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Because of the way light is projected in a camera the image can be darkened near the edges. My questions is, do modern cameras attempt to correct for this light fall off?

Description of the light fall off

To be precise, if you have a uniformly lit wall and you take a straight on photo of it, the projection from the wall onto the camera gets darker away from the center.

If I computed this correctly (I'm sick, I never trust computations when I'm sick)

In the corners of a 35mm photo the light is only 20% of what it is in the center.

In the corners of a 50mm photo the light is only 50% of what it is in the center.

In the corners of a 70mm photo the light is only 70% of what it is in the cneter.

As a rule you only get cos^2(A) of the full amount of light at angle A off center. These are for straight on photos of a wall.

Note: I am not sure how autocorrecting for this would look if it was applied to picture of a wall that was at an angle.

Extreme Wide Angle Lenses

On a specialized lens with a full viewing angle of 120 degrees the light fall off is very dramatic. Its actually 1/4 of the light at the center).

As a side note, this is why the various types of fish eye lenses use a different projection than the usual linear projection which gives them their distorted look. The different types of fisheye lens use projections that devote a CCD that is OFF center to a disproportionately large area of the wall. The further off center, the bigger the portion of the wall that is "seen" by that CCD sensor.

The math (if you wanted it)

To recognize this, think about the light that reaches a single CCD sensor. The area of wall covered by a single CCD sensor is the same no matter which sensor you choose. That is the nature of a linear projection.

However, different CCD sensors "see" different parts of the wall. The parts of the wall that are further away provide less light to the CCD because the intensity of light falls off like 1/r^2. Points at angle A off center are r = 1/cos(A) further away than the center.

John Robertson
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  • Most higher end modern cameras use CMOS sensors, not CCD sensors. – Michael C Nov 15 '14 at 23:21
  • It is not as simple as you present it because most modern cameras use compound, rather than simple, lenses. Some lenses demonstrate very little light fall off at the edges while others demonstrate quite a bit. – Michael C Nov 15 '14 at 23:24
  • @MichaelClark. No, in this case it is that simple. It has nothing to do with the actual choice of lenses used to make the projection. It is purely a property of the projection from outside world to photographic film or sensor. Ultimately no matter how many lenses you use they are put together to make a linear projection, and it is a property of that alone. – John Robertson Nov 15 '14 at 23:49
  • hint: image-space telecentric lens :-) of course, such design only reduces sensor-induced fall-off, and the root cause described by the OP can not be avoided, unless, somehow, the front lens becomes spherical and presents equal surface to all entry angles. – szulat Nov 16 '14 at 00:15
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    No one is saying there will be no fall off. But some lens designs will increase the amount of fall off when compared to another lens design of the same field of view. And the wider the FoV, the more pronounced it will be all other things being equal. A long telephoto lens that only has a 3°-4° FoV includes such a small part of your spherical projection that the difference between it and a flat plane becomes almost insignificant. – Michael C Nov 16 '14 at 02:07

2 Answers2

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Light fall-off depends on a particular lens at a certain aperture and for some focal-length. There are a number of cameras which can do this but they must be able to recognize the lens. This is a profile based approach and can usually be enabled some cameras with electronic lens mounts.

A few other cameras allow to apply an amount of compensation, probably based on averages. It can be enabled on a number of Nikon DSLRs while you can choose between levels of correction. A D750 for example offers 3 levels, Low, Normal or High.

Some modern fixed-lens cameras attempt to correct the problem via hardware: A micro-lens array in front of the sensor redirects light to correct for loss of light due to angle-of-incidence.

The bottom line, to answer your question, is yes, some do correct for light fall-off. They do not all do it the same way and some do not do it at all. Any correction done in software is available for JPEG or TIFF images only, while hardware correction applies to RAW files as well.

Itai
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Some cameras now have lens correction features built in, but many do not. And sometimes those corrections extend to light fall off, but not always. For example, the Pentax K-5ii and a few prior models correct for distortion and lateral chromatic aberration. The K-3 adds "Peripheral Illumination" to correct light falloff in the corners. More on this in the question Is it technically possible to build a camera body to correct for lens defects? (where the answer is, obviously at this point, "yes").

In any case, this in-camera correction is (always, I think) JPEG-only. However, most RAW software also includes lens correction modules, which do include this. For example, here's a guide for Adobe Lightroom. Most open source tools (like Darktable or Rawtherapee) do it using Lensfun.

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mattdm
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