6x17 and centre filter

[Charles Twist]

Dear Forum,
I come to you for advice. I have a 6x17 roll-film back which I often use with a Fujinon 120mm f/8 SW lens - which is not a million miles from the Nikkor 120mm f/8 SW lens, apparently. For most landscape applications, I can fire away with limited movements and get very decent results. I can see that the edges are a tad darker than the centre, but nothing unpleasant - quite the contrary as it gently draws the attention to the middle. For architecture, I use far more movements and end up with quite significant fall-off - maybe as much as a stop at the very edge of the frame. It's really noticeable when I scan the film and can look closely. There is no denying the results look better when I correct for the loss of exposure. So a centre filter beckons.
With the Nikon, BHPhotoVideo recommend a Heliopan 3x ND which is about 1.5 stops from centre to rim - sounds about right for my needs.
Question 1: does anyone know about Heliopan's quality in terms of neutrality and clarity? I would hope they are V.V.V.V good given the price . They're made with Schott glass in Germany so hopes are as high as the price tag.
Question 2: Can you buy the filter in the UK? I think Teamwork will be able to source them, but there is no sign of it on their website.
The big problem with the above filter and indeed many centre filters is the insistence on having a very wide outer filter diameter - 105mm in this case (to the 77mm of the lens). Now, I understand the need to give clearance, to avoid vignetting and that, but it's hardly convenient when one wants to mate the filter with a Lee system. I could get a special ring made to fit within the 105 mm and have the groove to accommodate Lee's system, but it's more faff.
Question 3a: is there anyone out there to manufacture a centre filter without the wide flange? What's their quality like? Do you think I could approach Heliopan or Schott and ask? Anybody with the contacts?
Question 3b: is there any reason for not putting a flangeless centre filter on the rear element of the lens? That strikes me as the best place from a mechanical point of view, but I am not sure about the optical considerations. There are slimline filters which won't allow you to screw further filters on, which would be a good format. I realise that means focusing with a dark lens (f/13 or something).
Any help will be gratefully received.
Thank you.
Best regards,
Charles

[George Hart]

Charles, I have nothing like your experience generally but I can chip in a bit about Heliopan filters of which I have quite a few. I have got mine from Teamwork. Delivery time can vary quite a lot—sometimes I have had to wait a few weeks, possibly for a new batch to be made. Although most of my filters are for B&W I have used warm-up jobbies and they have all been excellent.

I have no answers regarding your flange difficulties, but given the starting price of big centre filters, I anticipate that OEM specials would be bordering on the crazy!

It might be worth scouting around the Fotoman site for advice. I think that their 6x17 model is quite popular, and I'm sure that there will be some info/advice on centre filters there.

Most authorities give a thumbs-down on putting any filter on the rear element. In general with rear element filters, some speak of focus shift, and all refer to the optical deterioration from an element behind vs in front of the lens. As I understand it CFs are designed to be used a specific distance from the front element, in order to apply the stated graduation profile to the image at the film plane.

Best wishes,

George

[Apple]

Charles,

Just a few snippets to chuck in:

B&W make filters for wide-angle lenses that don't have front threads - I have a couple of polarisers from them like that. I don't know whether they make other series in the same style.

For custom filter holders, SRB-Griturn do bespoke stuff so it might be worth contacting them. You're almost wanting a standard filter with the front turned off in a lathe but I don't know how strong the rest f the filter would need to be to be held in the chuck or how much meat they'd have to actually grip one.

For the DSLR on LF you mentioned last weekend, there's an advert in most View Camera mags but I can't remember the company name (it may be Betterlight but they tend to do scanning backs.)

Here's some links to other versions / ideas.

http://www.camerafusion.com/

http://www.vistek.ca/store/ProPhotoLarg ... canon.aspx


Andrew

[Thingy]

I've got centre filters for my 58mm Schneider XL & 75mm Rodenstock Grandagon but haven't bothered with the 120mm/f8 Nikkor. The filters need to be mounted directly to the lens and their wide outer ring accomodates adding additional screw in filters, such as a UV filter. On both these lenses you could possibly attach your Lee filters using the push on filter holder & donut spacer. None of mine have an outer thread of 105mm, so I use the Gelsnap holder secured with a rubber band.

http://www.robertwhite.co.uk/product.as ... &PT_ID=501

[Emmanuel Bigler]

A few comments about Charles' questions.

