What will cause a washed out spot in the centre of an image?

I thought it was light leaking in through the viewfinder, but these were taken with it completely covered.

It only happens sometimes, and I've only noticed it when I'm trying to get some macro/extreme macro shots. I sometimes use extension tubes and reverse lenses.

For a while, I thought it was my cheap extension tubes. But I bought some expensive ones of much better quality. Every now and then it still happens.

Here are some examples; shot with a reversed Canon EF-S 24mm f/2.8 STM, on tubes (I know they're not great photos, but they illustrate the problem):

fly isopod spring

I've noticed it in regular wide angle shots too. Here's an example of a photo sequence in which the aberration occurs; taken with a Canon EF 50mm f/1.8 STM:

pre-spot spot post-spot post-spot2

  • What kit are you using??? – Digital Lightcraft Aug 20 '17 at 9:24
  • Various lens configurations. Canon EOS 650D DSLR body & MR-14EX Flash. Happens without the flash too. I thought it was a hardware problem at first, now I'm not so sure. – voices Aug 20 '17 at 9:31
  • I've seen similar spots when using a microscope with a T2 adapter. For some stupid reason the adapter was not matte black inside but shiny metal. Are your extension rings too shiny on the inside? – Gerhardh Aug 21 '17 at 11:30
  • if you are using a reverse lens, you should make a "reverse lens hood" in order to avoid light coming from the sides. – Noldor130884 Aug 24 '17 at 7:27
  • I don't always mount my lenses in reverse, but yeah, I use something like that when I do. – voices Aug 24 '17 at 7:37

Looks like classic "blue spot" where flat (or near flat) optical surfaces are reflecting light back onto the sensor which was reflected from the sensor itself.

It's pretty extreme which may be due to the fact you're using the lenses reversed (i.e. not as designed).

The MR-14EX ring flash would make things worse - very bright light source.

  • I can't find anything about this "blue spot" phenomenon. Can you point me to some information on it? – voices Aug 20 '17 at 19:39
  • That proved harder that I expected. There's a mention of it in a post on this dpreview.com forum in connection with an older Tamron 90mm macro lens (my old 90mm f2.5 had the same issue, BTW). Later versions had a curved rear element and this removed the problem. – StephenG Aug 20 '17 at 19:49

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