The mysteries and features of Sigma's sensors

The Japanese photographic equipment manufacturer Sigma is probably best known for its very fine lenses. These lenses are highly respected in the art and science of photography. Sigma make lenses to fit and work with just about every popular camera make.

Less well-known and certainly less common however are Sigma's cameras. You won't see Sigma cameras every day - or even every week - but they're certainly out there, and I believe their owners will defend them fiercely. I do - I love my two Sigmas, a dp0Q and a dp2Q. That's a dp0Q in this page's heading picture. I know what the very first impression newcomers to the camera will be - wow, what a strange looking camera. Never mind how it looks, it's what's under the bonnet (or hood, if you're one of those strange Yanks) that counts. I'll stick my neck out here and declare that you will not find any camera/lens combination which is capable of delivering such quality and, in particular, sharpness. I should qualify that by excluding medium-format digitals. I'm talking 35mm equivalents, in their various sensor sizes, from the tiny things found in phones, up to full-frame, as found in top-level pro DSLRs.

They're fixed lens cameras. The dp0 is wide angle and the dp2 is more 'normal' standard focal length. I believe that this approach must always guarantee better results. The lens is perfectly matched to the body. Interchangeable lens cameras must almost certainly cause some degradation of the image, as the match is never as close as in fixed lens designs. And forget zooms; they are almost by definition compromises.

Sigma don't only make the dp series (of which there are four in all, the 0 is ultra wide, the 1 is wide, the 2 is normal and the 3 is modestly telephoto). We're not here to talk about Sigma's cameras though. I want to discuss Sigma sensor technology, its oddities and its values, and compare the technology to the more mainstream sensor technologies, because Sigma's sensors are very different, and unique in the world of digital cameras. Apart from Fujifilm cameras, whose technology departs somewhat from the norm, no other camera manufacturer departs from a standard sensor technology.

So, what are the technologies?

The standard common camera sensor uses a Bayer system. Sigmas use either Foveon or Quattro. Foveon and Quattro are very similar, both very different from Bayer.

Let's first describe Bayer. I will here extract a snippet and a picture from the excellent Wikipedia, source of all human knowledge.

A Bayer filter mosaic is a color filter array (CFA) for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camcorders, and scanners to create a color image. The filter pattern is 50% green, 25% red and 25% blue, hence is also called BGGR, RGBG,[1][2] GRGB,[3] or RGGB.[4]

As you can see, red-sensitive, green-sensitive and blue-sensitive pixels live side-by-side on the surface of the sensor. If you take a picture of a blue object with a Bayer camera only a quarter of the pixels actually react to and record the data; the green and red pixels say "nothing here for me". Similarly, for any image taken, only a proportion of the pixels record any individual element in the picture.

Let's now look at the Quattro sensor (for this discussion we may consider the Foveon sensor to be similar, in broad terms). I quote from Sigma's own website:

Almost all other image sensors are mosaic sensors, which use an array of RGB color filters in a single horizontal plane to capture color information. Each pixel is assigned only one of the three colors and cannot capture all three colors at once. In contrast, the Foveon direct image sensor captures color vertically, recording hue, value, and chroma accurately and completely for each pixel.

Sigma have kindly pointed out the differences between Bayer and Quattro in this picture, reinforcing what I have already said about Bayer.

So what IS the difference?

Above I claimed that on a Bayer, only 25% (50% for green) of the pixels are capturing any given colour's data. That's a bit simplistic as, of course, if it's a purple teddy bear then the blue and the red sensors willl both play a part. But you get the idea.

But with a Quattro sensor the pixels do not live side by side; each of these three primary colours are represented on separate layers. Photograph a blue ball and the top layer says "Yo, these photons are mine!". But photograph a red ball and the blue and green layers, being transparent, let the photons through, saying "Hey, I don't want those photons." So, fundamentally, while Bayer captures every colour on one layer, ignoring various colours per pixel, the Quattro can capture every colour at every pixel location. Bayer: only a quarter of the pixels contribute to the image of the red ball; Quattro: every pixel in the image has recorded redness.

All of these sensor shenanigans simply lead to one conclusion: Quattro images MUST, by definition, be better! But there's a downside (isn't there always?). Any photographer with a decent camera and a modicum of knowledge will photograph in RAW mode, rather then JPEG. No need for a discussion of the differences here; suffice it to say that a RAW image is precisely that - raw. Uncooked; not in any way modified by what the camera's manufacturer will think the correct sharpness, colour temperature, exposure, etc, should be.

No the RAW image is bald and unprocessed; just coloured pixels which YOU, the photographer, can knock into shape with any of the many many photo processing application. Except Sigma RAW, that is. None of the mainstream applications understand Quattro RAW! Only one application (in effect - there is one other but it's crap) can handle a sigma RAW file - an X3F, as it's called.

But I'm paying a tenner a month for Adobe Lightroom and you're telling me it won't touch an X3F??

