Infrared lens hotspots.

Lens choice matters more in infrared than in any other kind of photography I shoot. A lens that is superb in visible light can be unusable at 720nm, and a cheap kit zoom can be perfectly clean. Manufacturers do not test or publish infrared behaviour, so it has to be found by testing.

What causes a hotspot

• Anti-reflective coatings. Coatings are tuned for visible wavelengths. Outside that range, their efficiency falls off and more light reflects between elements.
• Internal barrel and aperture-blade finishes. Matte black paint that absorbs visible light can be noticeably reflective in infrared, turning the inside of the barrel into a soft ring light aimed at the sensor.
• Sensor reflectivity. The sensor stack itself reflects some infrared back into the lens, which then returns it — a round-trip that concentrates on the optical axis.

Because the effect is centred on the optical axis, it always shows up as a central blob rather than an edge artefact — which is exactly why it is so hard to hide.

Aperture and wavelength change how bad it is

Two variables reliably move the needle in my own shooting:

• Aperture: hotspots are usually invisible wide open and grow as you stop down. f/11 and f/16 are where they bite. This is inconvenient, because landscape work wants depth of field.
• Cut-on wavelength: a lens that is clean at 590nm can hotspot at 720nm or 850nm. Test at the wavelength you actually shoot, not at a lower one. Wavelength comparison.

Hotspots matter at every wavelength and with every filter setup — a dedicated conversion, a screw-on infrared lens filter, or a clip-in filter on a full-spectrum body. The infrared lens filter comparison covers the filters themselves; this page covers the glass you put in front of them.

The ten-minute hotspot test

1. Pick a bright, sunny day and a flat, evenly lit subject — a clear patch of sky away from the sun, or a large sunlit lawn.
2. Mount the lens on the converted body and set the widest focal length if it zooms.
3. Shoot the same frame at every full stop from wide open to the minimum aperture, adjusting shutter speed to keep exposure constant.
4. Repeat at the longest focal length. Zooms often hotspot at one end only.
5. Open the files, apply the same processing to all of them, and push contrast hard. A hotspot that is subtle in the raw file becomes obvious once contrast is raised.
6. Note the last aperture that is clean. That number is your working limit for that lens.

Do this before a trip, not during one. Every frame in the archive was made with lenses that passed this test at the aperture used.

Working around a hotspot-prone lens

• Shoot it wider. If it is clean to f/5.6, use f/5.6 and focus-stack when you need depth.
• Use it for subjects with busy centres. A hotspot hides in dense foliage far better than in open sky or water.
• Switch to a lower cut-on. Moving from 720nm to 665nm sometimes clears a marginal lens on a full-spectrum body.
• Accept monochrome. Some hotspots are recoverable in black and white with a radial correction; almost none are in false colour, because the swap amplifies the colour shift.
• Buy a known-good lens. Cheapest path in the long run if you shoot infrared often.

I have found that older kit lenses typically perform very well for infrared — simple coatings and fewer internal surfaces often mean fewer hotspots. Kolari Vision maintains a community List of known good & bad IR lenses (affiliate link, opens in a new tab) that is worth checking before you buy any lens for infrared.

I also keep a searchable infrared lens database here on the site — hundreds of lenses sorted by camera mount and rated good, maybe or bad for hot spots, so you can check a lens before you buy it.

Common questions