Actual image of IR LED contamination from a 30 second exposure at ISO 10,000 (Sony A7S-II)
The streak in the top right corner seems to be a random artifact - possibly a reflection
If you shoot full spectrum photography with a converted Sony A7 II / III — or have been considering a conversion — there is one specific, poorly-documented issue worth understanding before you commit to long exposures and high-ISO work: internal IR LED contamination from the shutter mechanism itself. If you are still at the research stage, our Full Spectrum Camera Buying Guide covers what to look for when choosing a converted camera, and our article on why the Sony A7 is one of the best cameras to convert explains what makes it such a strong platform.
This is not a light leak in the traditional sense. It is not caused by a gap in the body shell, a poorly seated lens, or stray ambient light. The source is an infrared optocoupler — an IR emitter and light sensor pair — built directly into the physical shutter mechanism housing assembly.
What the LED is actually doing
Sony uses a tiny near-infrared LED, radiating at around 850nm, as an electronic diagnostic shutter monitor. Its job is to track the mechanical shutter curtains in real time, measuring their precise travel speed and timing to ensure exposure accuracy and to check for shutter degradation or errors. It is a functional component of the shutter assembly, not an incidental emitter — and it is active during every exposure.
In a stock A7 II / III, this presents no problem whatsoever. The factory IR-cut filter sitting over the sensor completely blocks near-infrared light, so the diagnostic LED is invisible to the sensor regardless of how long the exposure runs or how high the ISO is pushed. To understand exactly what happens to that filter during a conversion, read: Inside the Full Spectrum Conversion Process.
Why it becomes a problem after conversion
Once that IR-cut filter is removed for a full spectrum conversion, the sensor is fully exposed to infrared light — including the 850nm output of the diagnostic LED. Even though the LED is incredibly faint, it sits right next to the sensor plane inside the shutter block. During long exposures, and at high ISO settings, its light bleeds across the internal chassis and causes a distinct, localised fogging or purple/white glow. Its intensity increases as ISOs get higher and exposures get longer.
The contamination typically creeps in from the bottom-right corner of the frame, which corresponds to the upper-left of the physical sensor layout. It is consistent, repeatable, and entirely independent of the lens or scene — which is what makes it so confusing to diagnose if you don't know what you're looking for.
Can you bypass it in firmware?
This is where the A7 II / III becomes particularly frustrating. On later models such as the A7 IV, switching entirely to the electronic (silent) shutter turns off the diagnostic monitor loop and bypasses the leak — a clean firmware-level workaround that costs nothing and requires no physical modification.
On the A7 II /III series, however, Sony's firmware fires the optocoupler check regardless of which shutter mode you select. Mechanical shutter, Electronic First-Curtain Shutter (EFCS), or Silent Shutter — the diagnostic loop runs in all three. There is no menu setting, no hidden option, and no firmware workaround that disables it. If you are shooting with a converted A7 II / III at exposures much beyond one second and at elevated ISO, this contamination may become detectable. In reality this limit is far greater than one second, and we get to that at the end.
The physical fix
Because the firmware route is closed on the A7 II / III, the only reliable solution is physical shielding applied during the conversion itself. Professional conversion houses that specialise in these specific bodies apply highly opaque black masking material around the frame edges of the replacement glass filter and sensor carrier. The goal is to mechanically baffle the stray 850nm light before it reaches the active area of the sensor — intercepting it at the point where it would otherwise scatter across the internal chassis.
This is not something that can be applied externally or retrofitted without full disassembly. It requires the sensor carrier to be removed, the shielding to be precisely positioned, and the assembly to be rebuilt and tested. Done correctly, it reduces the contamination, but won't necessarily eliminate it entirely. Done incorrectly — or skipped — it leaves every long-exposure or high-ISO frame compromised.
What to look for if you already have a converted A7 II / III
If you are unsure as to what point the IR contamination might become visible, the test is straightforward:
- Put the lens or body cap on and shoot a 2–4 second exposure at ISO 3200 or above in a very dark space (you want to avoid any potential light leakage from other sources)
- Examine the lower right side of the resulting frame
- Any purple or warm glow in that area — on an otherwise black frame — is the diagnostic LED bleeding through
- Repeat at ISO 6400 and 10 seconds if the result is ambiguous at lower settings
Affected bodies
The Sony A7 II, A7S II, A7R II, A7 III, A7R III, A9, and A9 II use an IR LED as part of their electronic shutter monitor. Once these models are converted to IR, the internal IR LED may contaminate all images shot at exposures much greater than one second. Only the specific models mentioned above have this specific issue.
The base A7 II, although still affected according to our testing, is more immune to this IR LED leak compared to the "R" and "S" variants, and generally behaves much better enabling longer exposures without the contamination.
The A7S III is completely immune as it doesn't have the same internal hardware as the other variants.
The Sony A7 IV, A7R IV, and A7R V also use an IR LED as part of the electronic shutter monitor. Once these models are converted to IR, Full Spectrum, or H-alpha, the internal IR LED will possibly contaminate all images shot at exposures much greater than one second. However, switching to silent shutter will de-activate the IR sensor loop and the problem goes away as the responsible LED is off.
A note on other A7-series bodies
The A7 II / III series is the most commonly affected body in our experience (not every variant), but the underlying mechanism — an 850nm diagnostic optocoupler in the shutter assembly — is present across all the later A7 ranges (not the MK1). Whether it causes visible contamination post-conversion depends on the internal geometry of each specific body and how close the LED sits to the sensor plane. If you are converting any A7-series body and plan to shoot at exposures beyond one second, it is worth asking your converter explicitly whether they assess and shield for this issue as part of their standard process - we do!
The "Real World" Conclusion
The Sony A7 II / III series remains an excellent choice for full-spectrum conversion, offering superb image quality for daylight infrared, some astrophotography and general photography. While it has a known, minor internal IR LED contamination, it is generally considered a strong candidate for full-spectrum conversion, though it is not recommended for long-exposure, deep-sky tracked astrophotography with the exception of the A7S III.
The one second exposure rule is "debunked" as just a very safe guaranteed limit set by the big well known conversion specialists and, in the real world, you will be able to go much further to five or ten second exposures and even beyond, dependant on ISO's, without noticing any IR LED contamination - especially if the sensor has been shielded, but the actual limit can't be guaranteed as each individual camera can have slightly different characteristics.
It gets very confusing, and far too easy to over-think things when researching snippets of contradictory information. You really need to experiment with your own settings to find this limit. Most big conversion specialists will under-exaggerate the true long exposure limits for understandable reasons (their reputation), and it isn't an exact science.
Unless you are planning to do very long exposures at high ISO's, then it will never be a problem. If you are, then consider the first generation A7, A7R, A7S, A7-II (not completely immune), A7S III or the A7 IV / V models with their safe silent shutter settings.