Placing Filters Behind The Lens

Ian-Barber

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I have recently purchased a Cokin P Green filter which looks to be about 80x80mm square

Without going to the trouble of purchasing a holder and adapter to screw on the front of the lens, I was thinking of just resting it behind the lens in the bellows. Would placing filters behind lenses results in a focus shift due to the thickness of the lens ?
 

David M

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If you focus with the filter in place, then any shift will be accounted for. I've tried this and found the filter wasn't always secure and could fall down inside the bellows. If this happens after you've composed the shot, you might not find out until you see the neg.
 

Ian-Barber

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I've tried this and found the filter wasn't always secure and could fall down inside the bellows. If this happens after you've composed the shot, you might not find out until you see the neg.
Maybe a bit of blue-Tak between the filter and lens
 

David M

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That would work if you avoid Blu-Tank fingerprints on the lens. I'm sure you're more deft than me. Or a bit of masking tape?
 

Keith Haithwaite

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Putting something into the corrected image-forming light is never a good idea Ian but whatever method you use you need to be sure that the filter is exactly square to the lens otherwise you are going to introduce aberrations. Why not Blu-Tak it to the front of the lens barrel where you can keep an eye on it and where it isn't going to affect the image forming light?
 

David M

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I don't want to contradict anyone, but I can't see how putting a filter in front of the lens will remove any aberrations that it may have. It will still be in the light path from subject to film. I can't disagree with keeping it square.
 

alexmuir

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The filter itself should be optically flat, so shouldn’t affect focus. Assuming you are only likely to use it on your 4x5 camera, you could make a tube to be glued to the filter, and that is sized to be a push fit onto the rear section of your lens. I’ve often thought about using rear mounted filters, but haven’t got round to it yet. I have read somewhere that gel filters are useful for this. I think you could at some stage find filter holders with a spring mechanism to hold on to the rear of the lens.
It’s a similar idea to fitting an infra red filter across the film gate inside a 35mm SLR to allow normal viewing. It would need to be a gel type, but I haven’t managed to source suitable material.
Alex


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David M

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Alex,
I have seen gel filters permanently taped to the back of the lenses of food photographers. Many subjects can cope with slight colour shifts but we are particularly sensitive to the colour of food.
 

Ian Grant

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Alex, evidently a glass filter placed behind a lens can cause a focus shift, a Cokin filter would do the same. However it's only critical at wide apertures.

David's reminded me that I bought a lens a few years ago with a decidedly blue shift when I went to clean it I found a gelatin filter cut to fit inside, it was a tungsten to daylight conversion filter used to shoot in a tungsten light studio with daylight balanced slide film.

Ian, I'd make a temporary filter holder to fit on the front of the lens until you can get the relevant Cokin P holder and lens filter adapter.

Ian
 

Keith Haithwaite

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I don't want to contradict anyone, but I can't see how putting a filter in front of the lens will remove any aberrations that it may have. It will still be in the light path from subject to film. I can't disagree with keeping it square.
With respect David if you had read my post again you will see that I never mention aberrations in the filter itself. I intimated that if the filter was not square to the lens it could introduce aberrations into the optically corrected image-forming beam of light from the lens. :)

Over the years I have heard tales of this or that being placed between the lens and the film and my thoughts always turned to the lens designers/manufacturers who spent zillions of hours testing glass formulations; ray tracing diagrams, mathematical tables and slide rules trying to produce the best possible lens they could only to have some wise guy screw it up by inserting something questionable where something questionable should not be inserted. ;) Of course there are exceptions, the centre-spot graduated filter for WA lenses is one - but this is usually designed by the lens maker. :cool:

I have shot acres of daylight film under artifical light but I always used correction filters in front of the lens or over the lights where possible - but each to their own I suppose. :)
 

David M

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Keith,
My apologies. I have over-paraphrased what you said. I should have said something like "effects." I too tremble for those clever lens designers.
My understanding is that at any surface, the light is refracted and so light passing at an angle through a flat filter will be refracted both on entry and exit. We all remember the bent stick in water. The emergent ray is parallel to the incident ray but shifted slightly sideways and I understand that this moves the point of focus by about a third of the thickness of the filter, so a thicker filter would have a greater effect. If the filter is of high quality there should be no other visible effect.
When the filter is between lens and film, this might have a very small effect on critical focusing that might be significant in some circumstances, such as a close-up of a flat surface using a wide aperture. The effect would be more pronounced with wide-angle lenses.
If the camera is focused with the filter in place, all this will automatically be taken into account. I suspect that for general landscape shots, at the usual small apertures, any effect would be undetectable.
It might seem rather clumsy to focus, remove the lens, insert the filter and replace the lens, but it could be done. I've sometimes removed a lens when the camera was in an awkward place, to see the aperture markings.
For lenses on the outside of the lens, the current fashion seems to be to add grad filters, a polariser and a big stopper, which fills me with horror.
 

Keith Haithwaite

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No worries David and hear what you are saying, I've never seen it in sixty years behind a camera and I just wonder why anyone would do it when there are safer alternatives, heck, I've hand-held/shielded a filter in front of the lens more times than I care to remember so I'd never go down that route.:) I agree with you regarding stacking filters, to me it just demonstrates a lack of understanding and technique in many cases - perhaps trying to make a silk purse out of a sow's ear instead of waiting for the right time/conditions to make the picture. Still, without those who are prepared to push the boundaries there would be no progress - unfortunately my own purse was never full enough to over-experiment with film and employers/clients took a dim view of it. :p
 

David M

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Let's remember that this post was begun by a Yorkshireman who'd just opened his purse and probably needed to lie down with a nice cup of tea to get over the trauma.
For the record, I was born in Skipton.
 

Ian Grant

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Let's remember that this post was begun by a Yorkshireman who'd just opened his purse and probably needed to lie down with a nice cup of tea to get over the trauma.
For the record, I was born in Skipton.
Yes but if I drop a coin it hit's the back of my neck before the ground :D

Ian
 

martin henson

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Did thee err about a lad from Sheffield, ee thought of an idea to save money on film, rather than tekking two shots, ee double exposed each sheet to get four outat film older :eek:


Why dunt thee stick it behind lens and see wot appens.:p

Clyde Butcher does it
 
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alexmuir

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I have a series of books published by Sinar in the ‘90’s or early 2000’s. They are written by Urs Tillman. Book one is about equipment and camera basics. It covers filters and suggests that, at the time of writing, rear mounted filters were the preferred option. I’m assuming it’s talking about the practice of professionals, and possibly students. It shows a holder marketed by Sinar to fit up to three filters behind the Sinar ( behind the lens ) shutter. It states that focusing must be done with filters in place. The advantage it identifies is the practical elimination of flare. The book also talks about front mounted systems, an example of which was also marketed by Sinar. It advises the use of an efficient lens hood, such as a bellows type when using front mounted filters. I tend to use the adjustable bellows hood when working indoors to prevent flare from studio lighting. It would be a lot of hassle to use out and about.
The Sinar books are quite interesting, but quite pricey, so not really worth buying ahead of other popular titles.
Alex.


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Alan Clark

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I regularly use filters behind the lens with my Kodak Ektar 203mm lens on my home-made 5x4 camera. I have Kodak's custom lens hood for this lens, which incorporates a holder for Kodak no. 370 filters. I even have a couple of these, but not the orange or yellow I normally use. But I do have plenty of 49mm screw thread filters which I use with my OM1 camera. So I removed an unwantes UV filter from its mount, and glued the mount to the back of the lens board. This allows me to screw any filter behind the lens. This arrangement is only possible because the rear part of the Ektar lens is of small diameter, and doesn't stick far out of the back of the lens board.
I like this arrangement because it allows me to use the custom Kodak lens hood and is a better arrangement than blu tacking a square plastic filter to the front of the lens hood, where it would be unprotected from direct sunlight unless shaded in some way.
Not had any problems using the filter behind the lens, but I do focus with the filter in place.

Alan
 

mpirie

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Ian, if you find out what the filter thread size is, i'm sure one of us will be able to lend you an adapter ring and filter holder to try out.
 
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