vignetting ?

[ricstew]

Is there such a word?
this was taken at 150mm, f6.7, 750/1 this morning and it has vignetting. Its really obvious in LR with the series next to each other. Is this something to do with the crop sensor? The helicopter ones done yesterday did not have a vignette? They were 65mm, f9.5 at 1/3000....same lens, same camera. The only difference was I didn't have the hood on this morn ( hey it was early! ).
What influences the vignette?

http://farm3.static.flickr.com/2623/5694797446_d33c3e7211_b.jpg

[markjaffa]

Is there such a word?

According to at least some dictionarys there is! :th3:

My understanding is that the presence of a vignette is influenced by -
1. the aperture(wide open will often induce vignetting - stopped down will remove it)
2. the focal length you are shooting with on a zoom lens(some zoom lens' exhibit more vignetting a one focal length compared to another)
3. the presence of a hood - especially an aftermarket hood not designed by the lens manufacturer
4. (possibly) if the lens exhibit a focal shift?
Not too sure if 4 would lead to visible vignetting - the shift would be very small.

[Wobbles]

To be honest I can't really see it, but that might just be my "average" laptop monitor. There's a good explanation of vignetting here - http://www.the-digital-picture.com/Canon-Lenses/Canon-Lens-Vignetting.aspx Good news is it's quite easy to fix now in Lightroom/PS.

Cheers
John

[etherial]

I can certainly see it in this image, but it isn't a bit deal. All lenses will have it just to different degrees. I think your question has been answered above and the link is a good one. Only thing I'll add is that a crop sensor will actually be less prone to vignetting when used with an EF lens as they have a large image circle (designed for larger sensors) and a crop sensor is taking only a crop of the centre of the image circle the lens is showing so you are cropping out the vignetting. But EF-S lenses have a smaller image circle as they are designed for the smaller sensor so vignetting will be again be an issue depending on the factors Mark has mentioned above.

[arthurking83]

basically as markjaffa said.

major factors are aperture, and on a zoom lens, focal length.

Wider open aperture will always cause more vignetting, compared to a more closed down setting.
The degree of vignetting is dependent on the lens(design).

Another cause for vignetting is the use of filters. Some lenses vignette due to the physical presence of a normal sized filter itself and there are a few slim or ultra slim designs available to reduce or eliminate this.
Polarisers are different in some ways tho. Even if the lens is not prone to vignetting with a filter, the same sized polariser may still cause vignetting due to the fact that the polariser is 'altering' the exposure of various parts of the scene.
You can see this more so with an UWA lens if used in early morning/late evening light.

In this image the lens appears to be a Sigma 70-200/2.8 and aperture set to f/6.7. Generally at that aperture vignetting would have been eliminated(or you would have expected it to be).
There is some vignetting obvious in the upper and lower RH corners, and that's normal.
On the whole the image looks OK, but over areas of a single solid colour and what would normally appear to be evenly exposed(such as the blue sky in your image) vignetting is more obvious.

I think that the choice of metering mode(being spot) may have also had a slight influence on the vignette too, although it doesn't actually have any impact on the vignetting itself, it can cause the phenomenon to be more exaggerated.
Because you spot metered, the exposure was slightly darker than it otherwise would have(or could have) been. That is, the blue sky is more darkly exposed because the meter has accounted for the bright bird in the centre.
If the blue was brighter, the vignette will be less obvious(brighter exposures seem to give the illusion of less vignetting).
The vignetting is still there! That's a property of the lens, but it appears to be less pronounced.

exif doesn't reveal if you've used a polariser filter, but at 150mm I don't think it'd have made any difference.

I'm surprised that this lens vignetted so much at the lens values you have selected. :confused:

According to the PhotoZone review of this lens, it produce negligible vignetting at f/2.8 on a crop sensor.... and close enough to zero at f/5.6(and confirmed on DPR)
There should be no vignetting visible at all, especially at f/6.7

[ricstew]

Thanks for the advice guys. I think its the lens.
The exif data lies.....:confused013......its actually sigma ex 50-150mm. So I went and checked a few reviews and most say to expect some mechanical vignette at 150mm so thank goodness I can remove it in PP. Its not severe and the IQ makes up for it :) I was quite disconcerted at first.
No filter on this one....I haven't got the right size.
I wonder why the exif data lies? Would I need to update something?

[arthurking83]

......
I wonder why the exif data lies? Would I need to update something?

Probably an Adobe thing(bug)?? :confused013

I've had issues with LightRoom, where it reports focused distance as 4 million klms away(in all images), but never lens type.

Have you tried converting the image(if raw) via Pentax's own raw conversion software to see if that makes any difference?(to the reported lens in the exif).