Some Comments about Focus-Peaking - 21 Mar 2021
There are several ways to achieve sharp focus using the technique of focus-peaking:
One way is to first focus on the subject using the spotting scope, then attaching the camera and using the lens to obtain critical focusing through focus-peaking. Turning the lens focusing ring gives a magnified view of the subject and allows critical focus to be achieved. Pressing the shutter will revert the image to normal magnification and allow a sharp image to be captured. One some cameras the lens may need to be adjusted toward macro focusing, while on others (including my Sony cameras) the lens needs to be focused at or just beyond infinity.
I had mentioned in previous lens reviews that I had gotten sharper images with my Sigma 30/2.8 (APS-C lens) attached to my Sony a6300 (APS-C camera) than when I tried the much more expensive Zeiss 35/2.8 (Full-Frame) lens on the a6300. The reason, I found, was that the Sigma allowed me to focus just beyond infinity to achieve razor-sharp images while the Zeiss stopped at infinity (so images ended up being slightly soft).
Recently, I've just been setting the camera lens to infinity and focusing on the subject using the scope.
This raised a question from a friend asking whether the camera lens had to be focused "beyond infinity" to achieve maximum sharpness (if the first method required focusing beyond infinity). Great question! I decided to test it out. Short answer: it really doesn't matter.
It turns out that focus-peaking will give a sharp capture on the camera regardless of whether the scope or lens is at maximum sharpness as long as either technique achieves a sharp focus.
To test this, I moved the lens focusing to the 3m setting (approximately half-way to infinity focus) and focused on this American Robin using the scope after activating focus-peaking. The image was razor sharp even though the camera lens was nowhere near infinity.
Message here is that as long as focus-peaking gives a sharp image the camera will capture a sharp image.