More on the 500 rule

This topic contains 19 replies, has 9 voices, and was last updated by  Tobie 11 months, 3 weeks ago.

  • Author
  • #366114

    Robert Apple

    I see we discus the 500 rule fairly often here on Light Stalking and lately NPF calculations. One of the variables I have never seen discussed here, or on any blog related to this, is one I discovered a few years back that can make quite a difference in shutter speed. I found that the settings I used in the plains and got sharp pin point stars didn’t give the same results when used in the mountains. Well we live on a spinning globe, so in relationship to a fixed point in the sky the farther from the center we go the faster we are moving, 

    Freakin’ physics.

    My father has a friend that is a Astro Physicist at Ball Aerospace in Boulder Colorado and I posed the question to him of how to compensate for this and if it was a valid theory.
    His reply was of course its valid and immediately spit out the equalization. After some discussion we agreed on a functional number of .02% per foot or .0002 based on the 500 rule using the baseline of sea level.

    I have been using this a while with good results and the modified formula would be

    (( 500 / Focal Length ) / Crop Sensor Factor ) – ( Altitude in Feet * 0.0002 ) = Shutter Speed

    Shooting in the plains at around 5500 feet I would use a 6.0s shutter speed on my Olympus which has a crop sensor factor of 2 with a 35mm lens

    (( 500 / 35 ) / 2 )- (5500 * 0.0002 ) = 6.0

    But in the mountains at 8750 feet a 5.4s shutter speed was needed to eliminate star trails.

    (( 500 /35 ) / 2 )-( 8750 * 0.0002 ) = 5.4

    For those metric folks the magic number would be .00006 per meter

    (( 500 / Focal Length ) / Crop Sensor Factor ) – ( Altitude in meters * 0.00006 ) = Shutter Speed

  • #366118

    John Thompson

    Wow great information Robert.  Night shooting is something I want to try but I have to get away from this light source I live in called Las Vegas.  You can see some of the lights from space so I may have to go farther than I think!!! Lol.

    • #366120

      Robert Apple

      Yeah John the worst part of the city and light pollution, that I never really considered until Colorado got so bad,  was the light reflecting off the pollution , all the particulates in the air.

      As an example, in this shot, this is smoke from the forest fires in Yellowstone 250 miles away reflecting the light from Sheridan Wy . 40 miles away. Sheridan is a relatively small city of  18,000 people with not a lot of light pollution. Last I knew Vegas and the surrounding suburbs had over 2 million residents.

  • #366134


    Robert I believe the 500 rule may be way over complicated by analyzing irrelevant factors. The bottom line is ‘for how many seconds can I leave my shutter open before the stars start trailing in the image?’. The 500 rule says ‘divide the 35mm equivalent of your focal length into 500’. That’s it. There’s nothing more about it. And actually 500 is conservative. Lots of people are using 600 and instead of 500.

    I can see your point of ‘the closer you are to the stars, the more conservative you have to be’ but I mean – what is the difference between 500 million light years (OK, 4.3 light years to be more correct) compared to 500 million light years minus the height of a mountain? Nothing! Maybe if you printed out the image on a canvas the length and width of planet Earth but on a 17″ laptop screen…

    As far as NPF is concerned I may be missing something but I do not see the point of lowering the number of sec’s shutter speed until the quality of your shots are the same as that produced by the ‘old’ DSLR’s! I’d rather keep to the 500 (or 600) rule at much lower ISO’s and see many more stars and detail than shown by the ‘old’ DSLR’s. Basically NPF says: “On the latest generation of cameras the 500 rule will give you even higher quality and more detail”. That’s it!

    I guess this can probably classified as a rant but – that’s my 2c! 🙂

    • #366149

      Robert Apple

      The whole reason I even looked into this Tobie was the 500 rule was pretty close at 5000ft elevation but I got noticable star trails with the same setting at 8500 ft, now had I been photographing an area not that accessable to me or a place I would never return too, I just shot a bunch of photos I’m dissapointed in. If just this little factor saves a photo shoot it’s worth the one extra calculation for me anyway. Now if I was a Flat Earther I would wholly agree with you. ?

      • #366223


        LOL! Well, put it this way @robertapple: at 4.3 light years away (the closest star’s distance) all of the earth’s surface is relatively as smooth as a mirror. It will be quite interesting to know why your mountain shots were disappointing but my gut feel is that you’re looking at the wrong place if you blame the height above see level. Maybe the link in my post lower down sheds a little more light on the actual reason?

