Focus problem with the 56mm 1.2 in astrophotography


#1

Hello all, im a happy owner of an XT20 and a couple of lenses, and love to do some astrophotography. When i got the 56 1.2 i was quite excited to test it for obvious reasons. Unfortunately i have found that it has (at least with my xt20) a problem that i couldn’t solve yet with the focus, and it is that even with the lever in manual focus, it slightly changes the focus between shoots while doing the interval shooting. Here i show two consecutive shoots, of course i didnt touch the camera at all, and found this more than one time…Any of you have any clues??? i have used also 18-55, 55-230 and none of those present this problem.
Thanks a lot and happy 2019!!!

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#2

In the title you say you have an issue with autofocus, then in the text you mention a switch to manual focus.
With manual focus this shouldn’t happen so maybe try to run some other tests with a different subject as well. If you’re actually using the autofocus instead, that setting is also the cause of your problem: for astro you can’t really use autofocus since the camera doesn’t see a defined subject to “snap” the focus to so it will struggle at each and every shot and trigger this issue.
I can’t enlarge the image but to me it seems that even in the first one you’re not reaching the sharpness you could be reaching (but maybe that’s the jpeg compression).


#3

Hello fabienb! it is true that the title is misleading…, im sure the camera was in manual focus (written in the metadata), that´s why im surprised,its clear from the pics taken with the internal intervalometer that it is changing little bit the focus. When dealing with stars at f1.2 even the smallest changes will make a lot of difference…(in the series of 30 pics taken there are some that are in perfect focus, i put just two consecutive shoots to show the problem) i will try to see if doing the photos with an external intervalometer gives the same -useless- result. Thanks!


#4

What’s your firmware level? It could be a camera firmware bug. The X-T2 had a bug where it would slightly refocus the lens between frames with manual focus. (I noticed it mostly with a newer lens.) Looks like the X-T20 had the same problem at version 1.01:

The firmware update Ver.1.02 from Ver.1.01 incorporates the following issues:

1. The phenomenon is fixed that in the MF mode, repeated halfway shutter pressing can shift the focus point under a specific exposure condition.

If you’re already using version 1.02 or later, call Fuji support. They’re pretty good about returning calls and actually listening to what you have to say.

And I really need to make it to the Southern Hemisphere…


#5

Hello pldove!, thanks for the reply! will check my firmware version, i think it was the last one but dont remember! Thanks a lot! regarding your visit to southern hemispheres the best sky i ever seen was in Atacama desert! try to go there and you will be amazed!


#6

Hi fedvento,
I also have this lens and I use it with my X-T10. I did a lot succesfull shots with intervalometer in manual focus. From my practice, for doing astro it is best to set that lens to f1.8 as the coma dissapears altogether and if focus changes a little bit, there is still some room for error. I would also suggest that after setting focus you use tape on the focus ring so it doesn’t move.

Hope that helps!


#7

Thanks for the advise! its true that maybe 1.8 is a better option. Regarding the tape since its focus by wire (hate that) maybe u have some movement of the focus plane without the focus ring being moved (that wont happen in a mechanical focus ring), but will try it also, thanks!