Learn Hardcore Hebrew: האישון קטן, אבל חדות הראייה טובה. - The pupil is small, but the visual acuity is good.

Because this is a present-tense nominal sentence. In Hebrew, when you say that something is something in the present, you usually do not use a separate word for is.

So:

האישון קטן = the pupil is small
חדות הראייה טובה = the visual acuity is good

In past or future, Hebrew does use forms of to be, but in the present, they are normally omitted.

What does ה־ mean in האישון and הראייה?

ה־ is the Hebrew definite article, meaning the.

So:

אישון = pupil
האישון = the pupil

and

ראייה = vision / seeing
הראייה = the vision

Hebrew adds the article directly to the beginning of the word, instead of using a separate word like English the.

Why is it קטן and not קטנה?