Getting ready for a two week photo-shoot in Devon I was putting some of the equipment together and found the ND graduated filters and holder which has been there for a while. Checking out reviews of these square filters you find that there are divergent opinions on the subject of vignetting at wide focal lengths (say 18mm) so here are a few thoughts having tried and got some strange results.
First, the filters are fine plastic or glass (if you have the money). We have tried out Cokin and Hitech standard (not the pro versions). We found that Cokin made the skies a strong inky blue which you had to try to contain later in software. The Hitech kept the proper colour and are much preferable for the price. We suspect there is a small degradation in image quality but we couldn’t rule out the effect of distance and mist so we have yet to make a definitive conclusion about whether to use them for landscape or not. The more expensive Lee filters or the Hitech Pro series would be the best if you can afford it or find you really need them.
A a point of information if you need a filter holder the two (Coking B series and Hitech standard) will fit each other fairly well so if you find one is out of stock you can use the other and this includes external fittings (all except the filters themselves). The various filter threads seem identical.
OK, to get finally to vignetting – this happens the wider your lens is and it can kick in around 18mm (full frame). Note that this would be about 12mm on a DX type lens (meaning the camera has a cropped sensor).
vignetting caused by bad filter holder alignment
The big problem I found with vignetting was caused by the holder not being completely straight. Since the holder is circular and turns, there is a natural inclination to not worry too much about this turning as you zoom in and out especially if it is windy and the ground uneven. But this positioning of the holder is crucial since the image being taken is rectangular and it has sides and top and bottom which the holder must be aligned to. If you are messing about at higher and lower focal lengths the vignetting will come a go with misaligned holder so you may well think it is the focal length causing the problem and miss the fact that it is really the holder.
Vignetting caused by the holder being mis-aligned will typically have just two corners vignetted at opposite corners when the holder is slightly out of true. Real vignetting caused by too wide a lens would be at the four corners.
Yes, you will get ordinary vignetting as you get the lens wider but it can be emphasized or the shadowing completely caused by angle of the holder. There is a mark on front of the holder but it would be better still if it had a white notch there so you could align it more confidently.
So the advice is – get the holder straight before deciding where the vignetting kicks in on your lens.