I have a 58mm ND 1000 B+W Filter. I want to use it with my 77mm 24-105 lens. What problems will I face if I use a step down ring?
Severe vignetting at the widest apertures and shortest focal lengths. The step down ring will block the outer 9.5mm around the edge of your lens. That's if you can even find a ring that has the male threads needed to connect to the lens on the outer edge and the female threads needed to connect to the filter on the inner edge. Such types of rings do exist, but I'm not sure I've seen one with a jump from 77mm down to 58mm. (edit: such rings are available)
The amount of vignetting you experience will be influenced by a few factors.
- Selected aperture. The wider the aperture setting you use, the more vignetting you will see. Since the purpose of using a very dense filter such as your B+W ND 1000 is often to enable the use of a wider aperture, this may be significant for you. At very narrow apertures you may not see much, if any, vignetting.
- The selected focal length. You'll get more vignetting at wider angles than with longer focal length settings.
- The actual diameter of the lens' front element. Some lenses have glass that comes very close to the filter threads. Other lenses have front elements that are quite a bit smaller in diameter than the filter threads. Even if the lens' front element is 58mm or less in diameter, if the lens is used at fairly wide angles and the step-down ring is several millimeters in front of the lens glass you will still see vignetting of the edges of the frame.
If the lens in question is the EF 24-105mm f/4 L IS, the actual glass of the front element is about 60mm in diameter. At longer focal lengths and narrow apertures you might not see much, if any, vignetting. At shorter focal lengths and wider apertures you could see significant vignetting, depending on how far in front of the lens' first glass element the step-down ring is located when attached to the lens.
Most adapter rings that cover such a wide disparity (77mm to 58mm) are step-up rings that are made the opposite from what you would need: the inner threads are male to connect to the female threads on a smaller lens and the outer threads are female to receive the male threads of a larger filter.