Proper exposure for stars, no moon?

[chris jordan]

Hi guys, does anyone know how to shoot starts, with no trails, for maximum "starry night" effect? I imagine a pretty short exposure is required because otherwise the stars would move and start showing up as lines instead of dots. I'm envisioning using a very wide lens and getting the equivalent of what we see with our eyes-- tons of stars over a dark landscape. Any exposure tips?

Cheers to all,

~cj

[chris jordan]

oh hell, I meant STARS, not starts...!

[Ron Marshall]

See the table:

http://home.earthlink.net/~kitathome/LunarLight/moonlight_gallery/technique/LightAndCamera.htm

[jnantz]

hi chris

i have a black cat exposure guide that is kind of helpful ... but not exact ...

for example it suggests asa100 ( or 125) film at f4 it will be about a 1minute exposure
for a landscape and a quarter moon in the sky .. it didn't list starry night that was
the closest match to your "scene" that i could find ...

you might have to bracket a bit ( or use a digital camera as a metering tool + test exposures )

good luck!

john

[QT Luong]

A motorized equatorial mount used in astrophotography would be the best, and probably the only way to get good results with film. As the mount "follows" the stars, you can give them enough exposure without streaking. For an idea of the possibilties in the hands of a art photographer, see: http://davidelivaughn.com/

Otherwise the easy way is to use digital, and the maximum possible exposure, which is maximum or near-maximum ISO, white open (preferably f2.8 or less), and 30s or less. With their sharp wide-angle lenses and great high-iso capabilities, Nikons would probably yield superior results, but on Canon, the 24/1.4 shouldn't be too bad. This will not yield by far the level of quality of equatorial mount images.

[GPS]

It's useless to give him advice about f2.8 or f4... Stars being point light sources depend on the actual aperture opening dimension in their effects on the film. (a f4 on a 300mm lens is not the same as f4 on a 65mm lens, for a point light source.) Very few LF lenses, if any, have such an actual aperture opening diameter that they could yield the demanded "starry night" effect on 50 -100Asa films.
More studies on his side (there are plenty of good web sites for it) will be more useful and can give him all the info he needs than this fishing for useful info.

[eddie]

not much help here. but i shot some stars on my last trip to utah. i was trying to get them moving. i do remember that even with very short exposures i still had some movement in the star. i got plenty of movement at 15 min. i am going to guess that even down near 2-3 min they showed movements (my memory refuses to function well on info that old...)

[GPS]

The star trailing movement on the film depends on the focal length of the lens, among other elements. Short focal lengths are less prone to it but have smaller actual aperture dimension than longer lenses etc...

[Kirk Keyes]

You get two choices:
A) tonnes of stars with a blurry landscape using a tracking system like QT mentioned, or,
B) star trails with a possibly sharp landscape using a regular tripod mount for the camera.

Or, you can composite the landscape from Option B with the stars from Option A and make a photoillustration.

Look up "The Backyard Astronomers Guide" by Dickinson and Dyer.

[percepts]

1/film speed at f8 or wider should do it.

p.s. the earth rotates 1 deg every 4 minutes. You can work out how much rotation is acceptable for you.

Just remember that the image angle of view on film is what you are measuring the movement across so with a 150mm lens on 4x5 film you are looking at approx 43deg of view. Then decide what percentage of the circle of confusion is acceptable as movement and calculate from there. Also the bigger the enlargement you want, then the smaller the acceptable coc will be. And the longer the lens focal length, the narrower the field of view and therefore the faster the star will move across the film.

[Richard M. Coda]

I'm no scientist but I seem to recall hearing (from Rod Klukas) that exposures have to be less than 8 seconds or there WILL be noticeable movement. This was for the moon, so I'm not sure if that applies to stars as they are tinier.

[percepts]

Around dusk while sun is just up you can easily see stars against the blue sky. They are very bright and don't suffer from night time. That means you can expose for sunny 16 rule or thereabouts and the stars will appear on film. Fact that it is night is irrelevant since you are not photographing the blackness, you are exposing the bright objects.

The problems arise when you want to include the landscape in the image. That means you have one subject which is very bright and one which is very very dark, especially if there is no moon. Then what is the sky doing. Is it light polluted because if not then it will be black too. That means there is nothing to give a contrast edge to the land sky boundary. So in reality it depends what you are actually photographing, land and stars or, land, sky and stars or just stars.

On my rough calculation for 150 lens on 4x5 film, you will get around a 0.025mm of movement over an approx 2 second exposure. That is 25% of 0.1mm circle of confusion. All very approximate but the maths is simple to do. If you want very sharp stars then go with the sunny 16 rule but don't expect any land features to show up except the horizon if there is plenty of light pollution around you.

[percepts]

motors are required for telescopes because the angle of view is very small so the stars move across the field very fast by comparison to the sort of lens focal lengths we use.

[percepts]

And finally, I'm referring to the human visible constellations which we all know. If you are wanting to photograph the much dimmer non human visible stars, then of course a motorised unit is required.

[konakoa]

I've done this with my 4x5. With film at least, this kind of photograph (with pinpoint, sharp stars) can't be done with a single shot on a tripod.

There really isn't a fixed formula for photographing stars. There are too many variables. Wide angle or telephoto lens, maximum f-stop of the lens, sensitivity of your digital camera or film, portion of the sky you're photographing, and how much local light pollution you have are going to radically affect the results.

Generally, however, exposure times for stars are going to be several minutes. A regular tripod just won't work. You'll need a equatorial telescope mount to track the stars to keep them sharp.

The photo below I made with my 4x5 camera. It was done with two exposures on a single sheet of color transparency film; one for the foreground and sunset (on a normal tripod) and the stars at home with the camera riding atop a simple equatorial telescope mount. I'd be glad to go into detail if you'd like - just send me a PM.

[GPS]

As already said, in the astrophotography the f number is not decisive, the actual physical aperture dimension is, because of the point light source the stars are. Forget about any sunny f16 rule...
Also, the star trailing depends on declination (latitude) beside the focal length ect... There is a mathematical formula for this.

[Richard M. Coda]

That's a nice image, Danny.

[Alan Davenport]

I think the question is, how long an exposure can he get away with and NOT create star trails?

This might help: http://velatron.com/dca/Tripod/

Edit: Just found the rule of thumb for this: 600 / FL gives the maximum exposure time (in seconds) to avoid obvious trailing.

[Mark Sampson]

My experience trying to shoot star trails, 10+ years back, is that reciprocity failure will kill you if you use conventional b/w films or 100-speed color film. In my case, an f/8 w/a lens wasn't fast enough. Perhaps with 400NC, TMY, or TMX?

[Peter De Smidt]

Acros and G100x (Kodak color slide) films don't have reciprocity failure until exposure over 2 minutes.

[Marko Trebusak]

Chris, the formula is 600 divided by lens focal length (600/focal length). That's for 35mm camera and word is that for larger format you can get away with longer exposures. But in my experience it isn't the case. I tried 6x6 film with a bit longer exposure than that and on print the streaks are easily visible. So stick to that and try to experiment later.

Cheers,
Marko

[argos33]

Why does it have to be so complicated? I imagine those tracking mounts are not cheap. Plus you get a blurred image of the landscape as it tracks the stars. I would opt for the multi-image solution myself.

1. meter with DSLR cranked up to max film speed
2. compensate appropriate number of stops for speed of LF film and reciprocity failure
3. use higher speed film and/or push it to get within the appropriate exposure time (stars without trails)
4. take one photo metered for the stars
5. take another photo metered for landscape
6. scan film and combine the two via photoshop or HDR software

[GPS]

Why does it have to be so complicated? I imagine those tracking mounts are not cheap. Plus you get a blurred image of the landscape as it tracks the stars. I would opt for the multi-image solution myself.

1. meter with DSLR cranked up to max film speed
2. compensate appropriate number of stops for speed of LF film and reciprocity failure
3. use higher speed film and/or push it to get within the appropriate exposure time (stars without trails)
4. take one photo metered for the stars
5. take another photo metered for landscape
6. scan film and combine the two via photoshop or HDR software

It is complicated because of the celestial movements (actually the Earth's movement) and the optical facts involved in the matter.
You cannot use an exposure meter for a point light source like stars. As already said and re-said the actual aperture opening dimension is more important in this case than the f number...

[Marko Trebusak]

Chris, the formula is 600 divided by lens focal length (600/focal length). This of course is for exposure time in seconds.

Marko

[GPS]

This of course is for exposure time in seconds.

Marko

This formula is a very rough one, of course, as it doesn't take in account the declination. The declination can make a 100% difference in the calculated result...

[Jim Michael]

google 'barn door drive' for info on a non-motorized option.


And finally, I'm referring to the human visible constellations which we all know. If you are wanting to photograph the much dimmer non human visible stars, then of course a motorised unit is required.

