View Full Version : Lens flair caused by filter?

Neal Shields
26-Nov-2003, 10:40
The other day I got to wondering if I could make panoramic images by using my 90 mm Super Angulon f8 on my 8x10 and getting a 4” x 9” picture. The image circle is listed at 216 mm ( I hoped to squeak out a smidge more) and it was one of those things that sounded like a good idea at the time.

About everything went wrong that could.

I made a ½ dark slide so I could make two images on 8x10 but then found when I put that lens on an 8x10 camera, the bellows were completely compressed, so I had no rise. I would have to waste a full sheet of film.

I planed on creating a gradient mask in Photoshop and printing it on transparent material with an ink jet printer to use instead of a center filter when contact printing. I then found out that transparent sheets for ink jet printers sell for almost as much as film and are 50 to a box!

Then I put a fairly thick name brand screw on red filter on the front of the lens and made the mistake of allowing the sun’s disk filtered through heavy overcast to be in the edge of scene.

When I processed the negative I had a circular band of flair about ¾” wide from about the 1:00 to the 4:00 position toward the edge of the negative on the opposite side of the sun’s disk.

It started close enough to the center of the negative that it would have been on a 4x5. I have used this lens many times and never had the problem before.

I decided that this must have been caused by the front and rear surface of the filter?

I then thought that had I anticipated this, I should have put the filter on the back of the lens.

Now, Bob Solomon says in the prior post, never put the filter on the back of the lens. Could this be the one exception, or do I have my problem misdiagnosed completely?

Bob Salomon
26-Nov-2003, 13:34
No this is no an exception. There really are no exceptions for putting a filter on the back of a lens (save perhaps a gel).

However it sounds like the filter you used was either not coated or was not properly multi-coated on both sides. Without effective MC on both sides flare is very probable on each glass to air surface. That would include glass to air surfaces in back as well as in front of a lens.

David Richhart
26-Nov-2003, 13:37
Howdy Neal... I am surprised that you were adventurous enough, and went through all the planning and construction in your experiement... Now you are afraid to try rearmounting a filter just because "Bob said so"???

I have used filters on the back of my 10"wf Ektar with success. Bob is correct about watching for focus shift... and I am sure you make a good effort to keep your filters clean, so why not give it a try???

tim atherton
26-Nov-2003, 13:40
"No this is no an exception. There really are no exceptions for putting a filter on the back of a lens (save perhaps a gel)."

what about a lens that has the thread on the back but not one on the front.. :-)

Neal Shields
26-Nov-2003, 14:18
Two parts:

Given that the problem was caused by the filter:

1) Would the problems caused by putting the filter on the rear element be likely to be worse than the flair caused by having it on the front?

2) Is the problem with lack of multicoating worse with the dark red filter than it would have been with say a light yellow? (I might spring for one first class brand new filter but not a whole set.)

I will try it again, but first I need a new box of 8x10 Tmax and a shot that gives me simular light conditions and sun position. (Note: I ran accross a paragraph from Ansel Adams that suggested that flair was more likely on overcast days than bright sunshine because the light making up the image was closer in intensity to the flair causing light. He went on to say that the sun need not be in the frame. This may have been the case here.)

I am supprised that no one has jumped me for not using a shade but I figured that with 100 degrees of coverage, the shade wouldn't have been much more than a black pie plate.

P.S. This wasn't subtle, my guestimate is that the band of flair was at least a full stop darker (on the negative) than the area around it. I have to print it to scan it, and it didn't seem worth the sheet of paper.

Bob Salomon
26-Nov-2003, 16:26

Quite frequently threads on the rear are used for a retaining ring to screw into. Not for a filter.

Geoffrey Swenson
26-Nov-2003, 17:13
I don’t know what is the big deal of screwing the filter onto the back element (at the rear). It works for me, and dare anyone to show me the difference on the transparency. Of course, one focuses with the filter in place and keeps it clean as well!

“ A filter should never be used on the rear element (unless the lens was designed for a rear filter). ”

Yes, a lot of things are not designed in a certain way, but we still try to find different ways. That is why we have round wheels among others. Most photographers would wish that a minor image degradation were their only problem :-))

tim atherton
26-Nov-2003, 17:18
"Tim, Quite frequently threads on the rear are used for a retaining ring to screw into. Not for a filter."

Oh probably - but if it's the only thread you've got, it's the one you use... and it works just fine.

tim atherton
26-Nov-2003, 17:19
Doesn't HP sell a rear filter system? (who used to do that neat magnetic one that isn't sold anymore? It worked very well - but was, of course, discontinued...)

