Sigma 24mm f/1.4 Art announced

[PhotoByTrace]

Saw the announcements for this new lens: http://www.sigmaphoto.com/product/24mm-f14-dg-hsm-a

I'll look forward to reviews of its wide-open performance. If the coma is well controlled I may consider this for night sky photos. I'd prefer a little wider, but will take a closer look once more is revealed about the lens.

[NorthernFocus]

I've never done any of that type of photography. Is the wide open aperture an advantage to keep ss down and minimize star trails?

[PhotoByTrace]

I've never done any of that type of photography. Is the wide open aperture an advantage to keep ss down and minimize star trails?

Hi Dan,
Yes, is the simplest answer. The faster the lens and the shorter the focal length, the better, if you don't want streaks to be apparent, but would still like to see some of the surrounding landscape.

Most people suggest the "500 rule of thumb" to calculate the longest exposure before streaking. So 500/lens focal length (at 35mm equiv). That would be 20 seconds for a 24mm lens such as the Sigma above; the faster the lens the less ISO needed to get a good exposure on a dark night in that length of time.

Feel free to stop reading here
OR
If you wish for more details than you ever wanted to know keep reading, but remember that you were warned

My experience was that my photos look fine viewed online at about 1200px using the 500 rule. However I was seeing streaking when printing full-size and looking closely (as probably only photographers themselves would ever do ). Instead of simply deciding to lessen my exposure by a few seconds, I challenged myself to try to understand when streaking becomes visible. So I figured knowing how many seconds I had before the motion would pass beyond one pixel would be a good place to start and that considering the principles used for the circle of confusion might also help.

This is what I learned...

The time it takes for a star to streak when viewed at 100% depends on:

• the declination of the star in the sky (the further from the celestial poles, the faster "it moves"),
• your latitude on earth (how fast the earth is moving under your feet),
• the lens focal length (field of view),
• the "size" of your camera's pixels (sensor density / mm).

Most of the workings I've come across assume worst case scenario of being located at the equator, where the earth is moving fastest, rather than factor in a specific latitude.

The most commonly used (other than the 500 rule) equation for establishing star trail length is
length(mm) = [time(secs) x Focal length(mm) x cos(delta)]/13750 where delta is angle of declination

If we take the longest paths that occur at the celestial equator then delta = 0^o and cos(0)=1

13 750 is derived from the angular speed that the earth travels which is 360^o per 24 hours OR 2.Pi radians/86,400 seconds which is standardized as 7.27x10^-5rads per second.

On the full-frame D610 with 6000 x 4000px and a sensor of 36 x 24mm, I have an inter-pixel distance of 0.006mm. I can use that to establish a minimum length of distance before a trail might become apparent when pixel peeping.

(.006mm x 13750)/24mm = time (secs) to pass through a single pixel.

So for this Sigma 24mm it is 3.4 seconds to pass through a single pixel. That establishes a minimum for me.

If instead, the sensor distance for the circle of confusion is used of 0.025mm

(.025mm x 13750)/24mm = 14.3 secs and about 4 pixels worth of streaking.

To me that is a better rule of thumb and outcome than the 500 rule which gave me 20 seconds and 6 pixels worth of streaking. BUT it also means I need good high ISO performance on the camera and a fast lens. All of which is the VERY LONG answer

[JR1]

Saw the announcements for this new lens: http://www.sigmaphoto.com/product/24mm-f14-dg-hsm-a

I'll look forward to reviews of its wide-open performance. If the coma is well controlled I may consider this for night sky photos. I'd prefer a little wider, but will take a closer look once more is revealed about the lens.

News comes here first

http://sigma-rumors.com/2015/02/pre-...porary-lenses/

[NorthernFocus]

Thanks, Trace. I'm an engineer so equations are never boring when they apply to real life. Your explanation was pretty straight forward. One of these days I'm going to have to get myself motivated enough to get out and shoot the aurora. It's just so darn cold when they are prime for shooting

[PhotoByTrace]

News comes here first

http://sigma-rumors.com/2015/02/pre-...porary-lenses/

Thanks Jeremy.

Thanks, Trace. I'm an engineer so equations are never boring when they apply to real life. Your explanation was pretty straight forward. One of these days I'm going to have to get myself motivated enough to get out and shoot the aurora. It's just so darn cold when they are prime for shooting

Hi Dan, Thank goodness I offloaded that spiel onto an engineer

As for capturing the Aurora, I can't even begin to imagine quite how cold, and the extra precautions needed for person and gear if shooting outdoors for extended periods. My limited experience of that kind of cold is a couple of winter holidays in Finland and if we were outdoors we were active and moving and it still chilled my bones.
Have to admit I'd be pretty motivated to give the aurora a try though. I've considered heading to Tasmania a couple of times for the Southern Lights, though the photographers I know that have travelled to try haven't gotten lucky in their timeframes; need to be able to plan flexibly for the CMEs it seems.

[James G]

Excellent exposition Trace, I'm an ex Chemist & IT Systems designer.... Equations are good... More to the point I like the idea of being able to tie exposure calculations to latitude....

[DanK]

Trace,

A very helpful explanation. Thanks for posting it.

Dan

[JR1]

Thanks Jeremy.



Hi Dan, Thank goodness I offloaded that spiel onto an engineer

As for capturing the Aurora, I can't even begin to imagine quite how cold, and the extra precautions needed for person and gear if shooting outdoors for extended periods. My limited experience of that kind of cold is a couple of winter holidays in Finland and if we were outdoors we were active and moving and it still chilled my bones.
Have to admit I'd be pretty motivated to give the aurora a try though. I've considered heading to Tasmania a couple of times for the Southern Lights, though the photographers I know that have travelled to try haven't gotten lucky in their timeframes; need to be able to plan flexibly for the CMEs it seems.

Also note Canon rumors
Sigma Rumors
Nikon Rumors