Yes, that's absolutely and irrevocably right! Only one application will consider working with an X3F, and that of course is Sigma's own Sigma Photo Pro, or SPP as we shall call it.

DON'T PANIC! You can use Lightroom and Foveon X3F together, but we need to introduce an intermediate step via SPP.

Now, SPP is weird. It's rather unlike, yet hauntingly similar to many other applications. It will let you do all your photograph adjustments itself, and let you export a cooked and finished JPEG, but not in (personal opinion here) a very nice way. In the Sigma community there's a good deal of uncertainty about what it's doing to a picture. I'll have a nagging feeling that I'm missing something, some hidden adjustment which will give me even better results.

From my research I'll feel able to declare that very approximately 50% of Sigma users will do all their post-processing in SPP. The other 50% (Lightroom users pay attention!) will use SPP simply as a translator. Just like those people at the UN who listen to German and output Mandarin, SPP can listen to an X3F and output a TIFF. These 50% will have experimented and played at length with SPP to try to get the ultimate TIFF. In my case, all adjustments are zeroed out. No sharpening, no colour balancing, no lightening or darkening. I want a TIFF which conveys all the information the X3F does, without any beautification. For the record, a TIFF is most definitely a RAW file. A TIFF will be at the hidden heart of just about any non-Sigma / Bayer RAW file. TIFFs work very nicely in Lightroom.

But, again, how can we be sure SPP isn't doing something, no matter how subtle, to our X3Fs before TIFFing them? Quite simply, we can't. Nobody really understands SPP! We just have to believe that we're getting the most untouched picture in our TIFFs. Indeed, when you import a batch of X3Fs into SPP their thumbnails display on screen. You then opt to batch export the whole lot as TIFF - and then go and get a coffee, because SPP is slooooooooowwwwwww! You can opt for either 8-bit or enormous 16-bit TIFFs. Go for 16 then you know you're making your best efforts.

But finally, assuming you've exported the TIFFs into the same folder as the X3Fs, when the export is done, the TIFFs appear next to the X3Fs as thumbnails, enabling you to spot if there's any substantial difference. In my own zero-adjustment routine I'm pretty confident about this, as they look the same, to all intents and purposes.


If you're not up to the challenges of X3F I can reveal an astonishing postscript to all of this:

Sigmas (at least the dp series) have a menu option which will let you shoot DNG format! Wow, so these can go straight into Lightroom? Yup, they can. Delete SPP!  But please do read on and check out The Third Hand, below.

But we can still find some niggle room. In the Sigma community (again) there is mixed opinion: is a Sigma DNG really up to scratch? Does it contain all the quality and juiciness of an X3F/TIFF? Is DNG a compromise? Well, all I can say is that I don't know. These days I habitually use DNG and my Mac is not even wearing a copy of SPP. But in the past I've stood outside somewhere nice and photographed given scenes in both X3F/TIFF via SPP, and DNG. I've then studied the TIFF and the DNG to exhaustion in LR and in exported JPEGs and I'm blessed if I can see any difference. Each are highly editable in LR, and satisfactorily so.  But please do read on and check out The Third Hand, below.

Can we make any assumptions about how much "goodness" is in these files by looking at file sizes?
X3F : about 50 Mb
TIFF 8-bit: about 60 Mb
TIFF 16-bit: about 120 Mb
DNG: about 108 Mb
Exported JPEG: about 25 Mb
So, 16-bit TIFFs are huge, as are DNGs. Much bigger than native X3Fs. Not sure if we can conclude anything useful really.

So you, dear Sigma owner, have some work to do and some decisions to make!

But on the Third Hand . . .

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

Up there ^ I said, in an astonishing postscript, that X3F can be forgotten, because Sigma cameras can also shoot in DNG raw. 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 is the ONLY worthwhile and valid RAW tool 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. By DNG I specifically mean Sigma DNGs:

Luminar Both version 4 and subsequently the much anticipated AI version DO open the DNGs, but this is 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 is 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 great piece of software, and has pretty well become my go-to developer. 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 4 Nope. Won't open DNGs.

Apple Photos Free, and supplied out of the box with all Macs, but DNG - huh? What's this? I refuse to open it.

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! C One catches up and you've grossly overshot the mark. Besides that, C One seems to do something unpleasant when you try to bring down highlights, creating a dirty grey look somehow.

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.

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?

But in the latest news, December 2020: I was musing about the gradual demise of Capture One into the slow thing that is version 20. Its awful slothfulness made it effectively unuseable. Then I recalled how good version 10 - long deleted from my Apps folder - was. I searched forlornly and without much hope for the installer and Bingo! There it is, deep in the dark depths of one of my external drives. After ensuring that there was no remaining trace of version 20 on my Mac I installed version 10. This is unbelievable! All the old well-remembered speed is back. The overshoot is a thing of the past.

So I am promoting Capture One back to being one of my day-to-day RAW working products