a couple of questions regarding GND filters

[emanresu]

Hi Bob et al.,

How are things going? Have been busy lately, finally got some time to get back to photography.  I have a couple of questions regarding GND filters.

1. is GC-GRAY different than GND filter.  I searched for GND filters, but it came back with sunpak GC-GRAY.  so i am wondering if they are the same or not.  The N in GND stands for neutral, which is just another word for gray, but then again, why dont they just called it neutral like others?

2.  when using GND with a zoom lens, does the focal length affect the intensity of the darkness?  I am using a screw-on filter for a specific lens diameter, not the kind with a mount that can be used with many lenses and can be shifted.   What I am wondering is that on the filter, the part near the top is tinted darker than that towards the middle or lower (which is completely untinted).  But when you zoom (towards longer focal length), doesn't it just "magnify" the light that passes through the center part of the front element? so in other words, they don't pass through the darkest part of the GND filter?  does my question make any sense?

Thanks!

Thanks.

[Bob Atkins]

Technically "grey" and "neutral density" should be the same thing. However I've seen some plastic "grey" filters which had a color cast, so may need to be a little careful what you buy.

The circular screw in filters are generally less useful (but cheaper!) than the adjustable square filters. The transition from clear to dark runs right across the center of the frame, so you have to compose the image with that in mind, which can be restrictive.

To be honest I don't know for sure what happens when you zoom, but the gradation will still be symmetical about the center line, lighter on one side, darker on the other. How much of the filter pattern the lens will see really depends quite a lot on the lens design and I'd suggest you try out the filter in a store before buying (or buy from a vendor like Amazon or Adorama who take returns without giving you a hard time or charging a restocking fee).

[emanresu]

Thanks Bob.  Maybe that explains the fairly low price of the item -- plastic.  I think I will look for a square one.

[KeithB]

I was just about to post that color casts aren't too critical in these days of computer correction, especially with RAW images.  However, it can be problematic with a graded filter since you may have the cast in one part of the image, but not the other!

[emanresu]

thanks for the heads-up, Keith.  it would be nasty to get the color out, especially for me because i am not an avid photoshop user.

After looking around a bit, I didn't find too many square filters! The only one I saw is the Singh-Ray.  I think singh-ray filters generally do have good reputation, but that also means their filters are *VERY* expensive.  How durable are these square GND filters? I just don't want to spend a fortune on them but end up breaking them when jamming them into my camera bag.