Why do I get a white smudge in the center of photos with an old manual telephoto lens on a digital Canon?
Asked 4/10/2017
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I'm using a Canon 1000D with an older manual 55-225mm telephoto/macro lens mounted via an adapter/teleconverter. In telephoto mode, many images show a pale white smudge or bright patch near the center of the frame. In macro mode the lens seems fine. The effect varies with the scene, and it also appears with some other older manual lenses, while my normal Canon EF lens does not show it.
The lens looks clean to visual inspection, and the effect can appear even when the camera is in shadow, so it doesn't seem like ordinary front-element flare. What is this artifact, and is it caused by the lens, adapter, or shooting technique?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Some film era lenses could produce this effect on digital cameras, it's called a "hot spot". The reason is that unlike film emulsion, which is matte, digital sensor is glossy (as it's composed of thousands of micro lenses) and it causes the light to bounce back to the rear lens element. If this element is flat or if it's curvature is not enough to effectively scatter this bounced light, the light then bounces back to the sensor which, again, bounces it back to the lens and so on. This results in a hot spot.
As this hot spot is an internal lens flare, it depends not on your front element being in the light, but on the subject matter. High contrast, back lighted or highly reflective scenes usually produce a more noticeable spot.
One of the well known lenses suffering from this effect is Tamron Adaptall-2 90mm f/2.5 macro lens. It has a flat rear element and the hot spot has a purple tint because of the color of the lens coating. It also gets much more prominent as you close the aperture.
Originally by user62353. Source · Licensed CC BY-SA 4.0
user62353
9y ago
0
Generated from our catalog & community — verify before relying on it.
This is most likely a lens “hot spot” or internal flare from using some older film-era lenses on a digital sensor.
Unlike film, a digital sensor is reflective. Light can bounce off the sensor, reflect from the rear lens elements, and return to the sensor, creating a pale central smudge. That’s why it can depend on the subject and show up more in high-contrast or reflective scenes, even when the lens itself is shaded.
It could also be made worse by stray light entering through the adapter/teleconverter or from inadequate baffling, so checking for light leaks is worthwhile.
What to try:
- Use a longer/deeper lens hood; one user solved it with an extra-long improvised hood.
- Test for light leaks by shooting with the lens capped or covering the setup with a dark cloth/bag.
- Compare with and without the teleconverter/adapter if possible.
So this is probably not your technique, and not necessarily fungus or dirt. It’s usually a compatibility/optical flare issue with the older lens setup on a digital body.
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