Purple fringing under very bright conditions and maximum aperture

Started Nov 12, 2012 | Discussions thread
Mark Chan
Senior MemberPosts: 1,700
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Re: Purple fringing under very bright conditions and maximum aperture
In reply to Anders W, Nov 12, 2012

Anders W wrote:

Mark Chan wrote:

Before people read into this, for those that read before, this is a follow up of my previous photos relating to whether:

a. I was experiencing purple fringing as a result of using a variable ND filter from lightcraft on my 75mm f1.8 see http://forums.dpreview.com/forums/thread/3285500

or

b. the 75mm f1.8 exhibits purple fringing at maximum aperture, see http://forums.dpreview.com/forums/thread/3292256

Whilst this is not an academic test, I had the opportunity to take random pictures in similarly harsh light. I was under a large umbrella when taking these so there was no chance for stray light of getting in. I had my EM-5 and 75mm f1.8 and no ND filter. I will note that I was not going about trying to check this, as we did not expect the weather to be good so I didn't even bring the filter. But as I took pictures, I noticed issues and pulled the aperture down. Coming back, I saw that fringing does indeed occur at max aperture. It seems that pulling the aperture mitigates this considerable, I used f2.5 - 2.8 and the issue was reduced, and further down gone. Photos below.

It seems this occurs considerably easier where the subject is white / fair skin toned.

Stopping down minimizes the problem, as does darker colours too.

There are also situations with f1.8 that don't exhibit these characteristics, but often of darker colours. From this perspective, I am having to learn when and how fringing occurs and need to really experience how to minimize it. Is this a result of shooting with strong backlight? Will it occur with front lighting too? How far can I stop down to remove the problem and at what cost to the bokeh (when needed).

Given the 'instability' one has under this situation, it does seem that if one were shooting portraiture under outdoor very bright conditions, its important to take note of the 'disadvantage' m43 has in this versus the FF cameras, where you can pull down to f4 and still get better bokeh @ f2.8 m43.

I don't have the money to spend on an ultra expensive vary ND filter, but the above images I would expect it won't make much of a difference.

The primary cause of purple fringing in your shots is longitudinal chromatic aberration (LoCA). See here for an explanation:

http://toothwalker.org/optics/chromatic.html

LoCA can occur in in-focus area as well as out-of-focus areas. The 75/1.8 is well corrected for in-focus LoCA for being an ordinary achromatic (as opposed to apochromatic, commonly designated APO) lens and I see very little of it in those parts of your shots where focus is perfect. Nevertheless, it is likely to have a little bit of it when shot wide open or close to wide open.

Out-of-focus LoCA will manifest itself as purple fringes on parts of the image that are closer than the focus plane and green fringes on parts that are behind. Pretty much all lenses (except possibly well-corrected APOs) will show this to some extent even when stopped down. Again, I see little in your shots that I find surprising or alarming in this respect.

The easiest way to deal with in-focus as well as out-of-focus LoCA, in those cases where you find it troublesome, is to shoot RAW and use the defringe tool available in Lightroom from version 4.1 on.

LoCA becomes more visible the stronger the contrast between brightly lit and dark areas. Whether these strong contrast transitions appear because the subject is lit this way or that way doesn't matter much. It's the strong contrast per se that matters.

Anders,

I got that from your previous posts on the same topic - many thanks for showing and explaining the technical stuff in such detail.  In fact I took some crops that showed very specifically the purple in the foreground and the green in the background.

As you may well note, I am posting this as a follow up investigation whether its my ND filter or its the lens / limitations of physics.  There were those that were stating matter-of-factedly that its the ND filter, and my experience here suggests otherwise.

I have still to get my hands on LR 4 to deal with the fringing, and will do that in due course.

Happy photographing.

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