If you are shooting with a 590nm “False Color” filter, channel swapping is mandatory. Without it, your images just look red. But if you are shooting with a standard 720nm filter – often sold as a “Black & White IR” filter – you might ask: Is there any point in channel swapping?

The short answer is: Yes, if you want cleaner skies. The long answer involves the physics of glass and the limitations of digital sensors.

The Myth of the “Brick Wall”

When we talk about a “720nm filter,” we tend to imagine a magical barrier that blocks 100% of light below 720nm and lets in 100% of light above it.

In reality, optical physics rarely works like a brick wall. Most filters have a Transmission Slope.

A typical 720nm filter might start opening up at 680nm, reach 50% transmission at 720nm, and not hit full transmission until 750nm. This means a small amount of “deep red” visible light (~680nm–700nm) leaks through to your sensor.

The “Sepia” Problem

Because of this visible light leakage, and because digital sensors (via their Bayer filters) still differentiate slightly between “Deep Red” and “True Infrared,” a raw 720nm image is rarely monochromatic.

If you set a custom white balance on foliage:

1. The Foliage turns white/neutral grey.
2. The Sky, however, often renders as a muddy, dark brown or sepia tone.

This is where the Channel Swap comes in.

Why We Swap 720nm

Even though there isn’t enough color data to create the vibrant gold/blue of a 590nm image, there is just enough data to “clean up” the image.

By performing a standard Red/Blue channel swap on a 720nm image:

• The Brown Sky is mapped to the Blue channel. This shifts the muddy sepia tone into a pale, steel-blue or a neutral dark grey.
• The Foliage remains largely white/bright.

The Visual Difference

• Unswapped: Looks like an old, faded sepia photograph.
• Swapped: Looks like a crisp, cold winter landscape with faint blue tones in the sky.

Many photographers find the “Swapped” look much more pleasing, even if they plan to eventually convert to Black & White. The swap creates better tonal separation between the clouds and the sky, giving you more flexibility when you finally desaturate the image.

The Exception: Deep IR (850nm)

This rule falls apart if you move deeper into the spectrum.

If you use an 830nm or 850nm filter, the “Transmission Slope” is now located so far past the visible spectrum that no visible red light reaches the sensor.

• The Red, Green, and Blue pixels on your sensor are all recording the exact same pure infrared signal.
• There is no color differentiation. The image is truly monochromatic raw data.

If you try to channel swap an 850nm image, absolutely nothing happens. The Red channel and Blue channel are identical.

Summary

• 590nm: Swap is required for color.
• 720nm: Swap is optional, but recommended for cleaner, cooler skies and better contrast.
• 850nm: Swap is impossible/useless.

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