Nikon introduced the D750, the first full-frame DSLR to feature a tilting LCD and built-in Wi-Fi, in September 2014. In this in-depth review Shun Cheung discusses the ins and outs of this new offering...
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.
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).
SD/SDHC Card, Multi-Media Card
Image Sensor Size
20.7 X 13.8mm
SIGMA SA bayonet mount
Angle of View
Equivalent to approx 1.7x the focal length of the lens (for 35mm cameras)
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)
Li-ion Battery Pack BP-21, Battery Charger BC-21, AC adapter SAC-4(optional)
144mm/5.7"(W) X 107.3mm/4.2"(H) X 80.5mm/3.2"(D)
680g/24.0oz (without batteries)
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.