You wish to use a centre filter when you shift your lens to compensate for darkening in the corners for some subjects. As mentioned, by shifting you use rays entering at a wider angle. So you need a large flange if you really want to correct the "natural vignetting" of the lens. In fact "natural vignetting" combines with any source of mechanical vignetting when the filter flange is not wide enough.
So If you use the same filter diameter as the front ring diameter you simply miss the point i.e. compensating for darker corners ! By re-introducing mechanical vignetting after having compensated (at a nominal cost) natural vignetting, you do not gain anything...

Probably you could mount a Rodenstock 82/112 centre filter designed for the (discontinued) 6.8/115mm or the current 4.5/90 Grandagon-N. Those Rodenstock filters sometimes show up as second hand items...

I was lucky to find a second hand 55/67 Rodenstock filter for my 6.8/75 Grandagon-N and I am very happy with it, it is perfectly neutral for colour, -1.5 f-stops is not a big deal for static subjects.
Recently I got (second hand as well) an Heliopan in 67/86 size for my 6.8-90; not yet tested but I am 100% confident about it (colour rendition is an issue with centre filters ; and color rendition is an issue for center filters as well ) since the previous owner is an uncompromising professional large format slide user.

So you can't go wrong in termes of color cast either with the Rodenstock or the Helliopan centre filters. Or the Schneider, I know that they exist, they might be even easier to find that the Heliopan of Rodenstock but I don't have any echoes about them.

In order to adapt the 82/112 Rodenstock filter to your 77mm front thread you'll probably need a 77-to-82 step-up ring at the cost of a small mismatch between the filter and the lens.. which is already mismatched not being a 115mm Grandagon-N
This could work even if the filter is not designed for your Nikon.

On the technical aspect on how the centre filter works.
Wide angle lenses are almost symmetrically built and their entrance and exit pupils are located very close to the nodal points of the lens. Hence by the basic property of nodal points, the angle of an input ray aiming at the centre of the entrance pupil is exactly the same angle as for the output ray exiting from the centre of the exit pupil.
So a centre filter, in fact : the same filter, could work almost perfectly on the back side of the lens.
Centre filters are computed by taking into account the mean ray crossing the centre of the pupil.
So if two lenses have their pupils located differently, in principle you cannot swap filters from the original lens to another. Moreover the natural vignetting of wide angle lenses is already partly compensated by the so-called pupillar distorsion effect, effects that might be different from one brand to another for the same focal length.
In reality the matching does not have to be very tight between the centre filter and the lens; and Heliopan, a serious and reliable independant filter manufacturer does not specify for which lens brand it is designed !

Now there is a subtle question regarding what happens when you insert a plane and parallel piece of glass behind a wide-angle lens aiming at infinity.
First, a piece of glass behind the lens generates a focus shift by about 1/3 of the glass thickness. This not very important since you focus on the GG and not with an helical or a rangefinder. However one should expect that this focus shift changes with the wavelenth of light...
When placed in front of the lens, the image shift is strictly : zero, since infinity plus 1/3 of thickness = infinity.

Another subtle effect in addition to the focus shift : when the glass plate is located behind the lens, at a finite distance from the final image. The glass plate generates some amount of spherical aberration, the results being worse for higher incident ray angles. Whereas when it is located in front, with an object at infinity, no aberration of any kind is generated even with rays entering at plus or minus 50° (total =100) of the optical axis. This is a bit hard to explain since in both cases angles enter at a high angle...
So as summary : placed in front of a wide angle lens, there is no image shift and no aberrations of any kind with any filter, hence this is the preferred setting for landscape use.

--------------

There are actually two series of centre filters
- the 100° series minus 3x / 1.5 f-stop the one we are discussing here
- a 110° series that are hard to use with their filter factor of 5x, -2.5 f-stops... but this factor is really necessary if you really want to compensate natural vignetting up to 110°...

For the convenience of our readers
- the Linos-Rodenstock relevant web page for centre filters is here
http://www.linos.com/pages/no_cache/hom ... 571ffb8782
- the Heliopan catalogue, version 2009-01 is here in pdf
Centre filters are described P.13, Heliopan apparently no longer fabricates 5x filters for the 110° lenses, they only sell 100° / 3x filters.
http://www.heliopan.de/Preisliste.pdf

[IanG]

Charles, I use a 6x17 camera with a 75mm Super Angulon, perhaps a centre filter might be useful but I don't have a problem with B&W. With my 90mm a centre filter isn't necessary. 120 will be better still.

