What can cause double-vision effect in slightly out of focus areas?

Question

I have an issue with my Canon 70-200 F/4L USM. It is less sharp at 200mm than at shorter range and the objects that are slightly out of focus are rendered twice with horizontal offset. The shutter speed is not the problem, because the effect is similar both with tripod shots and 1/2500 handheld ones. Stopping down the aperture greatly weakens the effect, but it is still visible.

What can cause that effect? I don't know whther my copy is bad or is it my mistake.

The samples below are 100% crops.

Edit: I took test photos with and without the uv filter.

With the uv filter:

Without it:

Both images were taken with tripod from the same place and with the same focus distance (defocused intentionally). The difference is huge and the double-rendering effect is gone without it.

Do you have any filter on the front? It seems a bit much for that to be the cause, but let's eliminate it for starters. — Tetsujin, Aug 22 '23 at 13:59
Then I think now would be the perfect opportunity;)) See also https://photo.stackexchange.com/questions/57/are-there-any-downsides-to-using-a-good-quality-uv-filter — Tetsujin, Aug 22 '23 at 14:23
@Tetsujin You were right! I was doubting that because I use a very cheap UV filter with my 50mm and I have never experienced anything like that. I thought that UV filter might decrease some contrast or add more flaring, but this is horrible. — Bartek Kudzia, Aug 22 '23 at 16:18
It's something I learned, actually from this very site, in my first year of taking up photography. I now won't use any filter unless I absolutely need it - ND or polariser. I never need a UV filter, the camera already has one ;) — Tetsujin, Aug 22 '23 at 16:20
It may come down to the physical shape of the front element. My 300mm front element is virtually flat. My 50mm is a huge curved 'bubble' of a surface. One would imagine the chances for internal reflections going back down the barrel to be much more prevalent on the flat front. — Tetsujin, Aug 22 '23 at 18:32
Also related: is uv filter a must? (the linked answer has a large number of links to other "protective filter" questions here at Photography SE) and Are there any downsides to using a good-quality UV filter? and What is the downside of a cheap UV filter that is used solely for the protection of the lens? — Michael C, Aug 22 '23 at 22:06

Steven Kersting · Accepted Answer · 2023-08-24T11:58:23.090

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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.

edited Aug 24 '23 at 11:58

answered Aug 22 '23 at 18:42

Steven Kersting

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Can you provide any insight as to why OP only sees an horizontal offset image? As the effect is explained above, I would expect it to work on all directions. – Vladimir Cravero Aug 24 '23 at 11:46
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@VladimirCravero, I'll add that to the answer. – Steven Kersting Aug 24 '23 at 11:52
@VladimirCravero, for it to occur in all directions it would need to be more of a blur effect rather than an angular offset. – Steven Kersting Aug 24 '23 at 11:59
I have one vertical photo in which vertical lines are sharp and horizontal ones are double, so the direction of the offset depends on the orientation of the lens in my case. – Bartek Kudzia Aug 24 '23 at 15:35

score 4 · Answer 2 · answered Aug 22 '23 at 15:26

The optical system consists of multiple surfaces of polished glass. Each will reflect away a small percentage of the light. Some of these reflections will hit other lens surfaces and reflect again. Some of this stray light finds its way to the imaging chip or film and records. Lens coatings are the mitigating factor. Sometimes a double image is caused by an add-on filter or internal reflections from lens shade or metal inside the lens barrel or lens add-on lens shade. Ghost images happen.

What can cause double-vision effect in slightly out of focus areas?

2 Answers2