Question for the astro shooters

[Glenda]

We have astro coming up for next month's photography club comp. One of our members shot the stars with his D810 Nikon 14-24 at f2.8 ISO 1600 20 secs. Every star has a black dot in the middle. Very strange and I've never seen anything like it. The stars are sharp so it doesn't appear to be a focus problem. I've only seen a crop of one section but every star has this black dot in the middle. Any ideas what would cause that?

[paulheath]

not really something I'm really into....... but could it be they had Noise reduction set on in camera, and it has somehow interpreted stars as noise and tried to eliminate them ? just putting it out there...probably way off the mark

[Glenda]

No idea but I'll ask. It's really strange these little circles of white with a black dot in the centre of each.

[nardes]

Hi Glenda

I have seen astrophotos where each star is surrounded by a dark doughnut and this is usually the result of applying a Deconvolution function during the image processing phase.

My understanding is that when you attempt to process star fields using Deconvolution, the process/algorithm attempts to analyse the stars in the field and make an educated guess about how much they have been bloated or deformed by atmospheric conditions. Based on the analysis, Deconvolution tries to remove the bloating and return the stars to a cleaner state. If applied too aggressively, it can leave dark doughnuts around each star.

Therefore, I would guess that some processing (image stacking, NR, Deconvolution, etc.) has occurred where the application has decided that the data at the centre of each star is an unwanted artefact and has "deleted" that data from the centre of the star and "replaced" it with a dark artefact.

Cheers

Dennis

[arthurking83]

I'd also say it's a workflow process, rather than a camera or lens issue.

[Glenda]

Thanks Dennis and Arthur will pass this on. Could this have been caused in camera then, as he said the image he showed me was a crop from the raw.

[arthurking83]

Thanks Dennis and Arthur will pass this on. Could this have been caused in camera then, as he said the image he showed me was a crop from the raw.

The last bit of your comment has different meanings depending on what software he used.

eg. if he used non Nikon software to open the raw image, then I'd say more likely it's not a camera related issue, and more so the way the raw file was rendered by the third party software.
If he used Nikon's software to convert the raw file into a cropped jpg file, then more likely it may be a camera issue.

Can't say I ever remember any such issue on the D810 tho.

One quick way for the fellow to test what's caused the issue is, if he can place the raw file back onto the card, into the appropriate folder than the camera saved it into and look at the image at 100% zoom on the camera.
Does it still show the black dots?

[Glenda]

Thanks Arthur, I think he uses either Lr or ACR in Ps but not 100% sure.

[arthurking83]

I just did a search for such an issue, as I had a vague recollection that a particular camera did have the exact issue your friend seems to be having.
I was fairly sure it wasn't the D810 that had this issue(and was right).
It was the Sony A7SII that had such an issue of rendering black spots in highlights, which was well documented.
Sony issued a firmware update that had sorted the issue too tho.

So it's looking more and more likely that his black spot issue is software related.

The reason I ask about what software he used is that there are differences in the way a raw file is viewed.

So in using adobe software, you won't be seeing a 'Nikon' generated 'raw' file, what ever image you are viewing, it'll be an adobe generated raw file.
If he(or you) tried using Nikon's available software to view the raw file, it's a closer rendering of the what the camera captured as a raw file.

Adobe uses it's own rendering tech to convert the data of a raw file into an image you see on screen.
Nikon uses their own tech to do the same.

I've made this suggestion a few times over the years, but there is a great little program called Instant Jpg From Raw(IJFR).
It's not really a program as such as much as it is a shell extension for Windows.
When you install it, it just gives you a new right click option, when you right click a raw file, to extract the already made jpg file contained within a raw file.
This ready made jpg file is what you see on your camera as the preview image.
There are options to extract the jpg file to be as large as the native resolution that the camera has, or smaller if needed.

In reality this is about as 'raw' as any jpg image gets, as it doesn't go through any process other than what the camera allows(ie. picture controls on Nikon cameras) and whatnot.
So what does this mean:
Many raw file viewers converters usually add some rendering process, like Nikon's ViewNX2, no matter what you choose on camera, it will always use anti vignette routine on an NEF file. I have vignette control set to off in camera, yet VNX2 still adds that routine to the rendering(and can't be disabled)
Capture NX-D is better, but I think it adds some processing, but at least (if it did) it can be disabled tho.
Adobe always adds it's own idea of raw file rendering.

IJFR doesn't. It is in effect the same thing as shooting jpg in camera, but without the hassle of actually shooting jpg in camera. That is, as we all know, it's really only the raw file that we count on when we shoot photos. So why bother wasting the space and shooting speed shooting jpgs?
With a program like IJFR, you don't need to shoot jpgs on top of raw files. You shoot raw only, then once on the PC(can't remember it there's a Apple version) you simply rightclick the raw file(or folder full of them) and extract the jpgs.
So whatever settings have been set in camera for rendering the raw file, is what you get out in the jpg file when IJFR extracts it.

[Glenda]

Thanks Arthur for your advice and for going above and beyond with your research. I'm pretty sure he uses an Apple so maybe the IJFR won't be available but I have passed your information on. It certainly sounds like an interesting tool to have.