[percepts]

google 'barn door drive' for info on a non-motorized option.

You could do that or you could take the worst case scenario which is what I did, and within that case you can easily work out how much movement is acceptable to you.

The real problem with this question is that the OP has not specified the stars magnitude of brightness which he wants to capture or the focal length of the lens he will be using. Just saying I want to photograph stars doesn't cut it as that could mean visible to the eye or it could mean every non human visible star too. Without knowing that nobody can say what the exposure should be or what the best setup would be.

[Jim Michael]

Using a tracker and taking a time exposure of a static scene at night are orthogonal processes unless one chooses to composite a tracked shot with the static shot, easy enough in Photoshop I suppose. Just throwing the low cost tracker option out there - 2 boards, a hinge, and a bit of threaded rod, doesn't get much lower tech than that.

[Greg Miller]

This will not help you directly but should give a god reference point. of the big dipper with pin point stars, was taken with a Nikon D700 at ISO 1,600 for 8 seconds, f6.3 with a 17mm lens. Somewhere between 15 and 30 seconds there was noticeable movement in the stars.

As a side note, aperture is relevant in this discussion unless you limit the discussion to photos with only stars (no earthly objects).

http://gregmillerphotography.com/Adirondacks/images/N000861.jpg

[GPS]

In night time photography for maximum "starry night effect" with "tons of stars over a dark landscape" and no star trails the aperture number is relevant only for the sky fog treatment, the dark landscape falls where it falls - unless you want to widen the discussion to a low light landscape photography, to make a double composition or not to go for the "maximum starry night effect". In the originally wanted case is valid the proverb - you cannot have a cake and eat it.

[Greg Miller]

In night time photography for maximum "starry night effect" with "tons of stars over a dark landscape" and no star trails the aperture number is relevant only for the sky fog treatment, the dark landscape falls where it falls - unless you want to widen the discussion to a low light landscape photography, to make a double composition or not to go for the "maximum starry night effect". In the originally wanted case is valid the proverb - you cannot have a cake and eat it.

Even with a dark landscape you still need to consider aperture to ensure that the objects that you want in focus or out of focus render as desired. "...falls where it falls" is simplistic in that it only considers exposure. I'm not saying that the OP would not choose to lets landscape object go soft in order to yield a maximum starry sky. Just that as with everything in photography, everything is a compromise, and nothing is an absolute.

[amoebahyda]

For maximum starry night effect, you need to take photos with exposure as much as possible (practical limit by light pollution) such that stars with higher magnitude can show in the photograph.

Without a perfectly polar aligned Equatorial mount to hold your camera, the motion of earth limits how long you can expose. The limit depends on the focal length of the lens and the position of ther stars in the sky. Longer focal length will magnify the amount of earth motion and requires shorter exposure; the closer the stars to the celestial equator (lower declination), the faster they appears to move and thence requires shorter exposure.

The longest acceptable exposure is:

T = 1000 / (F Cos d)

Where T is the acceptable exposure, F is focal length in mm, d is declination of star

For perfectly sharp and round stars the exposure is:

T = 343 / (F Cos d)

These are from the point of earth motion, if you want to record dim stars you have to play around with high ISO and faster lenses. I used to record the milky way with ISO800 film and a Leica Noctilux-M 1:1/50 at F1.0 and 20sec exposure while pointing at zenith.

This is the equipment I used for shooting dim stars...a research grade cooled CCD with high quantum ratio, a sturdy equatorial mount with computer correct for motion error, and a RC telescope...

http://i294.photobucket.com/albums/mm100/amoebahydra/Photo%20Equipment/BRCSTL.jpg

[GPS]

Even with a dark landscape you still need to consider aperture to ensure that the objects that you want in focus or out of focus render as desired. "...falls where it falls" is simplistic in that it only considers exposure. I'm not saying that the OP would not choose to lets landscape object go soft in order to yield a maximum starry sky. Just that as with everything in photography, everything is a compromise, and nothing is an absolute.

The OP is interested in stars photography with "dark landscape". He would be an uttermost fool if he sacrificed the "maximum starry night effect" he expressly wants using his aperture to include some fictive foreground in his depth of field not reachable with an open lens. But of course, in the collection of advice given (a manual barn door, a star guiding mount - both unusable for the landscape, sunny f16 rule etc.) even this one - to use the aperture for depth of field regulation - is of the same kind. Nothing is absolute - someone said something is absolute? Hmm - sometimes it rains... sometimes not...

