why am i calling it that and not a F# Diminished?
arent they the same thing?
Classical Guitarist
Quote by Zep_shizzle
why am i calling it that and not a F# Diminished?
arent they the same thing?

It is the same thing as 'half diminished'.

Diminished is just a 3 note triad F# A C.
An F#dim7 would have a diminished 7 too, so you got F# A C Eb. The chord you've got is F# A C Eb.
I had a typo. The chord you got is F# A C E not a diminished one like F# A C Eb.
^ there is an edit button for that reason.
As correctly said a full diminished is:
Root b3 b5 bb7
Half dim is:
R b3 b5 b7

A full diminished chord is made by having each note a minor 3rd above the last.
a half is adjusted so that the notes all fit within the diatonic scale.
When naming a seventh chord there's two basic parts you need to define - the triad and the seventh - followed by any alterations or suspensions to the chord.

Things to remember:
The triad is assumed unless stated otherwise
The seventh is assumed minor unless stated otherwise.

This way by saying Cm7 you know the "m" for minor applies to the triad because the 7th is assumed minor but if you write CMaj7 you know the "Maj" applies to the 7th because the triad is assumed major.

Now there's problems when you come across the term diminished used in a seventh chord name because it could be referring to either the triad or the seventh.

There are two solutions to this problem.

1. The first is to use the term "fully diminished" or "diminished seventh" to refer to a chord in which both the triad and the seventh is "diminished". When just the triad is diminished and the seventh is minor we use the term "half diminished" and in doing so we view the chord as a diminished triad with a minor seventh.

2. The second solution is to reserve the term "diminished seventh" for the fully diminished seventh chord in which both the triad and the seventh are diminished. The "half diminished chord then is referred to as a "m7♭5" in this term the "m" denotes a minor triad the "7" denotes a minor seventh, and the "♭5" says we alter the fifth by lowering it a half step. So we consider the chord a minor seventh chord with a flat five.

Both ways of naming a seventh chord with a diminished triad and minor seventh are commonly used though you will probably see the m7♭5 used more often. There is no real difference between them other than convention and personal preference.

Here's some labels to be aware of
C∅ = C half diminished = Cm7♭5 There's no need to include the seventh because only a seventh can be half diminished
C∆7 = C Major seventh The triangle is shorthand for Major
C- = C minor the dash is shorthand for minor
Cᴼ = C diminished a C diminished triad
Cᴼ7 = C diminished seventh aka "fully diminished" = C diminished triad with a diminished seventh
C+ = C Augmented the plus is shorthand for augmented.

So anyway I hope that goes some way to explaining why you are calling it a m7♭5 instead of diminished or half diminished.