Star trail help

[Nass]

Looks to me like the trails are somewhat spot-like: I don't think that you used the correct shooting mode. What ISO, mm & f stop?

[victor]

Have checked the rather dark photo in CS5 and raised levels to produce this quick view. Yes star trails are there. Looks like an exposure problem. What ISO, Shutter Speed and F stop was used. Is it a single or Mutli Exposures stacked ??

[Mongwopman]

Canon EF-S 18-55mm f/3.5-5.6 [II]
Shot at 31 mm
Manual exposure, 15 sec, f/5, ISO 200

Multiple shots stacked using startrails.exe

[Nass]

Canon EF-S 18-55mm f/3.5-5.6 [II]
Shot at 31 mm
Manual exposure, 15 sec, f/5, ISO 200

Multiple shots stacked using startrails.exe

Hello, looks a bit on the low side. What I'd do if I were you is do 5 min experimentation at home with 31mm, f5, 30s, ISO 800 and also 31mm, f5, 30s, ISO 640, and make 2x5 minute trails. See if they become trails rather than spots. If they don't we'll try something different. Doesn't matter about the composition at this stage, it's about getting the exposure right =)

[Mongwopman]

Is there any post production that i could do on it to make it look better?

[victor]

Damien,

Re post production. Try a levels adjustment by moving the right hand slider way to the left to meet the shadows. There is not much else you can do as your histogram is stacked well to the dark side.. very black colours. This will give your photo a presence of star trials but will introduce noise.

You will then see the shadows of trees ? in the foreground see thumbnail below.



Regards

David aka victor

[Mongwopman]

Yeah, i did try to get the tops of the trees to add a bit of forground for depth, obviously this did not work well.

[Nass]

No, I think this one is a lost cause, chalk it down to experience and use it as an excuse to do some further testing & learning =)

[Nass]

The problem here was the ISO and exposure time

ISO ==> needs to be much higher, 640ish, like I said
Exposure time ==> use 30s like I said

I did a short comparison to show you tonight, trying to use same settings as you. At 200/15s I get the same, dots rather than trails. Not quite as dotty as yours, but still, clear gaps (imo). Remember that you want the "non photographing" gap between exposures to be as small a % of what you expose as possible and on 15s the 1/2s or so it takes between exposures is noticeable. Less so at 30s, they (almost) seamless. So at the suggested settings it works much better. The bottom one is resized to same dimensions as yours to show the continuity (all of a massive 1 min 30s worth!).



Other startrail information here. Those are without any levels or contrast adjustment, it's easy to take the bottom one and make it darker. But to take the topleft one and try to make the trails continuous or brighter introduces all manner of yuck as Victor showed you.

[Nass]

Here - you might also find this useful 400d startrails on Flickr. people usually put a blurb at the bottom of their shots which will give you an idea of what various ISO, MM & f stop settings should produce for you.

[Colin Southern]

Is there any post production that i could do on it to make it look better?

Did you shoot RAW or JPEG?

[Mongwopman]

I shot in Jpeg im afraid. Did it in jpeg so i would be able to stack in startrails.exe

[Nass]

Colin - Yes I see where you were going with RAW but my experience with startrails underexposed by as much as this is that even when you up the EV in a RAW editor, it still makes for a bit of a writeoff - the artifacts and noise start to deteriorate what you end up with to too great an extent, sadly =(. It also doesn't help eliminate what I consider the bigger flaw, the dottyness rather than continuousness - that's due to 15s vs. 30s exposures.

Mongwopman - For what it's worth, although I'm experienced enough with my settings to go straight with JPG nowadays, I used to shoot in RAW and then batch convert into JPG based on fiddling with one to get the right setting. It is VERY laborious though, and kept my computer churning away for many an hour processing.So what Colin says is spot on, if you've got them in RAW and you've got the software, fiddling about with on to get the right setting and mass converting the RAWs can tip something in the right direction if you're initially close enough.