  • #366148

    Rob Eyers

    I’m with you on this one Robert. It’s the velocity of travel through space relative to the stars that’s the issue, if you want round stars (no trails). Anyone who’s played crack the whip as a child would understand that the further away one is from the center of rotation the faster one travels. Altitude plays a part in this, relative to the earth’s center.

    There’s one other factor to consider. I recently had this discussion with a photographer buddy who lives near the equator. He shoots faster for night shots than I have to at a latitude of roughly 43 degrees north, even though he’s at sea level and I’m at about 650 feet. Here’s some googled data.

    At the equator, the circumference of the Earth is 40,070 kilometers, and the day is 24 hours long so the speed is 1670 kilometers/hour ( 1037 miles/hr). This decreases by the cosine of your latitude so that at a latitude of 45 degrees, cos(45) = .707 and the speed is .707 x 1670 = 1180 kilometers/hr. You can use this formula to find the speed of rotation at any latitude.

    These calculations can drive some people nuts but at least an understanding can get you in the ball park to start testing.  As a side line I’ve always thought it should be the 400 rule or less. Even that gets small trails were I live.

    Happy to listen if anyone can enlighten me otherwise.

    • #366155

      Bruce Gordon


      Interesting discussion! One has to love science.

      Don’t make fun of flat earthers, Robert! Their website used to claim they have members ‘from all around the globe.’ Sigh…That makes me laugh every time I think of it.


  • #366224


    Since this really fascinated me I searched – and searched, and searched for things that might affect shutter speed calculations for milky way shots. I bounced upon this very nice article (including an advanced calculator) which explains a few things we do not usually consider in calculating milky way shutter speeds.

    It also gives another perspective on the need for NPF calculations (sensor resolution & pixel tolerance) on some of our latest top-notch cameras.

    Advanced Astrophotography Shutter Time Calculator


    • #366227

      What Tobie says about newer cameras is spot on. My d810 needs to be SUPER still on a landscape or softness rears its ugly head.

    • #366244

      Rob Eyers

      Great article Tobie. Thanks for posting. This exactly confirms the results I’ve had with my camera. So the rule should be more like the “rule of 300” more or less.  I wish this was available in an app.

      • #366288

        @reyers – There is an app by our own @cw2323 on the app store!

      • #366360


        @reyers I guess the secret is to play with a range of shutter speeds within the 500 rule range and see which render the best result afterwards. Fact is, in terms of detail captured the longer you can leave the shutter open, the better. There’s another issue not even discussed yet and that is lens imperfection. Some lenses (even good ones) tend to form star trails in the corners. My Tamron 24-70 just starts showing that here where I (yes, wait for it!) applied the 600 rule (25s @ 24mm)! 🙂



        • #366369

          Rob Eyers

          Tobie I agree that all lenses tend to create star trails in the corners relative to the center.

          The stars in your image look good. Those are not the results I get at 25 seconds with my 5D Mk iv. When I shoot at 16mm the 600 rule says I should expose at 37.5 seconds. At that exposure time I have lines not dots for stars. Your latitude is around 30 degrees and I’m at 43 so that isn’t much of a difference. The calculator you posted would indicate a D600 at 24mm should expose at 12.4 sec and a 5D4 at 16mm should expose at 14 sec…..I’m confused as to why your image doesn’t have trails?!?

          Anyway here’s and example of what I get at 20 and 13 seconds. The size you’re viewing or printing at makes a difference in what’s acceptable but there’s no denying that 13 is the better between these two.


          • #366370

            Rob Eyers

            Here’s the full 13 second frame. It’s interesting how the slant of the Milky Way is different between the two hemispheres.

          • #366483


             The calculator you posted would indicate a D600 at 24mm should expose at 12.4 sec and a 5D4 at 16mm should expose at 14 sec…..I’m confused as to why your image doesn’t have trails?

            The only reason I can think of is your sensor resolution (you’re at 30Mpx and I’m at 24Mpx) but yes – according to the calculator I should be starting to see clear trails at 25s. Do you understand now why I believed the whole vibe around 500 rule and NPF to be an overkill? I never had these issues! Maybe it trails easier at your 16mm than at my 24mm? So much to wonder, so much to learn…

            I must admit I’m quite surprised a thow much detail you got at 13s above – it says something (good!) about the 5Div’s sensor quality!

  • #366269


    My head hurts now. 🙂

  • #366300

    Federico Alegria

    Oh Jeez…

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