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prince fritz first
20-Dec-2016, 18:14
Hi,

I just received my center filter and wanted to ask a question about a darker spot that I'm noticing in the very center of the filter... Can anyone confirm this is normal in theirs as well, please.

Thank you!158951158950158949158952

Mark Woods
20-Dec-2016, 19:04
Yes. The image from the lens is closer to the film at the center and the image has to travel further to the film at the edges. The filter is correct for very wide lenses.

djdister
21-Dec-2016, 05:27
The neutral density center filter should appear darker in the center of the filter and taper off to next to no neutral density at the edges, as this one appears. In this way, it counteracts the tendency of the wide angle lens to vignette at the edges of the lens. You should give the film extra exposure when using this ND filter - whatever the filter factor is (perhaps 1 stop)?

Bob Salomon
21-Dec-2016, 06:24
The neutral density center filter should appear darker in the center of the filter and taper off to next to no neutral density at the edges, as this one appears. In this way, it counteracts the tendency of the wide angle lens to vignette at the edges of the lens. You should give the film extra exposure when using this ND filter - whatever the filter factor is (perhaps 1 stop)?

You must stop down a minimum of two stops for the filter to work and then apply the filter factor.

prince fritz first
21-Dec-2016, 14:57
Thank you for your responses. I do understand how a ND filter works and what its’ appearance should be. This one seems to have a very defined darker spot of about a 1/4” in the very center of it. That’s the part I was inquiring about.
I'm attaching two images which should point this out best. Same image, just the second one has the mentioned darker spot encircled.
158986158987

Just found another filter someone is selling and here's a picture of it. It doesn't seem to have the same spot in the center. 158999

Lachlan 717
21-Dec-2016, 17:38
I can't tell the difference.

It looks like both of my CFs.

stawastawa
21-Dec-2016, 18:47
I dont recall the one I had having a central spot.
https://www.flickr.com/photos/stawastawa/25803762465/in/album-72157663351104743/

If you have a very good densitometer you could try and measure it... I imagine the spot is simply some sort of optical illusion. I do think I see it on yours, but barely, and not 100% of the time.

Corran
22-Dec-2016, 19:11
Okay I was going to write a description of what a center filter was but I see now there may be a tiny little center spot darker. Maybe. Have you tried just cleaning the filter? I doubt it would be at all a big deal in real use.

hiend61
23-Dec-2016, 09:48
Your filter has a normal appearance, and wil perform as it should. If you have a spot meter, take a measure of a white subject with the center part of the filter in front of the meter, then take other measure through the outer part of the filter, the clearest one. If the difference between the two readings is 1,5 - 2 F stops, your filter is OK.
And remember that the filter will work properly when you use F16 and beyond.

faberryman
23-Dec-2016, 10:20
I would send it back if there were any question. They are expensive. I wouldn't want something nagging me in the back of my mind when making images.

BetterSense
24-Dec-2016, 10:59
take a shot of blank wall onto a test sheet of film or paper. See if the spot shows up. It won't, because the filter glass is so out of focus on the film.

Daniel Unkefer
24-Dec-2016, 12:42
Specks on the glass will be completely out of focus and you will never see them

Shoot a test

prince fritz first
29-Dec-2016, 19:31
Thank you very much!

Jaguar52
9-Dec-2017, 15:38
I read this post because I was interested in purchasing a ND center filter for my Schneider super-angulon 90mm f/8 lens. After reading the posts there seems to be some confusion on terminology. The pictures taken by prince fritz first show the correct appearance for a ND center filter - darker in the center and gradually lightening to the periphery. An ordinary ND filter is uniformly "dark" from center to edge. These are different filter for different uses. Ordinary ND reduces light to allow longer exposures or bring very bright scenes closer to the exposure range of the film used. ND center filters are used because of the fairly dramatic light drop off of wide angle lenses from center to edge. These ND center filters even out the amount of light striking the film from center to edge. Think of these filters as graduated filters but instead of upper and lower gradation, these are center to edge graduated.

Paul Ron
10-Dec-2017, 05:49
+1


center nd filters are designed to be darker in the center and taper off at the edges.