So what opens Sigma RAW files? Yes, nothing is as easy as it may seem, especially when Sigma and its RAW files are concerned.

The native RAW format on Sigmas is the X3F file. Virtually nothing opens X3Fs, with the exception of Sigma's own Sigma Photo Pro - SPP. The normal route is to open a folder of X3Fs in SPP and then click Export, opting for TIFF format. Next, as these are regular TIFF files, just about anything will open them, some better than others. In my experience, virtually all of the software products below are lacking in some respect, and in honesty, I would only use Lightroom or Capture One to work with them. But please see below, where I talk about ApolloOne.

Digital Negative (DNG) is a patented, open, lossless raw image format developed by Adobe and used for digital photography. It was and is designed to obliterate the divides between the many many proprietary RAW formats. Indeed, Adobe supply a universal DNG translator, which will ingest many RAW formats and spit out DNGs. Need I say that these many RAW formats do not include X3F! But yes, Sigma cameras can in most cases shoot directly into DNG files, so problem solved what what!!

Nope. DNG, the supposed universal format, is not a ne plus ultra tool to reduce all cameras to a common output methodology. In other words, there's DNG, and there's DNG. Look back up above and you'll see that Sigma DNGs are an enormous 108 Mb in size, against X3Fs of around 50 Mb. Why? What's in them?

I'm not going to speculate about that question, but we do know that Sigmas capture colour information from three separate layers. Maybe there's really the equivalent of three DNGs in each DNG? Damn, I just speculated, after saying I wouldn't.

We have already established that Adobe Lightroom (LR) is happy with Sigma DNGs. I have already stated my opinion that LR really one of the ONLY worthwhile and valid RAW tools out there. I'm fortunate in that I have been able to purchase - or free trial - just about all of them, and have given them all Sigma DNGs to consider. I now state my findings. (Here, by DNG I specifically mean Sigma DNGs)


Luminar
Both version 4 and subsequently the much anticipated AI version DO open the DNGs, but they were a silly bug-ridden and much criticised, even mocked, piece of crap. Yes, you can process DNGs if you can be bothered to get to the nitty gritty of basic processing after wading through all the AI crap. Who really needs an artificial moon or a garish sky replacement? Who can watch the interminable export process happening. And why must my 16:9 ratio pictures be displayed in the Luminar editor squished to 6:4? No, Luminar was crap, as any quick trawl through forums will tell you.

In a late May 2021 update: those nice guys over at Skylum have obviously been very busy, since their latest Luminar version, 1.3, is improved almost beyond recognition. No longer is it a piece of crap! One tiresome thing I used to see was my Mac's memory disappearing, to the point of rude messages popping up; no more. The squishing of 16:9 images to 6:4 has gone; exporting is now much quicker. I still don't use the sky replacement or other so-called AI features, but as a general RAW editor it's turned into a rather good piece of software, and is pretty well becoming one of my go-to developers. One quite important omission remains however is that it doesn't seem to support using the NIK filters seamlessly, as in Lightroom, but I hope that, given the major improvements they've made, that feature will be along soon.
Affinity Photo
Affinity are decent. Their Designer and Publisher products are nice, clever, quick and pleasantly understated, and I use both. However, Affinity Photo will NOT open our DNGs, and I don't like their Persona approach.
DXO Photolab
Nope. Won't open DNGs.
ApplePhotos
Free, and supplied out of the box with all Macs, but DNG - huh? What's this? I refuse to open it. I must say, it's pretty crap stuff anyway.
RAW Therapee
Free. Weird. Unpleasant. I don't know if it likes DNGs because I couldn't get to them. RAW Therapee wouldn't let me get to my external drives, so in frustration I quickly abandoned it. Let's say it MIGHT open DNGs but merely looking at it, and trying to contact an external drive rapidly put me right off it.
Exposure X6
Admittedly, I didn't try it with DNGs, but processing TIFFs was an unpleasant experience (which I don't need to explain) so it was abandoned. RAW processing should be pleasurable; this wasn't.
Capture One
I've talked about this product elsewhere. It does open DNGs and can process them quite nicely. We're up to version 20 or 21 now, of a product that was once THE product for me, for a long time. But that was in the days of version 10. With each new version it's become slower and slower, to the point now where we can observe the overshoot syndrome. What's that? It's when you adjust a slider - exposure for example - and nothing seems to happen. So you adjust it a bit more and wham! COne catches up and you've grossly overshot the mark. Besides that, COne seems to do something unpleasant when you try to bring down highlights, creating a dirty grey look somehow. I am really not very sure about this product.
So there you have it. These wonderful cameras have this difficult downside if you want to shoot RAW. X3F and getting them into your RAW editor after wrestling with a TIFF intermediate route, or DNG but being able to use only Lightroom and not whatever your own personal favourite editor is?

