I have no experience with wider lenses such as the 90mm-75mm and their needs for center filters, which I also know little about their function. I have heard that the Schneider 80mm XL does n ot need a center filter because of its unique construction. Is this true
Another questions that affects my use of the 80mm XL: It would be used on my Toy o 45AX, is there any considerations I must take in regard to coverage limits based upon this particul ar camera
Gary: I used a 75mm f/6.8 Grandagon on a Toyo 45A with no problems. The bed did not intrude into vertical shots. Movements will be a bit limited, and you will probably want to use the 12mm recessed board to keep as much movement as possible.
I can't imagine that you will need a center filter. I use a 75mm Fujinon frequently and have never needed a center filter. On the ohter hand get much wider and you will likely need one. For example, in most lighting conditions I did use one most of the time with my 65mm Super Angulon.
Ted
The Super-Symmar XL lenses are wonderful, but this doesn't mean they are perfect. Their unique design does not improve the evenness of the illumination. The older design wide-angle lenses like the Super-Angulon, Nikkor-SW and Grandagon have more coverage and less light fall-off.
An example comparison from the datasheets on Schneider's web site: the 80 mm Super-Symmar XL has a circle of coverage of 212 mm diameter. The relative illumination at the edge of the circle is less than 15%. Scaling, the coverage of a hypothetical 72 mm Super-Symmar XL would be 190 mm. The 72 mm Super-Angulon XL has a circle of coverage of 226 mm. The relative illumination at a diameter of 190 mm is about 20%. So the Super-Angulon XL offers more coverage and better illumination.
You have to decide which factors are most important for your use of the lens: weight? maximum aperture? coverage? illumination?. Depending on your criteria, either lens might be the best choice. As to whether you need a center filter, the best thing is to take photographs typical for your use and judge whether the falloff is objectionable.
If you look at the flange focal distance for any of these wide-angle lenses you will realise that they are true wide angles of the the nominated focal length rather than retrofocus wide-angles such as are commonly used where space has to be provided for a mirror.
As a result of the nearness to the film palne they ALL suffer from the cosine effect of the light fall-off to the distant corners compared to the closer center of the image. A centre-grad will improve even illumination on all of them although using the lens close to on axis you can get away without them. However, once you apply a reasonable amount of shift, tilt or swing the effect of the cosine law will become increasingly objectionable.
In monochrome work some of this can be overcome somewhat in printing but the result will never match the seamless transition accomplished with a filter. IMHO.
Walter Glover
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