[GPS]

Sure Amoebahyda, the equations are right. Still there is the question of the actual aperture dimension to deal with the sky fog, the question of the wide angle lens he wants to use to complicate the things. The OP problem cannot be solved with a simple answer as the correct answer depends on many factors correlated together. Some homework on his side is necessary, unless someone wants to do it for himself.

[percepts]

This will not help you directly but should give a god reference point.

Well I suppose if you are photographing the heaven's at night with a long exposure, then praying might help. :D

[rdenney]

The point that was made (but is still being missed) is that stars (except the Sun, of course) have no measurable projected size. They are too distant for their disks to be resolvable from earth. Thus, the ability to record light from them is a matter of two things:

1. Aperture, and

2. Signal/noise ratio.

We are accustomed to thinking of aperture in terms of smaller=sharper. That's not the case here. If you make a star too sharp, it disappears. The larger the aperture, the more of its light you capture, but more importantly, the less of that light is damaged by diffraction. So, the larger the physical aperture, the better.

I've made a couple of usable images (usable being defined as prints no larger than snapshot size) at a minute or so with an APS-sized digital SLR and a 14mm lens. The Moon was up and behind the camera, and it lighted the landscape.

The first image was made at ISO 1600 and an exposure of two minutes at f/8, looking north. Star movement is not excessive, but in the corners you will see trails. Looking north helps, of course. The image has been excessively sharpened at the downsampled resolution to bring the stars out, but I did this five or six years ago and I was still enamored of those tools.

The second image was made using ISO800 and an exposure of 30 seconds at f/5.6--I had learned in that few minutes between them that the wider aperture was necessary more than the longer shutter speed. I think the second image shows stars of dimmer magnitude with less sharpening, even with half the net exposure. This image is a cropped sample from the center of the image, shown actual pixels (at a sensel density of 133/mm). At its f/3.5 maximum aperture, this particular lens might have been too soft and if so would have caused the dimmer stars to blur out. F/5.6 might have been the best this lens can do.

These images also show the effects of signal/noise ratio. The sky in these images was lit by a full moon, and by the lights from a small city about 25 miles to the northeast (whose lights are visible in the lower right). The stars are what they are, but light pollution can ruin the exposure even if you are able to capture the stars. Longer exposures would just have washed out more stars.

It should be noted that the stars are always moving in the field, so a really long shutter speed doesn't make them brighter on a fixed-mount camera. The exposure each star receives is a function of how long the star sits on that piece of film, and that is affected solely by the size of the airy disk on the film. That's the point of the tracked equatorial mount, even if used with a wide-angle lens--it's the only way to increase the exposure of each star greater than the time it takes the star to move the width of the airy disk on the film.

These images were made with a Canon 10D (when it was new), and technology has come a long way since then.

Rick "wider aperture is better" Denney

[Colin Graham]

This is the equipment I used for shooting dim stars...a research grade cooled CCD with high quantum ratio, a sturdy equatorial mount with computer correct for motion error, and a RC telescope...


nice rig!

[Kirk Keyes]

nice rig!

I'll second that!
But how do you get good shots indoors like that? ;^)

[amoebahyda]

I'll second that!
But how do you get good shots indoors like that? ;^)

It will eventually goes into here...

http://i294.photobucket.com/albums/mm100/amoebahydra/Photo%20Equipment/BRC250house.jpg

but in the meantime, it is how it was used...

http://i294.photobucket.com/albums/mm100/amoebahydra/Photo%20Equipment/BRC250inuse.jpg

[Struan Gray]

Nice toy amoebahyda :-)

Metrogons and Aero-Ektars were traditionally recommended by amateur wide-field astro photographers. Better mapping lenses are available at a price: they combine wide angles of view with large relative and absolute apertures.

I have known research projects which gang up a bunch of Canon 300 f2.8 lenses pointing in slightly different directions. If you're not a purist you can always tile-and-stitch multiple exposures from smaller cameras.

Personally, I'd stitch frames from Google Sky. "Time spent waiting for the light": 0 seconds.

[GPS]

Nice toy amoebahyda :-)

Metrogons and Aero-Ektars were traditionally recommended by amateur wide-field astro photographers. Better mapping lenses are available at a price: they combine wide angles of view with large relative and absolute apertures.

I have known research projects which gang up a bunch of Canon 300 f2.8 lenses pointing in slightly different directions. If you're not a purist you can always tile-and-stitch multiple exposures from smaller cameras.