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Darktable
This is a free open-source product, available not only for the Mac, but also for Windows (apparently, but as we all know, Windows is a steaming bloody pile of crap, which no sane computer user should touch) and just about any flavour of Linux. A new release was touted in January 2021, so I thought I owed it to my research to give it a try. It does open Sigma DNG files, which is a plus at least. It's deep and complicated, and features many many adjustments, a number of which are not present in the others. However, despite concentrated experimentation and much comparing with the best results I've seen from Lightroom and Capture One - never mind the others - I just couldn't experience any satisfaction with it. No matter what I tried, results look dull and unflattering, and it does a number of adjustments before you even get into personal to-taste adjusting. I quite soon exhausted my patience with it and declared it to be unsuitable for my personal preferences. Your mileage may vary, but I crossed it off my list quite quickly. Now it appears that I need to go through my DNG folders and delete all the .XMP files it's created in the folders, regardless of whether I "touched" the pictures or not.
Iridient Developer
This is a paid product, $99.99, a new version of which was released in March 2021. I suppose, to be honest, that their very poor website should have warned me - if they simply can't be bothered to create a nice-looking site then hey, let's not expect too much from their product, right? It claims to open Sigma DNG files (not X3F, which goes without saying) and will, of course, handle TIFFs too. Deleted immediately after trying it, which I did for about a minute. I opened a folder of Sigma dp0Q DNGs and started to make some adjustments. First of all: 9 seconds from clicking on a thumbnail to seeing the preview. 9 seconds. Well, for a start, that's just not good enough; LR displays them instantly. So then, let's try adjusting exposure. An exposure adjustment is the first and most important adjustment you'll make. So, the first photo I experimented with was a bit bright, so we'll reduce exposure. 7 seconds later, whoops, now it's too dark, less reduction required; 7 seconds later, hmm, not enough, a bit more ... 7 seconds later. To hell with this! How about some shadow brightening? Again, 7 seconds. And so on. Enough is enough. Close it and delete it: this product is simply completely unuseable.
ON1 Photo RAW
Awful. Just awful. It does, to be sure, open Sigma DNGs as well as TIFFs and all the usual JPEGs etc. But whoa, it is possibly the slowest and jerkiest software I've ever seen. I opened a folder of Sigma TIFFs, which was successful. I then witnessed the mouse cursor jumping about all over the screen as I moved the mouse. Meanwhile the CPU temperature whizzed up into the high 90% area. I gave it a while, made some coffee, in case it was digesting the images into its catalog, but no, ten minutes later the cursor was still hopping about like some sort of demented frog. It was impossible to settle the pointer onto, for example, the exposure slider. So I ditched that and opened a folder of Sigma DNGs. No difference whatsoever. So here is a product which - for all I know - may create the most beautiful JPEGs for export, but who knows, since it seemed impossible to get the mouse cursor to behave smoothly and progressively.
ACDSee Photo Studio
An odd one this. The Mac version is not an equivalent of three different PC versions. The Mac version most closely resembles the PC's Pro version. Initial impressions were very favourable. It imported Sigma TIFFs rapidly, and displayed them as thumbnails. Clicking on one, and then hitting Develop brought up all the usual controls. Each slider caused instant results in the viewing area, and export was really quick. However, importing a folder of Sigma DNGs was a different matter; although the thumbnails appeared nicely in the browser, clicking on one and hitting Develop resulted in: blackness; a black and empty screen. No apology, no explanation, just nothing. It was as though it just couldn't handle Sigma DNGs, but there was nothing in the code able to explain that. So if I was TIFF only I would be more than happy to recommend the product, but lack of DNG support renders the software useless - despite its speed and power. Regrettably, therefore, I must cross it off the list for Sigma DNG users.