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Eyes Vs Guiding


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#1 MarkM

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Posted 15 August 2016 - 03:07 PM

Beginner question here...
I've been following the 4-PAGE Milky Way thread. Fascinating and awesome shots!!!

If me and my tripod mounted camera are standing side by side, stationary on good ole planet Earth, looking at the stars, Why do I NOT see star trails when looking? Or vice versa, why does the camera see star trails (if over the 500-600 rule)?

Are my eyes/brain actually doing the "guiding" for me automatically?!

#2 TrailEx

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Posted 15 August 2016 - 03:12 PM

Pretending the human eye is electronic, its much more akin to a video camera than a DSLR. Our internal refresh rate is far too fast for stars to trail. Not even satellites will trail if you watch them.

 

And to answer the vice versa scenario, DSLRs become light buckets when the shutter opens up. It will record whatever it captures while the shutter is open. When you exceed the 500 rule, the sky will have moved enough in the frame that the camera will detect the movement. As an example, if you photograph the milky way with a 50mm lens for 5 seconds, you will not get trailing because the sky had not moved far enough for the sensor to register the stars on more than a couple pixels. When you use the same lens at 20 seconds, the sky will have moved enough to register on several additional pixels.



#3 rospondek

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Posted 15 August 2016 - 03:22 PM

Don't understand question :D

But I'll try to explain.

 

Pick a flash light. Point it in your face. First swipe it slowly from one side to the other. You will see only the point of light moving left to right. Now start waiving it very fast left to right continuously. You will see trails of light. Why? In first case your eyes refresh rate is so fast that this slow move is correctly registered and you see it clear. The second case eye won't 'refresh' fast enough and the light will blend into one line.

But now during that fast waiving start to blink very fast. You will again see the point of light. Your 'exposition time of eye' will be short enough to catch the instant frozen point of light. Just a trick I used as a kid to see a 'frozen time' when looking at wheels in motion ;)

 

Now the stars. They are moving (well not them but earth but not the case) constantly but they are moving very slow. Slow enough for the refreshing rate in your eye but fast enough for 300mm lens to see their move in only 2 seconds. It is like this flashlight but at the slower rate.

 

So eyes are not actually guiding. Their exposition time or refresh rate is fast enough for you to not notice the trailing. Even very fast ISS you will see as a moving point not a line or even a tiny trail.And it is bloody fast especially at the highest point.



#4 MarkM

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Posted 15 August 2016 - 03:57 PM

Excellent boys, the eyes have it😁That helps a lot! Thx




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