Sigma SD15 Review

by Bob Atkins, October 2010 (updated February 2011)

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The Sigma SD15 is the successor to the Sigma SD14. First announced at Photokina in 2008, it was finally released early in 2010. Both of these DSLRs, in fact all of Sigma’s DSLRs, are characterized by their use of a Foveon sensor. Unlike the sensors in most DSLRs, which use separate pixels to detect red, green and blue light, in a Foveon type sensor each pixel position generates a red, green and blue signal. The Foveon pixels take advantage of the fact that light of different colors can penetrate to different depths in silicon. By measuring the signal generated at different depths, the color of the light can be analyzed in terms of its red, green and blue components.

Bob Atkins

The SD15 (and SD14) sensor has 4.65 million photosites, i.e. light is measured at 4.65 million different positions. However, Sigma multiplies by three because each site generates signals for red, green and blue and thus 14.1 million different data points are generated, with red, green and blue signals from each position.Sigma and Foveon call this a 14.1MP sensor. However, you can’t really directly compare it with a conventional 14.1MP sensor (using a Bayer color matrix). Conventional Bayer matrix sensors measure the light at 14.1 million different positions on the sensor, but each position records only either a red, green or blue signal. Processing the data interpolates (calculates/estimates) the color at each position based on the signals recorded by adjacent photosites.

Tests generally show that a 4.65MP Foveon sensor generates higher image quality than a 4.65MP conventional Bayer matrix sensor, but really equal to a 14MP conventional sensor (at least in terms of resolution). More on this later.

The SD15 sensor is actually slightly smaller than APS-C sensors from Canon, Nikon, Sony, Pentax etc. It measures 20.7 × 13.8 mm (Canon APS-C sensors are around 22.3 × 14.9 mm while most of the others are around 23.6 × 15.8 mm). This yields a “digital multiplier” factor of just over 1.7x (Canon is 1.6x, the others are 1.5x).

SPECIFICATIONS
Storage Media	SD/SDHC Card, Multi-Media Card
Image Sensor Size	20.7 X 13.8mm
Lens Mount	SIGMA SA bayonet mount
Angle of View	Equivalent to approx 1.7x the focal length of the lens (for 35mm cameras)
Image Sensor	Foveon X3 direct image sensor (CMOS)
Number of Pixels	(Effective pixels) 14.06 MP (2,652 X 1,768 X 3)
Image Recording Format	Lossless compression RAW data (12-bit), JPEG (Super High, High, Medium, Low)
Continuous shooting speed	3 frames per second
Buffer	21 frames
Viewfinder	Pentaprism SLR viewfinder, 98% vertical, 98% horizontal, 0.9x magnification, 18mm eyepoint, -3 to +1.5 diopter adjustment
Metering Systems	77 segment Evaluative Metering, Spot Metering, Center Metering, Center-Weighted Average Metering
ISO Sensitivity	100, 200, 400, 800 and 1600 (ISO 50 and 3200 with Extended Mode)
Exposure Compensation	+/-3EV (in 1/3 steps)
Shutter Speed	1/4000 – 30 sec. + bulb (up to 30 sec., with Extended Mode: 2min.) sync at 1/180s
Built-in flash	Manual Pop up Built-in Flash, GN11  (17mm lens angle covered)
LCD Monitor	3.0" TFT colro LCD monitor, Approx 460,000 dots, Coverage 100%
Power Source	Li-ion Battery Pack BP-21, Battery Charger BC-21, AC adapter SAC-4(optional)
Dimensions	144mm/5.7"(W) X 107.3mm/4.2"(H) X 80.5mm/3.2"(D)
Weight	680g/24.0oz (without batteries)

Operation

The SD15 has the usual set of “push a button, turn a dial” controls, but it feels a little less well thought out than a camera like, for example, the Canon EOS 60D. For example, with the SD15, the dial that is used to change aperture, shutter speed, ISO, exposure compensation etc. surrounds the shutter release button. It’s almost in a horizontal plane, while the Canon equivalent is just behind the shutter button in a vertical plane. As it turns out, it’s much easier to rotate the vertical wheel of the Canon with your shutter release finger than it is to rotate the horizontal wheel of the SD15.

Changing ISO or exposure compensation requires two fingers (or a finger and thumb), since the ISO or EC button has to be held down while the dial around the shutter release is rotated. The setting is displayed on the top panel, but all other info disappears while ISO or EC is being set. ISO is only displayed while the button is held down.

