The Difference Between ZWO and QHY Binning, and How Software Pixel Binning Works

The previous article covered the hands-on procedure for software pixel binning (Bin); this one goes deeper into the principles: why the two camera makers, ZWO and QHY, have different Bin behavior, and why “doing Bin on the computer” is less likely to lose data than “doing Bin on the camera”.

ZWO and QHY Have Different Bin Defaults

Taking the ZWO 6200mm and the QHY600M as examples, the two actually use the same Sony IMX455 BSI sensor, but their default Bin behavior isn’t the same:

1. ZWO’s pixel binning only does “pixel averaging”. You can shoot with the same evenly lit flat panel and find that the brightness is roughly the same before and after Bin—that’s the signature of averaging.
2. QHY’s pixel binning defaults to “pixel summing” (similar to the behavior of traditional CCDs), though it can of course be switched to averaging.

Note that if QHY is set to summing, the values may exceed 16-bit, and in that case you need to adjust the readout command and enable 32-bit transfer to send the complete data out. Also, ZWO’s approach works for both mono and color cameras, whereas QHY’s summing Bin only works for mono cameras.

Why Whether to Bin on CMOS Follows Different Logic Than CCD

On a CMOS camera, setting software or firmware Bin at the moment of capture is mainly intended to reduce file size or raise the frame rate. But you have to watch the behavior of read noise:

• CMOS: after Bin, the read noise doubles;
• CCD: after Bin, the read noise actually goes down.

This is the fundamental difference between CMOS and CCD when it comes to Bin, and it’s why the incentive to “Bin at the camera” in the CMOS era isn’t as strong as it was for CCD. CMOS pixel binning is actually better suited to being done later, in software during post-processing, for better results.

The Pros and Cons of Doing Bin in Computer Software

Putting Bin into computer software has several real advantages:

1. It still improves signal-to-noise ratio, but the computation is more flexible

Software Bin likewise brings a 2x SNR improvement, and the computer’s computational efficiency is better, with more algorithms to choose from.

2. It doesn’t discard any data

This is the key point. So-called binary averaging isn’t really “adding them up and dividing by 4” (taking Bin 2 as an example). Adding four 16-bit binary numbers requires 18 bits (an overflow, because it exceeds 16 bits), and binary division by 4 is really adding them up and then discarding the lowest two bits (equivalent to shifting the decimal point two places to the left).

Therefore, if a ZWO user sets Bin directly on the camera, they lose this portion of the data; a QHY user needs to enable 32-bit transfer to send the complete data to the computer, otherwise the overflow portion gets discarded all the same. In contrast, doing Bin on the computer uses 32-bit computation, loses no data, and preserves the full well depth and bit depth.

3. The price is transferring and storing more data

Pixel binning sacrifices resolution and sharpness goes down, but the file size shrinks dramatically. Conversely, if you don’t Bin at the camera, you need to transfer more data and also spend more space storing the original files. This is the price to pay when you choose “Bin on the computer”—bearing the larger original files up front in exchange for complete, lossless flexibility during post-processing.

An Addendum

For instructions on enabling 32-bit transfer on QHY, you can refer to the official user manual. Note that this feature is only supported on a few of the newer models (such as the QHY600, QHY268, QHY411, QHY461, and so on).