Ian

[Peter B]

Charles, a couple of points/views.

Q1 - Heliopan get a good reputation and I seem to recall Speedgraphic saying they were better than B&W

Q2 - Centre filters are listed by both Teamwork and Robert White, but under the Rodenstock and Schneider brand names, e.g. http://www.robertwhite.co.uk/product.as ... &PT_ID=370 is the link for Schneider centre filters at RW, whereas they are listed under the LF lenses category at Teamwork. Whether they are in stock is a different matter! I have a nagging feeling that Heliopan are linked to either Schneider or Rodenstock, hence will probably make the centre filter under that brand name. I could be wrong on that one, though. Good luck, it's quite an investment!

[Thingy]

There is a secondhand centre filter currently on ebay for the Rodenstock Grandagon 75mm/f4.5 & 90mm/f6.8 lenses. You can buy it for £160 if anyone is interested. I recently bought one of these at the full price ( over £200)! Aaaaarrrrggggh!

http://cgi.ebay.co.uk/Rodenstock-Centre ... 240%3A1318

[IanG]

I paid less than that for my 75mm Super Angulon 2 years ago, which although supposedly used, came as new in it's original box, it had quite obviously never fitted to a lens board.

Ian

[timparkin]

Dear Forum,
I come to you for advice. I have a 6x17 roll-film back which I often use with a Fujinon 120mm f/8 SW lens - which is not a million miles from the Nikkor 120mm f/8 SW lens, apparently. For most landscape applications, I can fire away with limited movements and get very decent results. I can see that the edges are a tad darker than the centre, but nothing unpleasant - quite the contrary as it gently draws the attention to the middle. For architecture, I use far more movements and end up with quite significant fall-off - maybe as much as a stop at the very edge of the frame. It's really noticeable when I scan the film and can look closely. There is no denying the results look better when I correct for the loss of exposure. So a centre filter beckons.
With the Nikon, BHPhotoVideo recommend a Heliopan 3x ND which is about 1.5 stops from centre to rim - sounds about right for my needs.
Question 1: does anyone know about Heliopan's quality in terms of neutrality and clarity? I would hope they are V.V.V.V good given the price . They're made with Schott glass in Germany so hopes are as high as the price tag.
Question 2: Can you buy the filter in the UK? I think Teamwork will be able to source them, but there is no sign of it on their website.
The big problem with the above filter and indeed many centre filters is the insistence on having a very wide outer filter diameter - 105mm in this case (to the 77mm of the lens). Now, I understand the need to give clearance, to avoid vignetting and that, but it's hardly convenient when one wants to mate the filter with a Lee system. I could get a special ring made to fit within the 105 mm and have the groove to accommodate Lee's system, but it's more faff.
Question 3a: is there anyone out there to manufacture a centre filter without the wide flange? What's their quality like? Do you think I could approach Heliopan or Schott and ask? Anybody with the contacts?
Question 3b: is there any reason for not putting a flangeless centre filter on the rear element of the lens? That strikes me as the best place from a mechanical point of view, but I am not sure about the optical considerations. There are slimline filters which won't allow you to screw further filters on, which would be a good format. I realise that means focusing with a dark lens (f/13 or something).
Any help will be gratefully received.
Thank you.
Best regards,
Charles

Hi Charles,

I think you'll find using a grad filter will be enough to compensate for most of the vignestting you are seeing (i.e. if the centre of the lens aligns with the centre of the picture normaly. Once you move the centre of the lens to the left edge of the picture, most of the light fall off is in a single direction and can be compensated with a one stop grad. You might have to play with hard or soft though). I've used this when I've used a lot of rise on my 110XL and (a one stop soft) and it's worked a treat.

Tim

[Charles Twist]

Hello Tim,
Yes, with 5x4, that works fine. The problem with 6x17 is the 17cm of width. Because of the axis ratio, one can think of the image circle not as a circle but as symmetrical bands of darkness. Assuming the 17cm are parallel to the horizon, when I apply shift, I could probably go the route you suggest, but when I apply rise or fall, I end up with the bands of darkness getting closer together as I move away from the centre. There be the problem. And I am not much fancying the solution...
Best regards,
Charles