Rebel XT/Canon EFS 17-85: horrid purple fringing

[AussieBloke]

I bought the EOS 350D (Rebel XT) and EF-S 17-85 mm combination around end-November, and have noticed some pretty nasty purple fringing in my photos, not to mention general lack of sharpness. Now I know that this is no "L" lense, but I really was not expecting such poorly controlled optics. I have attached some pix to illustrate my problem.

First shot (2940): Av, f5.6, 1/50 sec, 17mm, ISO100.
Second shot (2974): Av, f4.0, 3.2 sec, 17mm, ISO100

Now please tell me that I have a dud lens, and Canon should repair this under warranty ASAP?? I am rather horrified by this, and am travelling overseas in a couple of months so I'd love to have this sorted out before I go!

Just wanted some extra opinions to ensure I wasn't expecting too much from this lens.
Many thanks,
Sean
Sydney, Australia

[AussieBloke]

Sorry, pix seem to have reversed order - first specs are for bottom photo, and vice versa.

[another view]

First shot (2940): Av, f5.6, 1/50 sec, 17mm, ISO100.
Second shot (2974): Av, f4.0, 3.2 sec, 17mm, ISO100

I see that this info is reversed - so the first shot is 3.2 seconds. The lack of sharpness could easily have a lot to do with camera shake, even on a tripod. Try using a shutter release or self timer to fire the shutter. How sturdy is your tripod? Was the tripod's center column extended? Was it windy out? All of these can affect sharpness.

Two other things that affect sharpness are lens aperture and of course the lens itself. Some lenses, typically less expensive ones, are less sharp at the edges of the frame than the center. A better lens will help this, but I think it's a long ways down the list. Work on the technique first and get the best, most consistent results you can from the equipment you've got before buying something better.

Color fringing shows up where a bright area meets a dark area with a very pronounced change (like the edge of a dark building against a bright sky). It's not helped by large apertures - stopping down to f8 may help this a little but you probably have ten stops or so of difference in light level from the shadow side of the building to the bright sky. That's a tough thing for any lens to handle.

Bottom line: With long shutter speeds, try a cable release or self timer so the camera doesn't shake when you hit the shutter release. Lower your tripod if possible; it will be much stiffer. Use a more moderate aperture like f8 and be careful of including high-contrast areas in the frame that can cause color fringing. If you have it, it can be removed in Photoshop.

[livin4lax09]

also, a lot of the times you see this effect on lenses with cheap filters. Are you using a UV filter on your lens? I know I saw a bit of this when shooting bright white jerseys against shadowed backgrounds with a UV filter on a cheaper lens.

[AussieBloke]

also, a lot of the times you see this effect on lenses with cheap filters. Are you using a UV filter on your lens? I know I saw a bit of this when shooting bright white jerseys against shadowed backgrounds with a UV filter on a cheaper lens.

Thanks for the tip. I am using a UV filter - a Hoya HMC. I thought that these were generally pretty well regarded and generally didn't have a major impact on image quality. I'm obviously reluctant to shoot without any filter on the lens at all - I like the insurance it provides, but I'll at least see if it improves things. Is the fringing in the above pictures more severe than you got? I know that the contrast in the examples will introduce some artefacts, but I didn't think it would be as pronounced as what I see.

[AussieBloke]

I see that this info is reversed - so the first shot is 3.2 seconds. The lack of sharpness could easily have a lot to do with camera shake, even on a tripod. Try using a shutter release or self timer to fire the shutter. How sturdy is your tripod? Was the tripod's center column extended? Was it windy out? All of these can affect sharpness.

My tripod is quite lightweight, and I agree that it probably contributed to the lack of sharness. I did use a self timer with mirror lock-up, but I guess it's all academic if the tripod isn't up to the task.

Color fringing shows up where a bright area meets a dark area with a very pronounced change (like the edge of a dark building against a bright sky). It's not helped by large apertures - stopping down to f8 may help this a little but you probably have ten stops or so of difference in light level from the shadow side of the building to the bright sky. That's a tough thing for any lens to handle.

Fair enough, but I am concerned about the severity of the purple fringing - I agree that its presence is unsurprising. I am interested to know whether, in you experience, purple fringing of this magnitude is consistent with a fault-free lens of this grade. I know it is not an "L" lens, but I wouldn't presonally regard it as a dirt-cheap lens either.

If you have it, it can be removed in Photoshop.

I am unsure how to do this at this stage - I may post another thread asking about this - thanks for the tip. Ultimately, if I can avoid this step in the first place then that would be great!

[another view]

It's always best to avoid having to do something like this in Photoshop, but sometimes that's how life is... I personally don't think it's that bad in the images you show based on shooting fairly wide open, at the edge of the frame, and the bright sky against the dark building.

It's probably easiest to correct if you're shooting RAW with Photoshop CS (or newer, I'd guess). The "advanced" settings on the file converter will fix this.

[AussieBloke]

It's probably easiest to correct if you're shooting RAW with Photoshop CS (or newer, I'd guess). The "advanced" settings on the file converter will fix this.

Thanks! I did take this in raw (now DNG) format, so I'll have a go at it and let you know how I go! Cheers.

[SmartWombat]

Thanks for the tip. I am using a UV filter - a Hoya HMC.

Hoya HMC comes in different grades.
Over here in the UK, HMC is lower grade, lots of reflections.
HMC Pro is nearly 2x the cost, and much better coatings and control of unwanted reflection.

[kalina01]

I recently bought the same 17-85mm lense as a kit with the Digital Rebel XT. I noticed the same degree of fringing (chromatic aberration) and decided to send the lense back for another copy. I used a tripod for my shots and NO filter. So the filter is not the problem. I've read reviews about this lense and some like it, and some don't. Being new to this (haven't bought a lense for an SLR in 20+ years), I am confused about the quality of lenses. It seems to vary from lense to lense within a specific model. I've heard that some people have sent their lenses back to Canon for calibration. Can anyone give details about how to do this? I'm awaiting a new lense from B&H and will let you know if it performs any better.