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12-23-2008, 07:33 PM   #1
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vignetting problem

Hello,

I am attaching a picture I took with my 18-250 mm lens. You can notice the vignetting problem. From what I read in this forum it is a common problem with this lens, however I would like to know how to fix the problem or even better how to anticipate this type of issues by setting up my camera properly. It occurs when I am close to the 18 mm focal distance. Currently to fix the problem I use a processing software. I shoot in RAW format.
Thanks

Detail of the shot:
body K10D
lens : 18-250mm pentax f3.5-6.3 DA
ISO 100
focal lenght 23 mm
F/4.5 1/90sec
no filter, no hood.

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12-23-2008, 08:21 PM   #2
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The only "fix" on-camera is to stop the lens down. Just view the lens as an f/8.0 daylight shooter that has "emergency backup aperture".
12-23-2008, 08:56 PM   #3
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I don't think there is any practical solution in-camera unless your vignetting is caused by a lens hood, in which case, removing it would probably eliminate the vignetting.

I get vignetting like that too with a couple of my lenses. So far, it's never been a real problem for me. It's very easy to clone out those dark corners with a photo editor. In your example pic it would be very easy. It's annoying for sure, but not a show-stopper.
12-23-2008, 10:00 PM   #4
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The quickest fix is shoot wider angle and crop out the vignette. I have the same lens too and at F11, I get no vignetting, but when it stopped up high enough, the vignetting shows itself. To me, it's not a problem.

In fact, I decided to use vignetting to my advantage in this pic. You either love it or hate.

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12-23-2008, 11:27 PM   #5
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Venturi,
Besides the hood it can be caused by a thick filter mounting ring.

Dave

QuoteOriginally posted by Venturi Quote
The only "fix" on-camera is to stop the lens down. Just view the lens as an f/8.0 daylight shooter that has "emergency backup aperture".
12-23-2008, 11:53 PM   #6
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Hi enrod666,

I have the Tamron version of this lens, and I, too, find the vignetting annoying in many situations.

The best bet is to stop down the aperture and avoid the extremes of the zoom range, for avoiding it in camera.

However, the Pentax Photo Lab software (and CS3/CS4's Camera Raw) both have vignetting correction which is easy to apply and completely correct the problem.

Personally, I just ignore the vignetting until I am printing or publishing the photo, then I use software to correct it.

-Chris
12-24-2008, 05:07 AM   #7
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QuoteOriginally posted by enrod666 Quote
I am attaching a picture I took with my 18-250 mm lens. You can notice the vignetting problem.
Is it normal that the vignetting does not appear to be symmetrical? Does this point to a decentering problem?

Most of the time, I don't seem to have a vignetting problem with my Tamron 18-250. Sometimes, though, mainly one corner shows the vignetting symptom.

I have checked the lens for decentering and it appears to have pretty much the same sharpness in all four corners.

Could the shake reduction mechanism be the culprit? When a lens vignettes and the sensor is moved out of its centred position, the vignetting should be non-symmetrical.

When I look through the lens (at the front element, at the light coming through the viewfinder) when stopping it down (with optical preview) I notice that one aperture blade is a bit "faster/earlier" than the others and one is a bit "slower/later". The resulting shape doesn't resemble a circle a lot. I didn't notice it from bokeh shots, though. I guess that an uneven aperture cannot cause such a "one corner vignetting" though.

I still have warranty on the lens. Would it make sense to send it in, pointing out the "one corner vignetting" and the odd aperture shapes? My copy seems to a good one (even corner performance), though, and I'm hesitating to have it butchered up by attempts to fix problem that don't really deserve attention.

Last edited by Class A; 12-24-2008 at 01:01 PM.
12-24-2008, 09:01 AM   #8
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QuoteOriginally posted by Big Dave Quote
Venturi,
Besides the hood it can be caused by a thick filter mounting ring.

Dave
True, but he stated in the OP he had "no filter, no hood".

12-24-2008, 09:39 AM   #9
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Stopping down and/or increasing the FL are about the only ways I know to fix it, besides PP.

Looking at your photo, you have a much worse problem than vignetting....SNOW! As a Minnesotan, I know the best way to fix that. Move south for the winter for crying out loud! Vignetting is much more tolerable in warmer climates!

Just joking....I love snow....till about 15-Jan....then I hate snow.
12-24-2008, 09:48 AM   #10
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Hi,

I just downloaded your image and tried to correct it the way I normally correct mine.

But now I see what you mean about the "uneven-ness" of the correction required.

I never experienced this with my Tamron. I usually just applied a moderate amount of correction and you'd never know it was there. Yours seems different.

-Chris
12-24-2008, 10:27 AM   #11
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I think vignetting is just something we end up having with wide angled lenses. I tend to have the most problems when I try to have a filter on my lens (e.g. a polarizer).

Recently, I've gotten myself used to shooting images a little wider than the final product I want (of course if you are already at the widest end of a lens this doesn't help). This has helped let me crop out the vignetting, but it has also helped me get the optimal crop out of an image...

I often find that I wish I could tweak the framing on my images slightly here and there, which is what led me to shooting a bit wider than what I want from the final image. Being able to crop out the vignette is a nice bonus of that method.
12-24-2008, 12:48 PM   #12
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Original Poster
Hi,

To get one stop down I can go either from f/4.5 to f/6 or from 1/90 to 1/125 correct? Is it better to adjust the aperture or shutter speed to fix the vignetting problem, or it doesn't matter as long as I go one stop down?
12-24-2008, 01:05 PM   #13
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QuoteOriginally posted by enrod666 Quote
Is it better to adjust the aperture or shutter speed to fix the vignetting problem, or it doesn't matter as long as I go one stop down?
You can address vignetting only by adjusting the aperture (increasing the f-ratio).

Increasing the shutter speed will not change the distribution of light within the frame. It won't help, on the contrary, it would exacerbate the problem as the aperture would have to become even wider to make up for the loss of light.

Last edited by Class A; 12-24-2008 at 07:12 PM.
12-24-2008, 10:04 PM   #14
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QuoteOriginally posted by enrod666 Quote
To get one stop down I can go either from f/4.5 to f/6 or from 1/90 to 1/125 correct? Is it better to adjust the aperture or shutter speed to fix the vignetting problem, or it doesn't matter as long as I go one stop down?
Don't *underexpose* it by one stop - you want to keep the exposure the *same*, but stop the lens aperture down. That means from f/4.5 to f/6.7, and then *slow* the shutter from 1/90 to 1/45 to compensate.
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