Personally, I'd stitch frames from Google Sky. "Time spent waiting for the light": 0 seconds.

Struan,
care must be taken so as not to confuse the "same" notions from two different fields. The wide-field astrophotography is not the same as photography with wide angle lenses and certainly not photography with "a very wide lens" that the OP asked for.
Thus Aero-Ektars and Metrogon lenses are not regarded as wide angle lenses (e.g. 7" Aero Ektar used for 5x5 film format) in photography. In the same way mapping lenses are not an equivalent of LF wide angle lenses in their focal lengths. This is a part of the OP problem as the LF wide angle lenses do not have a sufficient physical aperture to catch higher magnitude stars in the short time he can use for non trail stars photography.
Stitching is perhaps a possibility.

[Struan Gray]

I know, I know.

But photographers wanting a sky full of start are often less pernickety than astronomers. The easily available aerial lenses will let you photograph major constellations on large sheets of film without too much effort. Deep sky nebulea are another matter.

Of course, more complex, rigorous, accurate etc solutions exist. Michael Covington's "Astrophotography for the Amateur" is a good primer:

http://books.google.com/books?id=WTKcbdR-2h8C

(The pages Chris needs are partly available in the limited preview)

[Greg Miller]

The OP is interested in stars photography with "dark landscape". He would be an uttermost fool if he sacrificed the "maximum starry night effect" he expressly wants using his aperture to include some fictive foreground in his depth of field not reachable with an open lens. But of course, in the collection of advice given (a manual barn door, a star guiding mount - both unusable for the landscape, sunny f16 rule etc.) even this one - to use the aperture for depth of field regulation - is of the same kind. Nothing is absolute - someone said something is absolute? Hmm - sometimes it rains... sometimes not...

I guess you are just more confident than I am that we all envision precisely what the OP has in mind when he states "dark landscape" or what kind of foreground he has in mind that he may or may not consider as important to the overall image; or how much compromise he is willing to accept in any of these details. My own personal experience is that the same words or phrases mean very different things to different people. And beyond the fuzziness of the English language, aesthetic vision in photography varies - Some photographers accept featureless black shadows and other require rich detail in shadows. I prefer to offer ideas that can be used, or not, at the readers' discretion. Others prefer to take a different approach.

[GPS]

...
I'm envisioning using a very wide lens and getting the equivalent of what we see with our eyes-- tons of stars over a dark landscape. Any exposure tips?

Cheers to all,

~cj


...
Some photographers accept featureless black shadows and other require rich detail in shadows. I prefer to offer ideas that can be used, or not, at the readers' discretion. Others prefer to take a different approach.

Sure, I understand - when was it the last time you saw "tons of stars over a dark landscape" with a "rich detail in shadows"? To save the OP from the fuzziness of the English language and with the same kind of logic you can also offer him tips for a dark landscape with moving lights in it (frozen in motion or not) or - spaghetti con sugo...;)

[Greg Miller]

Sure, I understand

No - it is pretty apparent that you are not even close. And I am pretty happy to leave it at. Peace, out.

[GPS]

No - it is pretty apparent that you are not even close. And I am pretty happy to leave it at. Peace, out.

No - it is pretty apparent that you are not even close to happiness and peace about it...;)

[Drew Bedo]

The stars are point sources, so stopping-down or opening up won’t affect overall exposure. The number of dimmer stars that show up will depend on the physical maximum aperture of the lens. A 150mm F5.5 will have a maximum opening of 26.7mm (150/5.6=26.7) while a 210mm f5.6 will have a physically larger maximum opening of 37.5mm (210/5.6=37.5).

Overall exposure will depend on the amount of sky-glow; the ambient light that spills over from street lights. When the film begins to fog from the sky-glow the exposure is over. Use slower emulsions and bracket. Shooting at higher altitudes will reduce atmospheric interference.

Oh yes, and BRACKET.

[chris jordan]

Hi guys, thanks for all your input. I'm trying to organize a photographing trip to Midway Island in the Pacific in September, which is reputed to have amazing night skies (being 1500 miles from the nearest light source). I'm bringing a 60MP digital medium format back, which I know is not technically LF, but the results are somewhere between 4x5 and 8x10 in quality. In addition to doing lots of daytime photographing I'm hoping to try some nighttime blended-exposure HDR images that include starry skies, with foregrounds painted with light. Back to being a novice (again...)!

