View Full Version : ND Grads for Big Lenses

Jeff Moore
6-Jul-2004, 21:30
(Cross-posted on Photo.net LF forum.)

I am looking for rectangular ND grads for my LF outfit. Here is my problem: Two of my lenses, 90mm f5.6 Super Angulon XL and 300mm f5.6 APO Symmar, have 95mm and 105mm filter threads, respectively. These lenses are too big to use either the Lee system or other 4" wide filters. The only system for rectangular filters that I am aware of that will cover these two lenses is the Cokin X-Pro system, which is what I am using. However, their ND grads are not neutral at all, but are gray instead. In my tests, these filters are fine when used with B/W film, but definitely add a color cast when used with Velvia and Provia. Also the Cokin filters only offer a 1- and 2-stop grad in a soft gradation. No 3- or 4-stop, and nothing in a hard gradation.

Do you guys know of a source for true ND grads which will fit the Cokin X-Pro system or otherwise cover the two lenses mentioned above? At one time Singh-Ray made ND grads to fit the Cokin X-Pro system, on a special order basis, but they no longer do so.

Any suggestions?

Eric Woodbury
6-Jul-2004, 21:50
Not exactly answering your question, but I bet 4" gels or equivalent will work. If you are stopped down a little, then you are not using all that glass (diameter). I use 4" polyesters with a 72mm SA XL. For a holder, I cut a piece of foam core so that it just slides over the front of the lens and then attach the filter (in a cardboard holder) to the foam core.

Bob Eskridge
7-Jul-2004, 09:16
Why don't you mount them at the back of the lens?

Bob Salomon
7-Jul-2004, 11:00
The Heliopan 4x6" grads were glass and they fit the Heliopan Sliding Fiter Adapter which screws directly into 105mm thread lenses.

Unfortunately, the Heliopan glass grads are no longer made but there are still some 1 stop ones left in stock.

As to placing behind the lens, 1: that would be quite a trick if you want to rotate them. 2: what a great way to ruin the optical quality of the lens. 3: How would you make a rectangular one fit?

Jeff Moore
7-Jul-2004, 12:51
Bob Eskridge: You're kidding, right?

Richard Fenner
7-Jul-2004, 13:13
I've never tried mounting filters behind the lens, so don't know what can go wrong. So, Bob, how does having the filter behind the lens ruin the optical quality, but in front doesn't? Hopefully there's an easy link I can go to, unless the answer is obvious.

Bob Salomon
7-Jul-2004, 13:37
"Bob, how does having the filter behind the lens ruin the optical quality, but in front doesn't? "

First by adding a filter to the back of the lens it becomes part of the lens and will change the focus by an amount = to about 1/3rd the thickness of the filter. Then any dust, smudge, fingerprint, scratch, etc can materially effect the image as well as the lens has already done what it is designed to do when the light passes through it.

When the filter is in front there is no effect on focus and any defects or marks on the filter will not effect the image as they will when in the back.

Unless a lens is designed with a filter inside it or in back of it you should never place a filter anywhere except in front. Unless you are will to have a focus shift and degrade the image.

QT Luong
7-Jul-2004, 14:43
Bob is of course absolutely right about filters in the back. However, if you have to do so, you can avoid the focus shift just by focussing with the filter in place.

Bob Eskridge
7-Jul-2004, 15:31
Calumet sells an attachment that allows putting filters in the back. I have even heard of folks taping the filter to the back of the lens for temporary use.

If the filter is a gel or polyester filter the effect on focus will be nil and can be disregarded because they are very thin.

This is done all the time. Just keep them clean.

Richard Fenner
7-Jul-2004, 15:47
Thanks Bob. I understand what you're saying, but it didn't feel right, so I asked someone else as well! Their agreed with you, but added a small bit which might have been obvious to others but which passed me by. I was curious what difference it made to have the dust, scratches etc in the front of the lens rather than behind the lens - the point was that in front, the dust, scratches etc are wildly out of focus so have almost no effect on the image; behind, and they're perfectly-ish in focus, and have a more marked effect. Makes sense.

Bob Salomon
7-Jul-2004, 16:34
One other indicator of where a filter belongs is the filter thread size published for your lenses by the manufacturer of the lens. If they had meant to have filters mounted in the rear they would publish the filter size for the rear. Most do not. They publish front or drop-in sizes.

Jeff Moore
7-Jul-2004, 20:15
The most obvious (or at least I thought) problem with using a rectangular filter on the back of a lens on a 4x5 is simply fitting it inside a bellows. The Lee filters, for example, won't even fit inside the bellows on my Wisner 4x5, much less allow any room for vertical movement, which is why one would be using a rectangular ND grad in the first place. If you can pull this off you are a lot better magician than I am.

Richard Fenner
27-Jul-2004, 05:27
A small addition - if the lens was designed to take a filter at the rear, apparently focus shift is no longer an issue (so I've been told). That said, I'm curious how that plays with QTL's suggestion of focussing with the filter in place.