How to Avoid Star Trails by Following the ‘500 Rule’

Sirius Glass · Nov 30, 2017

https://petapixel.com/2015/01/06/avoid-star-trails-following-500-rule/

Skiver101 · Nov 30, 2017

Got me wondering if there is a similar >/< number rule that can be applied to the Moon's travel.

JP

Sirius Glass · Nov 30, 2017

Skiver101 said:
Got me wondering if there is a similar >/< number rule that can be applied to the Moon's travel.

JP

Yes, but I do not know the rule.

Maris · Nov 30, 2017

The moon appears to travel across the sky by its own diameter in two minutes. The size of the moon's image on the film is about the lens focal length in millimetres divided by 100. How much blur is too much? Depends.

AgX · Nov 30, 2017

First time I hear of this rule.
(Maybe I read it over, or it never was in my books.)

Punker · Nov 30, 2017

So for medium and large format I guess you first have to find the 35mm equivalent of your lens and then do the calculation.

Sirius Glass · Nov 30, 2017

Punker said:
So for medium and large format I guess you first have to find the 35mm equivalent of your lens and then do the calculation.

Yes, I believe so.

AgX · Nov 30, 2017

Or do the calculation and get an appropriate XXX-Rule.

Poisson Du Jour · Nov 30, 2017

Sirius Glass said:
Yes, but I do not know the rule.

See: http://calphoto.com/moon.htm
#The Moon is a Moving Target

Helinophoto · Nov 30, 2017

I have read something similar, and I find that chart in the link to be incorrect.

I often photograph the aurora with my 5D MK III (full-frame digital) and I normally use a 17-40 lens to do this.
I've found that if the exposure is any more than around 15-16 seconds, I start to get trails.

The article's formula says around 30, which could create god-awful images of the night sky.

Does anyone know if there are difference to which latitude you photograph from?

Sirius Glass · Nov 30, 2017

Poisson Du Jour said:
See: http://calphoto.com/moon.htm
#The Moon is a Moving Target

Thanks. I knew about the exposure starting with Moony 11 for a full moon and then compensating for the phase, but not the shutter speed specifically, just and estimate.

Jim Jones · Dec 1, 2017

Sirius Glass said:
Thanks. I knew about the exposure starting with Moony 11 for a full moon and then compensating for the phase, but not the shutter speed specifically, just and estimate.

As bright as the moon is, we would normally use high enough shutter speeds to stop the movement of the moon across the film. With my 1000mm f/11 lens and ISO 100, I would bracket the full moon exposure around 1/60 second. Other photographers generate other rules, often based on their tolerance for mediocre sharpness. Depth-of-field rules also often demonstrate this.

Another consideration: stars near the celestial pole move slower than those at the celestial equator. Ordinary rules don't adjust for this. Mathematically inclined photographers may enjoy generating their own rules that take into consideration the performance of their lenses, the film or sensors, the degree of enlargement of the image, the photographer's criteria for sharpness, the stability of the camera mount, and other factors.

Poisson Du Jour · Dec 1, 2017

There would be expected variations, quite large and ambiguous, to the point of questioning accuracy, given that Conrad's article was written for film (around 2000) before today's very, very sensitive digital camera sensors came into being. ISO, lens, sensor, brightness all play into the equation. Personally the stuff doesn't interest me as I carry out intervalometric star trails overnight while I am sound asleep in the sticks! Star trails always look different and colourful.

Bud Hamblen · Dec 2, 2017

Helinophoto said:
Does anyone know if there are difference to which latitude you photograph from?

Your latitude makes no difference at all.

The declination of the star does make some difference because stars near the celestial equator appear to move faster than stars near the celestial pole. It varies by the cosine of the declination. The declination is zero degrees for a star at the celestial equator and 90 degrees for one at at the celestial poles. The cosine of zero degrees is one and the cosine of 90 degrees is zero.

Include the landscape in your image to get a pretty picture, and be far from city lights.

How to Avoid Star Trails by Following the ‘500 Rule’

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