Why do slightly out-of-focus areas show a doubled image on my 70-200mm lens?
Asked 8/22/2023
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2 answers
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My Canon 70-200mm f/4L looks softer at 200mm, and slightly out-of-focus details appear doubled with a horizontal offset. The effect happens on both tripod shots and very fast handheld shots, so it does not seem to be motion blur. Stopping down reduces it, but does not completely remove it. I later tested the lens with and without a UV filter using the same setup and intentional defocus, and the doubled rendering was dramatically worse with the filter. What causes this?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
2y ago
2 Answers
11
A cheap filter (uncoated or poor coatings) can reflect some of the light which is always reflected off of the front element surfaces.
The primary reflection (blue in the drawing) will be stronger than the secondary reflection and it can be reflected back through the lens a second time by the filter (right side of drawing).
Because these reflected light rays have bounced off of the lens/filter at an angle, the secondary image is offset. And because the reflected rays have a slightly longer travel distance, they will be a little more out of focus.
Edit to add: The strength and orientation of the error will vary with the strength of the light, angle of the light, orientation of primary edges (hard/reflective), and the orientation/effectiveness of coatings; lenses are (almost) always coated, and even cheap filters tend to have some coating on one side. And even with the cheapest of filters you will not always see the effect.
Here is an example of a cheap UV filter causing a doubled image in the vertical plane.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
2y ago
0
Generated from our catalog & community — verify before relying on it.
The most likely cause is ghosting from reflections, especially from the UV filter rather than a fault in the lens itself.
Any lens has multiple glass surfaces, and each surface reflects a small amount of light. Lens coatings reduce this, but an added filter introduces two more reflective surfaces. If the filter has weak or poor coatings, light can reflect between the filter and the front lens element, creating a faint secondary image. Because that reflected light takes a slightly different path, it appears offset and a bit more out of focus, which matches your “double vision” effect.
Why it changes:
- More visible in defocused areas and around bright edges
- Often stronger at some focal lengths or lighting angles
- Reduced by stopping down
- Can be dramatically worse with cheap or poorly coated filters
Since your test shows the effect largely disappears without the UV filter, the filter is the cause. Try shooting without it, or replace it with a high-quality multi-coated filter if you need protection.
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UniqueBot
AI2y ago
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