Bob Atkins

There is no constant display of JPEG/RAW mode or white balance mode on the LCD or in the viewfinder. In order to see (and adjust) these settings you have to use the rear LCD. There is a “QS” (quick set) button, however, which makes access to RAW/JPEG selection, white balance, color rendition and image size selection fairly easy.

There are just four shooting modes: Program, Aperture Priority, Shutter Priority and Manual set by a dial on the right next to the top LCD display. A second dial on the left selects one shot, continuous shooting, self timer, autoexposure bracketing and mirror lock up, as well as turning the camera on and off.

Bob Atkins

One unique feature of the SD15 is that it uses a window right behind the lens mount in order to prevent dust and dirt entering the mirror box from the outside. This means that the sensor should stay much cleaner. Any dirt that deposited on the window wouldn’t show up in the image because it would be too far away from the sensor to cast a detectable shadow. The window is removable so the sensor can be cleaned if necessary. The window does mean that you couldn’t use a lens that extended into the camera body (as EF-S series lenses do on EOS bodies), but since there are no such Sigma lenses, it’s not really an issue!

This window is also the IR blocking filter (found on the sensor in most other cameras), so if it is removed the image will be formed by UV, visible and IR light. If you put a visible blocking/IR passing filter on the lens (such as a Hoya R72) you get a camera with high IR sensitivity. Most DSLRs with an IR filter over the lens require long exposures (measured in seconds) because most of the IR passing though the filter gets absorbed by the IR blocking filter in front of the sensor. This makes the SD15 very useful for those who want to experiment with IR imaging. UV imaging would also be possible with the right filter. Of course with any visible light blocking filter over the lens, you can’t see what you are pointing at, but as long as you don’t mind aiming the camera first and then putting on the filter, all is OK. IR focusing can be a little tricky but with some trial and error and a wide/normal lens (and a small aperture) getting good focus isn’t hard.

Operation isn’t particularly speedy. For example, in RAW mode you can shoot 21 frames in around 7 seconds, but then the camera goes dead for about 15 seconds and “busy” is displayed on the LCD. At that point you can shoot another frame, but even if you don’t the camera still spends the next 2.5 minutes writing data to the memory card.

Converting a RAW file to JPEG using the supplied Sigma software takes just under 30 seconds if you want the output to be 2640×1760 (4.65MP). There is an option to double the output size to 5280×3520 (18.6MP) via software and that took around 85 seconds on my PC (Dual Core 2.5GHz AMD processor). There didn’t seem to be any huge advantage in using the Sigma RAW converter to double the image size over a bicubic upsizing using an image editor.

Bob Atkins

There’s always the question of just how much resolution Sigma can squeeze out of the sensor with 4.65 million photosites on it. The lack of an anti-aliasing low pass filter should sharpen the image somewhat, though it’s still constrained by the same theoretical limit (Nyquist limit). Comparing shots taken with the SD15 and the lowest pixel count DSLR I had available (a 10MP Canon EOS 40D), it was quite clear that the 10MP Bayer matrix sensor outresolved the 4.65 Foveon sensor, but that’s really no big surprise. I suspect that the Foveon would have outresolved a 4.65MP Bayer sensor, but since I didn’t have a camera with such a sensor, I can’t say for sure. However, I think the test against a 10MP sensor is fair because even the most basic, entry level, current production DSLRs have a 10MP sensor. The difference was even greater when comparing the resolution of the SD15 with a basic entry level DSLR like the 18MP Canon EOS Rebel T2i.

Though the Sigma sensor doesn’t use an anti-aliasing filter (which does lower resolution slightly), that just can’t make up for the lack of (spatially resolved) pixels.

Performance

Autofocus was reasonably fast, but on careful examination of the images it appeared that the camera backfocused slightly. Although this wasn’t immediately noticeable when using a relatively slow lens, when using a fast lens such as the Sigma 30mm f/1.4 wide open, the images were soft. Giving Sigma the benefit of the doubt I’d assume this was an issue to the specific camera I was testing and that it is something that could be fixed. Nevertheless, it was somewhat disappointing that the camera didn’t get the maximum performance out of the 30/1.4 shot at wide apertures unless manual focus was used.

Metering tended towards the side of giving images more exposure rather than less, almost to the extent that I was inclined to dial in a little -ev exposure compensation sometimes, depending on the subject. Colors were good, but not really in any way that amazed me or suggested that the sensor was out of the ordinary.

Noise seemed higher than I’m used to seeing from my EOS 40D, especially at ISO 400 and over.

