Foveon verses Bayer Sensors

There is a behind the scenes battle going on in digital still photography between two major technologies that has some of the features of the battle between VHS and Betamax video tape battle of yesteryear.

It is between the Bayer sensor which is a monochrome sensor, used in almost all digital cameras  and the worlds first true colour sensor -  the Foveon. The Bayer system uses a colour grid array filter overlay and in camera software allocates a colour value to each photosite, to give an emulated colour image. The Foveon is a 3 layer sensor with true colour sensitivity for red, green and blue. Foveon (now owned by Sigma) claim that this increases colour detail and sharpness detail in the images over and above Bayer sensors. I will not detail the precise techo talk behind this as it is well covered elswhere on the net (Wikipedia has a good summary).

I am a newcomer to Foveon sensors and I  now have in my posession the Sigma DP1 and DP2s cameras. My main system is a Sony a850.

I have some observations and opinions to share abut Foveon vs Bayer sensors.

I have been impressed by the image quality from the DP1 and DP2 being such small packages. I have printed a 48cm by 33cm print from the DP1 camera which approaches (in sharpness) the image quality from the Sony alpha full frame.

I am comparing the Sony full frame sensor with the APS-C in the DP1 and DP2s, that I have in my posession. The comparisons were made over a few weeks with the 3 cameras shooting in various conditions. These are my principal observations:

1) The FF sensor outresolves the Foveon sensor in fine detail as seen in the test images in this article. The former posseses 155% more pixels so that is not surprising.

2) Despite the claim by Sigma that the APS-C Foveon sensors have 14 megapixel these sensors have only 9.4 million effective pixels (for reasons that I will not detail here). This gives an effective pixel image resolution indistinguishable from the old Sony DSC - R1 bridge camera which had almost the same sensor area as those in the Sigma DP range.

I confirmed this by cropping an identical screen area from the same sized image from each camera (Sony DSC-R1 and Sigma DP) and magnifying up the pixels until their sizes could be compared. They were the same dimensions. The Sony DSC-R1 sensor had 10.2 million effective pixels. I have not included that test here but it was conlusive enough to me.

It is correct then that the DP Foveon sensors equate to 9.4 million pixels.

3) The dynamic range of the Sony FF sensor somewhat surpassed the Foveon sensors which blew out highlights that the Sony managed.

4) The colour palete of the Foveon sensors was (to me) more likeable than the Sony FF sensor. But that of course is a matter of taste. Also a Foveon preset that I have worked up in Silkypix 3.0 (which has good colour management) emulated the Foveon look very well without some of the more tricky Foveon artefaects like magentarised reds. In many ways the emulation was preferable.

5) The shadow noise rendition of the Foveon sensor was more pleasing than the Sony FF sensor but a noise reduction suite fixed the Sony's Bayer sensor weakness (that is CHROMA noise). The Foveon noise was more luminance and the Bayer more chroma. This holds until higher ISOs where the Foveon becomes a blotchy, coloured mess.

 

INFORMAL COMPARISON OF THE TWO SENSORS

I have done two quick, informal test and share them with you here.

For thie FIRST TEST I used the Sigma DP2s  which has a 40mm equivalent lens. The Sony a850 was fitted  with a Minolta 24mm f2.8 lens. Both lenses are of high optical quality.

Post processing was done in Lightroom 3.4 with no sharpening and no change in colour balance. Only exposure and highlight / shadow values were altered. Otherwise the pictures are as the cameras saw, with only conversions to jpeg to upload here.

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 FIRST TEST

Sony a850 full frame	APS-C Foveon 3 stitch
Sony a850 centre crop	APS-C Foveon 3 stitch centre crop
Sony a850 corner crop	APS-C Foveon 3 stitch corner crop

         SECOND TEST

LEFT - Sigma DP2 RIGHT Sony a850 with Minolta 28-135 at 40mm Taken within 2 minutes of each other, cropped to near the same magnification.

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COMMENT

These tests are subjective comparisons and readers may wish to differ from my interpretation here.

That there is a high degree of  resolution from the Foveon sensor is not in doubt.  Whether Foveon outperforms Bayer pixel for pixel would require formal MTF testing. In theory they should as the Bayer array that causes slight blurring of the image is gone and there is no guesswork as to which colour is designated to which photosite in the colour sensitive Foveon sensor.

The Sigma DP cameras produce remarkable IQ and their size means that they are pocketable. They are of course in direct competition with the many offerings in the high end compact market, such as the Canon G series, Micro 4/3rd pocket camera and Fuji x100. These other cameras are more feature rich.

Some advice to Sigma might be to bring out a rangefinder successor to the DP range with a good optical viewfinder and interchangeable lenses, and make the system fully featured. If it were priced competitively then Sigma would have a camera that could sell well and earn the company a reputation for camera making as well as lens manufacture.

FOVEON DSLR

The Sigma SD1 DSLR  (being the latest in Sigma's  big camera line up) has 15.4 (read 30.8) million photosites  and is thus a big leap up from Sigma pocket cameras. However at 7500 USD it is seems grossly overpriced,  probably due to executive decision making at Sigma that went badly wrong. The detail and overall IQ of the SD1 is awesome according to some reviews, and this camera may well outperform full frame DSLR's despite its APS-C sized sensor. I have not tested the SD1.

Certainly in bringing out the SD1 flagship, with a hinted price tag of about 2000 USD, and then launching it at 10,000 USD Sigma has alienated many of its fans. If Sigma is serious about Foveon that kind of pricing needs to become historical.

SUMMARY

To me there is an out of the camera magic to the Foveon images.

The impression is of a sensor that gives a unique look and may offer meaurable advantages in colour accuracy and fine detail. After much deliberation and republishing this article twice now it is my opnion that  the Foveon sensor is impressive and Sigma would do well to develop it further. In particular Sigma needs to catch up with what surrounds their sensors.