Vignetting

[Clark Cook]

Just wondering what's causing this problem. The first shot is taken Tv 1/250 sec and the second is taken Av f/5.6. The problem is in the latter. Canon EOS 1D MKIIN, 100-400 IS lens. I would like to hear your expertise on this.

Clark

[garphoto.com]

Clark,
   I too have experienced this and it seems that the focal areas are often brighter than the surrounding sky or background, thus the area of our interest is "glowing" against the background and as if an angelic projection which is lighter while, the outer edges appear dark. The only way I have overcome this is by attempting use of filters to even the background, but in the same action I often dull my primary target.
   Personally, I like the natural (if it can be assumed to be so) vignetting as it gives somewhat of a heightened emphasis on my subject.

- 

[M.M.]

I'm sure this topic has come up before.  The vignetting you're seeing is due to light falloff away from the central axis of the lens.  It's present in all lenses as a consequence of optical laws, but in some lenses it's very prominent (and in others there's so little it's invisible).  It becomes more pronounced at wide apertures.  It's also more noticeable if you have a bright subject in the middle of the frame, as that increases the illumination in the centre of the image and shows up the relatively dark edges.  So if you'd shot a Blue Angel at f/8 rather than a T-bird at f/5.6, you probably wouldn't see any vignetting.  

I believe there are PS plug-ins that can fix vignetting....  Anyone?

-M.M.

[k5083]

I believe there are PS plug-ins that can fix vignetting....  Anyone?

Yes.  There's even a filter for it built into PS (cryptically called the "Lens Correction Filter"), but it isn't very versatile.  Most of the plugins are designed to correct the type of vignetting that results when you stack too many filters or use too long a lens hood, and don't allow you to get far enough into the center to address this light fall-off issue.  But a few months ago I found a terrific one, Vignette 2.0 from www.theimagingfactory.com, which allows you to "profile" your lens to establish the characteristic vignetting pattern that you are experiencing.  To profile your lens, you take a picture of a featureless monochrome field (e.g., a uniform patch of blue sky), then feed that to the plugin and let it build a profile.  You then save the profile, use it when you need to correct vignetting for that particular lens, and adjust the degree of correction for each image until it looks right.  It does a beautiful job.  For a zoom lens I suppose you should build several profiles at different focal lengths that you commonly use, as the vignetting pattern will likely vary as you zoom in and out.

A cautionary note, though -- vignetting correction is never perfect and it is very hard to get the corrected areas to have the same brightness, hue AND saturation as the central area, even though the better plugins like vignette allow you to manipulate all three.  The darkened area of your picture inevitably has less contrast or dynamic range, which represents lost information that can never be completely restored.  It is relatively easy when your background is a blank blue sky, but when you get a part of the aircraft projecting into the vignetted area, or if your shot has some runway or terrain in it, you start to have a lot more trouble.  Therefore, I agree with Gar that if you can convince yourself that the vignetting actually works for the picture aesthetically, you may want to leave it at least partly in place.

August

[DanG (m1)]

i dunno, i think it adds to that particular image. I once heard that the greats even purposely used vignetting in their work? It can make the subject more prominant think.

[FlyingKiwi]

It's quite possible with a zoom lens for the light falloff to be more prominent at different focal lengths. 

For instance, I suspect that the first photo was taken at a relatively short focal length in order to fit all of the aircraft in, while the second photo was probably at or closer to the 400mm end of the lens.

Richard.

[Clark Cook]

Thanks everyone. I too think that the fact I as "locked" on to a wide aperture, it may have caused the vignetting. When a wide aperture is used, the shutter speed is increased in the Av mode, this example 1/1250 sec at f/5.7. On the "good" shot, Tv was used at 1/250 sec and the f/stop set at f/11. This makes for more time to expose the shot with a narrower aperture, resulting in a more natural photo. That's my theory. I may go to the old rule of reciprocal focal length and shutter speed or narrow the the aperture a few stops. Come to think of it, the effect kind of looks cool.

Clark