[stein]

I recently bought a Sigma 28-300mm lens and yes, it was (relatively) cheap, and yes, it appears that I wasted my money. Today I tried to get the best out of nice weather and made a few shots with it. However, it seems that I can't get a single photo with it without getting the edges of objects turning into smudged pink-ish... well, I'm not sure if I can use the word starting with cr... and ending with ...ap...

Anyway... can I do something about this or is it better to give up? As long as I don't want the image to be perfectly sharp and instead of cropping use resizing (from 20mp to 1000pix on the bigger side) it seems to be bearable.

Here's the image resized (the problem is not visible)



And a crop from the center of the original, really visible (even though my camera gave me a signal that the focus is okay with a red square and a sound signal)



The kit lens (18-55mm DAL) seems to be better than this (regarding sharpness and aberration) but limited to 55m (not really a zoom lens) and the other lenses I have are prime fixed (50mm) lenses.

Both images are just either resized or cropped from the original, no further manipulations have been made.

edit: shot at 180mm, Pentax K-S1, 1/320sec, F11, ISO 200

[Venom3300]

Do you have photoshop lightroom? If so, there may be a lens profile there to help. Also a slider for purple fringing to help wit that.

[stevebrot]

What I am seeing is a very mild case of purple fringing in conjunction with general softness. It may be possible to defringe using a tool like Lightroom, though I am not confident of success. Both purple fringing and the related axial (longitudinal) CA are difficult to remove, particularly when the color is indistinct.


Steve

[RGlasel]

even though my camera gave me a signal that the focus is okay with a red square and a sound signal

If wasn't for the chromatic aberration, it would look much sharper. This is a really difficult scene for producing purple fringing, and even if you apply corrections in post-processing, you probably still end up with a blurry picture. It won't scare off really dedicated pixel-peeping zealots, but you can play with contrast and clarity settings (or whatever tool reduces colour gradients) in post processing to get some of the original sharpness back, but the really fine details will be lost forever. Downsampling after doing your post-processing will also help give you a more attractive image.

To be honest, I don't have many good pictures with more than 50 or 60 recognizable branches in them. It's too many distractions for our minds, I believe.

[wildman]

Without having a decent file it looks like there might be a WB problem also......

[Adam]

Photoshop / Lightroom both include aberration correction tools, both via profiles and manual adjustments.

In this particular case, I would simply de-saturate the purple color channel (i.e. via the Selective Color tool), apply a bit of sharpening, and you should be good to go

[Alex645]

If you have the patience, I would run a very thorough test using the focal lengths you think you're most commonly going to use. If you're uncertain, then perhaps wide at 28mm, long at 300mm, and then perhaps 50mm, 135mm, and 200mm.

With each focal length shoot the same bare tree wide open(f/3.5-6.3), then f/8, and then f/11. My guess is you're going to see most of it will perform average in terms of sharpness and CA, but at certain combinations it will be garbage or you might find a sweet spot. Of course, that will greatly limit the convenience of a zoom, but you will have mapped out exactly what you can expect with this lens. Also I'd suggest either focusing with Live View or manually zooming in, set the focus, and then zooming out to the test focal length.

Post processing will help, but you might as well get to know this lenses strengths and weaknesses instead of guessing.

[normhead]

What Adam said, since there is not natural purple will work... but, this is a very tough image for any lens. Pointing my lens up into the sky like that even my DA*60-250 has shown some purple fringing at times. You're just asking too much of the lens. But a couple of things you can do.. find a higher vantage point, walk around until the light is behind you. it's at the side for these images. The best way to avoid CA and purple fringing is to take shots that won't be prone to CA or purple fringing. I've attempted images like this a hundred times and have exactly zero worthwhile images to show for my efforts. Your best hope is a washed out looking image with little contrast or dynamic range on your subject.

[bar_foo]

You best hope is a washed out looking image with little contrast or dynamic range on your subject.

That, or black & white.

[stein]

Thank you all for the very useful and helpful sugestions! I thought that this is a lost war, now I see that I don't even have to lose even a single battle
I never used post processing except for the basic turning the image into B&W or the so called Sepia effect. I don't have the Lightroom or Adobe PS, or any "serious" image manipulation tool, only Gimp which is not really meant to be used for post processing, although it can do some interesting things. I limited myself to use the FastStone image viewer which actually has all the basic tools to work with photos. I'll look into it, but it will take time as I don't have any experience with LR and similar applications.

This Sigma lens is okay at 28mm and close objects, the problems start once I get closer to 150mm and over. But I don't need that bulky lens for if I limit myself to 50mm

I posted this particular image because the problem is really pronounced in cases like this. An image of a plain landscape, without a lot of small details does appear to be fine. But such images I can take with my Canon SX50HS and they also seem to be fine, so, why would I need a DSLR if I limit my shooting of images which I can take with a plain zoom camera... That's why I bought the Pentax - to make photos that you can't make with a cheaper compact.

Once again, thank you very much. Will need time to play with with this, but now I know where to look for the tools to fix it.

A special thanks to Wildman as I see that there is a way to fix it (the attached fix looks so much better)

[LensBeginner]

Purple fringing is a common issue with high-contrast scenes like that one, and is especially apparent when using lenses without ED glass I believe.
Defringing is hard and not always practical, esp. when you have real purples/reddish colors anywhere else in the picture (and defringing sliders can't really distinguish between the two...)

Also, be wary of all-purpose lenses... I often bring a set of MF primes instead of an AF zoom just because of that...
If you need something in the 55-300 range, look at the Pentax: it has ED glass and is quite effective against purple fringing, even though it's possible that you'll need to stop it down a bit from wide open

[desertscape]

Stein-- remember that most color fringing is produced from the marginal area of the lens, so that is why stopping down reduces fringing. You are truncating the poorly corrected marginal rays. So, stopping down will help prevent CA in the future.

[Alex645]

Stein-- remember that most color fringing is produced from the marginal area of the lens, so that is why stopping down reduces fringing. So, stopping down will help prevent CA in the future.

Note that his example is at f/11 and by the time he gets to f/16 or f/22, he would have other issues like diffraction, high ISO, and/or slow shutter speeds.