Can't get sharp Focus with DSLR Lens

Jasp05 (Aaron) · #1 12-01-2018, 12:04 PM

Hi guys,

I've been struggling to get really nice sharp focus with my 55-250mm DSLR lens.

It always seems to want to leave a small tail on the stars. I've attached a pic to show what I mean. (it was cropped a bit due to artifacts from the stacking process).

I use live view to focus the stars manually. What I want to know is this just an inherent problem with the lens, or is there something else I can do to achieve better focus?

xelasnave · #2 12-01-2018, 12:23 PM

Maybe make a focuser mask or try either side of what you consider to be in focus.
Alex

doppler (Rick) · #3 12-01-2018, 12:40 PM

Looks like lens aberrations to me, you can try using a higher f ratio, or make a cardboard circle to fit over the lens and cut a smaller hole in the center so you are not using the outside edges of the lens. Lens aberrations are quite common especially with fast lenses.

bojan · #4 12-01-2018, 01:03 PM

Yep, this is lens issue.
While being excellent for everyday photography, most lenses do no pass the ultimate test on stars, especially fast zoom lenses.
Try what Rick is suggesting, only one stop down could make a big difference.

Jasp05 (Aaron) · #5 12-01-2018, 02:39 PM

Ive already got it stopped down one step to f6.3. But will try another stop down. If thst doesnt work i will try the lens cover with cutout.

Thanks guys.

sil (Steve) · #6 12-01-2018, 04:12 PM

Quote:

Originally Posted by bojan

Yep, this is lens issue.
While being excellent for everyday photography, most lenses do no pass the ultimate test on stars, especially fast zoom lenses.
Try what Rick is suggesting, only one stop down could make a big difference.

a tiny tiny tiny amount of flex in the lens/mount is enough to get this too. its common aberation though on lenses, even expensive ones. corners often show it the most.

Jasp05 (Aaron) · #7 18-01-2018, 09:47 PM

Got an edited final version after adding some more data. Still not super happy with the overall colour. (keeps getting a really strong green after stacking, even though the individual subs don't show it.) I assume it has something to do with the light pollution from my backyard as I don't have a light pollution filter yet...

This is around 50mins worth of 2min, 60sec, and 30 sec subs at 800 iso 250mm FL, F7.1. (tried to stop it down to keep the abberations of the lens to a minimum). Darks added and stacked in DSS with a bit of touching in Photoshop. Cropped to remove some vignetting/ distortion at the edge of the frame.

But for my first image on the HEQ5 pro and 55-250mm lens on an unmodded dslr overall I'm happy with the outcome. Can only get better from here

jenchris (Jennifer) · #8 19-01-2018, 09:09 AM

Looks like lens droop there's a misalignment coma to the left of frame

ZeroID (Brent) · #9 19-01-2018, 10:54 AM

What make lens is this ? Because even in the centre the stars are not round.

redbeard (Damien) · #10 19-01-2018, 01:34 PM

Try an auto colour balance, might help with the green.

Cheers,

Damien

Jasp05 (Aaron) · #11 19-01-2018, 03:29 PM

its a Canon 55-250mm lens. When I try to focus the lens it never seems to give perfectly round stars. Always trys to leave a tail on them. Even when stopping down 2-3 steps in F# it doesn't seem to make any difference.

May just be a bad lens?