[Drew Bedo]

Hey Chris; When you get back...show us some of your shots and tell us how you did them, OK?

[percepts]

Well since its digital you are using then it becomes simpler to calculate what is acceptable.

Assuming your pixel sensor is 8000 pixels wide and your lens field of view on sensor is say 80deg, then knowing that the earth rotates 1 degree every 240 seconds, you get:

((8000/80))/240) = 0.42 pixels per second.

That is worst case scenario so you have a margin of error at the latitude you will be at. How much margin depends where you are pointing the camera (north horizon or south horizon or somewhere in between). But using that formula, you don't need to know that stuff. 1 second exposure won't show the dim stars but you won't get trails either.

[percepts]

I would add that since declination is the angle of a star north or south of the equator, then at the latitude you will be at and photographing a southern horizon, it is quite possible that you will be photographing stars with zero declination.
This is not the same as your declination which is your latitude and from my understanding, it is a combination of your latitude and the star declination which determines how quickly a single star in the sky moves. But you are not photographing a single star. You are photographing a large chunk of the sky and each star is moving across your field of view at a different speed. So if you want astronomical accurracy, you must first work out which star in the sky is moving fastest. i.e. the one nearest to zero declination. But then you have to factor in your latitude which none of the examples given have mentioned.

Hence the formula T = 343 / (F Cos d) is a fudge formula and assumes 35mm film format. Assuming 0 declination and 50mm lens, then that gives 6.86 secs. Thats for film. But use a 50mm medium format lens, then things change depending on the lens design. i.e. how much of the field of view is projected onto the useable camera sensor. So you must know what angle of view is projected onto the sensor area to make an accurate calculation. It all becomes a royal pain for the casual night sky shot. So unless you are serious about finding the correct star to work from, its declination, your exact latitude, the required level of accuracy for high density sensors, and doing all the calculations based on the actual field of view on your sensor, then a simple worst case scenario makes life very easy. Or the fudge formula adjusted to your field of view on the sensor but you will require trial and error to work that out.

Having said all that, since your camera is digital, you should have zoom software in the back to see the images in fine detail. Therefore why can't you just take some shots and check the trails or not at the time, so you can be 100% sure you have no star trails. Thereby negating the need for any of this astronomical stuff.

[GPS]

percepts,
you're talking total nonsense. If you have no astronomical knowledge you should avoid making up answers that, as in this case, are complete rubbish. You mix up the celestial and the geographical equator, stars declination and the geographical latitude, declination and azimuth and mistake one for the other, etc.

[percepts]

GPS suppose you give a worked example so we see that you are only quoting big words you found in a dictionary. And while you're at it, how about you explain the magic number 1000 or 343. And further more how about you explain:

"It's useless to give him advice about f2.8 or f4... Stars being point light sources depend on the actual aperture opening dimension in their effects on the film. (a f4 on a 300mm lens is not the same as f4 on a 65mm lens, for a point light source.) "

[GPS]

percepts,
sincere at last, you give the indication of the way you make up your posts. For the numbers you're interested in ask those who post them. As for my quote, it is self-explanatory and basic astronomical knowledge for astrophotographers. Not for you, understandably enough.

[percepts]

percepts,
sincere at last, you give the indication of the way you make up your posts. For the numbers you're interested in ask those who post them. As for my quote, it is self-explanatory and basic astronomical knowledge for astrophotographs. Not for you, understandable enough.

Just as I thought. You're not capable of backing up your claims. Only capable of dissing other people who are trying to help. Says it all.

[GPS]

I would add that since declination is the angle of a star north or south of the equator, then at the latitude you will be at and photographing a southern horizon, it is quite possible that you will be photographing stars with zero declination.
This is not the same as your declination which is your latitude and from my understanding, it is a combination of your latitude and the star declination which determines how quickly a single star in the sky moves. But you are not photographing a single star. You are photographing a large chunk of the sky and each star is moving across your field of view at a different speed. So if you want astronomical accurracy, you must first work out which star in the sky is moving fastest. i.e. the one nearest to zero declination. But then you have to factor in your latitude which none of the examples given have mentioned.

Hence the formula T = 343 / (F Cos d) is a fudge formula and assumes 35mm film format. Assuming 0 declination and 50mm lens, then that gives 6.86 secs. Thats for film. But use a 50mm medium format lens, then things change depending on the lens design. i.e. how much of the field of view is projected onto the useable camera sensor. So you must know what angle of view is projected onto the sensor area to make an accurate calculation. It all becomes a royal pain for the casual night sky shot. So unless you are serious about finding the correct star to work from, its declination, your exact latitude, the required level of accuracy for high density sensors, and doing all the calculations based on the actual field of view on your sensor, then a simple worst case scenario makes life very easy. Or the fudge formula adjusted to your field of view on the sensor but you will require trial and error to work that out.