Conclusion

The Sigma SD15 has three features, which are unique and which some may consider makes it better than most other (non-Sigma) DSLRs.

• It has a Foveon sensor
  
• It has a sealed mirror box
  
• It can be used for IR and UV imaging

On the other hand, it lacks a number of features found in most other current DSLRs:

• A high resolution (high photosite count) sensor
  
• Any form of Live View
  
• Any form of video
  
• The ability to simultaneously record RAW and JPEG files
  
• ISO settings over ISO 1600 (3200 in expanded range)

So do the former make up for the latter? Unfortunately, for 95% of potential users, I don’t think so. While the Foveon sensor is said by some to give better color rendition, sometimes described as a “almost 3-D effect”, I really don’t see it. Color rendition is good, but then so is color rendition from most conventional Bayer sensors. The limited photosite count (4.65 million) just can’t compete in terms of resolution with even what is today a “low-end” conventional Bayer sensor of 10MP. It’s probably better than a 4.65MP Bayer sensor would be, but there simply aren’t any current DSLRs with such low pixel count sensors. Even the least expensive entry level DSLRs currently have 10MP sensors. The Foveon sensor does allow the low pass filter (which is required with most Bayer sensors) to be eliminated, which in theory should sharpen the image somewhat, though that effect isn’t huge since it starts out with a significant resolution disadvantage because of the relatively few photosites on the sensor.

The sealed mirror box of the SD15 is nice. It does mean that dirt can’t get into the camera from the outside when the lens is removed, and it’s much easier to clean the sealing window than it would be to clean the sensor. If dirt does get on the sensor from inside the camera, the sealing window can be removed to reach the sensor itself. Removing the window and placing a filter over the lens does allow IR imaging with good sensitivity and even UV imaging should be possible.

Then you have to consider the other limitations of the SD15. No Live View of any type, no video recording, no simultaneous RAW and JPEG and the somewhat unsophisticated menu system and slightly awkward ergonomics.

The Sigma SD15 is also limited to the use of Sigma lenses. There’s nothing at all wrong with Sigma lenses, but with the Sigma body you have no choice but to use them. If you buy a Nikon/Canon body you not only get to choose from the manufacturer’s lenses, but you can also chose any Sigma lens, as well as any Tamron or Tokina lens (and a few more brands besides). Even with Sony and Pentax you can use many Sigma, Tokina and Tamron lenses, if not all of them.

And finally you have to factor in the price of the Sigma SD15, which is currently $989 and takes it well out of the entry level DSLR price range and puts it in competition with mid range cameras like the Canon EOS 50D and 60D and the Nikon D90 (maybe even the Nikon D7000)

Though the SD15 is capable of creditable performance, from a cost/features point of view it just seems to me that it’s not competitive today for general purpose photography. It’s really a slightly updated SD14 and that was introduced over 4 years ago (September 2006). It’s now showing its age, even compared to current entry level DSLRs.

The one group that might most appreciate the SD15 would be those inclined to experiment with UV and IR imaging, which is easier with the SD15 than conventional DSLRs with blocking filters attached to the sensor..

The good news is that Sigma has said that in 2011 they will be releasing the SD1, which will have a 46MP 1.5x crop Foveon X3 sensor. Though 46MP, that’s actually 15.3 million phototosites, each of which generate 3 color signals, so its resolution should be competitive with the current crop of 18MP Bayer sensors used by Canon. If they also address some of the other issues, the SD1 may be a competitive camera, though as far as I’ve heard, still no Live View or movie modes (at least initially).

Where to Buy

You help to support Photo.net when you purchase from one of our partners. The Sigma SD15, (compare prices) (review) is available along with a selection of Sigma lenses including the Sigma 30mm f1.4 for Sigma, (compare prices) and the Sigma 18-50mm f/2.8-4.5 SLD DC OS HSM for Sigma, (compare prices).

Image Samples

Bob Atkins

Sigma SD15 with Sigma 30mm f1.4 lens. 1/250s at f/5.6. ISO 100. Overall exposure is a little high, blowing out some of the background detail, though the car itself is well exposed.

Bob Atkins

Sigma SD15 with Sigma 18-50mm f/2.8-4.5 SLD DC OS HSM. 50mm focal length, f/16 at 1/30s, ISO 200. A closeup shot taken handheld using optical stabilization.

Bob Atkins

Sigma SD15 with Sigma 18-50mm f/2.8-4.5 SLD DC OS HSM. 18mm focal length. 1/100s at f/5.6, ISO 200. The Sigma 18-50mm f/3.5-5.6 DC AF provides a wideangle view, equivalent to 30.6mm on a full frame camera due to the 1.7x multiplier.

