🧱 Build & Design

• Lens type: Manual/AF telephoto prime, Nikon F mount
• Focal length: 100mm (effective 150mm on D300S due to 1.5x crop)
• Aperture: f/2–f/16, 9-blade diaphragm for smooth bokeh
• Build quality: Lightweight plastic barrel, metal mount — feels budget but usable
• Focus ring: Smooth but lacks tactile precision; not ideal for manual focus critical work

✅ Strengths

📸 On the D300S

• Effective 150mm reach: Excellent for tight portraits, isolating subjects, and compressing backgrounds
• Wide aperture: f/2 allows shallow depth of field and low-light shooting
• Bokeh: Surprisingly smooth for the price — good subject-background separation
• Color & contrast: Decent rendering, especially when stopped down to f/2.8–f/4
• Compatibility: Fully mounts and functions on the D300S, including autofocus

💰 Value

• Price point: Often under $150 — one of the cheapest telephoto primes available
• Entry-level telephoto: Great for photographers exploring longer focal lengths without investing in Nikon’s 105mm or 135mm primes

❌ Limitations

⚠️ Autofocus

• Inconsistent AF: Can hunt or miss focus, especially in low light or on older bodies like the D300S
• Noisy motor: Audible whirring during focus — not ideal for video or quiet environments
• No internal focusing: Front element rotates, complicating use with polarizers

🧠 Optical Performance

• Wide open softness: f/2 is usable but not tack-sharp; improves at f/2.8 and beyond
• Chromatic aberration: Visible in high-contrast edges, especially wide open
• Flare resistance: Weak coatings — prone to ghosting and flare in backlit scenes

🕰️ D300S Constraints

• 12MP resolution: Limits cropping flexibility — lens flaws are less visible but detail is capped
• No live view AF: Manual focus is harder without magnified preview
• DX crop factor: You lose the “classic 100mm” feel — becomes more compressed, like a 150mm

⚖️ Summary Table

Feature	Yongnuo 100mm f/2 on D300S
Effective focal length	150mm (DX crop)
Autofocus	Functional but unreliable
Bokeh	Smooth, creamy at f/2
Sharpness	Soft wide open, better at f/2.8–f/4
Build	Lightweight, budget feel
Value	High — telephoto reach under $150
Best use	Portraits, detail shots, compressed street scenes
Limitations	AF inconsistency, flare, chromatic aberration

Sources: Ian Kydd Miller’s review, DPReview forums

✨ Conclusion

The Yongnuo 100mm f/2 is a quirky but capable budget telephoto prime. On the Nikon D300S, it delivers creamy bokeh and tight framing ideal for portraits and compressed street scenes. Autofocus and optical quirks require patience, but for the price, it’s a compelling tool — especially if you embrace its limitations and shoot deliberately.

Verdict: A budget telephoto with character — imperfect but rewarding when paired with the D300S’s rugged charm. (Not a bad lens for the $50 I paid for it, needs to be used with a little care, AF can be a little haphazard).