What is the story with center filters?greenspun.com : LUSENET : Large format photography : One Thread
I'm trying to learn about some of the finer points of LF.
What is the story with center filters? In 35mm if a lens has bad light fall-off we say its a bad lens. We don't say "that's ok, just whack a center filter on it". Why does this happen in LF?
If a lens needs a center filter for even illumination, then why isn't it part of the optical formula? Like why don't they either include it with the lens or else tint the lens elements to give evenness?
Just to pick an example out of the air - The Schneider Super Symmar XL 150mm (according to http://www.cs.berkeley.edu/~qtluong/photography/lf/lenses-table4x5.html needs a centre filter, but the similar spec Nikon 150mm SW does not. Does that mean the Nikon is a better lens (at least in that respect)? Wow look at the price of the Schneider filter for the 150mm - $1039 - wow.
Are center filters only for perfectionists? Or are they more or less mandatory? Do the lenses on that web page not listing a center filter not need one, or are they just not mentioned? Should it be a big part of my decision which lens to get according to whether it needs a center filter?
-- Chris Bitmead (email@example.com), August 07, 1998
Well, that site doesn't say the lenses actually need a centre filter, it merely says they are available. I see that even the 210mm has one available.
I am sure you are aware of the cos^4 law, which basically means means that for a simple lens, light falls off as the fourth power of the off-axis angle. For 45 degrees, this amounts to 2 stops. However, wide-angles lenses do usually have features in the geometry to reduce this effect. The Schneider site (http://www.schneideroptics.com/) has graphs of the net effect for their lenses. However, I believe they over-estimate this fall-off. I regularly use the Schneider 47XL, and think the fall-off in the corners is only about 1 stop, rather than the 4 stops given by cos^4, or the 3 stops given by the graph. As I use B&W, I can easily compensate while printing.
Why does this happen in LF, but not 35mm? I suspect that it does happen in 35mm. My Nikon 15/5.6 gives a narrower field of view than the Schneider 47, but has (I think) greater fall-off (and certainly much worse flare).
Why don't they include it with the lens, such as by tinting the elements? Possibly because it would inevitably reduce the speed of the lens. If you are just using the central portion of the 210mm lens, as you are for 5x4 with no movements, why should you have to suffer a built-in centre-grad when it is not needed. There are also, of course, cost factors involved, besides these small fall-offs can be compensated during pre-press.
I assume that for a given lens focal length, the fall-off will vary between manufacturers, or between models within one manufacturer. For pictorial purposes, I doubt that it would trouble you. For scientific/technical work, such as photographing paintings, it might be a factor in lens selection.
-- Alan Gibson (firstname.lastname@example.org), August 07, 1998.
The 150XL needs a center filter if you are going to use it on an 8x10 or larger format with movements. Roughly speaking, a 150mm focallength lens on an 8x10 is the equivalent of a 75mm on a 4x5. Most 150 lenses, like the Nikon you mention are not designed to cover that big of format (however mine covers 4x5 easily, and will also cover 6x17 on my Vpan, with movements.)
Wide angles for 4x5 are designed much differently than wide-angles for 35mm. As I understand it most wideangle 35mm lenses fro SLR cameras are retro-telephoto designs, meaning that the nodal point is somewhere towards the front of the lens, in some cases in front of the lens. This is necessary so that the lens does not protrude into the mirror box area. Hence, a side benefit of this (and now I am guessing) is that the designers can also correct the natural light fall off (see Alan's post above). Designers for large format glass do not have this design restriction and hence the rear of a wide angle lens gets very close to the filmplane. The CWF is a brute, yet elegant way of solving the problem of light fall off. I have a Heliopan CWF for my 90mm f/4.5 Grandagon but I don't need it if I am shooting 6x9cm, occaisionally use it for my 4x5 work, and always use it for my 617 shooting.
Cheers from America. Elis
-- Ellis Vener (email@example.com), August 07, 1998.
Ellis is correct about the difference in lens design (35mm retrofocus design vs 4X5 non-retrofocus design) having an effect on light fall off. The rear lens element of a wide angle LF lens is often not much further fron the film plane than its 35mm equivalent and since the LF lens must cover much more area, the light reaching the edges of the film must travel many times farther. And we all know that means less light at the edges than the center.
I use a Schneider center filter designed for my 90mm f8 and it works great for outdoor shots on color film. It also works well for Schneider's 75mm f5.6 and 65mm f5.6 lenses but some falloff is still evident. IMO it has been worth every dime I paid for it! It works best with low contrast films like Kodak's VPS and E100SW. I did some shots with Pro 100 and when I printed them, the corners were actually lighter than the center of the print.
Why Nikon ignores this matter of physics puzzles me though. Rent some LF WA lenses and try them before you buy. You probably won't see much difference. Also, be sure the center filter you buy. Schneider's goes from 67mm at the lens to about 85mm at the fron to avoid vignetting when you use movements. It's really disappointing to compose, stop down, close shutter, add filter, take picture, then see vignetting in final product. Been there, done that, didn't buy the T-shirt.
Have fun, Jim
-- Jim Blecha (firstname.lastname@example.org), August 11, 1998.
Chris: Some comments on the Nikkor 150 SW, and the need for CFs in general. (I cannot comment on the 150 XL.)
The 150 SW is strictly an 8x10 lens. It produces an image circle of about 400mm, comparable to the 165mm SA or 155mm Grandagon. It's a great super-wide angle for the 8x10 format; I don't use it that often, but it's wonderful when needed.
I've never used or needed a CF with this lens, which is a good thing because the filter size is huge. This does not mean the 150 SW is "better" than the 150 XL, it just means it's a lens intended for the 8x10 format. On the other hand, many would say the Schneider 150 XL is a super sharp lens, probably better than the 150 SW.
With this much coverage, the 150 SW would be sheer overkill for the 4x5 format. Even if you wanted to, it is way too large and heavy to use with 4x5 field view cameras. In a pinch, I suppose it could be used with some 4x5 studio monorails/views with very large lensboards and very sturdy front standards.
As you've been advised by others (in other posts), for a normal lens for the 4x5 format, something like a 150mm Apo-Symmar, Nikkor W, Apo-Sironar, etc. would be most apropriate. None of these 150s would need a CF when used with 4x5. You might need a CF if they were used with 5x7, or any film size that would begin to exceed their usable image circles.
CFs are only necessary, or useful, when the image circle for the lens in question is insufficiently large to evenly illuminate the film size being used, all the way out to the corners of the negative. For example, a 90mm SA XL would not need a CF when used for 4x5; the same lens will benefit from a CF when used with a larger format, 6x17 or 5x7, which would tax the lens' covering power and cause uneven illumination in the edges/corners.
Good luck with your many choices, Sergio.
-- Sergio Ortega (email@example.com), August 13, 1998.