OCULAR OPTICAL SYSTEM

1 . An ocular optical system, for imaging of imaging rays entering eyes of an observer via the ocular optical system from a display screen, a side facing towards the eye being an eye-side, a side facing towards the display screen being a display-side, sequentially from the eye side to the display side along an optical axis, comprising first, second, third and fourth lens elements, each of the first, second, third and fourth lens elements having an eye-side surface facing toward the eye side and a display-side surface facing toward the display side, wherein:

the display-side surface of the first lens element comprises a convex portion in a vicinity of the optical axis;

the second lens element has a positive refracting power;

the third lens element has a refracting power;

the display-side surface of the fourth lens element comprises a concave portion in a vicinity of the optical axis; and

wherein G4D is a distance between the display-side surface of the fourth lens element and the display screen along the optical axis;

AAG is a sum of all three air gaps from the first lens element to the fourth lens element along the optical axis; and

G4D and AAG satisfy the inequality: G4D/AAG≤7.

2 . The ocular optical system of claim 1 , wherein

EFL is an effective focal length of the ocular optical system;

and EFL satisfies the inequality:

3≤250/EFL≤15.

3 . The ocular optical system of claim 1 , wherein

G23 is an air gap between the second lens element and the third lens element along the optical axis,

G34 is an air gap between the third lens element and the fourth lens element along the optical axis; and

AAG, G4D, G23 and G34 satisfy the inequality:

( AAG+G 4 D )/( G 23+ G 34)≤8.2.

4 . The ocular optical system of claim 1 , wherein

T1 is a central thickness of the first lens element,

T4 is a central thickness of the fourth lens element;

and AAG, G4D, T1 and T4 satisfy the inequality:

( AAG+G 4 D )/( T 1+ T 4)≤5.

5 . The ocular optical system of claim 1 , wherein

G23 is an air gap between the second lens element and the third lens element along the optical axis,

T4 is a central thickness of the fourth lens element,

T1 is a central thickness of the first lens element; and

G23, T4, G4D and T1 satisfy the inequality:

( G 23+ T 4+ G 4 D )/ T 1≤10.

6 . The ocular optical system of claim 1 , wherein

T1 is a central thickness of the first lens element;

and AAG and T1 satisfy the inequality:

AAG/T 1≤3.5.

7 . The ocular optical system of claim 1 , wherein

ER is a distance from a pupil of the observer to the eye-side surface of the first lens element,

T2 is a central thickness of the second lens element,

G23 is an air gap between the second lens element and the third lens element along the optical axis; and

ER, G4D, T2 and G23 satisfy the inequality:

(ER+ G 4 D )/( T 2+ G 23)≤6.

8 . The ocular optical system of claim 1 , wherein

ER is a distance from a pupil of the observer to the eye-side surface of the first lens element,

G12 is an air gap between the first lens element and the second lens element along the optical axis,

G23 is an air gap between the second lens element and the third lens element along the optical axis,

T1 is a central thickness of the first lens element,

T3 is a central thickness of the third lens element;

and ER, G12, G23, G4D, T1 and T3 satisfy the inequality:

(ER+ G 12+ G 23+ G 4 D )/( T 1+ T 3)≤16.

9 . The ocular optical system of claim 1 , wherein

SL is a distance from a pupil of the observer to the display screen along the optical axis,

EFL is an effective focal length of the ocular optical system;

and SL and EFL satisfy the inequality:

SL /EFL≤1.9.

10 . The ocular optical system of claim 1 , wherein

SL is a distance from a pupil of the observer to the display screen along the optical axis,

ALT is a sum of the central thicknesses of all four lens elements; and SL and ALT satisfy the inequality:

SL/ALT≤ 4.3.

11 . An ocular optical system, for imaging of imaging rays entering eyes of an observer via the ocular optical system from a display screen, a side facing towards the eye being an eye-side, a side facing towards the display screen being a display-side, sequentially from the eye side to the display side along an optical axis, comprising first, second, third and fourth lens elements, each of the first, second, third and fourth lens elements having an eye-side surface facing toward the eye side and a display-side surface facing toward the display side, wherein:

the display-side surface of the first lens element comprises a convex portion in a vicinity of the optical axis;

the second lens element has a positive refracting power;

the third lens element has a refracting power;

at least one of the eye-side surface and the display-side surface of the fourth lens element is aspherical surface; and

G4D is a distance between the display-side surface of the fourth lens element and the display screen along the optical axis,

AAG is a sum of all three air gaps from the first lens element to the fourth lens element along the optical axis; and G4D and AAG satisfy the inequality:

G 4 D/AAG≤ 4.

12 . The ocular optical system of claim 11 , wherein

ER is a distance from a pupil of the observer to the eye-side surface of the first lens element; and

AAG, G4D and ER satisfy the inequality:

( AAG+G 4 D )/ER≤2.5.

13 . The ocular optical system of claim 11 , wherein

T3 is a central thickness of the third lens element,

G23 is an air gap between the second lens element and the third lens element along the optical axis; and AAG, G4D, T3 and G23 satisfy the inequality:

( AAG+G 4 D )/( T 3+ G 23)≤6.5.

14 . The ocular optical system of claim 11 , wherein

T3 is a central thickness of the third lens element,

T4 is a central thickness of the fourth lens element; and AAG, G4D, T3 and T4 satisfy the inequality:

( AAG+G 4 D )/( T 3+ T 4)≤5.

15 . The ocular optical system of claim 11 , wherein

G23 is an air gap between the second lens element and the third lens element along the optical axis,

T3 is a central thickness of the third lens element;

T4 is a central thickness of the fourth lens element; and G23, T4, G4D and T3 satisfy the inequality:

( G 23+ T 4+ G 4 D )/ T 3≤10.

16 . The ocular optical system of claim 11 , wherein

G23 is an air gap between the third lens element and the fourth lens element along the optical;

and AAG and G34 satisfy the inequality:

AAG/G 34≤6.

17 . The ocular optical system of claim 11 , wherein

ER is a distance from a pupil of the observer to the eye-side surface of the first lens element;

T2 is a central thickness of the second lens element,

T4 is a central thickness of the fourth lens element; and ER, G4D, T2 and T4 satisfy the inequality:

(ER+ G 4 D )/( T 2+ T 4)≤4.

18 . The ocular optical system of claim 11 , wherein

ER is a distance from a pupil of the observer to the eye-side surface of the first lens element,

G12 is an air gap between the first lens element and the second lens element along the optical axis,

G23 is an air gap between the second lens element and the third lens element along the optical axis,

T1 is a central thickness of the first lens element,

G34 is an air gap between the third lens element and the fourth lens element along the optical axis; and ER, G12, G23, G4D, T1 and G34 satisfy the inequality:

(ER+ G 12+ G 23+ G 4 D )/( T 1+ G 34)≤20.

19 . The ocular optical system of claim 11 , wherein

TTL is a distance between the eye-side surface of the first lens element and the display screen along the optical axis; and TTL and AAG satisfy the inequality:

TTL/AAG≤ 7.

20 . The ocular optical system of claim 11 , wherein

TTL is a distance between the eye-side surface of the first lens element and the display screen along the optical axis,

ALT is a sum of the central thicknesses of all four lens elements; and TTL and ALT satisfy the inequality:

TTL/ALT≤ 2.9