Wednesday, September 3, 2014

Sigma DP Merrill & Monochrome

Abandoned Boles-Aero Trailer - Mojave, California
Captured using a Sigma DP2 Merrill.
I get asked camera questions from time-to-time. Sometimes the question is found in a blog post comment. Sometimes it is via social media. Sometimes I'm asked in person. Other times the question comes in my e-mail inbox.

Yesterday I received a question in my e-mail and I wanted to answer it right here. The question is regarding the Sigma DP2 Merrill camera. Are there any differences between shooting in black-and-white mode and shooting in color mode converting to monochrome later in post-processing?

First, I want to explain how Sigma's Foveon sensor is different from traditional Bayer sensors, and why the Foveon sensor is better for monochrome.

Sigma's Foveon sensor has three layers, one sensitive to green, one to red and one to blue. The image is captured fully in each color channel. Bayer sensors have only one layer, with pixels sensitive to either green, red or blue. Half of the pixels are sensitive to green, and the other half are split between red and blue.
On A Brighter Day - Tehachapi, California
Captured using a Sigma DP2 Merrill.
What this means for monochrome photography, when using a Foveon sensor, is that the color channels can be manipulated to simulate what color filters do to black-and-white film. For example, you simulate the look of using a red filter by using the red color channel.

You can also do this with a Bayer sensor, but you have far less data to work with. Instead of having a complete photograph available in the red color channel, you only have 25% of an image. The software has to grab data from the other color channels and manipulate it significantly to compensate. It doesn't take too long before degradation sets in.

This is a particular problem for Bayer sensors with low resolution, and less of a problem for higher resolution sensors. A 24 megapixel sensor has twice as much data to work with than a 12 megapixel sensor, and a 36 megapixel sensor has three times as much data to work than a 12 megapixel sensor.

Next, it is important to understand the differences between RAW, TIFF and JPEG file types. RAW files are unprocessed data and are not yet photographs. They are like undeveloped film. TIFF files are processed photographs, but where the unused data is still kept. I guess this might be like developed film that hasn't been rinsed. JPEG files are processed photographs where all of the unused data has been erased.
Broken Angels - Bodfish, California
Captured using a Sigma DP2 Merrill.
Here is another way to understand this. Let's say you have a plastic kit (car, airplane, train, building--it doesn't matter). RAW is all of the pieces and parts unassembled. They're sitting in a pile waiting to be constructed. TIFF is the kit assembled with a tacky glue that's easy to undo and the extra parts are saved. JPEG is the kit assembled with super glue and the extra parts are tossed in the trash.

So what does all of this have to do with the question? It is the foundation to understanding the answer.

I don't own a DP2 Merrill anymore, but, if I remember correctly, if you use the black-and-white mode on the camera it saves the files as JPEGs. You are letting the camera decide how the monochrome photograph will be, then you are throwing away all of the useful data that makes using the Foveon sensor great for black-and-white photography. You might as well use a Bayer sensor.

If I am wrong and the camera saves the image as RAW when in the black-and-white mode, then it is the same as using the white-balance monochrome conversion method in Sigma's software. If this is true, there is no difference between using color mode and converting later in post-processing or using the black-and-white in-camera mode. But I don't believe this is true--I'm 85% certain that using the black-and-white mode saves the files as JPEGs.

It is best to avoid using the black-and-white mode when using Merrill cameras so that you can take advantage of the three-layered sensor. Capture in color, then convert to monochrome in post-processing.

Update: Someone told me that I didn't explain very well how the whole RAW/TIFF/JPEG thing tied into the answer to the question. When using a Sigma Merrill camera for monochrome, save in RAW, do whatever editing you want in Sigma's software, save as a TIFF, do whatever other post-processing you want in the software of your choice, and then save as a JPEG. 

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