If you've been researching full spectrum or infrared camera conversions, you've probably come across conflicting advice. Some conversion services claim mirrorless cameras need no focus recalibration after conversion. Others insist it's essential. So who's right — and why does it matter?
The answer, as with most things in specialist photography, is nuanced. And understanding it could save you from a camera that consistently misses focus in ways that are frustratingly difficult to diagnose.
Why Focus Calibration After Conversion Is Not the Same for Mirrorless and DSLR
Camera conversions for full spectrum or infrared work require different approaches depending on your camera type. Mirrorless and DSLR cameras have fundamentally different autofocus systems, and they handle filter stack changes in completely different ways. If you're new to the process, our guide to what happens inside a full spectrum conversion is a good place to start.
The DSLR Problem: Off-Sensor Autofocus
A DSLR has two separate optical paths: one to the sensor (for the image) and another to an autofocus module via a mirror and prism. When you change the filter stack during conversion, you shift the focal plane. But the AF module measures focus at a different plane entirely—redirected by the mirror above the sensor. If these planes no longer match, the camera focuses correctly according to its AF module, but the sharp focus lands in front of or behind the actual sensor.
Result: systematic front-focus or back-focus, especially with fast lenses.
Solution: Recalibrate the AF module to align with the new sensor plane.
The Mirrorless Difference: On-Sensor Autofocus
A mirrorless camera has no mirror and no separate AF module. Phase-detection autofocus (PDAF) pixels are built directly into the sensor itself. When the camera focuses, these pixels measure phase difference at the exact plane where the image lands—the sensor plane. There is no offset between where AF measures and where the image forms.
When you change the filter stack, the focal plane shifts. But this is purely a mechanical problem. You simply adjust the sensor's physical position (flange distance, spacers) so light converges directly on the sensor. Once positioned correctly, the PDAF system works perfectly—no AF recalibration needed.
Critically: The adjustment must be precise enough that the camera can focus at infinity. If infinity focus is not achieved, the camera cannot focus beyond a certain distance and is essentially unusable for most work. For a detailed look at how this is done correctly, see our article on shimming for perfect infinity autofocus after a full spectrum conversion.
What This Means For Your Conversion
DSLR: Requires AF recalibration to align measurement and imaging planes.
Mirrorless: Requires precision mechanical adjustment of flange distance so the camera can focus at infinity. Once infinity focus is verified, the camera is done—no AF fine-tune menu adjustments needed.
If a converted mirrorless camera won't focus at infinity, the conversion wasn't completed properly. If it relies on camera menu AF fine-tune adjustments to work, that's a workaround for poor mechanical assembly, not a proper fix.
The type of conversion also matters here — a full spectrum conversion versus a dedicated infrared conversion involves different filter replacements, which affects how the flange distance adjustment is calculated.
Choosing the Right Camera for Conversion
Not all mirrorless cameras are equally well-suited to conversion. If you're considering a Sony Alpha body, our article on why the Sony A7 is one of the best cameras to convert to full spectrum explains what makes it such a strong candidate. More broadly, not every camera can be converted — some are far more practical than others.
If you're still at the research stage, our full spectrum camera buying guide covers what to look for when choosing a converted camera.
Ask Your Conversion Service
— How was flange distance verified and adjusted?
— Was infinity focus tested and confirmed post-conversion?
— Was focus verified with a fast prime lens at maximum aperture?
A properly converted mirrorless camera focuses accurately across all distances without any menu adjustments. That's the standard you should expect.