Shooting the stars
I was doing some experimentation last night, and I ran into a few problems. Aside from the too present ambient light from trying this in town, I could not get my 55-200 AF-S VR to focus at 200mm. I tried to manually focus, but the shots came out blurry. I was using a remote, and was holding the shutter open for about a minute, trying to get some streaking stars. I took the filter off, as it was causing some issues with my moon shots. How do I focus on a sky full of stars?
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#1. "RE: Shooting the stars" | In response to Reply # 0
JosephK Nikonian since 17th Apr 2006Sun 28-Mar-10 12:09 AM
The stars are too small and too dim for them to be any use to the AF system. You will need to go manual focus or AF on the moon or something big and bright.
Also make sure the VR is turned off, or it will try to remove the star trails.
Seattle, WA, USA
#2. "RE: Shooting the stars" | In response to Reply # 1
#3. "RE: Shooting the stars" | In response to Reply # 0
Actually, it is possible to AF on a very bright star or planet. I just focused tonight on Venus using a D700 and 24-70/2.8 and 70-200/2.8. It may not work with other bodies or lenses, especially slower aperture lenses. The key is to find the brightest star in the sky
I was victorious, and managed to image Mercury, despite some thickening clouds, which is what I was really after
Short of that, the best way is manual focusing using LiveView while aiming at a bright star. Zoom the Live View display in as far as possible and just make the star as small as possible
Last night was a near full moon, which had risen well before sunset. Not a good night to shoot star fields. Try a new moon or after the moon has set. The moon puts more light into the sky than most suburban locations.
One way to resolve the sky glow (ambient light) problem is to tighten up the black point. That will take out the fainter stars, of course, so you have to play around with it. As I understand it, the best exposure is one that actually generates a fair amount of skyglow in the image, followed up by a black point tightening or other contrast stretch. Also shoot raw and then set white balance using a gray point selection on the sky glow. That will at least get rid of the garish colors from the various artificial light sources
#5. "RE: Shooting the stars" | In response to Reply # 4
Fri 02-Apr-10 03:05 PM | edited Fri 02-Apr-10 03:57 PM by nrothschild
Be aware that, in general the wider the lens the more difficult it is to focus on and record images of stars and bright planets.
The reason for this is that point sources of light (infinitely small) record detail in a camera solely as a function of true aperture (diameter of the entrance pupil). I use the term "true aperture" to differentiate what we photographers refer to as "aperture", which is actually "focal ratio", a different thing.
As a result, a lens with 1 inch of aperture, for example, will record a star with the same intensity as any longer or shorter focal length lens with the same true aperture.
True aperture = focal length divided by focal ratio.
So, for example, a 100mm lens shot at F/4 (regardless of it's wide open aperture) has 25mm of true aperture.
A 28mm lens, shot at F/4, has a 7mm aperture, which means that stars will record almost 4 stops below that recorded by the 100mm lens at the same focal ratio.
Extended objects (not point sources) record an image intensity proportional to the focal ratio. For example, a galaxy or nebula records wit the same intensity at any focal length if the focal ratio is kept constant. And, of course, you want the widest focal ratio possible in that case
There is an interesting implication here. Sky glow records as any other extended object. It's like a galaxy or nebula that spans the entire sky . Because of this, for a given focal ratio (and you always shoot the night sky as fast as possible) longer focal length lenses will record more stars and less sky glow than shorter focal length lenses. That puts some limitations on wide angle sky photography in urban and suburban locations. It also explains why AF has a tougher time locking in on even bright stars at very short focal lengths.
The next two nights will be ideal for imaging Venus and Mercury in the dusk skies in the Northern hemisphere. The past few weeks Mercury has been moving "up" at dusk to slowly meet Venus. On April 3rd and 4th Mercury will be at about the same altitude, "next to" Venus and it should make for great landscape images! Venus will be very bright, you may have to squint to see Mercury. Use binoculars if you have them, the two will be about 3 degrees apart, within the field of most low power binos. Best shooting time will be 20-45 minutes after sunset, I suspect. Just be there at sunset and keep an eye on things and start shooting when you see Mercury. You will need a very low and very clear horizon although you can see Mercury through very thin white/light clouds, but not usually if the clouds are darkish. Although we are having unusually clear skies during the day here in Md, I doubt that the horizon will cooperate for this, at least for me
Although the next 2 days are the best, for a week or more you will will have a shot if the weather cooperates.
I took this image March
28th27th, while in the driveway of a friend's house, a get together with about 7 other friends. I did not have access to the water's edge. The conditions were marginal, with thin clouds thickening quickly nearer the horizon. We did not see Mercury with our unaided eyes but viewed it in Binos and my Questar telescope. As they were all pounding me to get out the door to go out to dinner I grabbed this shot, which is a fairly deep crop of the original. I thought they had left me there and gone without me . I should have used my 70-200 but by the time I figured that out and ran in to grab the lens and do the swap, Mercury had slipped into the deeper clouds and disappeared.
This will be much easier to do the next few nights when Mercury is as high as Venus. Venus is the bright star left of the mast. Mercury is a dim star 2/3 of the way down and slightly to the right, but still left of the mast. Hopefully it shows up in this image.
D700 24-70/2.8 @70mm
ISO 1600 F/4 1s
Attachment #1, (jpg file)
#6. "RE: Shooting the stars" | In response to Reply # 4
Fri 02-Apr-10 04:23 PM | edited Fri 02-Apr-10 04:25 PM by nrothschild
After sunset on April 15th, in the Northern Hemisphere, the alignment of Mercury and Venus will be very close to what I imaged above, with Mercury receding away from Venus and toward the sun and the brighter twilight.
However, the moon, on the East Coast, will be 1 day, 3 hours old, the "new moon". By coincidence the moon will be as old as last month's new moon, which I discussed and imaged here in my recent Landscape Forum thread, and included Venus in that image. Successive new moons usually have varying ages but not in this case. For time zones west of the East Coast the moon will be successive hours older, a little brighter, and generally slightly easier to shoot. It is one of the most challenging "causal astrophotography" situations you will ever face.
You can go out any clear night and shoot stars. Experienced dusk and dawn star photographers know that the sexiest images include conjunctions of bright planets and/or the moon because they are rare and quite ephemeral. In this case Mercury, always elusive, can best be shot only when it's greatest elongation from the sun occurs during a few Spring months, with less favorable results during greatest elongation in the Fall prior to dawn.
Here, on April 15, we have a reasonably favorable Mercury, with Venus (not always the case!), and the newest moon, with the thinnest crescent, that you are ever likely to see or image.
IOW if you like this stuff and live in the America's, and you get the right weather for this, you do not want to miss this! This demands that you find a great landscape scene with a low flat horizon!