How The Exit Pupil Affects Brightness in Binoculars
March 6, 2025 | Posted by Best Binocular Reviews 
Understanding the Relationship Between Lens Size, Magnification & Low-Light Visibility
In this article, I wanted to briefly go over the main points about the exit pupils on binoculars and just how their size affects the perceived image brightness in different light conditions. For a more in-depth version, you can also look at my complete guide to the exit pupil.
What is The Exit Pupil?
- On a pair of binoculars, the “Exit pupil is the diameter of the beams of light coming out of eyepieces”
- Formula: 📏 The size of the Exit Pupil (mm) = Objective Lens Diameter ÷ Magnification
Exit Pupil Sizes in Different Binoculars
| Binoculars | Lens Size (mm) | Magnification | Exit Pupil (mm) |
|---|---|---|---|
| 10x50 | 50mm | 10x | 5mm |
| 8x42 | 42mm | 8x | 5.25mm |
| 8x36 | 36mm | 8x | 4.5mm |
| 10x42 | 42mm | 10x | 4.2mm |
| 8x32 | 32mm | 8x | 4mm |
| 10x36 | 36mm | 10x | 3.6mm |
| 10x25 | 25mm | 10x | 2.5mm |
How The Exit Pupil Size Affects Perceived Brightness
Expected Pupil Size in an Eye of an Average, Healthy, Young Adult Human in Different Light Conditions:
| Condition | Avg. Human Pupil Size |
|---|---|
| 🌞 Daylight | 2-3mm |
| 🌅 Dusk/Dawn | 5mm |
| 🌑 Very Low Light to Dark | 7mm |
📌Key Concept:
✅ If Exit Pupil ≥ Eye Pupil → Brighter Perceived Image
❌ If Exit Pupil < Eye Pupil → Image Looks Dimmer
📌Best Binoculars for Low-Light Use
✅ 7x50 (Exit Pupil = 7.1mm) → Great for Very Low Light
✅ 8x56 (Exit Pupil = 7mm) → Great for Very Low Light
⚖️ 8x42 (Exit Pupil = 5.25mm) → OK in Very Low Light, Good in Low Light
⚖️ 10x50 (Exit Pupil = 5mm) → OK in Very Low Light, Good in Low Light
⚖️ 8x36 (Exit Pupil = 4.5mm) → Not Ideal, but also not bad in Low Light
🔻 10x42 (Exit Pupil = 4.2mm) → Not Great at Very Low Light, OK in Low Light
🔻 8x32 (Exit Pupil = 4mm) → Not Great at Very Low Light, OK in Low Light
❌ 10x36 (Exit Pupil = 3.6mm) → Poor Low-Light performance, Good in Daylight
❌ 10x25 (Exit Pupil = 2.5mm) → Poor Low-Light performance, Good in Daylight
📌 Conclusions:
- Most binocular configurations will appear bright enough in good light conditions ☀️👀.
- For low-light use (e.g., dense forests 🌲, twilight 🌆, or very overcast weather 🌫️), choose binoculars with a moderately large exit pupil (≈5mm) for better brightness.
- For very poor light conditions (e.g., after sunset 🌄, before sunrise 🌅, or deep jungle 🌳🌑), go for binoculars with a large exit pupil (≈7mm) to maximize perceived brightness.
Categories: Binocular Terms | 
Tags: Exit Pupil | 
Comments Off on How The Exit Pupil Affects Brightness in Binoculars
