a bit, but still not very well.

    Do you find you get a good enough quality final image if you stack with jpgs? or do you stack raw and then then convert to jpg and then stack?

    Thanks for the articles.
    Paul Caldwell

    1. Steven Christenson

      Well Paul, in the “Gaps in Star Trails” chapter, under “Camera Limitations” the bullet reads “Failing to turn off long exposure noise reduction.”
      But to state it in the positive: “Turn off long exposure noise reduction”. Photo 1: even describes what happened when I accidentally left LENR on.
      Sorry if this isn’t clear.

      On to your next questions:
      (1) Do you find you get a good enough quality final image if you stack with jpgs? or (2) do you stack raw and then then convert to jpg and then stack?
      1. Yes (but not always), and
      2. Yes.
      I.e. I do both as the mood or circumstance allows. The choice is easiest using RAW+JPG then I get to pick afterward.

  4. Paul Caldwell

    Steven, thanks for the response.
    I am still curious on your thoughts as to how to remove the small gaps that are created
    when stacking. From what I have been able to read, it seems these are created by the blending mode, not the slight 1 to 2 sec pause as the shutter closes and reopens. I am working with star tracer which seems one way to get these out, however it would be nice to have a way to do it in the actions that stack. Do you have any thoughts on this?
    So far, I tend to always try not to stack unless the moon is so bright that it gives me no choice. Most of my stacks are 30 to 45 seconds and the gaps show up. I guess if I go with longer stacks, the gaps would still be there, but not as close together.
    They only show when you attempt a print or view at 100%, on the web they never show, however my main purpose is printing.

    Paul Caldwell

    1. Steven Christenson

      The size of the gaps depends on the field of view of the lens, the direction the camera is pointed and the amount of delay between exposures. I have published work with those “gaps” in them, and in fact printed 20×30″ my Bristlecone Pine Image has those gaps too – but NO ONE notices them!
      (son of) Bristlecone Pine Star Circle

      Having a “less dense” sensor reduces the gaps, too.

      So my first answer would be to set your lens wide and your inter-shot delay as short as you can and not bother further. Second even though you can stack in screen mode and that will flow better – I don’t find the result all that pleasing.

      If you REALLY want to get rid of the gaps you can do what I’m attempting to do (it’s theoretical) – shoot two cameras with the same view offsetting the exposure time by about half. This method would have two huge additional benefits: the ability to dither which reduces noise, and the absence of gaps, but it’s not practical for most people.

      1. Paul Caldwell

        Steve, thanks
        Have you tried star tracer? It will also remove the gaps, takes a bit to figure out but does work. Single biggest problem I have with it is figuring out the correct FOV. I agree that unless the image is large you won’t see them especially on the web. I guess it’s a personal thing.

        Have you looked at Russell Brown’s two methods? He has a video using two sep scripts loading the images from bridge and then another video that uses a photoshop extension. I have not tried either one, but I am looking into them.

        I also just realized that CS5 has a star trails action that allows really anyone to “place” star trails into any image. Hate to see that because it really tends to make most folks think that the images that take so long to create are “fake”

        If you don’t mind, send me an email address and I will forward to you all the links of the items I mentioned.

  5. Paul Caldwell


    Thanks more info on the gaps. I agree that unless you are making a larger print, they are hard to see and for web viewing are next to impossible.

    Have you looked at Star Tracer? I have started to use it and have found it to be a nice addition. So far as I can tell, it won’t hard the color profile info as other tools seem to i.e. dropping everything to sRGB.

    Also I have found some interesting information from Russell Brown, he has two nice video’s showing some stacking solutions, the 2nd of which may not leave the gaps. I know of one other photographer here using this method.

    Of course, the sad issue is that Russell and Mark Johnson both have made the world aware of the star trails action built into CS5. This really allows people to just create them without much work at all, my worry being that most folks will just take the short cuts and not do the real work.

    If you are interested, send me an email and I will forward to you the links for the various videos and tools.

    Paul Caldwell

    1. Steven Christenson

      Paul, I have used Star Tracer and the photoshop action for making star trails. I found Star Tracer a bit tedious to use (though I’m generally a big fan of the utilities Max Lyons has published – Image Stacker in particular). While the Photoshop Star Trail action might seem interesting my experience is that those who use that method produce obvious forgeries (obvious to me, at least) because of the many inconsistencies that result. Photoshop allows for cheating in many other ways, so why worry about it? Indeed a more honest way to remove trail gaps is to flatten your image, slightly rotate it and blend it back with the original image. For reasons stated earlier I don’t bother to do this.

      NOTE: Andy posted an article explaining how to do this rotation thing (he uses radial blur). See the BLOG article.

  6. Paul Caldwell

    Thanks, I will have to try the rotation idea, never thought of that. I agree Star Tracer is a bit tedious. Image stacker seems to loose all the color profile and defaults everything to sRGB.


  7. Dan

    I just started using bulb mode and I notice immediately that bulb lets in alot more light. if I shoot a 30sec exposure at f3.5 in bulb mode its bright and washed out. If I shoot the same settings in standard D90 30sec mode my stars look mormal. Why is bulb mode washing so badly with the same time and settings?

    1. Steven Christenson

      The answer is one of two things: 1. Something is different from what you think (i.e. the ISO setting is different between the shots), or 2. The timing is incorrect. If, for example you’re shooting in time priority (T) mode for the 30 second shot you allow your camera to select f/stop and ISO whereas in manual mode you are specifying all of the settings. Make sure you’re in “manual” mode in each case and you should see identical (or nearly identical) results. If you see differences it is likely due to the errors in timing – e.g. the camera or intervalometer’s 30 second exposure might be 29 seconds or 31 seconds.

      Interesting bit of trivia… it’s called “bulb mode” because some of the first cameras to have an endless exposure mode used a pneumatic bulb to hold the shutter open.

  8. Pingback: Trouble with Long Exposures – Part 1 of 2. | Star Circle Academy

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