centre spot ND for Nikon SW 90mm f8

[kryn_sporry1]

Hi guys,

 

I am about to buy a SHen Hao 4x7 with a Nikon SW 90mm f8 lens. I was

wondering if I would be needing a centre spot ND grad fliter for

this lens, and if so, where I can get one.

Any ideas/suggestions?

[dmin-99]

People who have this lens say that it has less falloff than the Schneider or Rhodenstock lenses, it seems that a center spot is not normally required. If you think that you'll need one the Schneider IIIb (used on the 110XL) is a 1.5 stop and will fit (67mm) and should work fine.

[michael_briggs2]

<p>I suggest using the lens for a while and seeing whether you like the result without the center filter. There are a wide range of uses of the wide-angle lenses among LF photographers, and a wide range of tastes. One LF photographer will say that a particular lens "has no falloff", while another will say "that a center filter is necessary". So you might as well find out for yourself and save yourself money and trouble if you find the results satisfactory without the center filter.</p>

 

<p>My guess is that most 4x5 photographers don't bother with center filters for 90 mm lenses. But some use them.</p>

 

<p>Some LF photograhers think that the Nikkor-SW lenses have less need of center filters than the competitors' similar lenses. I think that this is probably a mis-conception. The optical design of the Nikkor-SW, Fuji-SW, Grandagons, and Super-Angulons are very similar, and the evenness of illumination should be very similar. For some business reasons, Rodenstock and Schneider choose to publish data on illumination and sell center filters, while Fuji and Nikon don't. I think that this causes some people to mistakenly deduce that the Japanese lenses work differently -- but light rays don't care what language was spoken by the designer of the lens.</p>

 

<p>B+H, Midwest Photo and Badger Graphics all sell center filters. I'd try any filter that has the right threads -- 67 mm for the 90 mm f8 Nikkor-SW. The filter from Heliopan

might be cheaper than one from a lens manufacturer (Rodenstock or Schneider).</p>

 

<p>Two past discussions: <i>Center filters</i> at <a href="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=005AC8">http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=005AC8</a> and

<i>Image circle and fall-off</i> at <a href="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=005gK2">http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=005gK2</a>.

[ben_diss1]

I did a bit of testing with this lens to answer the question when I posed it after acquiring this lens. In my neighborhood I have a handball court with a wall painted grey. It's consistent and even, close to a Kodak grey card in color and evenly lit by the sun. I shot about a dozen sheets of film and concluded that with no movements the light falloff is apparent, but only slightly. With movements it becomes more noticible and can be quite objectionable. For this reason, I use the center filter at all times. Do some testing and see for yourself.

 

-Ben

[dmin-99]

Ben,

 

Which filter do you use?

[michael_briggs2]

Testing against a uniform tone wall is the worst case. Maybe the falloff wouldn't be noticable in a typical scene. Did you compare to your other lenses? They have falloff too. A little dodging or burning (depending on negative or positive film) (or the digital equivalent) can take care of modest falloff. I suggest making a test print of an actual scene.

 

The uniformity from using a center filter is nice, but the drawbacks are the cost, hassle, and increased exposure time. Another drawback is that if you want to use a threaded filter (e.g., yellow, IR, polarizer), you will need a larger size to match the larger threads on the front of the center filter.

[michael_briggs2]

How much qualilty improvement will result from using a center filter also depends on whether you are using negative or transparency film. Negative film has a lot more exposure latitude, on the over exposure side. If you are using a lot of movement such as front rise that will result in two of the corners of the film receiving less illumination, I'd increase the exposure time to compensate, so that the corners receive a good exposure and the center is over exposured. With negative film, extra exposure is much better than under exposure.

[ben_diss1]

I'm using the Heliopan filter and Velia.

 

Good point about neg vs. slide film. If I were to switch to negative film I'm not sure I'd care to use the center filter. For slide film where we only have 5 stops or so of latitude I wouldn't want to lose 1.5 stops in a corner that may already be a dark portion of the image.

 

-Ben

[kryn_sporry1]

Well, I'll be using it in combination with a 6x17 back.

From what I hear a CS ND could be useful.

Ben, you saw light fall off. Can you tell me how many stops?

Should I try the suggested Schneider IIIb?

[ben_diss1]

With no movement, on 4x5, I saw 1/3 to 1/2 stop in the corners. It's when you use movements that the corners get dark. With a 6x17 back and no movements I wonder if it's needed at all. If you are going to get a CF, yes, the Schneider 3b would be the one to get.

 

-Ben

[michael_briggs2]

Re how many stops falloff for 6x17... Some of the links I gave before explain the optical theory for the lens design used for the Nikkor-SW -- the illumination should go as cosine to the third power. The graphs of actual performance supplied by Rodenstock and Schneider for lenses of the same desigh type (Grandagons and Super-Angulons) approximately follow this theory.

 

What is the actual diagonal of 6 x 17? Guessing 178 mm, the angle theta from the exit pupil of the lens to the corner of the film would be 45 degrees. (The half diagonal is 89 mm, and the inverse tangent of 89 mm / 90 mm is 45 degrees). This isn't that wide. The cosine of 45 deg to the 3rd power is 0.36, which is a falloff of 1.5 stops.

 

This may sound bad, but a lot of scenes have lighting variations of this scale, and some of your longer lenses may have this much falloff since they probably aren't using the tilting entrance pupil trick.

 

Applying the same calculation to 4x5 without movements, the angle to the corners is 40 degrees (diagonal 153 mm, inverse tan 76 mm / 90 mm), cosine to the third is 0.45, and the illumination falloff is 1.2 stops.

 

The density difference on the film will be less because film isn't developed to unit gamma, not even transparency film. I don't know the contrast of the film that Ben used, or how accurately he measured the density difference. So this simple calculation is probably in agreement with his measurement. The illumination from the lens may not exactly follow this idealized theory.

[andy_f1]

I have the same combination. The Schneider IIIb is perfect.