Purple flare: Causes and remedies

Started Mar 1, 2013 | Discussions thread
TrapperJohn Forum Pro • Posts: 16,474
A very effective, albeit less practical way, to eliminate purple spots.

Anders, your very detailed approach got me to looking back at my EM5 and EP1 shots that have slightly out of frame light sources. Bright ones, like... the sun. Especially with the 7-14, which sucks in everything, there is often a light source just out of the visible frame.

Purple spots? None. I have never seen one, not even with the 7-14, not even with the sun just out of frame.

However... I typically have 4/3 HG and SHG ZD lenses mounted, or my beloved 4/3 PL25 1.4. The 7-14 I have is the ZD 7-14, quite a bit larger than the Panny 7-14.

All of the high grade ZD and PL 4/3 lenses have a telecentric light path, which is to say they straighten the light out so it hits the sensor at a perpendicular angle. This was done to eliminate edge problems on the earlier deep well sensors, and is probably the reason the better ZD lenses are renowned for their uniform sharpness, edge to edge, even on an ultrawide like the 7-14. To a degree, the telecentric light path accounts for their larger size.

M43 uses a shallow well sensor that isn't so fussy about how light hits it. Consequently, M43 lenses are not telecentric, it wasn't needed, and wasn't desirable from a lens size perspective. In fact, the shallow well sensor was key to M43's short registration distance.

My guess is - the telecentric light path also suppresses reflections from out of frame light sources - the straight path knocks out reflections by getting them to hit the out of frame area near the sensor straight on, rather than at an angle where they can reflect off of internal parts.

My guess also is that one doesn't see purple spots as much on Panasonic bodies because Panasonic tends to apply more PP and correction automatically in body. That's a tradeoff - you get more consistent photos and you can cut lens development cost by correcting in body rather than optically, but you can also lose detail if you lean on that too heavily - depending on the body to correct CA rather than eliminating it optically will cost some detail.

So there is a way to eliminate purple spots optically, before they ever hit the sensor: use telecentric 4/3 PL and ZD lenses. Not necessarily a practical solution for everyone, but it does work.

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