One of the most technically demanding aspects of a full spectrum camera conversion is ensuring that autofocus remains accurate after the internal IR cut filter has been removed. When this is done correctly, the camera focuses perfectly at all distances — including infinity — across all lenses. When it's done incorrectly, or when a replacement glass is not precisely the right thickness, the result is a camera that front or back focuses, or that cannot achieve true infinity focus. This post explains why this happens and how it's corrected through a process called focal register correction. For a full overview of the conversion process, read: Inside the Full Spectrum Conversion Process. For a deeper look at why mirrorless cameras specifically require this mechanical adjustment — and why some conversion services incorrectly claim they don't — see our article on why a mirrorless full spectrum converted camera needs focus recalibration.
Why Focus Changes After a Conversion
The IR cut filter inside a camera is not just a filter — it's also an optical element that contributes to the overall optical path length between the lens and the sensor. This is known as the flange focal distance: the precise distance from the lens mount to the sensor surface that the camera and lens are designed to work with.
When the IR cut filter is removed or replaced with a piece of optically neutral glass, the optical path length changes if the replacement glass is not exactly the same optical thickness as the original filter. Even a difference of tens of microns — hundredths of a millimetre — can cause a measurable focus shift, particularly at longer focal lengths and when shooting at infinity.
The optical thickness of a piece of glass is not the same as its physical thickness. It depends on both the physical thickness and the refractive index of the glass. A high-quality conversion uses replacement glass (assuming that replacement glass is used at all as the other option is to use no replacement glass as the sensor already has a hard glass under the IR cut filter) that is precisely matched in optical thickness to the original filter, but in practice, achieving a perfect match requires careful measurement and, often, fine adjustment through focal register correction.
What is Focal Register Correction?
Focal register correction is the process of adding or removing extremely thin spacers — called shims — between the sensor assembly and the camera body to fine-tune the flange focal distance. By adjusting the position of the sensor by fractions of a millimetre, the technician can correct any focus shift introduced by the conversion and restore perfect autofocus accuracy across all focal lengths and distances.
Shims used in camera calibration are typically made from precision-ground metal or plastic and are available in thicknesses measured in microns — commonly 50µm, 100µm, 250µm, and 500µm (0.05mm to 0.5mm). The process of selecting and fitting the correct shim combination requires precision measurement equipment, technical expertise, and methodical testing.
How Focus Calibration is Performed
A proper focus calibration after a full spectrum conversion involves several steps:
1. Individual Sensor Assessment and Register Calculation
No two camera bodies — even identical models from the same production run — leave the factory with the same shim configuration. This is because the flange focal register of every unit is the product of accumulated manufacturing tolerances across multiple components: the sensor assembly, the sensor bracket, and the lens mount itself. Each of these varies by tiny but meaningful amounts, and Sony compensates for this at the factory by individually calibrating the shims at the three sensor contact points to bring each unit into specification. The result is a shim configuration that is unique to that specific body.
This has an important implication for conversion work. When the filter stack is removed, the register correction cannot be looked up from a table or applied as a fixed value — it must be derived from the existing shim configuration of that specific camera. Our process uses the factory shim values as the starting reference, calculating the required adjustment from the optical properties of the filter stack that has been removed. This preserves the sensor planarity that Sony established at manufacture while correcting precisely for the focus shift introduced by the conversion.
The outcome is a register correction that is as individual as the camera itself — calculated for that body, verified on that body, and confirmed with real-world focus testing before the conversion is signed off.
2. Register Adjustment
Fitting the shims requires care and precision. They must be seated evenly on the sensor mounting points so that the sensor remains perfectly parallel to the lens mount. Any tilt introduced at this stage would result in uneven focus across the frame — sharp on one side, soft on the other — regardless of whether the overall flange focal distance is correct. Each mounting point is checked and adjusted as needed before the camera is reassembled.
3. Infinity Focus Test
With the calculated register adjustment made, infinity focus is tested as the most demanding check of flange focal distance accuracy. The camera is pointed at a distant target and the autofocus is engaged. The resulting image is examined at 100% magnification to confirm that focus falls precisely on the target.
4. Multi-Lens Verification
Focus accuracy is then confirmed across multiple lenses at multiple focal lengths — a wide-angle, a standard, and a telephoto — and at a range of distances. This stage confirms that the register correction is performing as expected across real-world shooting conditions.
5. Final Sign-Off
Only when focus accuracy has been confirmed to be within acceptable tolerances across all test conditions is the conversion considered complete.
Why This Matters for Full Spectrum Photography
Focus accuracy is critical for any serious photography, but it's particularly important for full spectrum work for several reasons:
- Infrared focus shift — Infrared light focuses at a slightly different point than visible light. On a mirrorless camera with contrast or phase-detect autofocus, the camera focuses on what the sensor actually sees — including the infrared light — so this is less of an issue than on DSLRs. However, if the base flange focal distance is incorrect due to a poorly calibrated conversion, infrared focus shift compounds the problem.
- Landscape and infinity focus — Infrared landscape photography frequently involves focusing at or near infinity. A camera that cannot achieve true infinity focus is significantly limited for this type of work.
- Telephoto lenses — Focus errors are most apparent at longer focal lengths. A focus shift that is barely noticeable on a 35mm lens may be clearly visible on a 200mm lens.
What to Look for in a Conversion Service
Not all conversion services perform thorough focus calibration as part of their process. When choosing a converter, it's worth asking specifically about their focus calibration procedure. Our Full Spectrum Camera Buying Guide covers what to look for when evaluating a converter.
Micro-Adjustment as a Complement, Not a Substitute
Many modern cameras include an autofocus micro-adjustment (AFMA) feature that allows the user to fine-tune autofocus accuracy on a per-lens basis. This can compensate for small focus errors, but it has important limitations: AFMA adjusts focus at a single test distance and cannot correct focus errors that vary with distance, which is a symptom of an incorrect flange focal distance. AFMA is a useful fine-tuning tool for small residual errors, but it is not a substitute for proper shimming and focus calibration during the conversion process. For a full explanation of why mirrorless cameras need mechanical correction rather than menu-based workarounds, see our article on focus recalibration for mirrorless full spectrum cameras.
Our Approach
Every full spectrum conversion we carry out includes comprehensive focus calibration as standard. We test focus accuracy with multiple lenses at multiple distances, including infinity, and carry out focal register correction as required to achieve accurate focus across all scenarios. We don't consider a conversion complete until we're satisfied that the camera focuses correctly — because a converted camera that doesn't focus accurately isn't a tool, it's a frustration.