Bob Atkins

Sigma SD15 with Sigma 18-50mm f/2.8-4.5 SLD DC OS HSM. 23mm focal length. 1/25s at f/9.0, ISO 100.

Bob Atkins

Sigma SD15 with Sigma 18-50mm f/2.8-4.5 SLD DC OS HSM. 50mm focal length. 1/800s at f/5.6. ISO 800. An exposure compensation of -1 stop was used for this image.

Readers' Comments

Add a comment

William Wilgus , January 25, 2011; 10:18 P.M.

Where in the world did you get the determination that the resolution of a 10MP Bayer is 'significantly superior' to the SD-15's?  A Cracker Jack Box?

Mike Maas , January 25, 2011; 11:07 P.M.

What enlargement technique was used to compare the uprezzed sigma with the canon image?  I've enlarged SD14 images to match the Canon 5D and at 100 iso I couldn't tell the difference.  I used the same lenses for both, a set of xenotars, through a bellows and lanczos for uprezzing.   Those results were for iso 100.  

I wouldn't claim the SD14 is as good of an all around camera as the 5D but when used carefully it can equal the 5D in many situations.

Mike

 

Jessica M. , January 25, 2011; 11:39 P.M.

William - DPReview said the same thing. In their test of the DP1, which uses the same sensor that's in the SD15, they said:

"While it outresolves the downscaled Nikon D60 image and almost matches the resolution of the ten megapixel Ricoh, it cannot really compete with a modern 10 megapixel DSLR such as the Nikon D60."

And it can't.

Michael James , January 26, 2011; 02:04 P.M.

I think your review is fair and on target.  In fact my Sigma 30mm f/1.4 also still has back focus issues despite being sent to Sigma repair 3 times.  I think that is a dud for Sigma in the AF arena.

One area of your review I got different results than you.

I shot a Canon 5D (original) along with SD14 side by side in 2007 and 2008 and nobody could guess correctly which was shot by which camera for both prints and large online galleries.... and the 5D is a 12+mp full frame camera.  So I'm not sure why the SD15 which is basically just a SD14 with bug fixes wouldn't be able to match a 10mp camera for you.

That combo again was:

5D + Canon 16-35mm f/2.8L II

versus

SD14 + Sigma 10-20mm f/4-5.6

Comparing was easy because the focal lengths matched given it was 10-20mm X 1.7 crop factor for the sigma.

The reason I haven't upgraded to the SD15 is that I have heard from other Sigma users that the dynamic range of the SD15 went down.  Users reported shooting them side by side and the SD14 would retain more highlight data.  So I opted not to upgrade and wait for the next gen body.

I'm pretty excited about the prospects of the SD1.  Sigma hired some engineers at the time they bought FOVEON so I am hopeful that many of the issues that held back the SD14/SD15 will improve in the upcoming SD1.

Alan Rockwood , January 28, 2011; 01:46 A.M.

Based solely on the Nyquist limit, along with how the physical light detection sensors are arranged on the sensors, a 4.65 megapixel Foveon sensor should equal the resolution of the green channel of a 9.3 megapixel Bayer sensor, and it should out-resolve the red and blue channels of a Bayer sensor by a factor of 1.4. The reason is a 9.3 megapixel Bayer sensor only has 4.65 megapixels in the green channel, and half that many sensors in each of the red and blue channels.

 

 

By the way, don't talk to us about interpolation to increase the resolution beyond the Nyquist limit. It is mathematically impossible. Any attempt to do so will be, at best, an educated guess and subject to various artifacts and distortions.

 

One more comment. In my opinion this camera would be a lot more interesting if it had an EOS mount rather than a Sigma mount. Sigma is capable of building a camera with this mount, since they did it for a DSLR once sold by Kodak.

Abbas Haider , February 03, 2011; 11:43 P.M.

Good comparison however I have canon 7d, Nikon D60 and sigma SD14.

When you shoot stretch the shot to its max size then you will know the difference. There is certainly diff. between SD14 & D60.

The only thing with SD14 is you have to drop the Hue abit (-7 tp -3) to get the similar reddish colors

Finallt the drawbacks of SD14 are

Slow writing to the memory which I can live with it and it requires high lighting which I am using it in day lights shotting 

Sattva Karma , February 05, 2011; 10:33 A.M.

