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I sometimes get unsharp results out of my Tamron 100-400mm F/4.5-6.3 Di VC USD, with a "doubled image effect", especially visible in the out-of-focus area (bokeh).

But the rest of the time, I get really sharp results and I'm quite happy with those.

This first image was taken at 400mm f/6.3 1/320 ISO1600 with VC activated. The pony isn't quite sharp and we can clearly see a "doubled image" on the white electric fence behind the pony as well as "lines" in the overall bokeh. 1

This second image was taken at 400mm f/6.3 1/1250 ISO1000 with VC activated. It is totally sharp and there is no "doubled image" artifact in the bokeh. 2

Here is the 100% crop comparison: 3

Both images were taken with my Canon 70D.

I can't identify what is causing this, it can't be a focus issue since this issue is present on the whole image, in and out of focus. It is not motion blur because I have seen this issues in images with shutter speeds as fast as 1/800, which should be more than enough even without VC.

I have suspected VC to be the cause, I have read that it could happen when releasing the shutter too fast after enabling the VC, but I always half press the release button at least one second before shooting.

I have tried to recreate this on purpose by taking pictures as fast as possible after starting VC, by shaking the lens before the picture to see if maybe some element could get misaligned, but I couldn't recreate this.

I would be grateful for some insight because I have been really happy with the overall performance of this lens. Thank you.

  • 1
    Do all images of the wire fencing have the doubling effect? The duck photo is not a good comparison image because there is no wire fencing in the background. What happens when you use a tripod (with and without VC)? What do plain bokeh balls look like? Are you ever shooting through anything when this occurs (fencing, leaves at the edge of the frame, etc)? – xiota Aug 22 at 0:55
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I seem have found the culprit, my single coated 67mm Hama UV filter.

I put it on as some point between these two pictures and didn't think it could be the cause because I have never had issues with filters before with any lens.

Here are before and after pictures with and without the filter, both at 400mm f/6.3 1/400 ISO250 on my 70D. 2 3

And 100% crop comparison: 4

EDIT: Since comments pointed it out, I did move a little bit to the left between those shots, was using autofocus, and was handholding my camera. This doesn't make for the most rigorous comparison but focus can't be blamed since we clearly see a difference in the bokeh which are of course both out of focus by definition. The "doubled image" effect isn't flagrant at the plane of focus, we can mostly see a more blurry image, but we clealy see some diagonal lines in the bokeh that are cause by this "doubled image".

Here is another comparison, this time mounted on a tripod and without changing the focus between shots, only removing the filter: 1

I am relieved that my lens is not faulty but am surprised of the quality deterioration that this filter is bringing. I have often heard that cheap filters could degrade IQ, but I had never seen it before with any of my lenses. I will be returning this filter, hoping it was a faulty one, but I found funny to find this in the product description:

Impact on the pictures: Clear, sharp photos Reduces haze and prevents fuzziness

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  • I was just coming back to this question to ask if you had a filter attached, as I recalled during this morning's outing with my cheap CPL that it's not worth the IQ loss; which seems to get worse the longer the lens is set.:\ Glad you figured it out. Also see other questions on here regarding the waste of space that is a UV filter on a DSLR. Send it back & get your money back. You don't need one. – Tetsujin Aug 22 at 10:55
  • I agree I am under the impression that this issue was worse at the longer focal ranges and focus distance. I tend to agree about the no-filter argument, but I share most of the gear with my dad, and he is an advocate of the other side, but now I have a good argument to show him. – pocpoc47 Aug 22 at 13:14
  • If both of these shots were taken at 400mm from the same position, why are the size ratios between the subject and the background objects so radically different? Was the orange pot to the right of the one holding up your subject moved between the two shots? It also appears the second photo was taken from a position to the left of the position used for the first shot. – Michael C Aug 22 at 17:39
  • Are you using careful manual focus for both shots? The 70D (which has the same AF system as the original 7D) could be more variable than that within a ten shot burst. – Michael C Aug 22 at 17:42
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    "Bad" filters will always look worse on longer focal length lenses than the same filter on shorter focal length lenses. (Assuming the filter is mounted in front of the lens). This is one (of several) reasons why very high-end long focal length lenses use drop-in filters placed near the rear of the optical path after most or all of the positive magnification has already been done. – Michael C Aug 28 at 5:06
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As you have found out, filters can cause issues.

Every time light hits a reflective/transparent surface some light is refracted through the surface and some light is reflected off of the surface. The coatings on lenses and filters are meant to suppress those reflections.

Single-coated is better than uncoated, and multi-coated is better than single. The light source (direction/size/etc) does affect this, and you can think of multi-coated as having a multi-directional coating... i.e. more effective in more situations.

The issue could be the lens coating, the filter coating, or a combination of the two. But a better filter (or no filter) will certainly be better than what you've got going on.

The first diagram shows this effect with a filter showing the reflected component at both surfaces, and then with a filter/lens interaction (I didn't add the secondary surface reflected components for the lens element). The secondary reflected light path travels a slightly greater distance; therefore it has a different focal distance and it will focus slightly in front of the primary light path. This is exactly the same effect as focusing on a reflection in a mirror... the focus distance is not the distance to the mirror, and it is not the distance from you to the object; instead it is the distance from you to the mirror plus the distance from the mirror to the object being reflected. I.e. The focus distance is the total distance the light has to travel.

enter image description here

The result of this secondary light path being focused slightly in front of the primary light path (short of the sensor/image plane) is a loss of image sharpness/detail/contrast and nisen bokeh... the double line bokeh shown in the horse image. You are basically recording two images stacked, and one is slightly out of focus.

The takeaway should be to avoid using filters when they are not needed. Add-on optics (filters, diopters, TC's, etc) are not part of the original lens design; and therefore there can be no correction incorporated into the lens design for any optical errors they add... they (almost?) always adversely affect IQ to some degree.

But sometimes the tradeoffs of using add-on optics/filters are worthwhile; or even necessary. So if you are going to use a filter, avoid using really cheap ones (uncoated/single coated).

However, you do not always get what you pay for with filters; except in the case of very cheap ones, those are almost always bad/worse. And a filter can also introduce its' own optical errors; and you can get a bad copy of one as well. Roger Cicala did a nice review/test of 20 protective filters (UV and clear) which shows the variability/issues: https://www.lensrentals.com/blog/2017/06/the-comprehensive-ranking-of-the-major-uv-filters-on-the-market/

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