Originally Posted by
Doremus Scudder
Bob is correct in everything he says.... however, optimum performance is not always necessary or possible. You need to find the right system for the type of work you do.
A few thoughts about the issues raised:
Light fall-off: You can't avoid this with a 90mm (or shorter) lens, but it may not be that objectionable or correctable. If you plan on shooting black-and-white, then much of the fall-off problem can be corrected at the printing stage (the same with color neg and digital processing). You just need to be sure you give adequate exposure (i.e., be aware that the edges and corners are getting less exposure than the meter reading!). If you are doing critical work with transparencies, then a center filter might be worth investing in. FWIW, I and many others use a 90mm regularly without a center filter.
Vignetting: One thin screw-on filter on the front of a 90mm lens should not be a problem with regards to vignetting. Two will start to impinge on the image circle, but you can avoid vignetting by not using the extremes of movements that would show vignetting. I like the smaller f/8 version of the 90mm for its size and weight, and I have done my share of vignetting with it, both from filters and from simply using more movements than the (somewhat smaller than the f/4.5 90s) image circle can handle. One way to avoid filter vignetting is to buy a step-up ring and use a couple sizes larger filters on you 90mm. You could just leave the step-up ring in place. This is more expensive, of course, but a viable solution. (If you get a center filter, it acts as a step-up ring as well, forcing you to buy larger filters). Check for vignetting by stopping down to taking aperture and making sure you can see the entire aperture from the cut-corners of your ground glass (or see all corners when looking back through the lens). One more comment here since you're stacking filters: If you don't use movements, two screw-in filters will work fine; it's when you need to use the edges of the image circle that you'll get into trouble.
The filter system you linked to looks alright to me, kind of like a poor-man's mat box. It's hard to tell if it's well built, but if so, then the issue (since you need to stack filters) would be to ensure that 100mm filters will be large enough to not vignette that far away from the lens. You'd have to try it out.
Placing filters behind the lens: Yes, as Bob says, you risk degrading the image. The amount of degradation depends on the optical quality of the filter you use and it's flatness when mounted behind the lens. I have used good quality coated glass filters behind the lens (B+H or Heliopan) and had good results. I could see no noticeable degradation in 11x14 enlargements. Keep in mind that you'll have to focus with the rear filter in place, especially if it's glass, as it will cause a focus shift. Doing this with a stronger ND filter might present difficulties. Optical gels behind the lens are best in regard to focus shift, but they must be mounted flat and parallel (they used to make gel holders for the back of lenses...) and kept meticulously clean. I have a bunch of gels that I used to use in the studio, but they are just impractical for the kind of field work I do now. I've settled on good-quality screw-in filters.
If you need the faster f/4.5 lens or not: If you aren't shooting with lots of movement, you can easily get by with one of the slower f/8-f/6.3 lenses (the Nikkor has the largest image circle in this category). The image quality is equal if not superior. I find focusing with the slower lenses to be no problem (I have a host of smaller slower lenses including the Nikkor M f/9 lenses, the Fujinon a f/9 lenses, etc.). The upside is that the lens will be smaller, lighter and take smaller (and thus less-expensive) filters. Stepping up to larger filters to avoid vignetting would be cheaper too. If you plan to use a lot of front rise or tilt/swing (e.g., for architecture or if you really needed that extra stop for some reason) then maybe the faster lens would be justified.
Hope this helps,
Doremus
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