Brennvidden er lang.

Because Norwegian usually puts the definite article on the end of the noun.

• brennvidde = focal length
• brennvidden = the focal length

So English uses a separate word the, but Norwegian often uses a suffix instead.

Because lang is not directly modifying the noun inside a noun phrase here. It comes after er and works as a predicative adjective.

Compare:

• en lang brennvidde = a long focal length
• Brennvidden er lang = The focal length is long

So before the noun, lang is attributive; after er, it is predicative.

Adjectives in Norwegian agree with the noun in gender and number. Here the subject is singular and non-neuter, so the adjective stays in its basic form: lang.

Compare:

• common gender singular: lang
• neuter singular: langt
• plural: lange

So Brennvidden er lang is correct.

Because in predicative position—after verbs like er—the adjective does not take a special definite form. It only agrees for gender and number.

So:

• Brennvidden er lang = correct
• lange would be wrong here

You do get -e on adjectives in many attributive definite phrases, such as den lange brennvidden.

Yes, it is grammatical and understandable. But in real use, Norwegians will often talk about the thing that has the focal length:

• Objektivet har lang brennvidde = The lens has a long focal length
• Det har lang brennvidde = It has a long focal length

So the given sentence is fine, but it can sound a little technical or dependent on context.

Usually, no—not if you want the normal optics expression. With brennvidde, Norwegian normally uses lang and kort:

• lang brennvidde = long focal length
• kort brennvidde = short focal length

Stor brennvidde might be understood, but it is not the usual phrasing.

The normal opposite is kort.

• Brennvidden er kort = The focal length is short
• Objektivet har kort brennvidde = The lens has a short focal length

In a yes/no question, Norwegian usually puts the verb first:

• Er brennvidden lang? = Is the focal length long?

That verb-first pattern is very common in Norwegian questions.