Steve J Murray
26-Nov-2003, 22:28
Calumet sells their Zenophon rear filter holder. I use tabs of industrial strength velcro on the backs of my lens boards to hold on the Zenophon. You typically use 3 inch gels with this holder. Works great!

tim atherton
26-Nov-2003, 22:57
that's the one I was thinking of Xenophon - for some reason I was thinking it was a heleopan.

The one I used to have worked with magnets

Doremus Scudder
27-Nov-2003, 04:04
Filters behind the lens (or between lens and negative in the enlarger) cause a focus shift. Therefore, focusing must be done with the filter in place. This can be a pain if the filter is strong and reduces light intensity by a large amount. Otherwise, I can see no other optical disadvantage to using the filter behind the lens. (If there are other problems, maybe someone can clarify??).

However, mounting the filter behind the lens is not going to change the fact that you have a light source in the scene. Flare is flare, whether from the filter or lens surfaces. That said, the lens has a better coating usually (especially if you use uncoated filters!), and a curved surface, which may reduce flare in certain situations in comparison. Lens shades help a lot in this situation.

Good luck

Bob Salomon
27-Nov-2003, 04:34
No Tim we do not sell rear mounted filter systems. We do sell a drop-in polarizer for Nikon lenses that fits in the filter draw.

As for the question from someone else about what happens when a filter is rear mounted other then a focus shift? Just put a good big smudge or fingerprint on a rear mounted filter and take a shot of a scene with good detail. Then shoot it with the filter in front.

CP Goerz
27-Nov-2003, 16:27
Perhaps the entire point is being missed..a 90mm lens used to cover too big an area possibly? A 90mm lens is being pushed to the theoretical limit of its usefulness on a 5x7 let alone an 8x10. Why not use a 110mm F18 Protar or similar etc, its all a matter of perspective anyway so who's to know?

CP Goerz

neil poulsen
27-Nov-2003, 23:23
I've heard that the another effect is that rays can reflect off the lens surface, then off the filter and back into the camera. I'm wondering if you saw some of this effect in your image.

I use gels behind the lens with a 4" Zenophon. Works fine. I would be reluctant to use glass behind the lens with a wide angle lens. While one can correct for the focus shift on axis, the rays go through more glass off axis, and this can introduce an abberation. (I believe, spherical abberation.) One could try using glass filters behind the lens with a long focal length lens, provided that they focused with the filter in place.

Michael S. Briggs
28-Nov-2003, 00:46
"Otherwise, I can see no other optical disadvantage to using the filter behind the lens. (If there are other problems, maybe someone can clarify??)."

Doremus, Neil has the answer to your question. In a parallel beam of light such as from very distant objects, a perfect filter has no optical effect. By "perfect" I am assuming that the filter is flat, of uniform thickness and transmits all of the light that it is supposed to. In a non-parallel beam of light, the filter changes the angles of the light rays depending on their original angles of incidence. One case of a non-parallel beam is the converging beam exiting from the rear of a lens, i.e., when a thick filter is used behind a lens. The beam is converging because each point on the film receives rays from all over the exit pupil of the lens. This effect causes spherical abberation -- rays from the rim of the lens will focus differently than rays at the center.

With a long lens stopped down to a small aperture, this effect from a thick filter used behind the lens should be very small to non-existent because the beam will be almost parallel. It might be important for a wide-angle lens used with a large aperture. I don't know how serious this effect is in practice.

Another case of a non-parallel beam is from the object to the lens in macro photography -- here each point on the object is emitting a diverging beam towards the front aperture of the lens -- so in this case the effect also applies to a thick filter used in front of a lens. So here too there might be some spherical abberation, except typically one stops down for depth-of-field, thereby reducing the problem.

If you use a thick filter behind a lens, or a filter in front of the lens for macro photography, it is best to focus with the filter in place. The reason is that the filter, because it is thick and has a different refractive index than air, changes the effective optical distance. This effect is unimportant for a filter in front of the lens focused on distant objects because the change in distance, which is about 1/3 the thickness of the filter, is very small compared to the depth of field. Used behind the lens, 1/3 of the thickness of the filter is usually significant compared to the depth-of-focus. Used in front of the lens for macro-photography, 1/3 of the thickness of the filter might also be significant compared to the depth-of-field. If the filter is too dark, focusing could be done with a lighter filter of the same material and thickness.