Zoom lens system

An image pickup apparatus comprising:

• a camera body;
• and
• a zoom lens system coupled to the camera body, wherein the zoom lens system includes, in order from an object side:
• a first lens unit having a positive refractive power;
• a second lens unit having a negative refractive power that moves for magnification-varying;
• a third lens unit having one of a positive refractive power and a negative refractive power that moves for magnification-varying;
• and
• a fourth lens unit having a positive refractive power, wherein:
• when an Abbe constant of a positive lens Lp having a smallest Abbe constant among lenses constituting the first lens unit is denoted by νp, a partial dispersion ratio thereof is denoted by θp, a refractive power thereof is denoted by φp, an average value of Abbe constants of positive lenses except the positive lens Lp is denoted by νap, an average value of partial dispersion ratios of negative lenses of the first lens unit is denoted by θan, a refractive power of the first lens unit is denoted by φ1, and a refractive power of an entire zoom lens system at a telephoto end is denoted by φtele, the following expressions are satisfied:
• −0.03<(θp−θan)/θan<0.15;
• 0.005<(1/νp−1/νap)/(φ1/φtele)<0.030;
• and
• 0.05<φp/φ1<0.4,
• provided that, when a refractive index for g-line is denoted by Ng, a refractive index for C-line is denoted by NC, a refractive index for d-line is denoted by Nd, and a refractive index for F-line is denoted by NF, the following expressions are satisfied;
• Abbe constant ν=(Nd−1)/(NF−NC)
• Partial dispersion ratio θ=(Nq−NF)/(NF−NC).