6 replies to this topic

[Michal1]

After three years I photographed the northern part of Cygnus with a 200mm lens again:

f/5.6, 2x35min, Kodak E200 push 1

The my older attemp can be seen here. This time, I decided to try dividing the exposure into two shorter pushed exposure, as successfully practiced by Dave Kodama. The total exposure time is approximately the same, the f-number and the place of photographing too. I changed the Takumar version of the lens for the newest SMC version and the films was developed by different labs. The newer photo is definitively sharper. But it is also much more grainy. Don't be confused by the darker look of the older image, it's only a matter of processing. Maybe I will reprocess the older image, as some parts of it look completely black on my monitor (maybe it's getting old).

Have you tried stacking several shorter exposures instead of taking one long continuous? After this experience, I'm not going to keep practicing this technique. I would rather consider combining several long exposures, if I wanted to reduce grain.

[Nebhunter]

Michal - just a thought with the push 1. If you stack does that increase the grain effect to equal a push 2 factor? I've shot everything from 0 push with longer exposure to +2 push and shorter exposure.

The 0 push gave the smoothest for grain and nicer stars, but lacking some of the contrast and features in the nebulae. Push 2 gave the most grain, 'noise" and contrast, but it I did not like the end result.

It might take more experimenting with your sky conditions as well, but you might want to consider stacking exposures each with a different push factor? I've found that push 1.25 to 1.5 at 55 - 60 minutes with f/5.6 to give great overall balance. Hope you find a good combination, and that others will have some input.

igor

[Michal1]

Igor, thank for sharing your experience with pushing.

just a thought with the push 1. If you stack does that increase the grain effect to equal a push 2 factor?

I think the answer is hard to guess due to the nonlinearity of film. Is the magnitude of noise (grain) in a push 2 frame double than in a push 1 frame? Is the amount of signal in a sum of two short exposures the same as in one frame of equal exposere time? The most reliable way is to make a test and see. I agree that more test should be done, but I'm afraid I'm seriously running out of my stock of E200.

The other thing is combining of two frames with different exposure times. I have some from the last summer. I've started to proccess one pair and the result looks very promising. You will see soon.

[whwang]

Hi,

Very nice image, Michal.

If the goal is to get the best S/N, the rule of thumb is to make long single exposures, and then stack as many exposures as possible. This old picture of mine is a stack of a total of 12 hr exposures:
http://www.astrobin.com/79387/

Many years ago, I had made some tests of S/N on E200, Provia 400F, and Centuria 400 for various exposures. I did not push them though. To properly answer the question, we need to re-do the experiment and measure S/N vs. exposure, both pushing and not pushing. This will tell us what pushing really does to S/N. Another important measurement is the slope of the S/N vs. exposure curves. If the slope is steep, then a single long exposure will be better than stacking several shorter exposures (if the total exposure is kept the same). On the other hand, if the slope is shallow, then it is better to break long exposures into shorter pieces and stack them. I would say, we should expose long enough that the sky fog is at least at Zone III.
For Centuria 400 (and probably 800 too), exposures longer than making Zone IV sky fog would be pointless, as it does not increase S/N at all. For other films, passing Zone III, I do not see any good quantitative measurements that tell us what is the best aim point.

Cheers,
Wei-Hao

[Nebhunter]

What a great film exposure of this galaxy. Usually I just give a quick glance at M31, but not this time. The smoothness of the frame is clearly visible. Interestingly, in the bottom right corner of the frame when enlarged, there are letters visible in the patterns of the stars. I see a t, g, H,. Is it just me?

Igor

[whwang]

hmmmm..... I only see the copyright water mark....

[Michal1]

I have reprocessed the photo on a new monitor, which is able to show substantially darker colors, and empoyed my new processing techniques: