I've been using a 90mm Super Angulon for awhile and have started learning about the need for a center filter. Of course a center filter is quite an expensive item when you have only so much income and a need for other things such as film, chemicals, paper etc. So taking this under consideration I started wondering if there was not a more economical way to achieve almost the same result but at a lesser cost. Here are a few questions that I have that all seem inter-related yet have not been answered completely in the archives.

1. Center filter on enlarger lens: Would it not be possible to achieve the same effect or result at the printing stage by placing a center filter on the enlarger lens? If so, might it be cheaper to make a center filter in the 40mm size rather than the 67mm size?
2. Diffuser made for light stage:Would it be possible to make a diffuser out of acetate or something similar that could be placed in the filter drawer of an enlarger to achieve increased even light distribution across the whole negative?
3. Center Filter Negative:Like an "unsharp mask" would it not be possible to produce a 4x5 negative that is "center weighted" to be used in a sandwich to even out the illumination from the center to the edge? There was a thread that mentioned using something like "PhotoShop" to try to achieve this result.
4. Larger Enlarger lens: I read in a previous thread that a 135mm enlarger lens will barely cover a 4x5 negative. Does this mean that there is additional light falloff on the edges of the 135mm lens? If so, then would somebody using say a 200mm or large lens gain by having less light falloff using the increased focal length for printing?
5. Is the light falloff accumulative? :In previous threads there were frequent mention that an enlarger lens can suffer up to 1 full stop of light falloff at the edge. Now taking into account for the possible "one step" falloff from a super wide lens, do you now have possibly as much as a two stop light problem to deal with during printing?

Hopefully I did not ask too much all at the same time but if I did then I apologize to the group. I thought this would all be better addressed as one thread. Thanks in advance for considering my question.

Regards

GreyWolf Phillips

-- GreyWolf Phillips (grey_wolf@telusplanet.net), August 22, 2001

Answers

Have you noticed uneveness in your negatives? I have a Schnider 90/8 and a Nikkor 65/4.5 and although everybody tells me I absolutely have to use a center filter, I don't see any uneveness in my negatives. I tested the even developing and exposure with a densitometer and my variations from edge to edge are of 0.03 units max. So maybe is the way I develop or something, but my advice is, if you don't see a problem, don't create one! as for your choices I would be more inclined to use any method that is above the negative, like the unsharp mask you mention. The way I would go about it is to take a pic of a gray wall with the lens unfocused or focused at infinity (e.i. if you are using a 90 mm lens then you should have an extension of 90 mm from lens nodal point to film plane). Then I would develop it and see the difference in density from edges to the center and I would make an unsharp mask of THIS negative to place on top of the negative to be printed. Save the original negative so you can make more masks as the one you use becomes more scratched etc. I still think you should beg borrow or steal a densitomere, measure the variation from the edges to the center and really see wether you need a center filter or not. Good luck!

-- Jorge Gasteazoro (jorgegm@worldnet.att.net), August 22, 2001.

a center filter is used to even the illumination on film. this unevenness changes with lens movements. you would have to adjust the position of any printing stage center filter to match, unless of course you don't use lens movements.

if the illumination is not evened out on film there is a greater possibilty of highlight and/or shadow detail problems. this cannot be fixed later - that is, although it may be possible to even the illumination of the print you cannot get (good) detail out of underexposed shadows near the edge of the image circle or vice versa from overexposed highlights at the center of the i.c.

the solution is simple and easy - use a center filter. it works. you'll save money on aspirin.

-- adam (asfberg@hotmail.com), August 23, 2001.

Grey, If you are shooting neg film you should not require a CF with your 90mm. The CF becomes important if you shoot transparencies where the exposure latitude is much narrower. If you are using black and white film and decide to use a wider lens, consider dodging the edges to correct for any fall off if necessary.

Your first option you outlined wouldn't really work unless you DON'T use movements on your camera. With movements, the hotspot will be in a different location every time.

I use a 155 lens with 810 without a CF and with B&W the effect is barely noticeable.

-- Dave Anton (daveanton@home.com), August 23, 2001.

A way to correct an unevenly exposed slide or neg when enlarging is by dodging and burning. You can do that easily by mounting a circular piece of black cardboard on a wire handle. Placing it under the lens, you give an up and down or circular movement during exposure time to retain light from the center of the image while giving more exposure to the edges and corners. If it's a neg, use a sheet of cardboard with a hole in the center. There are also some methods for digital corrections in Photoshop. I tested the method explained on Michael Reichmann's web site and it works quite well. Of course the best results are attained by using a CF at the time of taking. This will avoid any of the color shifts that happens when a slide is overly corrected. I have used my Super-Angulon 90/5,6 for years without CF and exposures where even enough, unless I used movements. But I have now a CF for it and the shots I take, especially when it's indoor, are much better with it.

http://www.luminous-landscape.com/panorami.htm

-- Paul Schilliger (pschilliger@smile.ch), August 23, 2001.

By the way, it's good to remember ;-)

http://www.luminous-landscape.com/parable.htm

-- Paul Schilliger (pschilliger@smile.ch), August 23, 2001.

I have used a graduated 4x5" N.D. filter sucessfully on both 65/4 and 90/4.5 lenses for extensive shifts or rises on architecture. Basically one places the darkest part of the filter on the inner side of the frame, thus helping to make the whole scene even density.

-- Richard Stum (info@kinesisgear.com), August 23, 2001.

Actually, while on the topic, could someone explain why these filters are so expensive? There are smaller production runs than with 58mm UV filters obviously, but are the production costs much higher than say a graduated (top/bottom) filter? Also daunted by the prices,

-- Eric Pederson (epederso@darkwing.uoregon.edu), August 23, 2001.

Much more expensive.

Screw-in type graduated ND filters are usually computer made. A computer is told the density and the size of the glass. A holder then puts the filters into the ND material for a controlled period of time and then removes it. The computer knows how far to insert the glass and for how long.

These types of filters are all one density + clear. One area with density, one without.

Graduated Center Filters are quite different. They range from cler edge to dense center and the density varies across the radius of the filter. This is quite difficult to do and is expensive to do. Plus there tends to be a lot of rejects which further increases costs.

Even trying to do the work of a center filter by a computer program like the one by Cam Dynamics is expensive ($700.00 projected selling price).

While it may appear simple a center filter is everything but. Especially if you want it to be as neutral as possible and as unabtrusive as possible.

After all a good center filter only makes the image look normal. A graduated ND filter usually emphasizes something.

A graduated center filter works regardless of camera movements and there is virtually no line where the effect becomes noticeable.

A graduated ND filter can, and usually does, leave a line seperating the ND from the clear area.

-- Bob salomon (bob@hpmarketingcorp.com), August 23, 2001.

Thanks to everyone who took an interest and offered possible solutions. In response to Jorge’s question regarding light fall-off, yes I have noticed a bit when measured on my enlarger easel. I do not have a densitometer so I need to rely on my spot meter readings and I need to take that into account in regards to accuracy. This is also why I asked about a 135mm enlarger lens (which I am using) to see if anybody thought it would make a difference to move to a 150mm lens.

Also thanks to Adam for pointing out that under exposed edges or conversely over exposed center area on a negative cannot be compensated for at the printing stage. A very important point that slipped by my first evaluation of the problem.

Thanks to Dave for suggesting that it is probably much more important to use a CF when shooting chromes than it is for color negatives. I do shoot chromes as well as B&W so this may still be a concern for me. Also Dave’s pointing out that movements could affect one of my potential solutions is quite an interesting fact. This was once again something that I had not originally taken into account.

The idea that Paul suggests about doing a dodge& burn may help to make some improvements but I probably do need to consider a CF in my future. I also appreciate that Bob took the time to explain why the cost seems quite high for a center filter but it does make sense.

In conclusion I wish to add that it is not only the cost that has me asking these questions but I also the fact that I do this LF for “pure enjoyment”. After thinking about Ansel Adams and other great photographers whose works came from an earlier period I began to wonder if there was not other alternatives that would be worth exploring and evaluating.

Once again thanks to everybody who both read the question and those that responded.

-- GreyWolf Phillips (grey_wolf@telusplanet.net), August 26, 2001.