Resolution

How sharp is a monochrome converted camera?
Original RAW file image from color a7.

  This page will demonstrate the difference between using a color camera to produce B&W images, or producing them directly from a monochrome converted camera . If you want to produce a great B&W image, it’s best to forgo the color portion of the image capture process and use a converted camera.

  Below are images taken in succession using two Sony a7’s. The first two images are the same image taken with a color a7 (the second image is the color image desaturated in LR), and the last two were taken using the monochrome a7(m) with and without a Color Correcting filter. This camera is a full spectrum converted camera; meaning that there are no filters over the sensor. The original color image is shown at left. Please take into consideration that most all lenses are sharpest near their center. But the sample here is taken from near the edge of the frame. And yet, it still resolves quite well the details in the image.

  The same lens was used for all images, and set to the same f-stop and the focal length was locked in place. The lens used: Nikon 50mm 1:1.2 @ ƒ8. The shutter speed was changed to achieve equal exposures of +1 EV indicated in the histogram. This setting maximizes exposure without clipping—ETTR. Equal sharpening was applied in post processing for all images. No other enhancements or adjustments were used on the images.

 

Factory stock Sony a7 color camera.
Sony a7 color image, converted to B&W in LightRoom.
Sony a7(m) monochrome converted camera using a color correcting filter.
Sony a7(m) monochrome converted camera without color correcting filter. Full spectrum UV-VIS-IR.

  Take note of the reflections in the side window of the car. There is a reflection of a roof with standard roof tiles. You can see the tiles in the monochrome converted images, but not in the desaturated color image—and forget the color image, it’s just muddy. The reflection of the tree off of the rear window has more detail also. Now imagine the results if I were using the a7R(m). I used the a7(m) for this test because it mimics the Leica Monochrome in resolution and sensor size. The a7 is a prime candidate for monochrome conversion and is reasonably priced on the used market.


Why does the image without the CC filter look fuzzier?

  The extra IR light is what causes the loss of resolution, as in the last image. The cause for this is the IR light does not focus at the same focal plane on the sensor as does the Visible light due to the limitations of the lens’ design and its coatings—it was designed to focus visible light. This is a form of aberration, chromatic aberration, and occurs when a lens attempts to focus all colors (wavelengths) onto one imaging plane. By limiting the wavelength range entering the optical system with a CC filter, significant resolution improvements can be made using a conventional lens. General rule here is to use a CC filter when there is a large amount of IR light present, such as during a bright sunny day as pictured above in the examples. When shooting indoors, at night or with flash, you can leave off the CC filter and gain extra sensitivity.

  There is no trickery here; these are the actual image files. Contact me if you would like the unaltered RAW files to inspect.