Changing f-stop on a DSLR

10 May, 2017 13:20
I would say I'm still pretty new to deep sky imaging and even wide field so please bare with my newbie question. What are the benefits to changing the f-stop? Most of the articles I read suggest using the lowest f-stop possible to increase the light gathering ability. I have noticed most people on here step theirs down to say f/4 or some other value. Is this to increase the depth of field and in turn make focusing easier or are there other benefits I'm missing? Thanks!
10 May, 2017 14:25
To say it simple, stopping down will show the objects sharper, it will reduce lens aberrations, but it depends on the quality of the lens (and personal taste),how many steps down are required, perhaps a few lenses are showing good results wide open.
Edited 10 May, 2017 23:39
10 May, 2017 14:54
if I do wide field images I will always use 1 stop above fully open. For example my zoomlens at 55 mm is f5.6 so I stop it down to f6.3 otherwise too much lens aberrations will appear at the sides. In short you will have to do some trials to find the best f-value for you, but if you stop it down too much, much longer exposure times will be needed.
10 May, 2017 16:46
First to reduce the aberrations but also to create diffraction spikes around the stars (1/3 to 1/2 a stop is sufficient there).
10 May, 2017 20:06

Closing down some lens improve the edge sharpness and can reduce the coma and vigneting.
some lens can be used wide open, like the samyang 135.
it depends of your taste : catching more light (signal), better sharpness, stars with spikes… smile
11 May, 2017 22:16
Thanks for the help guys!
20 May, 2017 12:55
I have up to this point not stopped down certain lenses for EX: Nikon 180mm f2.8 because I absolutely HATE diffraction patterns in my images and running the lens wide open has given me excellent results . Some lenses I have for example my Pentax 28mm f3.5 is so crappy around the edges I have to stop down or crop off the final image which defeats the whole purpose of a wide angle or fish-eye.
20 May, 2017 17:57
Stopping down primarily increases sharpness while also reducing certain aberrations.  If you do not like diffraction spikes, like myself, you can eliminate them while stopping down by using step down rings on the front of the lens (to match the desired aperture diameter).
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