Lens Issues of the purple kind...

[KenRexach]

Hi, I just got the 85mm f1.8 for a job and it exhibits some really nasty purple fringing on extreme highlights. Is this normal for this lens? I tested the 17-40mm wide open and had very very little fringing and the 50mm macro also very little and the 24mm TSE none (all tested wide open).

On the 85mm the fringing is worse at f1.8 and gets better progresively till f5.6 where it has very little or none.

I tested using the 1D mark 2 and the 10d and on both cameras the results were identical.

Here are some samples:

85mm at f1.8 whole frame: http://kardesignphoto.com/img/photos...p/fringing.jpg

85mm at f1.8 , 100% crop: http://kardesignphoto.com/img/photos...ingingcrop.jpg

17-40mm at 40mm f4, whole frame: http://kardesignphoto.com/img/photos...ringelarge.jpg

17-40mm at 40mm f4, 100% crop: http://kardesignphoto.com/img/photos...p/nofringe.jpg

is this an issue with digital only?

[Eric_Chan]

Ken, this fringing is common for the 85 1.8 (and I've also seen it with the 1.2) when shooting wide open. However, in most cases you should not be seeing it beyond 2.8. Part of the problem is the overexposed highlights, but sometimes this can be addressed in the raw conversion (assuming you shoot raw). Some raw converters actually do a very good job of reducing the fringing. For instance, recently some people discovered that some of the CA artifacts seen on the 1Ds II in DPP disappeared when they used C1. So if you're shooting raw, it may be worth fiddling with different raw conversion parameters or with a different program.

Eric

[KenRexach]

I always shoot Raw on critical jobs. The samples were JPG's though, il try and do some tests converting raw files on C1. Its a shame that the 85mm f1.8 has this fringing because otherwise its excellent, sharp in the center at all appertures and all across the frame very very sharp by f4. I wouldve thought the 85mm 1.2 wouldnt have any fringing at all. Amazing that the L zooms dont have any though. The color and bokeh of the 85mm is very nice also.

[LinEvans]

Whether this is a "solution" for you or not, there are some excellent PhotoShop actions available for a very reasonable price which can remove this purple fringe with a couple keystrokes.

This problem happens with both film and digital, but the microlenses on some sensors seem to exacerbate the condition with certain lenses. The problem is most serious with some of the higher resolution prosumer cameras such as the Sony DSC-F828 where using the actions is frequently necessary for critical applications where there are strongly contrasting areas.

At the source, the problem can be greatly ameliorated by stopping down, but obviously we don't buy fast lenses to stop them down. Also you can minimize this by dropping back a bit on the exposure compensation on shots where blown highlight reflections are anticipated such as in these shots.

Here's a link to Shay Steven's site where you can evaluate his purple fringe removal actions.

Lin

http://www.shaystephens.com/

[diglloyd]

[ QUOTE ]
H
On the 85mm the fringing is worse at f1.8 and gets better progresively till f5.6 where it has very little or none.

[/ QUOTE ]

So you're comparing f1.8 to f4? What about the 85mmf1.8 at f4 vs the 17-40 at f4? That is an apples-to-apples comparison.

It's not obvious to me that the purple fringing is any sort of lens defect. It looks like CCD "blooming" to me. Or it could be an interaction between lens and sensor. Maybe that's relevant, maybe not, depending on your application.

I also don't think it's chromatic aberration. It doesn't look like it, and when I've tried to correct similar problems with Nikon equipment using Photoshop Camera Raw, it had no effect. Also the purple stuff doesn't seem to be sensitive to position within the frame, as CA typically does.

The fact that it improves upon stopping down may be due to the fact that at small apertures (large f stops), there is a much more point-source illumination of the pixels versus a more diffuse source at a large aperture (small f stop). Possibly this interacts with the sensor in some way. Certainly dust shows up more and more as you stop down.

[LinEvans]

Hi Lloyd,

It's definitely not CCD sensor blooming since both the 1D Mark II and 10D are CMOS based and CMOS sensors are immune to blooming per manufacturers. This problem has been hashed over for several years on dPReview and the best estimates are that it's caused by some combination of transverse chromatic aberration and specific lens characteristics which are exacerbated on some sensors by micro-lenses. It appears both on film and on digital, but it's definitely more common to see it with digital sensors. There have been a nuber of experiments which indicate that even UV may light may exacerbate the condition. Certain lenses with certain cameras will produce it every time there are highly reflectve surfaces or strongly contrasting backlit scenes. It's been noted that stopping down can ameliorate the situation as can exposing for the reflective areas. I suspect it's something we must simply live with. I've seen it with nearly all my 30 odd digital cameras at one time or another and that includes all my Canons (D30, 10D, 1D, 1D Mark II, 1DS), my Sigma SD10, my Kodak DCS-760 and Kodak Plus Pro Back. The best solution of course is prevention, but when that fails PhotoShop actions such as Shay Stephen's (link in earlier post) are a rather simple and quick solution.

Lin

[Joe_Tree]

Check this out:



100% crop / 1D MKII / 100-400L IS / 5.6 / 100th second

Best part of £4500 that cost me...

But seriously - it is very odd and not a problem I've had before, but I remembered reading this thread so thought I would post it. I exposed for the building and the sun was behind it, so the sky is way over exposed.

This crop is from the corner of the frame and other blown out highlights closer to the centre don't demonstrate the same problems. That suggests to me it must something to do with CA, surely? If it was sensor blooming, that would surely apply evenly across the whole image?