[Colin Southern]

Colin - Yes I see where you were going with RAW but my experience with startrails underexposed by as much as this is that even when you up the EV in a RAW editor, it still makes for a bit of a writeoff - the artifacts and noise start to deteriorate what you end up with to too great an extent, sadly =(. It also doesn't help eliminate what I consider the bigger flaw, the dottyness rather than continuousness - that's due to 15s vs. 30s exposures.

Until I knew if it were shot in RAW then I can't tell if what we're seeing is true under-exposure, or just poor processing. F5 @ ISO 200 should have been fine (or at the outside, perfectly recoverable). Exposure duration doesn't affect the exposure because the light source is moving.

With regards to 15 sec -v- 30 sec exposures, I wouldn't do either - better to just make it one continuous one using bulb mode; noise isn't an issue because the scene is so high contrast anyway, that it's easy to kill any noise by raising the black cliping point.

[Nass]

What user-selectable 'scene' type settings does a Canon 400d use to process the JPG, like "bright" or "velvia" or "vibrant" etc? That will have a bearing.

In terms of ISO, I know what works for me with stacked startrails. Higher ISO gives you a stronger signal on the faint data but it'd blow the image out if it were a single exposure. On 30s there just isn't the buildup of excessive sky light. So your 200 ISO approach is probably rooted in that. 200 ISO stacked startrails definitely do exist but the prevalent modus operandi for stacked, typically 30s, is usually 400-640. Of course if you have a lower f stop lens then you can also have a lower ISO.

It's a lot more work doing it stacked though, but it's also safer - if you have a problem you might lose one of the sequence rather than write off the entire image. Ie aircraft in sky, clouds, torch, battery running out etc.

[The Blue Boy]

Johan,

Finally had a bit of clear sky to practise this technique. Worked a treat. I only stuck the camera setup through the kitchen window, so the actual picture is rubbish but the technique is sound. My only other concern was then to stack them. I thought it might take forever using Startrails.exe. Again, worked a treat and only took about a minute to stack 160 images. I'm a very happy bunny. We're off to Crete at the end of the month and can't wait to shoot some real sky.

[Colin Southern]

Higher ISO gives you a stronger signal on the faint data

Yes and no. In terms of potential image quality, it's all about signal to noise ratio ... higher ISOs get you more "signal", but you also lose a stop of DR for each stop the ISO is increased (ie the noise floor increases by a stop) (and partial ISO stops are also a double-whammy in that the ISO gain is generally set to the lowest full stop, and then the data is adjusted digitally for the remainder, which also increases the noise more than it would had one selected the next higher ISO full stop).

Startrails are extreme contrast scenes (or they can be pushed that way in post), so one can do things that one can't normally get away with.

[Nass]

Yes, of course higher ISO increases noise, but if it's a choice between seeing virtually nothing and seeing something the latter is usually prefereble. Ie between the topleft and either the middle or topright of the comparisons above.

Damien, I've done a diagram for you which explains something that I'm often asked, the direction of the startrails that you see. So if you're pointing north you'll see a small trail with centre (so your exposures actually need to be longer) like the top middle pretend view of grass, tree and sky.



I wasn't 100% happy with it so I've done a better one, I'll put that on the main startrail thread in due course. Also more details like it matters where on our planet you are etc.

[Colin Southern]

Yes, of course higher ISO increases noise, but if it's a choice between seeing virtually nothing and seeing something the latter is usually prefereble.

I find it doesn't actually make a lot of difference to be honest; Perhaps a little (to see what you've captured, using the camera's LCD), but I find startrails are something I tend to do "by the numbers" - the real results are only seen in post-processing, and to that end, the same amount (or more) info is probably captured at the lower ISO. Because it's relatively high contrast, it's relatively easy to drive the signal and the noise apart.

I'm not trying to argue by the way - I think in many ways it doesn't make a big difference - just saying that in Damien's case the apparent under-exposure shouldn't have been an issue with a RAW capture. With a JPEG capture it's beyond recovery though, because too much of the needed DR has been discarded. Or put another way, "If one shoots RAW then ISO selection is relatively unimportant" but "if shooting JPEG it'll make a much bigger difference" but "RAW is going to win over JPEG" everytime.