لا يضبط قوة العدسة إلا إذا كانت وصفة النظارة جديدة.

It is the Arabic pattern for restriction.

Literally, لا ... إلا means not ... except. In smoother English, that often becomes only or only if.

So لا يضبط قوة العدسة إلا إذا ... means that adjusting the lens power happens only if the condition after it is true.

إذا by itself means if or when.

But إلا إذا means something like except if. Because the main clause is negative with لا, the whole structure gives the sense of:

it does not happen except if...

That is why English often translates this kind of structure as only if. In some contexts, it can also feel close to unless, but literally it is except if.

In normal Arabic writing without vowel marks, يضبط can be ambiguous.

The same spelling can represent:

• an active reading: he/it adjusts
• a passive reading: is adjusted

The context or the given meaning tells you which one is intended.

This is very common in Arabic: short vowels are usually not written, so sometimes you cannot know the exact grammatical reading from the bare spelling alone.

Arabic often leaves the subject unstated if it is understood from context.

So if the verb is read actively, يضبط can mean:

• he adjusts
• it adjusts

The exact subject is not named in the sentence itself. It could be a person, a machine, a doctor, a technician, or something else already known from context.

English usually wants an explicit subject, but Arabic often does not.

This is an iḍāfa construction, often called the construct phrase.

It is made of:

• قوة = power/strength
• العدسة = the lens

Together, قوة العدسة means the power of the lens or lens power.

A few important things about iḍāfa:

• the first noun usually does not take ال
• the second noun tells you whether the whole phrase is definite
• since العدسة is definite, the whole phrase is definite

So even though قوة has no ال, the whole phrase can still mean the lens power.

Because the noun it refers to is وصفة, and وصفة is feminine singular.

So Arabic uses the feminine singular form:

• كانت = she/it was

The full subject is وصفة النظارة. The head noun is وصفة, and that is the word that controls agreement.

So:

• وصفة is feminine singular
• therefore كانت must also be feminine singular

It is feminine because it agrees with وصفة, which is feminine singular.

So:

• وصفة = feminine singular
• جديدة = feminine singular

It does not have ال because it is the predicate of كان. In Arabic, the predicate in this kind of sentence is often indefinite.

So Arabic says:

• وصفة النظارة جديدة
• كانت وصفة النظارة جديدة

Even though English says the glasses prescription is new, Arabic normally leaves new without ال.

This is a very common Arabic pattern.

After إذا, Arabic often uses a verb form that looks past in order to express a general condition or a future condition.

So إذا كانت ... is often best translated as:

• if ... is
• when ... is

not necessarily if ... was

This feels unusual to English speakers at first, but it is completely normal in Arabic.

Because ن is a sun letter, the ل of ال is assimilated in pronunciation.

So النظارة is pronounced roughly as:

an-naẓẓāra(h)

not al-naẓāra

In writing, however, it still keeps the normal spelling ال. This is just a pronunciation rule, not a spelling change.