starry nights with wide angle/short fl

[max10]

Hello all,
First post as a newbie.

I'm a member of an astronomy forum, and a member there takes amazing pictures of the night sky with the Milky Way as a prominent subject. Here is the equipment he uses:

Camera: Canon EOS 600D (rebel T3i)
Lens: Tokina SD 11-16mm F2.8 (IF) DX AT-X Pro
Shutter Speed: 30 seconds
Aperture: F2.8
ISO: 3200
White Balance (WB): 4000K

Remote: Canon RS-60E3
Tripod: SLIK U8000
Editing Software: Windows Live Photo Gallery

His pictures are from a static tripod with 30 second exposures, and there are no star trails! (Long exposure shots of the night sky from a static mount will always show the motion of the stars as star trails).
When I asked him about it he said there were no star trails because he used a wide angle lens at a short focal length.
Here are his exact words:

"...why there isn't any star trails on the pictures that I've taken, and they all are 30 seconds or 1 minute exposure is because of the REALLY wide focal length on the lens. The photograph in this thread was taken with my Tokina lens at 11mm. But, since my 600D (Rebel T3i) is a crop camera and not a full frame camera, the real focal length of the lens is 17.6mm at 11mm. 11 x 1.6 = 17.6 So you really need a wide angle lens if you want to get no star trails when taking a long exposure picture..."

Can anyone explain to me how a wide angle lens, and a short focal length can simply "cancel" 30 seconds' worth of star motion?
If you could explain it so a dummy can understand, it would be appreciated.

[PhotoByTrace]

The wide angle lens/ short focal length is to do with your field of view.

Assume the earth's rotation "moves" the star 1 degree during your exposure. With a wide angle lens your field of view can be more than 100 degrees. So moving one degree covers a small percentage (1%) of that frame. Whereas, with a long tele lens, say 200mm the field of view is usually 10 degrees. So the star still "moves" 1 degree in the same amount of time but that is now about 10% of your frame.
So if you are trying to avoid visible star trails, the wider your field of view the better.

Lots of places quote the 600 rule... 600s / focal length (35mm equiv) will give you a maximum time for your exposure. And is a good rule of thumb.

However it also depends a little on the location of the stars and your earthly location, as the earth being a sphere those at the equator move further than those at the poles as it rotates. Stars very near the pole are almost stationary in the sky, while stars closer to the equatorial plane move quite fast (hence the circular/oval shapes to star trails).

All stars move about 15 degrees per hour in right ascension ("horizontally"), but do not move in declination ("vertically") [Sorry I know that isn't exactly plain english ]. But essentially it means that there are also more complicated equations for exposure that will also consider the star's declination from the celestial equator/pole.

[ajohnw]

I took a series of 10 sec exposures recently and the figures may be of use to you. Viewed at full resolution the stars were elongated by a factor of about 2. That was with a 50mm lens. You should be able to scale those figures but remember this is at 100% fill size resolution. I would suggest that you take some trial exposures as the "rule" that is usually quoted doesn't make much sense. ISO ratings also needs looking into. Some say max but noise characteristics of cameras vary.

Reduction reduces apparent trailing this for instance is a 1/5 size reduction where the elongation is about a factor of 2. Straight from raw and no noise reduction or anything else. This aspect has a direct bearing on how things finally look.


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[max10]

The wide angle lens/ short focal length is to do with your field of view.

Assume the earth's rotation "moves" the star 1 degree during your exposure. With a wide angle lens your field of view can be more than 100 degrees. So moving one degree covers a small percentage (1%) of that frame. Whereas, with a long tele lens, say 200mm the field of view is usually 10 degrees. So the star still "moves" 1 degree in the same amount of time but that is now about 10% of your frame.
So if you are trying to avoid visible star trails, the wider your field of view the better.

Lots of places quote the 600 rule... 600s / focal length (35mm equiv) will give you a maximum time for your exposure. And is a good rule of thumb.

However it also depends a little on the location of the stars and your earthly location, as the earth being a sphere those at the equator move further than those at the poles as it rotates. Stars very near the pole are almost stationary in the sky, while stars closer to the equatorial plane move quite fast (hence the circular/oval shapes to star trails).

All stars move about 15 degrees per hour in right ascension ("horizontally"), but do not move in declination ("vertically") [Sorry I know that isn't exactly plain english ]. But essentially it means that there are also more complicated equations for exposure that will also consider the star's declination from the celestial equator/pole.

Thanks Trace,
That was very informative and I actually understood everything.
I've been an amateur astronomer for a while, so RA and DEC are concepts that I understand, but I hadn't thought of the Earth latitude aspect, and you are right.
This fellow is in Finland, so his rotation factor is much less than if he was closer to the equator.

So if I understand everything, it means that his pictures do have star trails, but being only around 1% of the total image, they are in effect, unnoticeable.
That is really interesting.

Thanks again.

[Bobobird]

This is an interesting article

http://blog.starcircleacademy.com/2012/06/600-rule/

[PhotoByTrace]

Thanks Trace,
That was very informative and I actually understood everything.
I've been an amateur astronomer for a while, so RA and DEC are concepts that I understand, but I hadn't thought of the Earth latitude aspect, and you are right.
This fellow is in Finland, so his rotation factor is much less than if he was closer to the equator.

So if I understand everything, it means that his pictures do have star trails, but being only around 1% of the total image, they are in effect, unnoticeable.
That is really interesting.

Thanks again.

Glad it helped, and yes, his pictures have trails. So when the image is at low res and small for a computer screen, the 1% is very small and most likely not noticeable. A 100% crop of his image or printed large at full res may show those trails more obviously. The article that Bobo posted talks about this in more depth too and that gives more insight into John's photo above and the information John gives to accompany the photo.

[max10]

Glad it helped, and yes, his pictures have trails. So when the image is at low res and small for a computer screen, the 1% is very small and most likely not noticeable. A 100% crop of his image or printed large at full res may show those trails more obviously. The article that Bobo posted talks about this in more depth too and that gives more insight into John's photo above and the information John gives to accompany the photo.

Thanks. All the answers to my post as well as that very pertinent article were most helpful.
It's much appreciated.

Cheers.