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PostPosted: Wed Jan 24, 2007 1:22 am 
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Hello! To kick off our new Technical and Scientific photography forum, here’s some basic tips on taking wider shots of the night sky with normal lenses.

The shot below of the Milky Way from the Southern Hemisphere was taken with relatively normal equipment: a Canon EOS 400D / Rebel XTi with the Canon EF-S 17-55mm lens. The camera was set to Manual mode with a 30 second exposure, f2.8 aperture and 1600 ISO sensitivity. The camera was mounted on a tripod, the lens manually focused to infinity and the shutter fired using the self-timer.

Image

Here’s more detail on the kind of settings you should try out for similar shots.

1: Manually focus your lens to infinity, and check that star images are pin points by playing back and zooming in afterwards.

2: Open your aperture. Astrophotography is normally about gathering as much light as possible, so set your aperture wide open using the smallest f-number. That said, many lenses perform better when slightly closed, so if the object is sufficiently bright, you may wish to experiment by slightly closing the aperture - for example, an f1.8 lens may produce sharper, more pin-pointed star images when closed to, say, even just f2.0. It'll also be slightly more forgiving on your focusing. But the downside is less light gathered.

3: Use long exposures. Again, to gather plenty of light, manually set your shutter speed to several seconds. There are a few things to watch out for though. Due to the rotation of The Earth, the stars appear to move across the sky and will leave a trail on long exposure photos. The effect can be quite attractive, but to avoid it, either keep exposures sufficiently short, or if you're really into astrophotography, use a special motorised mounting which follows their motion, thereby effectively keeping them still.

Assuming you're shooting from a fixed tripod though, I generally find you can avoid star trails by keeping exposures under 20 seconds when using an effective focal length of 50mm. At 25mm, you could therefore do 40 seconds, and at 100mm, 10 seconds. As you take pictures further from the pole though, the trailing effect is greater, so you’ll need shorter exposures to avoid it.

Obviously exposures of this length require a steady tripod and either the self-timer or a cable release to take the shot without wobbling the camera.

4: Increase the ISO. Another way to maximise the light entering the camera is of course to increase the ISO, although this will in turn increase undesirable noise levels. If you want to keep exposure times short though or capture particularly faint objects, higher ISOs are normally necessary.

5: Set your white balance to daylight, as Auto settings can produce unfaithful colours.

6: Experiment! Try a variety of exposure times, focal lengths and ISO settings to see what produces the best results for the object you’re shooting under the conditions of the day (or evening!). Unless you live (or are photographing) miles away from civilisation, you will have to deal with light pollution from street lamps which will turn your sky orange or green. If this is plaguing your photos, you may have to reduce the exposure time or ISO setting to retain a dark-looking sky.

Have fun!

Gordon Laing


Last edited by Gordon Laing on Sat Nov 15, 2008 7:55 pm, edited 1 time in total.

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PostPosted: Wed Jan 24, 2007 9:40 pm 
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Nice shot Gordon. I am very envious. Thanks for the tip about setting the white balance to daylight. I must give it a try. I'd also like to put a few rifle rounds through my local sodium street lights but that's another story...

As you point, out a remote switch is essential to avoid camera shake at all but the shortest focal lengths. The RS 60E3 Remote Switch fits the bill for my Canon 400D perfectly as it is fly by wire.

My more modest efforts so far have been at the prime focus of my Televue Pronto with a motorised mount but your shot has made think again about my original intention to buy a fast fixed focus wide angle lens and do some big sky astrophotography as well.

The EF 35mm or 24mm f1.4 L USM lenses will have to remain on hold until the lottery win so I guess the EF 35mm f2.0 looks a hot contender. Does anyone have experience with lens this or have recommendations for something else?


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PostPosted: Wed Jan 24, 2007 11:37 pm 
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Hi Bob, glad you like the pic!

As you know, a fast bright lens is invaluable when it comes to astrophotography. Most people know an f2.8 lens can gather twice as much as one at f4, or four times as much as one at f5.6, but it's easy to take it all for granted in normal photography. After all, doubling your exposure time from 1/500 to 1/250 or even quadrupling it to 1/125 isn't that big a deal for most static or daytime subjects.

But if you're talking about doubling or quadrupling a 10 or 30 second exposure - well that makes a big difference. Especially since the stars begin to trail at a certain point and you're already at the highest ISO.

Then there's the dark frame removal for noise reduction on modern DSLRs, which normally take the same time as the exposure itself. Again not a big deal if the exposure is only a few seconds, but after taking that five minute comet photo in my other post, I had to wait a further five minutes just for the NR to be applied - and the comet was setting fast! Having one extra stop on my lens would have halved that time. It was certainly a big difference going from the 85mm f1.8 to the 17-40mm f4.

This is why specialist photographers spend so much on bright lenses! Same with sports and wildlife where you need a certain shutter to freeze the action, but can't increase the available lighting! Once you're already at your highest ISO (or the highest you're willing to use) the only way to get more light is a lens with a faster, brighter aperture.

I actually used to have the Canon 35mm f2.0, but never used it for astro photography so can't comment on the quality; it was good for general use though! Stars are tough challenge for all lenses, especially towards the corners of the frame. I tried the pricey 35mm f1.4L for the Aurora shot in my 5D review - see:

http://www.cameralabs.com/reviews/Canon5D/page5.shtml

It was certainly a very bright lens and invaluable for capturing the faint aurora, but the stars weren't perfect dots in the corners, even once you'd taking trailing into consideration. This is often par for the course on a wide angle though. I was impressed by the 85mm f1.8 though.

