AZSteve wrote:
I do wish people when referring to corner coverage would be careful to elaborate. One can imagine illumination of the corner with minimal definition, suitable for OOF foreground and sky but not detail in focus, and illumination (or partial illumination, corrigible in post) with good definition.
Yes. And yes.
So, a better way to refer to this, and 'sides' and 'corners' in general, is to divide the general image circle into concentric rings - Zones A to D. These correspond to vertical stripes on MTF charts. For more on that, please see here: Fuji GFX Adapted Lenses
Also, discussions involving the GFX have revealed the need for a more precise definition of 'coverage'. Perhaps the clearest definition of coverage would involve the absence of a 'hard' (whatever that means) vignette caused by a mechanical obstruction. Where a soft vignette begins and ends is - literally - a grey area.
As has been pointed out, if we apply too strict a definition, many native lenses would not qualify as 'covering' their own format. A particularly heinous example of this is the Sony 18-135mm which at 18mm is completely inadequate for APS-C.
I think we have to regard illumination as the fundamental, but aberration as a distinct, secondary property. White wall tests effectively show whether a lens 'covers' GFX, but it doesn't quantify deleterious aberrations in Zone D. It's also true (noted earlier) that vignetting inconsistently differs with focal length and aperture.
To give an example: wide open, it appears that both the Sigma 50mm Art and Tamron SP 45mm cover GFX with only a soft vignette. The Sigma throws a better illuminated image circle at f1.4 than the Tamron does at f1.8. However, only when stopping both down and observing their different behaviour in Zone D do we see that the Tamron reaches a peak of performance at f8-f11 in which aberrations are largely absent. Whereas stepping through the full range of apertures with the Sigma Art, it is shown in extremis that its image circle is actually smaller, inasmuch as it's Zone D is never aberration-free.
AZSteve wrote:
I do wish people when referring to corner coverage would be careful to elaborate. One can imagine illumination of the corner with minimal definition, suitable for OOF foreground and sky but not detail in focus, and illumination (or partial illumination, corrigible in post) with good definition.
Yes. And yes.
So, a better way to refer to this, and 'sides' and 'corners' in general, is to divide the general image circle into concentric rings - Zones A to D. These correspond to vertical stripes on MTF charts. For more on that, please see here: Fuji GFX Adapted Lenses
Also, discussions involving the GFX have revealed the need for a more precise definition of 'coverage'. Perhaps the clearest definition of coverage would involve the absence of a 'hard' (whatever that means) vignette caused by a mechanical obstruction. Where a soft vignette begins and ends is - literally - a grey area.
As has been pointed out, if we apply too strict a definition, many native lenses would not qualify as 'covering' their own format. A particularly heinous example of this is the Sony 18-135mm which at 18mm is completely inadequate for APS-C.
I think we have to regard illumination as the fundamental, but aberration as a distinct, secondary property. White wall tests effectively show whether a lens 'covers' GFX, but it doesn't quantify deleterious aberrations in Zone D. It's also true (noted earlier) that vignetting inconsistently differs with focal length and aperture.
To give an example: wide open, it appears that both the Sigma 50mm Art and Tamron SP 45mm cover GFX with only a soft vignette. The Sigma throws a better illuminated image circle at f1.4 than the Tamron does at f1.8. However, only when stopping both down and observing their different behaviour in Zone D do we see that the Tamron reachs a peak of performance at f8-f11 in which aberrations are largely absent. Whereas stepping through the full range of apertures with the Sigma Art, it is shown in extremis that its image circle is actually smaller, inasmuch as it's Zone D is never aberration-free.
Jan 25, 2024 at 04:13 PM
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