Compliments on a very well written review. Writing 'anything' about Foveon sensor cameras always seems to invite 'passionate' discussion :) ... and i have learnt that actual photo images speak louder than words! So i will leave it at that!

I personally did not feel very inclined to upgrade from my SD14 to the SD15 as technically there was not much that has been improved...things like faster in-camera image processing i was not very kicked about as my PC can do all that number crunching comfortably. I was actually hoping for better low light performance in the SD15!

As noted by you...the basic camera ergonomics need some attention and a more stringent quality control check for the lenses will be invaluable.

Lets see what the SD1 has in store for everyone! Cheers!

Yakim Peled , February 11, 2011; 05:08 P.M.

The sensor looks great but I won't be buying any SA mount lenses (low resale value). I really wish they'd offer their cameras with EF or F mount. Kodak did it once......

 

Happy shooting,

Yakim.

Scott Kennelly , March 16, 2011; 01:07 P.M.

I think there is an advantage of the SD15 that you gloss over. The 1.7 crop factor. For many people who shoot nature (particularly birds), the 150-500mm Sigma lens is the KING. It's very affordable, and it's capable of producing very good quality images. With a 1.7 crop sensor behind it, you get that much more magnification. Compare that with what the Canon 7 D and the Nikon D7000 produce with the same lens. The SD15 will give comparable image quality with a slight advantage. The reason I say it will give comparable image quality is because the Sigma will produce red images at about 4.5 megapixels or blue images at about 4.5 megapixels, and the other cameras will produce about the same resolution for red images and for blue images. When comparing green images or images with a combination of colors, the more expensive cameras will have a slight advantage. My guess is that the Canon 60 D user will find a slight advantage in value when using the same 150-500mm Sigma lens. The upcoming Sigma SD1 will eliminate any advantage gap there, and will place Sigma in the game with the Nikon D300s and Canon 7 D (primarily for its weather sealing and image quality).

 

There is one more thing you seem to just gloss over. The Sigma SD15 has a HUGE buffer for shooting in RAW mode. I don't know of any other camera in its price range that offers such a huge RAW buffer, something necessary when shooting a quick action series of two mating birds, a group of kids having fun, or a surfer riding the wave of his life. If I run into a 6 frame limit while shooting such an event, my camera is NO GOOD TO ME. I can't stress the importance of a very large RAW shooting buffer enough. I bought my Canon 5 D because of its large 14 frame buffer for shooting RAW. The Sigma SD15 has a 21 frame buffer.

 

One major disadvantage of the SD115 is its slow 3 fps shooting speed (something I sometimes felt hindered with when shooting my Canon 5 D). I wish the SD15 could shoot 7 fps. I also wish the new SD1 will shoot at that speed, like the latest competitors in its upcoming price range.

John T, June 20, 2011; 02:34 A.M.

I have been looking at alot of photos from the SD15, is it just my vivid imagination or are the colors captured seem brighter than other competing cameras? Any thoughts?

Don Park , June 22, 2011; 10:50 P.M.

I've switched from Canon EOS world to Sigma Forveon sensor world. After using SD15 for a month, I sold all the Canon equipment, because I was absolutely sure that I would not need them any more. I do admit most of the reviewer's points as they are mostly true.

However, I have a quite different viewpoints form the reviewer's. Let me ask each of you for what you buy cameras/lenses/etc. and what you do with those. In my case, I want good to great pictures to see and to share. That's the utmost crucial criteria on buying optical equipment. Images as a final result are kind of "must to have", and convenience features such as Live View mode, RAW+JPEG, HD-class motion framing, etc. are "nice to have" consideration to me. Some may desperately needs non-still pictures, then he or she makes a decision based on his/her own criteria.

We get analog pictures finally out of digital works.

In short, I am just satisfied with the SD15 + Sigma-provided software(Sigma Photo Pro 5.0) + Apple Aperture 3 + Nik Software plug-ins. I can produce a Ruben's painting like pics, Leica-produced picture like images, of course can produce Canon conventional pics at my discretion. I admit I am not a great photographer, but I do have a tool to make it happen someday.

Lastly, I do ask a favor to the reviewers. There are two groups of people, (1) who seek picture quality like photographic artist and (2) who seek optical quality, build quality, features like a mechanic. The review should show two facets, and let the readers take the points valuable to them.

 

Kevin Sanders , February 09, 2012; 09:16 P.M.

