X-T100 /Rokinon 12mm focus

david bosdet wrote:

In most circumstances, you don't need to focus it at all. What you need to know is the hyper-focal distance of the lens. With the Samyang, that's 3m. So assuming you're not using it wide open, from f4 or so that means everything from about 40cm or so to infinity will be in focus.

The subjective term here is "in focus".

In the days of film, people expected some things, and now in the day of digital many expect much more. Of course, what was fine a long time ago could be fine today, depending on the subject matter.

It has to do with how much sharpness you want as a photographer. If you want details at pixel level, your parameters will be much more stringent. If you are doing a print and the main subject is in focus with pixel-level clarity, the rest of the image is likely not going to be critically important.

The circle of confusion (CoC) is useful to show that what some would call blurred is in the mind of others not blurred. It all depends on what you want:

https://en.wikipedia.org/wiki/Circle_of_confusion

From this article, "If there is no enlargement (e.g., a contact print of an 8×10 original image), the CoC for the original image is the same as that in the final image. But if, for example, the long dimension of a 35 mm original image is enlarged to 25 cm (10 inches), the enlargement is approximately 7×, and the CoC for the original image is 0.2 mm / 7, or 0.029 mm."

I'm viewing my monitor screen at a distance of 40cm. It is 55cm wide with 1920 pixels along this dimension. Pixel pitch is 1920 pixels/550mm = 3.49 pixels/mm. This supports about 3.5 lines/mm of image resolution (White vertical line, then black vertical line, then white vertical line, then almost half of another black vertical line). This screen is pretty clear and sharp for me, but I can still see the pixel dots, so a finer pitch would give me images without visible pixel dots. From the Wiki article, when they say "5 line pairs per mm" at a viewing distance of 25cm, it might be nearly what I have now.  This might require me to increase my screen resolution to 2750 pixels horizontally.

Do I need pixel clarity all the way across my existing screen? For text, yes but for photos maybe not. It's a judgement call.

So, "hyperfocal distance" is also a judgement call.

Here is a link to the Wikipedia article:

https://en.wikipedia.org/wiki/Hyperfocal_distance

Quoting from the first paragraph of this article (underlining added by me), "The hyperfocal distance is entirely dependent upon what level of sharpness is considered to be acceptable."

Interesting stuff, but much too subtle for me.  I usually want sharpness at the location in an image that I am most interested in, and then I usually slack off about the rest.

For a high quality landscape image of course you would want maximum sharpness at every pixel.  For a portrait, maybe not.

With the Rokinon 12mm F2, I usually get good results at F8 most of the time.  But, I find it necessary to manually focus at the shooting aperture to get the best out of any lens.  Fuji focus aids work well for this.  Even at F8, focus with this lens requires some real precision and high focus magnification.