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Fujifilm GFX100 for astrophotography - Game changer? YES but...
Jul 11, 2019

I recently purchased the new Fujifilm GFX100 as my main camera. I literally sold all my video and photo gear to step up to this system. It offered everything I needed in a hybrid photo / video camera. So no need to own two different systems anymore. Upon initial announcement of the system a few months ago I knew this would be the system for me. The only thing I wasn't 100% sure about was how it would hold up shooting astrophotography. That many pixels on a sensor, at first glance, should yield not so great results. But this is a Sony made, backside illuminated sensor so on paper, this actually should be very good for low light conditions. I searched everywhere online and could find no images or information on its low light abilities. Immediately after getting mine I raced out to shoot some astro photos with it and the GF 23mm F4. I could not wait any longer. I HAD to know. 

GFX100, 23mm F4, 25 sec, 6400 ISO. Download the RAW here

So there I was, on a mountainside by myself in the middle of the night doing my first ever night exposure. 23mm, F4, 6400 ISO, 25 seconds (my normal Milky Way exposure time). The shot came back and there it was, the Milky Way over Washoe Lake in glorious 100 megapixels. You can see the light pollution from Carson City in the middle of the frame. Check out how much you can recover those highlights in the raw file! Is there noise? Yes of course. Is there any more noise than say a D850 or A7r3? I don't believe so. So we have a similar noise floor as the best cameras on the market but in a 100mp 16 bit file. Add 30-40 of luminance noise reduction and you're good to go. You will notice that 25 seconds is too long. We are getting star trails. I bumped it down to 15 seconds and they became much better!

But, I wanted better. I knew there was an opportunity to do more. To get a very clean 100mp file that I could print 40x60 is my end goal. So I went back out with my friend Abe Blair to do some more testing two nights ago. This time there was a bit of moon in the shot which helped illuminate the scene very well. This time I shot 23mm F4, 12,800 ISO, 15 sec. I shot 7 frames to stack later with a program called Starry Landscape Stacker (available on the app store). This program combines all the exposures and averages out the noisy pixels in the frame yielding a MUCH cleaner result. 

GFX100, 23mm F4, 15 sec, 12,800 ISO, 7 images stacked for noise reduction. Download full res .jpg file here 

As you can see, stacking is the way to go. It greatly reduces the noise in the image which is going to allow a beautiful 40x60 to be made from this file. 

This camera system is not perfect, however. There are two major hurdles and they are both linked together I believe. The first and most important is that F4 is far too slow for astrophotography. We are only able to expose for 15 sec @F4 VS. full frame cameras with a Sigma 14mm F1.8 can expose for almost 30 seconds @F1.8. That is 3 stops more light hitting the sensor. This yields a much cleaner result simply because we are not starving the sensor of light as badly as we are with the GFX. Our signal to noise ratio is 3 full stops better. One thing a lot of people don't understand about dual gain, ISO invariant sensors is that anything past the second base ISO is simply a digital gain of whatever the amount of light was that hit the sensor at that ISO. For example, if you expose one image @ 15 sec, f4, 800 ISO (second base ISO for GFX) and another at 15 sec, F4, 6400 ISO you are still having the same amount of light (signal) hitting those pixels. Bumping the ISO is simply a GAIN of that signal. When we have a weak signal and add lots of gain to it, we inevitably introduce noise and artifacts. Exposure signal on modern cameras is determined only by your aperture and shutter speed when exposed at the second base ISO. ISO is irrelivant to the total amount of photons hitting each pixel (signal). So the only way to get more signal to those pixels is faster glass or longer shutter speeds. This brings me to the second issue. If you download the image below and look at the trees in the direct foreground at 200% you can see the banding from the phase detect autofocus pixels in the sensor array. This is a common issue with modern sensors that have PDAF. The a7r3, a73, Z7 etc all have this issue. Those trees were incredibly under exposed (my guess is 5 stops). I stacked 11 images to get a cleaner result but stacking does not remove fixed pattern artifacts like this. Stacking only averages out the random noise in an image. In hindsight, I should have done 11 exposures for the sky at 15 seconds each, and then another 11 images for the foreground at a much longer exposure time of say, 1 min each. This would have given me much more signal and would have alleviated the PDAF artifacts. 

GFX100, 23mm F4, 15 sec, 6400 ISO, 11 images stacked with Starry Landscape Stacker. Download full res .jpg here

In closing this camera system is in my opinion the best one you can buy right now for all around landscape work. Mid day, sunsets and sunrises this thing sings. It's only in the night time shots that I am still finding my way in producing high resolution, noise free images. It's totally possible and I will keep refining the workflow to accomplish my goal. I will keep you updated when I learn more! Thanks for reading and have a nice day. 

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