Stopped-down Focusing
When utilizing continuous autofocus on the a7 III, one thing to bear in mind is that when using smaller apertures, the camera
does not open up the lens for focusing. Because this means that the autofocus system is getting less light, and decreased phase separation between 'left' and 'right' (or 'up' and 'down') looking pixels, it can result in an image that is inaccurately focused. The example below is back-focused, but the focus inaccuracy can just as easily result in
front-focus. This is unfortunately exacerbated in backlit situations, where the AF system is already stressed by a drop in subject contrast.
This image was photographed in AF-C, but for depth-of-field, the photographer wanted to use F5.6. The individual's shoulder is clearly sharper than his nose, and as such, is back-focused. A similar shot resulted in
front-focus. Stop-down focusing should
theoretically increase focus accuracy, but in low light or low contrast situations, it can ironically decrease accuracy.
Sony FE 24-105mm F4 G | ISO 2000 | 1/125 sec | F5.6
Photo by Dale Baskin
In our controlled testing with both the Sony a7 III and a7R III, we found that there is no native Sony FE lens that focuses wide-open in AF-C. For reference, wide-open focusing is standard practice on DSLRs (as well as Canon and Olympus mirrorless), and allows the AF system to consistently get the most light available, and the best phase separation possible, regardless of the user's settings. Look-up tables in the lens can correct for any focus shift that results from stopping the lens down.
Unfortunately, there's no workaround for this other than to revert to the slower AF-S,
1 which focuses wide open.
2 A commonly held misbelief is that setting 'Live View Settings Effect' to 'off' remedies the issue, but it doesn't for AF-C: all it does is force the
initial focus acquisition to be done wide open, but then stops the lens down immediately thereafter, potentially throwing off focus - or causing hunting from reversion to CDAF - if continuous focus struggles at the smaller aperture. Furthermore, this disables one of the eminent advantages of mirrorless: being able to preview your exposure or depth of field.
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KNA (10 September 2020 - 02:53) писал:
