Fuzzy Stars with focus at infinity

Started Dec 14, 2004 | Discussions thread
Jack O'Neill Senior Member • Posts: 2,290
Re: Fuzzy Stars with focus at infinity

Prinzpaul wrote:


at infinity mark you wont get sharp photos!

Sure you can, if the infinity mark just happens to line up with where you infinity focus actually is. You see you can move your focus past the infinity mark on the lens because the true infinity focus point changes with the temperature of the lens. So they make the lenses capable of going past the infinity mark to compensate for changes in the infinity point caused by temperature of the lens.

Its pretty hard to focus in stars with f2.8 because the focus depth
ist within a fraction of 1/100th millimeter.

Wrong, it is just as easy to focus on a star at f2.8 as it is at f16 because there is in effect no depth of field. This is because to a camera lens or even a high power telescope all the stars and galaxies in the sky are so far away that they are all at infinity. Since they are all so far away the camera sees them as if they were all at an equal distance. If you use Fcalc program you will see that if your using a 50mm lens(this does not change much larger lens at these distances) and say a subject focus distance of 10 million miles (much closer than even the closest star our sun and most if not all planets) and you set your aperture to f2.8, then everything from about 42 meters to infinity will be in focus. Stopping down the lens to f8 you then get everything from about 7 meters to infininty in focus, not much difference at all and if the true numbers of even the nearest stars( 4 light years) are used and even a large telephoto, the difference will even be smaller, so f stop does not matter at all when focusing on or getting stars in focus. The main thing is to not focus beyond infinity( as viewed through the lens, not the reference mark on the lens)

A good approach to get sharp stars is to use the AF on a very
bright star then shut the focus down and make your picture. This is
not the most prcise way but much better than putting the lens on

I agree, but the best way is to just manually focus on a bright star or even the moon and then everything in the night sky will be in focus.


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