Vector Optics show the differences between the Direct Bulb and Diaphragm Bulb used for red dots... "Direct Bulb and Diaphragm Bulb reticles offer distinct advantages for optics users. The Direct Bulb provides consistent brightness and simplicity, ideal for straightforward shooting scenarios. In contrast, the Diaphragm Bulb excels in adaptability, offering adjustable brightness that suits various lighting conditions. Each design caters to specific needs, ensuring users can select the option that aligns with their preferences and shooting environments. Understanding their differences is key to making an informed choice for your optical equipment."