Reciprocity Failure?

[Speed]

This is a fairly long (2.5 hours) exposure of Polaris, the North Star. I have taken several shots of Polaris over the last month or so, and I have noticed that in my 10, 20, and 30 minute exposures, the sky is blue. In my exposures of 60 minutes and longer, the sky is green.

I will be taking more shots of Polaris in the future, and I intend to take some shots from darker sites. I'll be taking shots without any trees in them, and I'll be taking some shots using slide film as well.

So my question is, is this reciprocity failure, or is this green cast from the trees in the bottom of the frame, or is it the culmination of all the sodium vapor lights in Havelock (which is north of me)? I will find out eventually through experimentation, but in the meantime, give me your best guess.

K1000, Superia Xtra 400, 50mm lens @ f2.0

Would you believe it was handheld? No? ;-)

Bogen 3011 tripod

[another view]

Yep - reciprocity failure strikes again. Some films do this more than others - my experience with Provia 100F and 400F is pretty good for long exposures but then these are slide films. The longer the exposure, the worse the problem.

If it were High Pressure Sodium lighting, you'd have more of a pinkish glow to it. Mercury Vapor or Metal Halide would be close to this green too, though. Only way to get rid of it is Photoshop. BTW - are you scanning the negative or the print?

[Speed]

Yep - reciprocity failure strikes again. Some films do this more than others - my experience with Provia 100F and 400F is pretty good for long exposures but then these are slide films. The longer the exposure, the worse the problem.

If it were High Pressure Sodium lighting, you'd have more of a pinkish glow to it. Mercury Vapor or Metal Halide would be close to this green too, though. Only way to get rid of it is Photoshop. BTW - are you scanning the negative or the print?

Are a possibility as well.

I know this is much more of a concern (night shots looking like day) with print film, and most films suffer reciprocity failure. I've got some Provia 100F, Provia 400F and Sensia 100 that I'm going to try some star shots with as well. :-)
From what I've read, these films do not suffer reciprocity failure like the majority of films do. Provia 100F has been the standard astro-photography film for years, and is being replaced with CCD and CMOS sensors today. :-(

Thanks for the comments. I do appreciate it!

Half Hour Photo scans my negatives onto CD's for me.

[another view]

For a 2-1/2 hour long shot, the trees were lit by something a long ways away! I saw a shot once of a house with star trails that was probably shorter (maybe an hour?). One room was lit like a regular 75 watt light bulb was on - it was only a night light!

Provia "F" is great film for this stuff. I've done it with 400F because you get bigger star trails and colors show up more than with 100 speed film. Still haven't gotten anything great - but haven't done too much with it either. Didn't realize that it was the standard - it's just my regular slide film!

The longest digital shots I've done are 30 sec. I don't plan to try star trails for a lot of reasons - first off, you'd about need a car battery to power the thing that long! Noise can be a problem with long exposures too(not 30sec in my experience with what I'm shooting).

[SmartWombat]

If it's from streetlights, then you can get astronomical filters to remove the specific wavelengths emitted by those Sodium (orange) and Mercury (greenish) lights.
Because they are not incandescent lamps, but excited vapour lamps, they emit at known, fixed, wavelengths that you can block and let the rest of the light through.
Becuase you are only blocking a very specific "shade" of orange, you don't lose all the orange colours for example.

[Speed]

If it's from streetlights, then you can get astronomical filters to remove the specific wavelengths emitted by those Sodium (orange) and Mercury (greenish) lights.
Because they are not incandescent lamps, but excited vapour lamps, they emit at known, fixed, wavelengths that you can block and let the rest of the light through.
Becuase you are only blocking a very specific "shade" of orange, you don't lose all the orange colours for example.

There are filters out there to block those wavelengths. I've seen them in the Orion Catalouge. (http://www.telescope.com/jump.jsp?it...GORY&itemID=22)
:-)

Some folks who take night shots use filters to correct for reciprocity failure of their favorite film. Certain films shift colors toward the red or the blue, and it's just a matter of matching the filter to the correction you want.

Thanks for pointing this out, as some here may not have realized that there are ways around these little problems.