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Diane Maher
13-May-2009, 11:52
Is using a center filter any different than using any other filter? I will soon have a Schneider SS-XL 150mm and the center filter for it. I have never used a center filter before.

Bob Salomon
13-May-2009, 11:54
Yes, a center filter will not work unless the lens is stopped down at least 2 stops.

Nathan Potter
13-May-2009, 12:20
Bob, I've not heard this. The radial light falloff is a function of aperture at least in the case of the 150 SS-XL and at large apertures? Does this mean these Schneider center filters are designed to function best at certain apertures? Interesting; or are we splitting hairs.

Nate Potter, Austin TX.

Bob Salomon
13-May-2009, 12:41
Bob, I've not heard this. The radial light falloff is a function of aperture at least in the case of the 150 SS-XL and at large apertures? Does this mean these Schneider center filters are designed to function best at certain apertures? Interesting; or are we splitting hairs.

Nate Potter, Austin TX.

All center filters need the lens to be stopped down at least 2 stops for the lens to "see" the effect of the filter.

See third paragraph of the attached Schneider instruction sheet from a new Schneider center filter.

Jeremy Moore
13-May-2009, 13:12
All center filters need the lens to be stopped down at least 2 stops for the lens to "see" the effect of the filter.

See third paragraph of the attached Schneider instruction sheet from a new Schneider center filter.

Bob, that's a really vague description and doesn't really explain the "why" of why you need to stop down other than to have the "full compensation effect". Do you know if there is a white paper or something similar for this? Here is the third paragraph Bob is pointing to:


When using these Center Filters exposure time should be extended three or resp. four times. Furthermore full compensation effect of the filters is obtained by closing down the lens by two stops at least.

Bob Salomon
13-May-2009, 13:43
Bob, that's a really vague description and doesn't really explain the "why" of why you need to stop down other than to have the "full compensation effect". Do you know if there is a white paper or something similar for this? Here is the third paragraph Bob is pointing to:

No, Rodenstock states that the center filter works at the recommended aperture of the lens and stopping down further will not improve the fall-off.

Archphoto
13-May-2009, 14:11
What I understand from the above: the center filters are designed for lenses at their optimum aperture.
That means that using a center filter used at full opening will have a diferent effect, because it is not designed for it.
Makes sense.

Peter

Jeremy Moore
13-May-2009, 14:25
What I understand from the above: the center filters are designed for lenses at their optimum aperture.
That means that using a center filter used at full opening will have a diferent effect, because it is not designed for it.
Makes sense.

Peter

Yes, the general sense of it makes sense, but I'm just curious if anyone has a pointer to a description of the physics involved. I find the science of optics to be incredibly fascinating. Maybe I'll just dredge out some optics book and see if I can piece a precise explanation myself this weekend.

Bob Salomon
13-May-2009, 15:01
Yes, the general sense of it makes sense, but I'm just curious if anyone has a pointer to a description of the physics involved. I find the science of optics to be incredibly fascinating. Maybe I'll just dredge out some optics book and see if I can piece a precise explanation myself this weekend.

Jeremy,

The filters are made for photography so white papers are not part of the support. Photographers want to know how to use them, when to use them and how much light they will lose when they are used. This is the kind of information that is published for users.

timparkin
13-May-2009, 15:24
I imagine that at large apertures the fall off is greater so the centre filter may not completely compensate.. At some point the decrease in fall off by closing down the aperture stops happending .. this is probably the aperture at which the CF works best..

Tim

John Brady
13-May-2009, 16:21
Is using a center filter any different than using any other filter? I will soon have a Schneider SS-XL 150mm and the center filter for it. I have never used a center filter before.

Getting back to the original question, it is very simple to use a center filter. I would recommend composing and focusing your image and then screwing in the filter. This will make it easier since the intended purpose of a cf is to restrict light in the center of the image and then gradually go to clear at the edge.

While holding the filter take a meter reading with a spot meter of an evenly lit area through the center of the filter and then compare another reading without the filter. I suspect it will be around 1 to 1.5 stop difference. You will need to adjust for this every time you use the filter (just like other filters).

Are you using the 150 on 4x5 or 5x7 or 8x10? For 4x5 you probably don't need a center filter with this lens, especially for black and white. For 8x10 (does it cover?) you probably do.

I use 2 stop center filters with extreme wide angle lenses, like 47xl on 4x5 or 90xl on 8x10.

I hope this was the kind of info you were looking for.
jb
www.gladesgallery.com

Diane Maher
13-May-2009, 19:41
I plan to use the 150 mm on whole plate (6 in. x 8 in.), 8x10 and 5x12.

Nathan Potter
14-May-2009, 10:54
Ah, thinking about this now maybe I know what you're saying Bob. It's an issue of Depth of Field. As one stops down the lens the actual center filter begins to show a real image of itself at the film plane while with the lens wide open the center region is simply a full blurr. Thus the recommended two stops down or so is a compromise for partial imaging of the filter at the film plane while still utilizing the maximum resolving power of the lens. Beyond f/11 or f/16 maybe the maximum effect of the radial density compensation is already achieved. This would be a bit of suttlety I've never thought much about.

Nate Potter, Austin TX.

Bob Salomon
14-May-2009, 12:06
Ah, thinking about this now maybe I know what you're saying Bob. It's an issue of Depth of Field. As one stops down the lens the actual center filter begins to show a real image of itself at the film plane while with the lens wide open the center region is simply a full blurr. Thus the recommended two stops down or so is a compromise for partial imaging of the filter at the film plane while still utilizing the maximum resolving power of the lens. Beyond f/11 or f/16 maybe the maximum effect of the radial density compensation is already achieved. This would be a bit of suttlety I've never thought much about.

Nate Potter, Austin TX.

The other way we phrase it when someone calls and asks is that you have to be stopped down at least 2 stops for the lens to "see" the filter. Same thing you just figured out.

Steve M Hostetter
14-May-2009, 12:33
center grad filters are graduated filters but unlike regular grad filters need compensation to work

Diane Maher
14-May-2009, 12:40
So, if I read all of these replies correctly, I'm going to need to apply a filter factor for not only the center filter, but also for any other filter I choose to put over the lens.

Thanks for all of your replies. As soon as the center filter gets here, I hope the weather is nice. The lens arrived yesterday and I'm looking forward to using it.

Bob Salomon
14-May-2009, 12:43
So, if I read all of these replies correctly, I'm going to need to apply a filter factor for not only the center filter, but also for any other filter I choose to put over the lens.

Thanks for all of your replies. As soon as the center filter gets here, I hope the weather is nice. The lens arrived yesterday and I'm looking forward to using it.

Correct, and the center filter has to mount directly to the lens and any other filters mount to the front of the center filter. So you will need other filters the size of the front of the center filter. Not the size of the lens thread.

JimL
14-May-2009, 13:51
Regarding the 2 stop thing, I would think that at 2 stops down, only the optical falloff is present (assuming no camera movements are used); at full aperture there is additional falloff due to mechanical vignetting.