Apertures can be de-oiled, de-greased and rebuilt.
Many classic rangefinder lenses suffer from some focus shift. Often, it’s not too noticeable. Specific designs, like the 50mm Sonnar and 35mm ASPH Summilux V1, it’s so pronounced, it can be annoying unless the user knows how to compensate for this.
We can calibrate lenses that have noticeable focus shift, to a particular aperture for you. On Sonnar lenses, f/2.8 calibration allows for the smaller apertures down to f/16-22, to all be in focus. However, f/1.5 and f/2 become front focused then. Choosing f/1.5 instead causes f/4 and f/5.6 to be back focused. With f/1.5 Sonnars, we’ve had a lot of luck choosing f/2.2 as the calibration point.
This causes f/1.5 to be sharper than the f/2.8 calibration, but then also causes f/4-5.6 to be sharper than the f/1.5 calibration, almost a happy compromise. Equally, we can do f/1.5 or f/2.8, which are the classic calibrations also if you prefer.
To prevent rangefinder lenses from having too dramatic of a focus shift, manufacturers often utilise what is termed ‘ninja star apertures’ at the mid aperture zones, from about f/2.8 through f/8. This ninja star pattern, allows the lens to drop in light transmission to achieve its next f/stop rating. However, the pointed stars on the aperture also cause the natural aberrations of the lens not to be reduced as much. Almost as if the lens had utilised a perfectly round aperture.
This purposeful leaving of the aberrations partially present until the depth of field can overtake the focus shift is the reason many classic rangefinder lenses have these type of apertures.
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