Star Trails Tips

[elcud35rc]

Most RF come with a BULB mode.
I got really interested in star trails.
What are the do's and don't of star photography?

I live in a suburb of Los Angeles--Do I need to drive out to the Mojave Desert to get awsome star trails?

Thanks

 

[ClaremontPhoto]

The desert?

For the stars I'd go to a nice restaurant in Venice CA.

 

[Skinny McGee]

Do it at night .... The day is much to bright...






I am sorry I had to say it ... I just woke up and I oculd not help myself... The Mojave should be a great spot...I think I saw a book at the Library the other day that had all the exposers in it... I also think you can find exposer charts for free on line..
It would ber cool to see some of them whn you get them done

 

[naturephoto1]

Pick up a Black Cat guide for night time photographs. It will give you some guide for exposure. The film speed determines the aperture. Shutter speed is not affected but determines the length of the trails themselves. Try to think of compostional elements that you wish to include in the photo.

You might like to see the Old Faithful and Star Trails Photo on my website:

http://www.nelridge.com/picturepages/oldfaithful.htm

 

[dostacos]

Most RF come with a BULB mode.
I got really interested in star trails.
What are the do's and don't of star photography?

I live in a suburb of Los Angeles--Do I need to drive out to the Mojave Desert to get awsome star trails?

Thanks

Orange County Astronomers [ http://www.ocastronomers.org/ ]
deep sky location is in Anza, probably closer than Mojave, There is also a big annual astronomy meeting that happens up in big bear too.

Dan

 

[pvdhaar]

Here are some hints to get you started:

Use a large physical aperture.

Because you're registering point light sources, the f-number is not as important as the diameter of the aperture. A 50/2 outperforms a 35/2 in light gathering ability.

Pre-flash your film to elevate the base exposure.

Film is not linear (reciprocity), so if you pre-flash your film to about -3EV, you've made a base exposure that sensitises the film for any further exposure. Even the smallest stars will now register. Pre-flashing can be performed using a grey-card held in front of the lens, and hitting it with a flash. Make sure the distance of the flash to the card large enough to get an evenly illuminated card.

Interrupt the star trail near the end to make individual stars visible.

When you're doing a one hour trail, then about 5 minutes before you'll stop, cover the lens. Wait a couple of minutes, and remove the cover. Expose for a minute. The stars will now be separated from their trails, and you'll be better able to make out the constellations.

And most important: dress warm.

The best weather for star trails is when the sky is clear, and that's when temperature drops considerably at night. You'll be sitting still in a cold environment for quite a while, make sure you're properly dressed.

 

[ChrisN]

Pre-flash your film to elevate the base exposure.

Film is not linear (reciprocity), so if you pre-flash your film to about -3EV, you've made a base exposure that sensitises the film for any further exposure. Even the smallest stars will now register. Pre-flashing can be performed using a grey-card held in front of the lens, and hitting it with a flash. Make sure the distance of the flash to the card large enough to get an evenly illuminated card.

Great information!

Can I ask a silly question? Can you expand on the pre-flash business? I assume this is done in the dark? (I said they were silly questions. 🙂 ) Grey card I understand, -3EV I am "in the dark". 😛 I gather I have to control the flash exposure onto the card?

 

[pvdhaar]

Great information!

Can I ask a silly question? Can you expand on the pre-flash business? I assume this is done in the dark? (I said they were silly questions. 🙂 ) Grey card I understand, -3EV I am "in the dark". 😛 I gather I have to control the flash exposure onto the card?

Correct,

you set up your camera on the tripod in the dark, lenscap still on, shutter open on B, lens focussed on infinity (not on the card!).

Now hold a grey card that is large enough to fill the frame at least 2ft from the lens. Remove lens cap and trigger the flash that preferably is kept at least 2ft behind the lens (that's four feet minimum total distance between flash and card). If your lens aperture is f2, then set the flash auto aperture to f5.6, and make sure it reads the reflection off the card.

Alternately, if your flash doesn't have an A-mode, you can put the flash in manual and use the aperture=guidenumber/distance rule measured between flash and card. Close the aperture three stops from what the rule says, pre-flash and open the aperture again.

Now withdraw the card, and the star trail exposure can start..

Note that pre-flashing decreases contrast, but increases the amount of dim stars that you register. Some may find that minus 4EV gives a prettier end result.. less stars, but more contrast.

FWIW, I've also tried post-flashing instead of pre-flashing, and that doesn't work.. It just decreases contrast and doesn't bring out anything..

 

[pvdhaar]

Pre-flashed

Pre-flashed

Here's two examples..

On the left hand side, Lyra, preflashed. Note there are many dim stars recorded (this scan doesn't do it justice though, the print has increadibly more detail), and also that the base exposure of the grey card has taken away some of the green (I assume residual fluorescent light) background. No interruption of exposure.

On the right hand side, Ursa, no preflash. Less detail, more contrast, green cast. Exposure interrupted and resumed for a tad too long to bring out individual stars. They're elongated into small trails on the right hand side of the main trails again.

 

[ClaremontPhoto]

60 at f2 is always 60 at f2. 35mm lens or 50mm lens.

That is why handheld exposure meters don't have any input for focal length.

 

[pvdhaar]

60 at f2 is always 60 at f2. 35mm lens or 50mm lens.

That is why handheld exposure meters don't have any input for focal length.

No, it's not when you're dealing with what in essence are point light sources. The physical diameter of the aperture determines the amount of photons that are captured.

In ordinary photography, when the field of view becomes smaller (longer focal length) and you thus have more physical aperture (while having the same numerical aperture), the magnification increases proportionally. The effects w.r.t. exposure are inverse, so f2 gets the same results independent of focal length.

This doesn't hold for point light sources, where changing focal length does not change magnification (zero times a number remains zero). It's also the reason that you'll see decent stellar telescopes listed with physical aperture only, you'll never see them listed with something like f numbers.

 

[Hektor]

Hmmm, I think 60 at f2 maybe different between a 35 and a 50 because the area of the aperture is different, and we are dealing with a point source (star) not a large extended object (sky)

 

[ClaremontPhoto]

Thank you. It's where art meets physics isn't it?

 

[Hektor]

Hmmm, even art is ruled by the laws of physics.

Physicists know that, but artists think they are "creative" and "original"..........Hmmmm.