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External field monitor for manual focus?


ajurjans

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Hi, All.

I have a Fuji X-T4 and shoot mainly for my own fun.
I am slightly interested in astrophotography. Don't have the fancy trackers, etc, just my lenses., so I would do some aurora timelapses, some comet shots, and so on.

I have spent several nights out there shooting timelapses only to discover at home that the focus was off.

Fuji only offers 6x magnification in focus peaking mode, which is not enough for accurate manual focus during night, due to the high noise level and the size of stars even when magnified. I have been trying different tricks, but that is always a trial and error process, and the result can only be seen once home. Bringing a laptop with me is not really an option.

I know that some external hdmi monitors have their own focus peaking features, and was thinking if this would help. However, it will probably not be very useful, if the camera outputs only what is on the display to the monitor.
So, my question is - has anyone used an external field monitor for fine focusing on stars during night and would it be helpful for me? Does the X-T4 output clean HDMI? I don't do video, so I know nothing of this.

Edited by ajurjans
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This is one place where having 10x zoom would really, really benefit photographers.

The hdmi output can go up to 4k resolution, which is 3840 x 2160. A portable monitor can easily handle this, but whether or not this will enough for what you are wanting to do is tricky. You may be able to haul a 4k tv / monitor outside, hook it up and try some test shots to see if the idea in practice is as good as it sounds. If it is, portable monitors  are not nearly as expensive as they used to be, but you will (usually) need to bring an external power source, usually another battery.

An alternative is to get Carey or Bahtinov masks for your longer focal length lenses and use them to focus; take a shot, hit the play button, zoom in and decide how much you need to adjust the focus, and repeat until the focus is as good as can be. These external masks come in two kinds, the most common one that fits over the end of the lenses and ones that snap into the lens. Carey masks tend to be sold more for telescopes, so finding a snap in kind usually means having a 3d printed one made.

Farpoint Astro makes a lot of snap in Bahtinov masks for camera lenses, the idea is to dig up one of those clear UV/IR “filters” that resellers give away with camera gear, put a  snap in mask in it and set it off to the side until you are ready to use it. Screw it onto the lens’ filter threads, focus, then remove it and get the photos. Just a reminder, if the temperature drops more than five or so degrees while imaging, your lenses may get focus breathing, i.e. lose focus a bit, so you may need to check the focus every so often.

https://farpointastro.com/products/farpoint-55mm-camera-filter-bahtinov-mask

If you have a lot of lenses, just get the set for your largest lens and use step-up rings with your smaller lenses.

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I should have asked which lens and what exposure times are you using? If you are using  exposures longer than a few seconds with longer lenses, then your stars may very well be in focus, but you are seeing the trailing effect, the camera is catching the “trail” the stars leave as the earth spins around, or thought of another way as the stars in the sky seem to move across the sky, their movement leaves a trail in a long exposure image.

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@jerryy, I am well aware of the trailing effect, no worries :) I use different lenses for different scenarios.

I do have a bahtinov mask as well, and while I have only tried it with a mirror-type 500mm lens, the results were not too good. My guess is that the mask I have is too coarse. But thanks for reminding that I have one, will give it a try next time the sky is clear.

Edited by ajurjans
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I mentioned that the 4K video is tricky, a lot of folks doing planetary imaging use video feed captures and use software such as AutoStakkert, etc, to convert and stack the video into the incredible single images or short videos they show us. For them, a 4K feed like what your X-T4 has is a dream come true. They can focus with the settings they are working in, using telescope or camera lens. But they are not trying to combine it with a landscape background as you are in those aurora shots. They are also usually using a tracking mount.

The masks I mentioned can help for longer lenses if you are quick enough to get the shot, crank up the ISO and use sort shutter speeds, and quickly take the shot as fast as you can after moving the focus ring. You can relax after that while hitting the play back button to zoom into the shot to see how close you are. Speed is needed because once you get past 50mm or so, the star’s motion over the X-T4’s small pixel sites will make it seem like what should be a round star is instead sort of egg shaped which is a problem if you hesitated getting the shot.

For wider lenses, the old technique of finding the lens’ true long distance infinity focus point during the daytime, focus on something miles and miles (kilometers and kilometers) away off in the distance and mark the lens so that you can come back to that setting later at night can work well. In optics, three feet can be considered infinity when it comes to focusing, so trying for very long distances away instead of just a few hundred feet or meters works out better.

Edited by jerryy
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1 hour ago, jerryy said:

I mentioned that the 4K video is tricky, a lot of folks doing planetary imaging use video feed captures and use software such as AutoStakkert, etc, to convert and stack the video into the incredible single images or short videos they show us. For them, a 4K feed like what your X-T4 has is a dream come true. They can focus with the settings they are working in, using telescope or camera lens. But they are not trying to combine it with a landscape background as you are in those aurora shots. They are also usually using a tracking mount.

The masks I mentioned can help for longer lenses if you are quick enough to get the shot, crank up the ISO and use sort shutter speeds, and quickly take the shot as fast as you can after moving the focus ring. You can relax after that while hitting the play back button to zoom into the shot to see how close you are. Speed is needed because once you get past 50mm or so, the star’s motion over the X-T4’s small pixel sites will make it seem like what should be a round star is instead sort of egg shaped which is a problem if you hesitated getting the shot.

Amazing! Thank you! Very valuable insights. Did not know about astrophotographers using video to create an image.
I'll stay off the field monitors until I decide to get a tracking mount and play with the mask more.

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  • 3 weeks later...
On 2/7/2023 at 10:41 PM, jerryy said:

But they are not trying to combine it with a landscape background as you are in those aurora shots

Ok, so, in my last post I said I'd avoid the field monitor.

Boy, did I forget that I had an ebay sniper set up for some already? :D

So, I got a ESDDI F5 monitor (5inch, fhd, can receive 4k signal). And last night there were favourable conditions for Aurora Borealis in my location.
And I am happy to report that the monitor helped me get the focus spot on! At least far better than I would have been able to.

I switch the camera to video mode (hdmi otuputs 4k in video and playback modes, fhd in photo mode), found a star, zoomed in on the focus point and then zoomed in on the display itself a few times. When the focus was reached, switched the camera back to photo and this is what I got.

I am quite happy with the result.

And, funny enough, the in-camera focus sclae was showing around 4 meters distance - waaay off the infitiny mark. Not even touching it. Had this confirmed with both 14mm and 10-24.

Welcome, dear visitor! As registered member you'd see an image here…

Simply register for free here – We are always happy to welcome new members!

Fujinon 10-24 @10mm, F4, ISO 1250, 15 second exposure.

Edited by ajurjans
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@jerryy

Glad you like them.

I am too far South to get the really spectacular auroras, but the one last night was cool.

Welcome, dear visitor! As registered member you'd see an image here…

Simply register for free here – We are always happy to welcome new members!

14/2.8 @F4, ISO 800, 20 seconds

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