Estimate the exposure time of this image?

[stephenb]

I have found this image from my archives and wondering if there is anyway to estimate or calculate the exposure length of the photo?

The shot of of Orion (obviously) and my initial thoughts was to factor in the distance of the trail, measured against the known separation of the stars in the constellation. I've been trying to work it out but it's doing my head in :rolleyes:

Before you ask, I do not know any info about the lens.

Can anyone help? :thanx:

(ah, the days of long exposure film, no computers, no "stacking", no processing) ;)

[stephenb]

Just to add to this, using Stellarium, I have estimated that the star trails are 21 minutes long.

[asimov]

A program called Neatimage will tell the EXIF information (including exposure) if it's an *original image*.

[stephenb]

John this is a scan from a film print I took circa 1990, and slightly cropped.

[Octane]

My estimate would be between 15 and 20 minutes. :)

H

[stephenb]

Thanks H. Am I not seeing the forest for the trees here? A movement of 20 minutes RA means a 20 minute exposure? Has the answer been staring me in the face all this time?

[leon]

Yep about 20 minutes I did an exposure like this one and the trails are about identical in length.

Leon

[stephenb]

Thanks Leon! I have drawers of prints I have no clue what the exposure duration, date or times are. Here is a lesson in noting your image data at the time. Although these days with the DSLR storing the data as the shot is being captured, I guess that problem is minimised.

[john_drummond]

Hi Stephen,

I measured the distance from Rigel to Bellatrix (14.8 degrees seperation) as 182mm on my screen and then measured the distance that Mintaka (0* dec) had travelled - 61mm. Stars at the equator travel 15 degrees in one hour. 182 / 61 = ~3. 60 minutes / 3 = 20. Therefore the exposure time is pretty much bang on 20 minutes - unless my logic is flawed...

Cheers

[stephenb]

Hi John, Your logic is perfect to me :thumbsup: I was thinking about it too hard. Most of my old film star trails were either 10, 20 or 30 minutes, so 20 minutes is the one.