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  #1  
Old 9th March 2020, 01:44 PM
Martin Aislabie's Avatar
Martin Aislabie Martin Aislabie is offline
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Default 1.5 Stop 67mm Centre Filter

Hi guys

I've been made aware that there is a 1.5 stop 67mm Centre Stop filter for sale in EBay.

https://www.ebay.co.uk/itm/HELIOPAN-...-/254529483369

Anyone who has a lens shorter than 90mm should really be use a Centre Filter.

No commercial Centre Filter will exactly match the light fall off of any lens precisely - its much better to be close but not exactly right by using a Centre Filter than be way off by not using one.

Martin
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Old 9th March 2020, 04:48 PM
Mike O'Pray Mike O'Pray is offline
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Just curiosity on my part but what's the difference between a ND centre filter and a normal ND filter that makes it better for use.

Is this large format photography we are talking about?

Thanks

Mike
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  #3  
Old 9th March 2020, 05:51 PM
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Martin Aislabie Martin Aislabie is offline
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Hi Mike

Yes it is Large Format we are talking about.

All lenses suffer from light fall off as we move away from the central axis of a lens.

The light fall off is in proportion to the cosine of the angle away from the lens centre line raised to the power of 4 (cos^4)

Therefore, the drop off in illumination is almost insignificant around the centre line, so long lenses give very even distribution across the entire area of the negative.

However, on short lenses with lots of camera movement and on very short lenses the corners of the negative are at a significant angle from the centre line.

For example :-

10 deg from the centre line - cos10 = .985 > cos^4 = 0.94

20 deg from the centre line - cos20 = 0.940 > cos^4 = 0.78

30 deg from the centre line - cos30 = 0.866 > cos^4 = 0.563 (aprox 1 stop)

40 deg from the centre line - cos40 = 0.766 > cos^4 = 0.344 (aprox 2 stops)

50 deg from the centre line - cos50 = 0.643 > cos^4 = 0.171 (aprox 3 stops)

To correct this fall off in illumination, large format lens manufacturers make Centre Filters to try and correct or at least minimise the fall off of illumination away from the centre line.

What you end up with is in effect a neutral density filter at the centre line of the lens and a clear filter around the edges.

These Centre Filters were made by grinding away neutral density glass around their edges and mating them with clear glass ground away in the centre to make an assembly which were both flat and void free.

Centre filters with their progressively increasing density towards the centre are things of beauty – IMHO

Apparently, the scrap rate was huge - hence why centre filters were so expensive to buy - so most people didn't bother parting with their cash.

If you have a 5x4 negative and a 90mm lens, the corners of the negative are at about 40 deg from the centre line.
When you look at a negative taken without a centre filter nothing looks to obvious – the fall of in illumination is gradual and progressive.
However, when you compare it to exactly the same short this time taken to one with a centre filter fitted you see a much more evenly illuminated negative and the light fall off of the non-centre filter negative becomes apparent.

Small format lens manufactures never bothered about the illumination fall off on their lenses for whatever reasons – cost/manufacturing difficulty/usability/bragging rights.

Hope this helps.

Martin
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  #4  
Old 9th March 2020, 05:54 PM
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A plain ND filter has an even pigment right through the filter. Graduated filters have clear parts, usually in a straight line across the face of the filter and then the pigment increases in intensity, either gradually or more abruptly to provide a fuzzy edge. A centre filter is a sandwich of clear glass and a pigmented layer. The clear glass is then ground concave, and the pigmented part convex. Then they're glued together, giving the clear edges and gradually darkening centre. Or they can be made by spraying a metallic coating onto a plane piece of glass.

All of which is rather difficult to do.

As for their uses. They are used in Large Format photography for the wide-angle lenses of 90mm or less for 4 x 5 cameras, but they were also available for the Hasselblad/Fujifilm X-Pan/TX-1. And they were useful on medium format panoramic cameras such as the 617s from Fujifilm and Linhof as the coverage is so large that light fall-off at the edges is noticeable.
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Old 9th March 2020, 05:59 PM
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Many thanks Martin for explaining this in very clear terms.

I have used a 90mm on my 5x4 without a centre stop and noticed some fall-off which I just accepted and used it creatively - I quite liked the effect. I am not sure if I would want to correct this but I can now understand how and why it is done if I need to do so.

Neil.
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Old 9th March 2020, 07:15 PM
Mike O'Pray Mike O'Pray is offline
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Thanks Martin and Nick I am a complete beginner on LF lenses - well any lenses really and had never heard of a centre filter.

Is there a filter factor to take into account when judging the correct exposure?

Mike
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Old 9th March 2020, 07:33 PM
Lostlabours Lostlabours is online now
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There's no such thing as a Centre filter, they are Center ND filters

New they are around £400 to over £1,000. I was lucky to buy two last month one for £30 and the second for £35 both NIB Hoya Center ND filters. I bought the first to use with my 75mm f8 Super Angulon which is fitted to my 6x17 camera, as 17cm is just short of 7" there's noticeably fall off.

The second came from the same Welsh photo store (both via ebay) once I realised they had two, it'll fit my 90mm f6,8 Gandagon N or 75mm f5.6 Super Angulon.

With 5x4 and my 90mm GAndagon N I noticed slight darkening of corners with E6 film but nothing with B&W and it's a lens Iuse a lot, more recently I've also added a 75mm f5.6 Super Angulon and gain with B&W not seen a problem with B&W, however my 65mm f8 Super Angulon suffers more noticeable fall off.

Neil alludes to "the effect" and we must remember many burn in the corners and edges of prints, so this is no different.

I've not seen so many Center ND filters listed on Ebay before, they don't get listed taht frequently.

Ian
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