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The Digital Sensor Reflection Effect

Digital camera sensors are highly reflective objects - much more so than film. Some lens systems, under some conditions, can actually pick up the glow of the image falling on a sensor and reflect that glow right back onto the sensor. The size and intensity of the reflection will depend on the size and curvature of the lens elements (especially the rear elements), the lens surface coatings, the proximity of the rear element to the sensor, the lens aperture in use, the focusing distance, and the distribution of light and dark areas in the scene. If and when a reflection happens it typically takes the form of a small bluish-white soft circle appearing in the center of the shot. This is generally regarded as a bad thing, so there is some importance in understanding the phenomenon in order to avoid it when possible.

Not all lenses will produce these reflections. Lenses specifically designed and built for digital cameras, either interchangeable or non-interchangeable, should never show the effect. If they do, the manufacturer has screwed up. Lensmakers are expected to use optical and mechanical designs that eliminate the formation of sensor reflections in digital-specific lenses.

Other lenses, such as interchangeable lenses designed and built for use with film cameras, might or might not produce reflections. In most cases these lenses were manufactured before digital cameras existed, so they were not designed with that possibility in mind. Some might show the effect frequently; some might show it only under peculiar circumstances; and others might be not be vulnerable at all. The only way to know if such a lens might display the reflection effect is to try it under a variety of conditions and check the results.

When it shows up, the effect may be either obvious or subtle. Here's a fairly obvious one - a lens cap on a light table at f/22:


But I first saw the effect while looking for something else entirely. The two shots below were taken by Ed Rice in order to compare his Minolta 50mm f/1.4 autofocus lens to his Minolta 50mm f/1.4 MD manual focus lens using a third party MF/AF converter. They were taken with a Konica Minolta 5D, and both shots were made at f/8. I, like Ed, was very interested in the performance of the MF/AF converter, so I studied the shots when he posted them online. One of the things I noticed was the obvious hot spot in the center of the AF shot. When I asked Ed, he had no explanation for it. I'm now quite certain that hot spot is a result of the sensor reflection. The shot with the manual focus lens shows a much fainter hot spot as well if you look closely. That reflection is most likely coming off the elements in the MF/AF converter.

Minolta 50mm f/1.4 AF at f/8, focus distance of 52 inches:


Minolta 50mm f/1.4 MD at f/8 with generic MF/AF converter, focus distance of 60 inches due to converter magnification factor:


When I later got my own 5D, I did additional testing. I used a Minolta MD 50mm f/1.4 and MD 50mm f/2 in combination with the same type of MF/AF converter that Ed used. Both of these produced the reflection effect under some circumstances. Here are two shots looking toward my back yard fence. Look carefully to see the small central area where the top of the fence appears to be faded. The fence itself is not faded there. That's a sensor reflection, again probably caused by the MF/AF converter.

Minolta 50mm f/1.4 MD at f/16 with generic MF/AF converter:


Minolta 50mm f/2 MD at f/22 with generic MF/AF converter:


I also built a macro setup using a Minolta 35-70mm f/3.5 MD lens in reverse position for making digital copies of my 35mm slides. For this purpose I used a modified MF/AF converter with the glass removed, which turns it into a simple MF/AF mechanical adapter/short extension tube. The front element of the 35-70mm is big and nearly flat, and with the lens reversed it ends up directly in front of the sensor. The photos below show that the lens reflects the sensor image quite well at apertures of f/16 or smaller.

Minolta 35-70mm f/3.5 MD reversed with glassless MF/AF adapter:

So - What does it all mean?

You decide. You might enjoy years of picture taking and never notice sensor reflections from any of your non-digital lenses. But if you don't check them for yourself in advance - at all distances, apertures, and focal lengths in the case of zooms - you might one day find yourself wondering why there's a mysterious blue-white spot in the center of your latest photo.

Which (Konica) Minolta/Sony AF lenses are known to be vulnerable?

Right now there are three that I know about: the 50mm f/1.4 mentioned above (the Sony branded version of the lens, which I owned briefly and tested, is also vulnerable), the 17-35mm f/3.5 G (reported by Michael Hohner), and the 50mm f/2.8 Macro, which I have tested (see the detailed results below). I can also report that the 50mm f/1.7 is not vulnerable, since I have tested it under a wide range of conditions and have not been able to produce any reflections.

Detailed Reflection Testing of the (Konica) Minolta 50mm f/2.8 (D) Macro Lens

This table shows a series of test shots made with the (Konica) Minolta 50mm f/2.8 (D) Macro lens and the 5D camera. It is my understanding that all four versions of this lens have the same optical construction, so these results should also apply to the original and restyled Minolta non-D versions and the current Sony version as well. The test scene was a black plastic lens cap on an illuminated light box. The original images look something like the sample shown at the right. I have cropped and resized the center portions of all the images below so that they show roughly the contents of the yellow box. This was just one test run of several that I have done. In all such tests, the lens showed the most pronounced reflections when set to a focus distance of 15 inches (1.25 feet on the distance scale) and an aperture of f/32. The sharpness and intensity of the reflections fade as the focus distance is changed, and as the diaphragm is opened.

f/32 at 10"

f/32 at 11"

f/32 at 12"

f/32 at 13.5"

f/32 at 15"

f/32 at 16.5"

f/32 at 18"

f/32 at 20"

f/32 at 22"

f/32 at 24"

f/22 at 10"

f/22 at 11"

f/22 at 12"

f/22 at 13.5"

f/22 at 15"

f/22 at 16.5"

f/22 at 18"

f/22 at 20"

f/22 at 22"

f/22 at 24"

Images are not shown here because reflections are barely noticeable at distances of less than 15 inches and apertures of f/16 or larger. I might publish some more test shots in this range when I have more time.

f/16 at 15""

Images are not shown here because reflections are
barely noticeable at distances of more than 15 inches
and apertures of f/16 or larger. I might publish some
more test shots in this range when I have more time.

f/11 at 15"

The results show that the 50mm f/2.8 Macro can produce a very pronounced reflection effect under specific conditions. The surface of the rear element is concave, so the reflection is apparently being precisely focused on the image when that element is at a particular distance from the sensor plane. Keep in mind that this test was done under unusual conditions of lighting and composition, and intentionally represents the worst possible scenario. And fortunately, the reflection effect falls off quickly as the lens is moved away from the 15" mark and/or the diaphragm is opened to f/11 or larger. The frames highlighted in orange text might be regarded as the distance/aperture "danger zone" where a reflection is most likely to be visible in the image.

For reference, I am including one more image taken with this lens where I was not attempting to generate a reflection. The shot below was made at f/8, and although I didn't record the exact focusing distance, it was certainly very close to the 15 inch mark. The black battery was shot against a white paper background in open shade with fill flash. The image has been resized to 1/5 its original dimensions, but otherwise it has not been processed in any way. Note that there is no visible sign at all of a reflection. I'm guessing that the background (white paper as opposed to a light source) wasn't bright enough to make it happen.

(Konica) Minolta 50mm f/2.8 (D) Macro, f/8 at approximately 15 inches:

Apart from the vulnerability documented in my tests, the f/2.8 macro is a superb lens, and I like it a heck of a lot. But if at all possible, I will make it a point to always use f/11 (my usual aperture for macro use anyway) or larger apertures when I work within the 12 to 18 inch distance range.

Text 2006-2010 Ray Lemieux / Images 2006-2012 Ray Lemieux, Ed Rice