Lens flare is caused by light reflecting off of (or improperly refracting through) lens elements.
Lens manufacturers go to great efforts and expense to reduce contrast-robbing flare (typically by using special coatings on the lens elements), but flare still exists in even the best lenses when the right light source is present. While at times lens flare is desired for effect, it is more frequently detested as it can be very difficult to remove in post processing.
The Flare Light Source
In real life, the light sources that cause flare can vary dramatically - and it can even be caused by a photographer's own flashes/lights. When developing the flare comparison tool, I tried numerous light source options - including some very expensive scientific ones. But, overall, the sun created the nicest results - and the sun is probably the most common flare source in real life photography. So, it made sense to use the sun as the flare source for these tests.
Variability of Testing Conditions
There is a lot of atmosphere between us and the sun, so I have to restrict testing to clear days - and even all clear days are not identical. The sky also shows gradation from horizon to horizon on clear days. RAW capture using MF (focused to infinity), Av / Evaluative Metering and the Neutral Picture Style is the source of the test images. Add lens vignetting to the equation and overall, there is a bit of variability in this test - but it still provides a good a data point for lens evaluation. Lens hoods are in place for this test.
Light Source Framing
Flare test shots are framed so that the sun is just barely visible in the corner of the Live View display on the test camera.
Comparing Vastly Different Focal Length Lenses
Longer focal length lenses must be positioned at a closer angle to the sun and they magnify the sun's rays a greater amount. This leads to a stronger flare response - and to possible camera damage.
Don't Try This at Home
I was testing a Canon EF 600mm f/4 IS USM Super Telephoto Lens on a Canon EOS 1Ds Mark III Camera body. After completing a short test procedure, I removed the 600mm lens from the body - only to be greeted with a very-dishearteningly-noticeable amount of smoke. I obviously had the 600mm lens framed too close to the sun for too long of a period of time - which I'm guessing was a minute or so. Here is a look at the blistered plastic in the 1Ds III.
Imagine taking an $8k lens off of a $6k camera and seeing a plume of very-black smoke come out. Wish I had video'd that! The good news is that the camera is fine - only cosmetic damage was done.
Maximum Test Focal Length is 400mm
Due to the deteriorating results of longer focal length lens tests and the potential for damage (I'm adverse to this), future testing will be restricted to 400mm focal length and shorter lenses.
What Is The Small Gray Frame In The Flare Sample Pictures?
The small gray frame seen in full frame lens results represents the APS-C/1.6x Field of View Crop Factor (FOVCF) found in many of Canon's Digital SLR Cameras. Nikon's APS-C sensors have a 1.5x (or 1.55x?) FOVCF, so the APS-C frame is slightly tight for the full frame Nikon lenses. Keep in mind that a full-frame-tested lens is not directed as closely to the sun as a same-focal-length APS-C-only lens.
Back to the Lens Flare Comparisons