But even after all that aperture envy, I've taken many fun shots of the milky way just using budget DSLRs with the kit lens set to 18mm, the aperture wide open at f3.5, the focus manually set to infinity, the camera ISO to 1600 and the exposure to 30 seconds, triggering it with a self-timer to avoid wobbles. If the sky's dark, I can't resist giving it a shot!

Show us some of your pics taken with the Pronto!

Gordon


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PostPosted: Thu Jan 25, 2007 12:57 am 
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Thank you for the link to the 35mm f1.4L aurora pictures. A lovely lens but not for me, especially with so few really clear nights to justify the price. Nearly at the end of January and we seem to have had precious few frosts here in the UK this winter.

I'll decline your kind offer to post any of my pictures. I really meant it when I said modest! My astronomical interest has always been on the theoretical side and that would have been my career if I hadn't got side-tracked. Playing about with the telescope and camera is just fulfilling something I started 40 years ago and gave up when I left school.


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PostPosted: Mon Apr 09, 2007 9:36 am 
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There were beautiful night skies in New Zealand, but unfortunately I didn't manage to get one decent shot.
Tried ISO 1600, 30 sec, F3. All the bright stars shine up and are sharp BUT no colorful milkyway, no nothing. Not even with postprocessing. I'm so disappointed!

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PostPosted: Mon Apr 09, 2007 9:16 pm 
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Hi Thomas, sorry you didn't get any milky way shots!

You will need a fairly dark location and moonless sky for the kind of shot at the top of this posting, but otherwise you should be able to get it with the settings you used. If it's not that dark, or your eyes aren't adapted to the light, it's easy to slightly mis-position your camera and miss the milky way - are you sure it was passing directly across the frame?

I'd also recommend only shooting at wide angle unless you have a mounting which can track the sky.

Gordon


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PostPosted: Tue May 15, 2007 8:51 am 
I was wondering what happens if you use sensor-shift stabilization when doing astrophotography. I'm not sure how exactly sensor-shift works, if it calculates shifts based on the frame or uses a gyrometer of some sort. Would you ultimately get a sharper image of the stars (no trail caused by earth rotation) or would it have no effect?


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PostPosted: Tue May 15, 2007 10:26 am 
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That's an interesting question.
I assume, they use gyro-sensors and cannot track the movement of an object.
If you move the sensor based on picture information it would mean you have to detect "pixel-shift", while shooting. That would certainly lead to at least one pixel blur over the complete image.

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PostPosted: Tue May 15, 2007 11:22 pm 
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Hi Rune, that is an interesting proposition, as in theory a CCD-shift system *could* counteract the apparent motion of stars for a short period, but sadly it's not possible in practice.

These anti-shake systems are controlled by internal gyroscopes and motion sensors, so they rely on the actual camera body being moved.

Also, to track the stars in astro-photography you need to rotate the camera on an axis which is aligned with the pole - and the rotation needs to be very slow - one revolution every 23 hrs 56 mins and 4 secs to match the Earth's rotational period.

So astrophotography tracking mounts are quite specialist pieces of kit, and the operator also has to perform the polar alignment. But some mounts are relatively affordable and you certainly get better at polar alignment with practice.

Gordon


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PostPosted: Tue May 29, 2007 2:24 pm 
You can get that amazing shot with only a 17-55? what about a 18-55. can you still get that same picture?


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PostPosted: Tue May 29, 2007 3:44 pm 
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There is almost no difference between 18-55mm and 17-55mm. So yes, you can make the same amazing shot!

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PostPosted: Tue May 29, 2007 9:17 pm 
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Hi Kriss, in terms of wide angle your 18-55mm will offer virtually the same magnification as my shot, but the 17-55mm lens I used had a maximum aperture of f2.8, whereas yours will probably be f3.5.

Doesn't sound like much difference, but f2.8 is around two thirds brighter than f3.5, so to match the result I got at the same ISO, you'd have needed an exposure two thirds longer. Mine was 30 seconds, so you'd need 50 seconds if we both used the same ISO. I used 1600 ISO.

Of course your D40 can go to 3200 ISO, which is twice as sensitive as 1600, so it can halve your exposures! So instead of 50 seconds at 1600 ISO, you could try 25 seconds at 3200 ISO! Although beware, the quality goes down as you increase the ISO.

One big advantage you have though is the skies around Nevada are extremely dark - much darker than where I took that photo. Utah and Arizona are also great for this. So next time you're away from the city, try a night shot.

Zoom your lens to wide angle, put your camera to manual, set the shutter speed to 30 seconds and set the aperture to the smallest f-number (probably f3.5 for the 18-55mm kit lens). Also try an ISO of 1600 to start with, but consider increasing this to 3200 if you want brighter stars.

If the camera can't auto-focus on the stars (and it will probably have difficulty doing it), you'll need to manually focus the lens yourself.

Finally you'll need somewhere steady to put the camera. A tripod is ideal, but if you dont have one, balance the camera pointing upwards on a fence. Aim for the milky way, which is the faint band of stars crossing the sky.

Then use the self timer to take the photo so you don't wobble it with your hands.

Good luck!

Gordon


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PostPosted: Mon Aug 06, 2007 4:31 am 
i tried 1600 iso, with the 50mm f/1.8 and my pictures comes out ALL PINK with 1-2 stars, 30 sec exp.


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PostPosted: Mon Aug 06, 2007 4:55 am 
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That could be light pollution from street lamps, but is more likely to be orangey than pink...

Gordon


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PostPosted: Thu Sep 06, 2007 12:36 pm 
Looks like the stars are either in the Northern or Southern Hemisphere, past few weeks.

Nothing in between as its pitch blank or dark ? - no stars outside tonight again at the Equator zone ! At least not raining tonight.. :cry:


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