Center filter question - usage on different formats

[Drew Wiley]

Generic center filters are an option, but those basically try to copy the most common possibilities, and are designed for similar amounts of falloff, which you should know in advance with respect to your particular lens before selecting one. This topic has been discussed over and over again. Look up some or Bob Salomon's posts on the subject; he's fairly expert at it, and would instantly contradict the notion that you can just go out and buy anything that fits the filter thread and walk away with a correct result.

[xkaes]

... you can just go out and buy anything that fits the filter thread and walk away with a correct result.

Like any piece of new-to-you gear, you should run some, at least minimal, tests to see how it works with your gear & approach. This is especially true with CND filters since it will probably come without any instructions:

http://www.subclub.org/fujinon/center6.htm

[Drew Wiley]

Huh? I said ... CONTRADICTS the notion that you can just go out and buy something, and think it's gonna work properly. Please don't chop my sentence in half and quote it out of context. Center filters can be expensive, and aren't the kind of thing you find at a local camera shop to test at a whim. It's important to do some homework in advance, or better if possible, buy a dedicated filter from the same manufacturer as your lens. But there is enough similarity between the designing of popular super-wides among the big four manufacturers that substitution can be realistic, especially since Nikon and Fuji don't market CF's of their own. All my own test involved over-developing the b&w sheet film to exaggerate any remaining falloff, if it were there; and then the corners of the sheet compared to the center was measured with a densitometer. In my case, the Nikon SW behaved exactly like the Schneider, and the Schneider CF worked perfectly.

[xkaes]

Sorry. I was trying to agree with you.

[Drew Wiley]

Yeah, bad day for me as far as misunderstandings go. Weather is starting to change and I had half my garden plans interrupted this afternoon. Oct is ordinarily our hottest month here, but the climate is showing showing signs of schizophrenia, with our first mountain snowfalls due in a few days, and hopefully rain here on the coast. So have tinkered on the computer too long today instead, though I did get some print toning done.

[xkaes]

I've found my darkroom to be an excellent escape many times myself. My blood pressure always drops the instant I turn on the safelights and the music!

[rawitz]

Suppose I have a center-filter designed for a certain focal length LF lens. Will I get similar results using that center-filter on a shorter lens and a smaller format if the angle of view for the two focal lengths on the two formats is roughly equal? This question goes the other was as well, moving to a longer FL on a larger format.

Here's a specific example to help clarify my question. A 90mm lens on 4x5 and a 50mm lens on 6x7 (rollfilm) have quite comparable fields of view. So would a center filter meant for the 90mm on 4x5 properly correct a 50mm on 6x7? I'm aware the lens must be well stopped-down.

The answer is yes and no! If the angle of view is the same for different film sizes (which means the lens focals are proportional different), a (filter-size) matching centerfilter will work equal on both. But as every lens-construction with same angle of view has different light-falloff curves, the result will not(!) be the same. LF wide-angle lenses are very different in construction than 35 or 120-film lenses.

In theory, and that has been said here, each WA lens needs a special centerfilter to correct its special light-falloff, and even more for each f-stop it needs a different filter, if a 100% correction is needed or wanted.

In practice, not even the lens-recommended CFs correct the light-falloff perfect, but only "approximatly". When I compare lens specs, for example the Super-Symmar XL 150mm light-falloff at the border of 385cm IC is more 4 f-stops for aperture 8 and 3 f-stops for aperture 22, but the recommended CF IVa has a correction of 1,5 f-stops (exposure 3x). So all avaiable CFs are undercorrected, and using different (but size-matching) CFs on the same angle-view lenses in sheetfilm or rollfilm, will only change the factor of undercorrection. So if you think you need a CF, every size-matching is better than nothing, but none gives perfect correction.

regards
Rainer

[Alan Klein]

It's said that color needs center filters more than BW. The falloff is more noticeable with color.

[xkaes]

The appearance of light fall-off also depends on the subject (some subjects look better with it), the amount of cropping under the enlarger, and other factors.

[rawitz]

It's said that color needs center filters more than BW. The falloff is more noticeable with color.

Oh, YES I see this in my LF color-negs, with are more sensible in densities than slides. What I found is not only radial light-falloff, but also radial color-shift what in BW is not an issue. I found ways to correct it in post, but have no theoretical explanation for this. Maybe anyone here has ...