If you own a full spectrum camera and want to use it for standard colour photography, you'll need an IR cut filter — a filter that blocks infrared light and restores the camera to normal behaviour. But not all IR cut filters are the same, and choosing the wrong type can result in images with a colour cast, reduced sharpness, or unexpected colour vignetting. This guide explains the difference between the two main types — hot mirror filters and absorption filters — and helps you choose the right one for your needs. For a broader overview of all the filters available for a full spectrum camera, read: Essential Filters for a Full Spectrum Camera.
Why You Need an IR Cut Filter with a Full Spectrum Camera
A full spectrum conversion removes the camera's internal IR cut filter, allowing the sensor to detect UV, visible, and near-infrared light simultaneously. This is exactly what you want for infrared or UV photography — but for standard colour photography in daylight, it creates a problem. Sunlight contains a significant amount of near-infrared light, and without an IR cut filter, this infrared light contaminates the image, causing a strong red or magenta colour cast and inaccurate colour rendering. For more on why this happens, read: Why Does My Full Spectrum Camera Have a Colour Cast?
An external IR cut filter screwed onto the lens solves this problem by blocking infrared light before it reaches the sensor, restoring the camera to standard colour behaviour. The quality and type of this filter matters more than many photographers realise.
Hot Mirror Filters
A hot mirror filter uses a thin optical coating applied to a glass substrate to reflect infrared light away from the lens. The coating is designed to reflect wavelengths above approximately 700nm while transmitting visible light with minimal loss.
Advantages of Hot Mirror Filters
- High optical quality — Good hot mirror filters are made from high-quality optical glass with precise coatings, and they introduce minimal distortion or loss of sharpness when used on quality lenses.
- Neutral colour rendering — A well-made hot mirror filter produces very neutral, accurate colour rendering across the visible spectrum, closely matching the results you'd expect from a standard unmodified camera.
- Thin profile — Hot mirror filters tend to have a relatively thin profile, reducing the risk of vignetting from the filter ring on wide-angle lenses — but there is a different type of vignetting on wide-angle lenses mentioned in the next section.
Disadvantages of Hot Mirror Filters
- Reflections and ghosting — Because hot mirror filters work by reflecting infrared light, they can sometimes cause internal reflections or ghosting when shooting in bright conditions, particularly if there are strong IR light sources in or near the frame.
- Angle sensitivity — The reflective coating on a hot mirror filter can be angle-sensitive, meaning its cut-off wavelength shifts slightly depending on the angle of incidence of the light. On wide-angle lenses this can be seen as a colour shift vignetting around the corners and edges of the frame which is not desirable.
- Quality variation — Hot mirror filters vary significantly in quality between manufacturers. A cheap hot mirror filter may have an imprecise cut-off wavelength, uneven coating, or poor optical quality that degrades image sharpness.
Absorption Filters
An absorption filter uses glass that is chemically formulated to absorb infrared wavelengths rather than reflect them. The IR-blocking properties are built into the glass itself rather than applied as a surface coating.
Advantages of Absorption Filters
- No reflection issues — Because absorption filters absorb rather than reflect infrared light, they don't suffer from the internal reflection and ghosting issues that can affect hot mirror filters. This makes them more reliable in challenging lighting conditions.
- Consistent performance across angles — Absorption filters perform consistently regardless of the angle of incidence, making them more predictable in use on wide-angle lenses.
- Robust and durable — Without a delicate surface coating to protect, absorption filters are generally more robust and less susceptible to damage from cleaning or handling.
Disadvantages of Absorption Filters
- Colour rendering differences — Some absorption filters have a slightly different spectral transmission curve compared to a standard camera's internal IR cut filter, which can result in subtle colour rendering differences — particularly in reds and skin tones. This varies between filter manufacturers and formulations, and some are more neutral than others.
- Thickness — Absorption filters can sometimes be thicker than hot mirror filters, which can increase the risk of vignetting on very wide-angle lenses, but in practice this rarely happens.
- Less common — Absorption-type IR cut filters are less widely available than hot mirror filters, and the range of sizes and thread options may be more limited.
Which Type Should You Choose?
For most full spectrum camera users who want to shoot standard colour photography, a high-quality absorption filter is the recommended choice. It offers neutral colour rendering, a thin enough profile, and a good one should be a close match to the original camera's internal filter. A hot mirror filter is worth considering if you don't intend to use it on wide-angle lenses, but best not to buy too cheap.
What to Look for in a Quality IR Cut Filter
Regardless of type, here are the key quality indicators to look for when choosing an IR cut filter for your full spectrum camera:
- Precise cut-off wavelength — The filter should block wavelengths above approximately 680–700nm cleanly, without a gradual or imprecise transition that allows some IR contamination through.
- High visible light transmission — The filter should transmit visible light with minimal loss — ideally 95% or above — to avoid any exposure penalty.
- Optical glass substrate — Avoid filters made with plastic or low-quality glass substrates, which can degrade sharpness and introduce distortion.
- Anti-reflection coatings — Multi-layer anti-reflection coatings on both surfaces reduce flare and ghosting and improve overall image quality.
- Correct thread size — Ensure the filter matches your lens's front thread diameter. Using step-up or step-down rings is possible but can increase the risk of vignetting.
A Practical Note on White Balance
Even with a high-quality IR cut filter, you may notice subtle differences in colour rendering compared to a standard unmodified camera — particularly in auto white balance behaviour. Setting a custom white balance or shooting in RAW and adjusting white balance in post-processing will give you the most accurate and consistent colour results. For a full guide to white balance with a full spectrum camera, read: White Balance and Exposure Settings for Infrared Photography.
In some cameras (such as the Sony A6000 and A7 series, amongst others) you can simply adjust the settings of the actual "Auto White Balance" and then just forget about it.
The Bottom Line
The IR cut filter is one of the most important accessories for a full spectrum camera owner who wants to shoot standard colour photography. Investing in a high-quality filter — whether hot mirror or absorption type — will make a meaningful difference to your colour accuracy, sharpness, and overall image quality. Don't let a cheap filter undermine an expensive camera conversion.
If you're unsure which IR cut filter is right for your camera and lenses, get in touch — we're happy to advise on the best option for your specific setup.