Focus problem - nd grad filter?

[Christopher Hansen]

I just my film back from the lab from my Labor day weekend shots. The one shot I made with no filters is razor sharp everywhere. However, every shot I made with nd grad filters seems to be generally sharp, but have one spot (roughly 1" square) on the right center of each frame that is out of focus. I don't get it. Could it be the filters? Why is is localized to one spot? Is this a common problem? What can I do about it?

I thought I saw something like this before, but I thought it was insufficient depth of field issue with a front tilt. Looking back on it, maybe it was the same problem. This time I used no tilts at all.

All my equipment is decent quality - Singh-ray filters, schneider 180 lens, toyo AII camera.

Thanks.

[C. D. Keth]

Filter do, on occasion have bubbles or uneven spots. Check your filters. Lay them flat on a page of printer word. That will make any uneven spots very obvious.

[Christopher Hansen]

Thanks. I just checked my filters and they look fine. No bubbles or scratches. I haven't used them very much. I'm wondering if it is flare off the combination of 2 filters and the front of the lens.

[C. D. Keth]

Any way you could post one of the images?

[Christopher Hansen]

I don't have a scanner at home that can handle 4x5 film, so I trimmed a section out of one frame and scanned it with my medium format scanner. I scaled it down and sharpened it a bit in Photoshop to make the problem stand out.

Look right above the big rock. You will see a section of the frame that is fuzzy. Any ideas? It just doesn't make sense to me.

[William Stone]

Perhaps a water drop on the filter ... ? Bill

[Christopher Hansen]

That would make sense, except that I was up high above the water - like 50 feet up - where I took the shot. This was taken from the walkway at Julia Pfeiffer Burns State Park in Big Sur, CA. I don't remember getting anything wet.

I'm wondering if down at the surf there was more mist in the air. It really doesn't look like lens / filter flare.

[Glenn Thoreson]

I have gotten defective filters in the past. The worst one was a new UV filter that made it impossible to get a sharp image. It was not just bad, it was horrible. I would return the filter.

[Shailendra]

Check the inside of your film holder as well, make sure its clear of any dust or something that could make the film not completely flat.

[Uusilehto]

Hard to say from a small picture like that but it sure looks like something was distorting the optical path. I think I've gotten something similar with waterdrops and a large aperture (on small format but large format has a narrow DOF by nature). Too small an area to be a smudge or a bump on the film.

Are you absolutely sure it isn't a smudge on the scanner?

[C. D. Keth]

I'm wondering if down at the surf there was more mist in the air. It really doesn't look like lens / filter flare.

I said before. There is no way for flare to defocus an image. it just reduces contrast by filling in the shadows of the image.

That looks like water or a smear on the filter to me. The further from the lens a water droplet is the stronger its optical effect would be so a drop on the filter would be noticeable. On the lens, not so much.

[Christopher Hansen]

The small water drop theory seems to make the most sense. It is definitely on the film - I saw it first on a light table, so I know its not my scanner.

Thanks everyone.

[Ole Tjugen]

The longer the focal length of the lens, the more critical is the optical flatness of the filter.

An unevenness that is invisible to the eye can give significant errors with the longish lenses typically used in LF photography.

I suspect the filter.

[Ben Chase]

I don't have a scanner at home that can handle 4x5 film, so I trimmed a section out of one frame and scanned it with my medium format scanner. I scaled it down and sharpened it a bit in Photoshop to make the problem stand out.

Look right above the big rock. You will see a section of the frame that is fuzzy. Any ideas? It just doesn't make sense to me.

A couple of things stand out to me here. It looks though the blurred part of the image is rectangular and symmetrical. Are you sure the out of focus part is just on the right of the image, or does it extend across the entire frame? Do you mount your Singh-ray's in a holder, or do you hand hold them over the lens? If you hand hold them, did you maybe angle one of them instead of hold it flat against the lens barrel?

[Christopher Hansen]

I mounted the singh-ray filter in a cokin filter holder. I actually had 2 of them, a 2 stop soft transition and a 2 stop hard transition.

Yes - it does look like a rectangular band that is out of focus. I'll go back and check the film on my light table. Unfortunately, I'm on a business trip right now and don't have it with me.

[Ben Chase]

I mounted the singh-ray filter in a cokin filter holder. I actually had 2 of them, a 2 stop soft transition and a 2 stop hard transition.

Yes - it does look like a rectangular band that is out of focus. I'll go back and check the film on my light table. Unfortunately, I'm on a business trip right now and don't have it with me.

In the past, when I have stacked filters in damp or wet conditions, a drop of water can sometimes get sandwiched between the two filters - not saying that's the case here since the filters are held apart slightly as a result of the Cokin holder - but you never know!

Good luck!