>>3482453The native ISO of 400IR is 400, which is what you'd use when shooting with visible light. For infrared only you must increase the exposure by about 4 stops, which makes it ISO 25 equivalent. When you use 25 when taking a reading with visible light, you have the approximate correct IR exposure. Likewise when I shot ADOX HR50 (pic related) which is ISO 50, I metered it as ISO 4 for IR exposure.