Having said all that, since your camera is digital, you should have zoom software in the back to see the images in fine detail. Therefore why can't you just take some shots and check the trails or not at the time, so you can be 100% sure you have no star trails. Thereby negating the need for any of this astronomical stuff.

Stars declination does not change with observers Earth's latitude. Therefore mathematical equation that calculates the stars apparent movement on film for a given focal length and given star declination is independent of the observer's latitude.

[percepts]

Stars declination does not change with observers Earth's latitude. Therefore mathematical equation that calculates the stars apparent movement on film for a given focal length and given star declination is independent on the observers latitude.

Yes that was error on my part, I meant longitude but everything else applies.

[GPS]

Stars declination does not change with observers Earth's longitude either. Therefore mathematical equation that calculates the stars apparent movement on film for a given focal length and given star declination is independent of the observer's longitude.

[GPS]

GPS suppose you give a worked example so we see that you are only quoting big words you found in a dictionary. And while you're at it, how about you explain the magic number 1000 or 343. And further more how about you explain:

Go to the post #42, click on the link, find the page 10 and read the explanation of one of your "magic" numbers.;) Just don't expect this service on my side all the time...:)

[GPS]

GPS suppose you give a worked example so we see that you are only quoting big words you found in a dictionary. And while you're at it, how about you explain the magic number 1000 or 343. And further more how about you explain:

"It's useless to give him advice about f2.8 or f4... Stars being point light sources depend on the actual aperture opening dimension in their effects on the film. (a f4 on a 300mm lens is not the same as f4 on a 65mm lens, for a point light source.) " GPS

Study more, my son, it's the post #...!;)

[percepts]

"It's useless to give him advice about f2.8 or f4... Stars being point light sources depend on the actual aperture opening dimension in their effects on the film. (a f4 on a 300mm lens is not the same as f4 on a 65mm lens, for a point light source.) " GPS

Study more, my son, it's the post #...!;)

You have a talent for being unhelpful. There's always one...

[GPS]

You have a talent for being unhelpful. There's always one...

No, I'm a kind boy, believe me...:) The post # 47 hints at the reason...;) Cheers!

[percepts]

No, I'm a kind boy, believe me...:) The post # 47 hints at the reason...;) Cheers!

So oh smart one, where in the equation does the fact that on the equator at midnight looking vertically up at a declination 0 star, the star will be moving across the film or sensor at 1 deg of subject view every 4 minutes. 6 hours later, the star will be moving across the sensor at 0 velocity. And in between those two times the star will be moving at maximum speed to start off with and slowing down as you get to 6 hours from midnight. Declination hasn't changed so what parameter has changed and what spatial coordinates are important in calculating that change?

[GPS]

So oh smart one, where in the equation does the fact that on the equator at midnight looking vertically up at a declination 0 star, the star will be moving across the film or sensor at 1 deg of subject view every 4 minutes. 6 hours later, the star will be moving across the sensor at 0 velocity. And in between those two times the star will be moving at maximum speed to start off with and slowing down as you get to 6 hours from midnight. Declination hasn't changed so what parameter has changed and what spatial coordinates are important in calculating that change?

Sorry percepts,
I'm kind but not so much that I'd serve you as a teacher of the basic astronomy that you need to study. Just a detail - looking vertically up in the night (at the zenith) you don't look at a declination 0 star. This time your observing latitude plays roll...:) Happy studies!

[percepts]

Sorry percepts,
I'm kind but not so much that I'd serve you as a teacher of the basic astronomy that you need to study. Just a detail - looking vertically up in the night (at the zenith) you don't look at a declination 0 star. This time your observing latitude plays roll...:) Happy studies!

Well I did say equator which is close enough to celestial sphere for this discussion. So what declination is zenith from celestial sphere equator.

You haven't answered previous question.

[GPS]

If a star that's moving on the celestial sphere after 6 hrs stops to move it's the time to take pictures of it - no star trails, what bliss! Or maybe it's a time to take sleep, who knows what could happen in the next 6 hrs.

