500 rule an crop sensor

Discussion in 'Beyond the Basics' started by BrentC, Oct 6, 2017.

  1. BrentC

    BrentC Been spending a lot of time on here!

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    So this weekend I am going up north to my uncles for Thanksgiving. He is far away from any noise pollution so I figure I will try my hand at photographing the Milky Way. So when using a crop sensor do I use my FL or do I use 35mm equivalent? I'll be using my 17mm so not sure if I use 17 or 34 in the formula?


     
  2. snowbear

    snowbear Big Furball Supporting Member

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    Not sure what formula you re referring to. It's still 17mm focal length; view angle is 35mm but objects will be the size of 17mm.
     
  3. BrentC

    BrentC Been spending a lot of time on here!

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    The 500 rule you divide 500 by the fl to get your exposure time so you don't get start trails. I thought I read somewhere before that crop sensors have shorter exposure times when you start seeing Star trails.
     
  4. snowbear

    snowbear Big Furball Supporting Member

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    No clue. If you don't get a definitive answer, just try it both ways and see what happens. I might try something here with the brighter stars - I can shoot at full frame and again in cropped mode.
     
  5. BrentC

    BrentC Been spending a lot of time on here!

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    Ok I found it out. Max exposure time is 500/(fl x crop). So in my case 500/(17x2) = 15 max exposure time.
     
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  6. snowbear

    snowbear Big Furball Supporting Member

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    OK.
     
  7. kalgra

    kalgra TPF Supporters Supporting Member

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    Just some food for thought. The "500 rule" shouldn't really be called a "rule" at best its a very general starting point. Going by the 500 rule you will still see some trailing which may or may not be acceptable to you. Without going way overboard on the technical crap there are several other things that will play in as well such as the sensor resolution and where the lens is pointed with respect to the celestial equator.

    I personally like to start with a "400 rule" instead which I find results in better pinpoint stars across the entire image and adjust from there. Sometimes I drop to 350 if Im using really fast glass (f1.8-f2). Of course fudging on the 500 rule like this depends much on your gear such as your lens' widest aperture, camera ISO capabilities, etc. If you can only open to f4 you may need that extra exposure time. If you truly want the best shutter time for the least amount of star trailing check out the calculator below. All the rest will then depend on your widest aperture, ISO, and how much noise you are willing to introduce. If you get the chance try out some shorter exposure times and bump the ISO a bit to see what ends up working out best in post. You may find that sacrificing some exposure time to get less star trail is not worth it overall due to the noise introduced with increased ISO or in post to get the level of detail you want.

    Advanced Astrophotography Shutter Time Calculator

    One other bit of advice I found helpful to me. Find your lens' true infinity focus point before you go out. The distance marker on many lenses is not always accurate and it crucial for this type of thing. To do this I like to set my camera up on a tripod and use live view to magnify as far in as possible, then focus on some object far off in the distance like a mountain, tree, whatever. I then use a piece of gaffers tape or masking tape on the focus ring of the lens and mark with a pen where true infinity actually is. Sometimes if I know i'm not going to use the lens for anything else I will just tape the focus ring in place so I dont have to worry about bumping it in the dark.
     
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  8. BrentC

    BrentC Been spending a lot of time on here!

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    Thanks. I realized the 500 rule was just a starting point. But thanks for that link it gives me a better understanding then what I have already read. And good tip on setting up focus ahead of time.
     
  9. petrochemist

    petrochemist No longer a newbie, moving up!

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    I'd heard of it being treated as a '600 rule' but hadn't realized that was talking it the wrong way for pinpoint starts. I guess the variation was done to makes the maths easier when shooting in minutes. Both probably date from film days where prints were frequently only 6x4 so star trails would have been less noticeable.
    Perhaps using it as a 300 rule & ignoring the crop factor would work well for APSC :)

    Thanks for the tip on infinity focus marks - that could make a lot of difference with some of my lenses!
     
  10. dunfly

    dunfly No longer a newbie, moving up!

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    Help me with this. I don't understand why you would apply the crop factor to the focal length. A 17mm lens on a full frame is the same focal length as a 17mm on a crop sensor, only the field of view changes. The exposure on any single point on the sensor would seem to be the same whether you are using a full frame sensor or a crop sensor. The only reason you would use the crop factor would be if you were looking for an equivalent field of view, in which case you would not be using the same focal length lens. On the full frame you would be using a 25mm lens and on the crop factor you would be using a 17mm (assuming a 1.5 crop factor). If you load the actual focal length in the formula you would get a different answer but you would not have to apply the crop factor to the focal length, you would just use the actual focal length. Am I missing something?
     
  11. TCampbell

    TCampbell Been spending a lot of time on here!

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    BTW, this "rule" was originally the "600 rule" (not the 500 rule) and the "500" was to be a bit more conservative because if you look close at a shot taken with the 600 rule, you'll notice the stars are starting to enlongate due to the motion of the Earth.

    But this rule assumes the angle of view from a 35mm film camera or "full frame" digital camera. For anything else, multiply it by the crop-factor of the camera (usually that's 1.5 for an APS-C camera).

    You can either multiply the 17mm focal length of your lens by 1.5 (arriving at 25.5) and divide that into 500 (500 ÷ 25.5 = 19.6 seconds (call it 20 seconds)) -or- you can divide 500 by 1.5 (e.g. 500 ÷ 1.5 = 333.3) and then divide that by the focal length (e.g. 333 ÷ 17 = 19.58 (so again... call it 20 seconds).

    While that example happened to work out to 20 seconds... and you probably can set your camera to take a 20 second exposure... often it will work out to some oddball number (e.g. Suppose you had a 14mm lens. That would work out to about 24 seconds) and you can't actually set the camera to those odd numbers. In those cases, it's nice to have a manual shutter release so you can control the exposure time (put the camera into "bulb" mode and use the manual shutter release to control the exposure length.)

    If you go longer you'll start to see elongation in the stars (star trails) and if you actually want that effect then you can go longer (usually you just take as many 30-sec back-to-back exposures as you want... possibly for a few hours... then use computer software to combine them all.)

    You can also go longer (and use narrower lenses) if you have a "tracking" head on your tripod (e.g. Sky Watcher brand "Star Adventurer" head or iOptron brand "Sky Tracker Pro" head, etc.) but if land is visible in the frame you'll get sharp stars with blurred landscape.
     
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