Aquila dark nebulae - two views
Posted 18 June 2013 - 06:13 PM
Pentax 6x7, Takumar 105mm, f/4, 48 min, Kodak E200.
Takumar 400mm, f/4, 34 min.
The photo is cropped, because the left side was degraded by vignetting. Note also the spot of soft focus on the left-down from the E nebula. It is normal for this lens at f/4. Until I cropped the image, the spot was exactly in its center. It is better to stop the lens down to f/5.6. Here is the original unprocessed scan.
Posted 19 June 2013 - 11:30 PM
Nice shots. If you are getting soft focus spots, check to make sure the film is not bowing when being scanned. Check the frame with a magnifier directly. Also make sure the film is being held flat during the exposure. If possible, use a vacuum back. The problem is not always with the lens.
Posted 25 June 2013 - 04:04 PM
Dave, I've checked the frame once again under loupe. The frame really is blurred in the centre, as well as the other frame I took with this lens at f/4. Maybe the infity mark is not the spot at which the lens is best focused on infinity. I strongly consider to replace the lens by the 300mm EDIF. I'd love to have a vacuum back, but I can't find any. If you, or anybody else, know about one, which is not more used, I'd be very thankful, when you help me to get it.
Posted 26 June 2013 - 12:59 AM
It does sound like it's the lens. I think it's worthwhile trying to find the exact focus point by testing. If you haven't done it before, try finding a field with just a few medium to dim stars, misalign the mount to cause drifting, then change the focus in about 8 steps, covering the lens for a few seconds in between each focus step. This will give you a piece of film with dashed star trails which should tell you which focus position was best and whether or not it was best for the center as well as off center.
I'll keep an eye out for vacuum backs but there haven't been very many swap meets around here locally. Watching online might be your best option.