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Pawlowski6132
15-Dec-2010, 11:00
So, I have a Schneider convertible lens: 210/370. I leave both cells on (on either side of the shutter) for the 210mm length. The filter threads on the front cell are hosed and I've just been using a type of Lee system. My question is, can I screw a filter on to the cell which is inside the bellows? I don't see why not. And, I'll try it but, was wondering if someone had some scientific reason for why this would or wouldn't work.

thanx in advance,

Joe

Mark Woods
15-Dec-2010, 11:11
This is called a Behind the Lens Filter, BTL for short. It's done all the time. With LF, it's no problem, with fixed lenses with a given flange focal distance, you can alter the focus to the image plane. Again, not an issue with your system.

Jason Greenberg Motamedi
15-Dec-2010, 11:15
As Mark said, yes, it is fine, but you need to be aware that you must focus the camera with the filter in place, as a slight focus shift occurs when you place something between the lens and the film. This shift is some proportion of the thickness of the filter, so it would be pretty negligible for a thin polyester filter, but significant for a glass or resin filter.

Bob Salomon
15-Dec-2010, 11:39
You should also be aware that anything on the filter and the quality of the filter will effect the image as a filter behind the lens becomes part of the lens. These imperfections in filter quality or marks on the filter have far less effect when mounted to the front of the lens (unless the filter was designed to be in or behind the lens). In addition, as noted earlier, any glass or acrylic behind the lens will create a focus shift equal to 1/3rd the thickness of the filter.

Filters belong in front of the lens. Why not get the threads fixed or buy a clamp-on holder for the front?

Also, those are not filter threads in the back. Those are the threads for the retaining ring of the front group and may not be:
1: standard filter thread and pitch
2: not designed to have things mounted and unmounted which can damage fine threads if not carful. Filter threads are usually a coarser thread.

Pawlowski6132
15-Dec-2010, 18:35
This is called a Behind the Lens Filter, BTL for short. It's done all the time. With LF, it's no problem, with fixed lenses with a given flange focal distance, you can alter the focus to the image plane. Again, not an issue with your system.

Awesome.

Pawlowski6132
15-Dec-2010, 18:36
As Mark said, yes, it is fine, but you need to be aware that you must focus the camera with the filter in place, as a slight focus shift occurs when you place something between the lens and the film. This shift is some proportion of the thickness of the filter, so it would be pretty negligible for a thin polyester filter, but significant for a glass or resin filter.

Hmmmmm.

Pawlowski6132
15-Dec-2010, 18:36
You should also be aware that anything on the filter and the quality of the filter will effect the image as a filter behind the lens becomes part of the lens. These imperfections in filter quality or marks on the filter have far less effect when mounted to the front of the lens (unless the filter was designed to be in or behind the lens). In addition, as noted earlier, any glass or acrylic behind the lens will create a focus shift equal to 1/3rd the thickness of the filter.

Filters belong in front of the lens. Why not get the threads fixed or buy a clamp-on holder for the front?

Also, those are not filter threads in the back. Those are the threads for the retaining ring of the front group and may not be:
1: standard filter thread and pitch
2: not designed to have things mounted and unmounted which can damage fine threads if not carful. Filter threads are usually a coarser thread.

Crap.

Mark Woods
15-Dec-2010, 18:57
Hello Bob,

I hate to disagree with you, but any optical element, either in front or behind, becomes part of the "lens." Filters are in the optical path Where BTL filters are a problem are when they are placed directly in front of the film aperture, as is done with Panavision Cameras. At that point finger prints, dust, whatever shadows or smears become part of the image. When the filters are on the lens the affect of the filter BTL is the same as it would be on the front of the lens (with some exceptions depending on focal length, but not really applicable to LF). I've used BTL filters for over 30 years -- including ones placed at the aperture directly in front of the film -- and never had a problem. Many of the center grads for extreme wide angle lenses are designed for BTL use. Finally, Sinar cameras have a holder for BTL gel filters. This is a well accepted practice and I don't understand why you don't support it. No offense meant, and I hope this doesn't come across as a flame. I value your contributions to this forum and hope you continue.

Kind Regards,

Mark

Bob Salomon
16-Dec-2010, 03:08
Mark,

First, a filter behind the lens shifts the focus point.
Second, the lens has processed the image as the rays pass through the lens. Placing something not designed specifically for that lens, in or behind that lens will effect the image as the rays pass through the filter. When the filter is in front the lens does its thing with the light that passes through the filter. A totally different thing.
Third, the threads behind the lens are usually not filter threads. The lens manufacturers expect the filtyer to be in front and that is where they put the threads.

You can do lots of things with products that the product was not designed for but that doesn't mean that it is as effective as using the item for what it was designed for. How many of us have used a table knife as a screwdriver in a pinch? Jury rigged a can opener?

Simply stated, for best optical results, the filter belongs in front. Period.

Yes you can put it in back but that does not mean that you do not effect the lens' performance.


Hello Bob,

I hate to disagree with you, but any optical element, either in front or behind, becomes part of the "lens." Filters are in the optical path Where BTL filters are a problem are when they are placed directly in front of the film aperture, as is done with Panavision Cameras. At that point finger prints, dust, whatever shadows or smears become part of the image. When the filters are on the lens the affect of the filter BTL is the same as it would be on the front of the lens (with some exceptions depending on focal length, but not really applicable to LF). I've used BTL filters for over 30 years -- including ones placed at the aperture directly in front of the film -- and never had a problem. Many of the center grads for extreme wide angle lenses are designed for BTL use. Finally, Sinar cameras have a holder for BTL gel filters. This is a well accepted practice and I don't understand why you don't support it. No offense meant, and I hope this doesn't come across as a flame. I value your contributions to this forum and hope you continue.

Kind Regards,

Mark

jb7
16-Dec-2010, 04:09
The filter threads on the front cell are hosed and I've just been using a type of Lee system.

You could try to find a step ring, and epoxy it to the outside of the rim...