I am using a 75mm Sironar (Rodenstock) on my Toyo 45CX. It's well nigh impossible to see anything outside of the centre of the groundglass. Is there a solution? Thanks in advance Yaakov Asher Sinclair

-- Yaakov Asher Sinclair (sinclair@actcom.co.il), October 08, 1999

Answers

All wide angles look dark in the corners in LF, especially extreme wides like a 75 or 65.

What I find is that I can only see one smallish area of the image on the ground glass at any one viewing position. So I move my head (that's the head with my eyes and what passes for my brain) so I'm looking directly into a corner of the viewfinder. I repeat this to check all the corners since I'm sick of having tripod legs show up in otherwise nice 65 mm images.

Along with all this checking allow yourself some time under the darkcloth to let your eyes to get used to the dark. My 65 is an f8 but it looks like a couple of stops below this because the light is is so "diverged" (While my Nikkor 300 f9 M seems brighter than my 210 f5.6 because the light is so "concentrated".) In any event a little time under the hood will brighten up the image considerably.

-- David Grandy (dgrandy@accesscable.net), October 08, 1999.

Contact Bill Maxwell at Maxwell Precision Optics. You can find info by going to the focussing screens subject archive and following the thread by Howard Slavitt "I Found a Great Focussing Screen" ...

-- Carlos Co (co@che.udel.edu), October 08, 1999.

Obviously there is some confusion.

Rodenstock never made a 75mm Sironar. The shortest Sironar is the 100 which doesn't cover 45.

They do make 2 different 75mm lenses. The Grandagon N 4.5 and the Graandagon N 6.8.

So we can assume you actually mean the Grandagon N or its' predecessor the Grandagon.

The 4.5 covers a 195mm circle while the 6.8 covers a 187mm circle. Both being wide angles suffer from cosign failure which means that the center of the image is brighter than the edges as the light rays to the edges have to cover a greater distance. To reduce this effect you would use a center filter. In addition if your camera does not have a fresnel lens on the back then the fall off is intensified when viewing the screen.

For best results a center filter should be used and for best viewing a fresnel screen should be used. These will give you the most evenly exposed frames and the most evenly illuminated screen for viewing.

-- bob salomon (bobsalomon@mindspring.com), October 08, 1999.

I have read lot of different strings about the light fall off with wide angle lens such as the 75mm. I own a Nikkor 75mm f4.5 and have not noticed this at all when focusing or in the final print. I also have no problems with focusing in the corners as some claim. Perhaps I have an untrained eye, or perhaps Nikon has done something different with their 75mm to correct for this.

Maybe it is because the lens is a f4.5 lens which is very bright, but I still have not noticed any drop off in my prints. One thing I do differently is that I do not tilt my 75mm lens, but rather tilt the entire camera body and then plum the back standard. This accomplishes the same thing and keeps the image in the center of the coverage area. Plumming the back standard does not distort the image as long as the subject is plum as well.

-- Stephen Willard (willard@lvld.hp.com), October 08, 1999.

No they haven't done anything different. ALL wa lenses have cosine failure. Including Nikon.

And all 1050 lenses have the same pattern as do all 1100 lenses and all 1200 lenses, etc.

Obviously a lens formulated for 1050 coverage will fall off differently than one for a different angle of coverage (not angle of view).

The Nikon, like the Rodenstock, like the Schneider, like the Fuji, etc. is app. 1 1/3 to 1 1/2 stops center to edge.

Some shooters use the fall off as a wide angle effect and are not bothered by it as they feel it draws the eye into the scene. Some shootrs don't have open light expanses in the corners and don't see the fall off.

Some shooters overlight the edges of the scene to compensate for the fall off.

Some shooters simply dodge and burn in printing to correct the fall off.

Some shooters use a center filter to help correct the fall off.

But your Nikon lens has the same fall off and it occurs 70% out from the center of the frame.

Not using base tilts will make the fall off more symetrical and using shifts and rise fall will postition the fall off torwards one edge.

Not using any movements at all will result in the most symetrical pattern.

But proper technique allows for the most even illumination with or without movements and for most that means a center filter.

By the way, stopping a lens down further does not eliminate the fall off. And using a lens wide open or one stop down with a center filter won't correct the problem

To use the filter the lens must be stopped down at least 2 sto

-- bob salomon (bobsalomon@mindspring.com), October 08, 1999.

Yaakov,

Is your camera equipped with a fresnel lens on the focusing screen? If not, adding one will help a lot. The fresnel changes the direction of the rays going to the corners of the screen from oblique to approximately perpendicular, allowing you to see more of the image from any one point of view.

-- Sean Donnelly (smd@alumni.rice.edu), October 08, 1999.