Actual measurement of the real impact of lens dust, fungus, and scratches on imaging: Which flaws can be ignored and which ones you should never buy

1. Dust Inside the Lens: In 99% of cases, it has absolutely no impact on image quality!

Almost all zoom lenses (due to the bellows effect) will develop tiny dust particles inside after a few weeks of use:

1. Geometric optical depth of field and defocus blur limit:
   • Even if there is a noticeable dust particle inside the lens, because it is extremely close to the camera sensor's focal plane (in an absolutely extreme out-of-focus state);
   • In the light path, the entire light beam collected by the front group of lens elements will diffract around it; the shadow of this dust on the sensor is blurred into an extremely faint scattered light that covers the entire image by billions of times;
   • Measured conclusion: As long as there is not a thick layer of dirt inside, a few tiny dust particles visible to the naked eye have an effect on sharpness, contrast, and clarity that is practically 0%, and in second-hand transactions, this is completely considered normal and acceptable!

2. Front Element Scratches vs Rear Element Scratches: Worlds Apart in Optical Fate

Can you buy a lens with scratches? The key is whether the scratches are on the front or rear elements:

1. Front Element Scratches: Minimal impact, a bargain for cost performance:
   • The front element has a huge diameter, and a 2mm fine scratch covers less than 0.1% of the light-transmitting area;
   • When shooting with the light source in front, photo sharpness does not decrease at all; the only effect is a slight radial flare around the scratch edges when facing strong backlight;
   • If you find a lens with slight front scratches but very cheap, it is a perfect cost-effective option for those on a tight budget!
2. Rear Element Scratches: Absolute deal-breaker, never buy!:
   • The rear element is extremely small in diameter and very close to the camera sensor; the light converges intensely here;
   • Even a micron-level scratch on the rear element can refract light, directly casting a clearly visible dark streak or severe flare on the image, which cannot be hidden even at small apertures!

3. Fungus and Haze: Irreversible Critical Damage

• Why fungus must never be left?: Fungal mycelium feeds on the organic anti-fungal coating on the lens surface, and the acidic substances they excrete permanently corrode the optical glass surface; even if the fungus is cleaned and wiped away, the lens surface will be left with uneven “etched pits”;
• Haze: Haze inside the lens will cause the entire photo to look like it is covered with a white veil, with backlit shadows appearing completely black with no contrast, which is an irreversible damage.