North Star Long Exposure Help?

[H4X1MA]

So I'm trying to create one of those north star spirals, but I have no idea how to set this up. Last night I went out, did a 13 minute exposure at f16, ISO100 and ended up with black and a few speckles. Upon bringing it into Camera RAW I could salvage a spiral, but not without insane grain (pushing the image limits?). I plan to try this again tonight, or tomorrow, and since my last trip was a complete fail; I gained no experience.

I know that a higher ISO would be better, but I also know that a higher ISO is going to introduce more grain.

Maybe my location is just bad; I'm on top of a mountain, overlooking a pond, with little to no light pollution. I would think that this would be ideal, but maybe I'm just wrong?

Can some vets give some pointers?

 

[TCampbell]

Her'e a great article: How To Photograph Star Trails - Photography by James Vernacotola

 

[KmH]

Open up the lens aperture.
You don't need f/16.
The stars are effectively at infinity, so DoF is infinitely deep even when using wide lens apertures.
Use the widest aperture that gives the sharpest focus.

Your location sounds like a good one.

Digital images don't have 'grain'. They have image noise.
Long exposures cause the image sensor to heat up and a lot of the image noise is thermal noise.
Consequently, many star trail images are now made by 'stacking' a series of much shorter exposures.

How to photograph star trails - Bing

 

[20belowphotography]

Hey,

Much like KmH mentioned, DoF is not important, but that of course doesn't mean that your focusing is not important. Lately, I've been almost exclusively shooting star trails. You can check out my portfolio at 20belowphotography.com. I've always found that stacking shorter exposures works out much better for me than doing a long exposure. With a single, long exposure it's kind of hit or miss, whereas if you do shorter exposures, you can have a better feel for how it's going to turn out. Here's a link to the tutorial where I learned how to do the stacked method. It might take you a few tries, but in the end you'll get it.

 

[H4X1MA]

Thanks everyone, I'm going to go with the guide in TCs post as a ref. The only thing that I don't get is how he manages to get a decent exposure at 4 seconds, and then adjust accordingly. It seems like at 4 seconds you would end up with darkness and nothing much else.

Secondly, it seems that the author said that a 3 second delay between shots was too much.. but on my little D3100 it takes longer than 3 seconds to process the image before it will shoot again. This may pose a problem, or maybe I am understanding it wrong.

We'll find out.

 

[KmH]

It's taking longer than 3 seconds because of features you have enabled on your camera.
Like Long Exposure Noise Reduction, which makes a 3 second exposure with the shutter closed so the image processor CPU can map the dark current image noise and subtract it from the original 3 second exposure. When Noise Reduction is 'On' - high ISO settings or exposures longer than 8 seconds cause the above.

When LENR is being done you will see "Job nr" flashing in the camera viewfinder.

See page 134 of your D3100 users manual.

 

[TCampbell]

Thanks everyone, I'm going to go with the guide in TCs post as a ref. The only thing that I don't get is how he manages to get a decent exposure at 4 seconds, and then adjust accordingly. It seems like at 4 seconds you would end up with darkness and nothing much else.

Secondly, it seems that the author said that a 3 second delay between shots was too much.. but on my little D3100 it takes longer than 3 seconds to process the image before it will shoot again. This may pose a problem, or maybe I am understanding it wrong.

We'll find out.

His 4 second exposure was a "test" exposure. We do this in astrophotography all the time. What we WANT to know is if we've nailed the focus and exposure. So we CRANK the ISO to the max setting (which is normally horrible image quality) to test our framing, focus and exposure. THEN... when we're happy that we've got a good exposure, we run the ISO back down to something reasonable, but we also count the number of "stops" (e.g. suppose you went from ISO 6400 down to ISO 800... that'd be 3 stops (ISO 6400 -> ISO 3200 -> ISO 1600 -> ISO 800). That means we have to change the shutter speed by the same number of full stops (4 seconds -> 8 seconds -> 15 seconds (a round-off of 16) -> 30 seconds.) So a 4 second ISO 6400 exposure and a 30 second ISO 800 exposure are basically "equivalent" exposures but the ISO 6400 exposure is going to have a "helluvalot" -- that's a technical industry term ;-) -- of "noise".

Don't shoot at 4 seconds... TEST at 4 seconds. When you're done with "frame, focus, and exposure" testing... go to a reasonable ISO and adjust the shutter speed longer by the same number of stops that you reduce the ISO sensitivity.

Don't forget... we definitely want to check out your results (good or bad... no cheating!) Let us know how this is working out for you.

 

[H4X1MA]

Well I just got back from trying, I'll throw it together and try to post results tomorrow. I'll have to shut off noise reduction next time like KmH said to. My main issue tonight was trying to focus. I tries my 8-16mm first so I could get a mountain view with the stars, but I couldn't even make out stars in the setup. Then I tried my 70-200, but aiming at Polaris would give me no scenery (still only managed to focus it once in 20 mins). I landed back on the 8-16, jacked it all the way open with 4 seconds on HI2 (blah). Ended up at 30 seconds, all the way open, ISO 400. Not sure how that's going to do, but we'll see tomorrow

 

[kja6]

There is definitely some great information in this thread. OP, hope your stuff turned out this time around!

 

[DGMPhotography]

Well I just got back from trying, I'll throw it together and try to post results tomorrow. I'll have to shut off noise reduction next time like KmH said to. My main issue tonight was trying to focus. I tries my 8-16mm first so I could get a mountain view with the stars, but I couldn't even make out stars in the setup. Then I tried my 70-200, but aiming at Polaris would give me no scenery (still only managed to focus it once in 20 mins). I landed back on the 8-16, jacked it all the way open with 4 seconds on HI2 (blah). Ended up at 30 seconds, all the way open, ISO 400. Not sure how that's going to do, but we'll see tomorrow

Once you've got everything in focus, switch it to manual focus so it won't change when you try to shoot again, if you're not in manual already, that is.

 

[20belowphotography]

agree with above posts. i test at iso 3200 to get my (manual) focus set correctly and make sure my composition is the way i'd like it. don't rely on trying to see it through your viewfinder. as a general rule of thumb that i've come to find when i do my star trails is to shoot wide open for 30 seconds or 1 minute at iso 200 (and no in-camera noise reduction) and adjust accordingly. i don't like to shoot above 1 minute (because i stack them), so if i need it brighter i'll raise the iso (but not above 400 as that's even too grainy for me). the first step if you're doing this method is to only expose for the sky. after your allotted stack of exposures is complete you then take one photo at the same settings with the lens cap on to cancel out the noise reduction. then, turn your in-camera noise reduction back on and expose only for the foreground. in your computer you'll stack the photos and use a layer mask to get the foreground. if you're still having a hard time, send me a PM and i'll video the steps next time i do one

 

[H4X1MA]

Here's attempt #1. Kind of noisy, I think due to processing the RAW files in order to make the stars show better. I also lack a decent subject, so I'll have to try to find a new spot. I will say that it came out better than I thought it would.