[percepts]

If a star that's moving on the celestial sphere after 6 hrs stops to move it's the time to take pictures of it - no star trails, what bliss! Or maybe it's a time to take sleep, who knows what could happen in the next 6 hrs.

I din't think you got it. Very noisy discussion.

[GPS]

Don't care about that, it's not important. Cheers!

[Kirk Keyes]

And further more how about you explain:
"It's useless to give him advice about f2.8 or f4... Stars being point light sources depend on the actual aperture opening dimension in their effects on the film. (a f4 on a 300mm lens is not the same as f4 on a 65mm lens, for a point light source.) "


Well, I'll bite...

300mm/4 = 75mm
65mm/4 = 16.5mm

The 300 mm f/4 will gather much more light as it has a greater aperature than the 65mm f/4. As was said earlier, recording point sources like stars is dependant on the absolute aperature/diameter, and not the f/number of a lens. The 65mm lens will perform much worse at reacording star trails than the 300mm.

[GPS]

...
Having said all that, since your camera is digital, you should have zoom software in the back to see the images in fine detail. Therefore why can't you just take some shots and check the trails or not at the time, so you can be 100% sure you have no star trails. Thereby negating the need for any of this astronomical stuff.

Absolutely right, percepts!

[QT Luong]

http://bb.nightskylive.net/asterisk/viewtopic.php?f=9&t=14503

"My night sky/landscape photographs which are my trademark have traditionally always been single frame shots of both the night sky and landmarks in one single exposure. In the olden days like for comet hale-bopp, the longer time exposures with tracked camera to follow the stars would always leave a tell tail sign on the landscape rocks as they would blur if lit or they would cast a shadow against the background stars if you lit them momentarily like with a flash. Now a days, all is different. Anybody with a decent digital camera like canon 20D and 24mm lens with high iso like 1600 at f/1.6 can record deep detail in the Milky Way in just 20 seconds and 10 times more stars than the eye can see." Wally Pacholka

[GPS]

While it is true what Pacholka says, he does not say the whole truth. These digital starry night pictures have their very typical tell tail sign too. Using high ISO sensitivity they register not only a lot of stars but also a lot of sky fog too. Therefore they look like taken on a very light night but with an unusual number of stars - very artificial.
Some photographers even try to present these pictures like those of the reality seen with your eyes, which is nonsense, of course.
I once saw a picture of the zodiacal light taken by a physics professor that claimed how easy it is to see it. When I emailed him saying that his picture is a digital vision never available to a human eye in nature he was furious...

[Brian K]

http://bb.nightskylive.net/asterisk/viewtopic.php?f=9&t=14503

"My night sky/landscape photographs which are my trademark have traditionally always been single frame shots of both the night sky and landmarks in one single exposure. In the olden days like for comet hale-bopp, the longer time exposures with tracked camera to follow the stars would always leave a tell tail sign on the landscape rocks as they would blur if lit or they would cast a shadow against the background stars if you lit them momentarily like with a flash. Now a days, all is different. Anybody with a decent digital camera like canon 20D and 24mm lens with high iso like 1600 at f/1.6 can record deep detail in the Milky Way in just 20 seconds and 10 times more stars than the eye can see." Wally Pacholka

There's no way that's one exposure. He claims that he works with a 24mm at around f1.6, ISO 1600, but if you look at the degree of DOF he has objects a few feet from the camera, and the stars completely in sharp focus. f 1.6 is not going to allow that. Also if you look at the horizon it is a harshly sharp line where it meets the sky, it looks cut. Also the horizon sky is usually brighter then the higher sky.

[Marko]

There's no way that's one exposure. He claims that he works with a 24mm at around f1.6, ISO 1600, but if you look at the degree of DOF he has objects a few feet from the camera, and the stars completely in sharp focus. f 1.6 is not going to allow that. Also if you look at the horizon it is a harshly sharp line where it meets the sky, it looks cut. Also the horizon sky is usually brighter then the higher sky.

It isn't a single exposure.

He is simply using 24mm/1.6/ISO1600 as an example of what was technologically possible using the technology of the day a couple of years or so ago.

Here's what Wally says himself about the image on this site (http://www.twanight.org/newTWAN/photos.asp?ID=3001638):




[...]

Since the cave is so large, even with my 24mm lens I was not able to get the whole cave in view without doing a panoramic image with 4 shots. During the exposures the crescent Moon lit up the canyons and I artificially lit the inside of the cave.

[...]



A four-shot stitch and a fill-in light, presumably flash.

Still a rather nice image, much easier to diss than to make.