I would love to have a Canon mount on my SD15, but it isn't going to happen. Why? In a word, 'Licences'. Sigma don't have them. When they make Canon-fit lenses they do it by reverse-engineering a Canon lens. Canon would never grant a licence to Sigma (or anyone else, I suspect) to use the Canon mount on a camera. Kodak may have obtained a licence from Canon and Nikon for their brief visit to the DSLR arena, but I think that was because they were Kodak.

I'm not a very good photographer, but I love the technology and I love 'having stuff'. Besides the SD15, I shoot a Canon 1D3, a 40D and a Nikon D200. I have a Sigma SD14, but I'm selling that. The SD15 is so much better.

If I've learned one thing in the five years I've been attempting photography, it is 'buy good glass'. It doesn't necessarily have to be expensive as there are some great bargains out there. On my SD15 I have a Sigma 300mm F4 APO HSM lens that is awesome. It even has FTM and macro close focussing. Pictures taken with the Canon 300mm F4 L IS on the 40D are no better (and, I have to admit, no worse) but the Sigma lens cost less than £200 on eBay.

The Foveon sensor is great. In the coming years, if Sigma can develop it well enough, it could be the only thing to have in a camera. As for the mega pixies, Sigma "lies" about resoultion because everyone else does. Nikon's new D800 is probably more like 18Mp than 36, but it has got 36 million photo sites. Sigma has only 15.3 in the new SD1, but they are three deep so sigma's claim of 46Mp is probably just as valid as Nikon's claim of 36. Don't get me wrong, the new Nikon is a far better camera than the SD1 in so many ways, but the image quality from the Foveon sensor is undeniable. If IQ is what you want...

I bought my Canon 1D3 for the frame rate. When the 1D4 came out, the frame rate wasn't that much better so I decided that I would buy a reasonable camcorder to give me the one thing the 1D3 can't do. The 1Dx, though... "we wantssssss it,, my preciousssss". I digress. Again. what I'm trying to say is that you have different cameras to do different stuff. No one would say that the SD15 (or even the SD1) is a sports camera, or one that will get you birds in flight, but you have other cameras for that. If you can only afford one camera, you have to go with what you do most. Rarely will that be the Sigma, but sometimes it will. Just saying.

It may not be your way, but it is a way. Happy shooting!

Mike Maas , February 10, 2012; 03:01 A.M.

Donald,

Are you aware that almost 100% of the electronic protocols between the camera and the lens are identical on both cameras along with the sensor face to camera flange distance?  The major exception on lenses is stabilization which will not work on a canon lens that has been converted to a sigma mount.  

While a canon body flange will not in all likelihood be made by Sigma at least one private individual has done the modification in a very precise way.  Others have also done so but with perhaps a little less precision.

Many have changed out the lens flanges however and there is no reason at all that the same conversion cannot be done for almost any canon lens.  Moreover, Conorus can convert the Contax N lenses to sigma just as easily as they can to canon.  

Mike

Kevin Sanders , February 10, 2012; 04:57 A.M.

SD15+300mm F4 APO HSM macro

Mike,

If you are implying that Don was wrong to sell his Canon lenses because they could have been converted, I have to disagree. Sigma Canon-fit lenses can be converted to Sigma-fit and vice-versa very easily, but converting a Canon lens to Sigma-fit would not only be difficult, it would reduce the value of the converted lens considerably. If I misunderstood your comment, I apologise.

One of the coolest things lens-wise about Sigma-fit currently is that there are loads of old (but good) 'film' Canon-fit Sigma lenses which no longer work correctly on Canon digital cameras, but work perfectly when restored to their "mother mount" :o) My 300mm F4 macro is one such and I get great shots with it on my SD15.

The Sigma cameras that have been converted to Canon-mount by skilled engineers could not be created in the factory by Sigma because to do so would require a licence from Canon. I don't think that Sigma would want to pay for, nor Canon grant, such a licence. Hence the Sigma fit, which, as you rightly pointed out, is nearly 100% Canon. Nearly, but not quite.

Mike Maas , February 10, 2012; 11:39 A.M.

No, I wasn't suggesting he was wrong to have sold his lenses.  I wasn't sure from his letter if he was aware of just how similar the two mounts are.

As to difficulty of conversion that is, as is beauty, in the eye of the beholder.  Perhaps a screwdriver and a soldering iron may not be all that is needed but if one wants to use a t/s 17mm on a sigma as some may, an additional trip to a machinist will be a relatively small part of the total expense of conversion and lens.   

As to loss of value, all such conversions should be readily reversible except for the residual soldering that will have been done.

I would only encourage conversion of lenses for which there are no close equivalents available from Sigma though, like the T/S lenses.

Mike