Kornelius: What are the facts about reverse curl sharpness problems?

[ernie_gec]

Much opinion has been expressed in medium format sites about whether

cameras like the hasselblad suffer a diminishment of sharpness in

frames that have been advanced after sitting for a while, taking on

the "set" of the reverse curl in these types of magazines. The idea

being that the film bulges away from the backing plate, somewhat, thus

creating resolution problems.

 

Some photographers always waste the next frame, in this situation, in

order to avoid these alleged problems.

 

Has Zeiss done any objective research into this matter and do you have

any advice? Is this totally overblown paranoia or a legitimate

phenomenon for concern?

[andrew_schank]

Hope you don't mind me answering, as I found this link: <a href="

http://www.smu.edu/~rmonagha/mf/flat.html#IDCC ">roll film flatness problems</a> on film flatness to be superb in covering all the aspects involved from a to z as many other articles by Mr Monaghan do.

[kornelius_j._fleischer]

The problem is real! This is what I wrote about it in Zeiss Camera Lens News (you can access CLN at www.zeiss.de, select English language):

 

Is rollfilm 220 better than 120 in terms of film flatness?

 

Zeiss has recently developed a new measuring system to evaluate film flatness in medium format photography.

 

The new system is based on an computerized microscope that can automatically scan and focus on multiple points of a film frame in a medium format camera magazine. The obtained focusing data are recorded by a computer and evaluated by a propriatory Zeiss software. The result is a mapping of the film topography with an accuracy of one millionth of a meter (1 micron), according to the developer of this system.

 

The purpose of this new device is to find out how well film magazine mechanics are designed in today's medium format camera systems, how precise they position the film and how well they hold it flat. From these findings Zeiss can draw conclusions about the field flatness required for medium format lenses and Zeiss can also trace causes for lack of sharpness in customer's photos. This is particularly interesting since more than 99% of all customer complaints about lacking sharpness in their photos can be attributed to misalignments of critical components in camera, viewfinder, or magazine, focus errors, camera shake and vibrations, film curvature, and other reasons.

 

So far, Zeiss has found that film curvature can have a major influence as a source of unsharpness. This has also been known by Zeiss' camera making partners Alpa, Hasselblad, Kyocera (Contax) and Rollei. Since Zeiss' evaluation program is not completed yet, we would like not to draw too many conclusions prematurely. But two things can be stated already as hints to enable sharper photos with medium format cameras at wide open apertures, since exactly those are invited by the high level of aberration correction in Zeiss lenses:

 

1.

220 type rollfilm usually offers better flatness than 120 type by a factor of almost 2. This is an advantage with fast, motorized cameras like the Contax 645 AF, Hasselblad 555 ELD (and previous motorized Hasselblad cameras) and Rolleiflex 6000 series cameras.

 

2.

Film flatness problems are mainly caused by the combined influence of two factors: the rollers in the camera or magazine that bend the film, and the time a certain part of the film is bent by such a roller.

 

Camera manufacturers usually space the rollers in a way that bent portions of the film will never be positioned near the center of the image. Therefore only marginal regions of the image should be affected by sharpness problems due to film flatness errors.

 

Since the photographer cannot alter the geometry and mechanics of his camera, he can only influence the other factor: time. A film run through the camera without much time between exposures should result in good flatness and hence sharpness. Five minutes between exposures may be some sort of limit, depending on brand and type of film. 15 minutes are likely to show an influence of bending around rollers. Two hours definitively will.

 

As a rule of thumb: For best sharpness in medium format, prefer 220 type roll film and run it through the camera rather quickly.

 

Camera Lens News No. 10, Summer 2000

[Quit]

[artie]

To Kornelius;

Am I correct in assuming this flatness problem is only a problem at wide open apertures.

[kornelius_j._fleischer]

It is a problem not only at wide open apertures. There, it can easily ruin your picture. But even with stopped down lenses, say f/11 or f/16 you may experience focus shifts that you cannot tolerate. This problem has more impact with wide angle lenses than longer focal lengths. Of course, it is worst wide open. The medium format film magazine that we found gives the best film flatness is the rather new 4560 reversible magazine for Rollei 6000. We also found why and how the new Mamiya 645 AF magazine produces the sharpness problems discussed elsewhere in this forum recently.

[jeff_green2]

Hello,

 

I'm curious as to any evidence specifically to larger formats like 6x7? I have an RZ67 and wonder how this specific camera fares in the flatness issue?

Jeff Green

[mike_rott]

So how to we go about getting film manufactures to offer more film

types in 220? How about a 100 speed B/W film in 220 like delta 100.

Does zeiss have any influence in this regard

[stefan_arend1]

Kornelius,

 

I have run into problems using 70mm Film in the 70mm magazine ( Agfa Aviphot Pan 80/200 - ultrasharp aerial films). The films are only 0.10mm thick. What I have found is that the film bends towards the lens, wich makes it impossible to focus accutately. Any Ideas ?

[kornelius_j._fleischer]

70 mm film: What camera and magazine do you use? And what lens? At which apertures?

 

220 film: The only influence that ultimately works with film manufacturers is $$$$$$$. Only if they experience a rising demand for 220 film, on the basis of actual sales, they will be interested to offer more variety. This is why I spread the word about the benefits of 220, equip the medium format cameras at Zeiss with 220 backs, and I order all rollfilm for Zeiss in 220. On top of that, I keep telling my contacts at Agfa, Fuji, and Kodak that I am doing so and why I am doing this.

 

Don't forget one important consideration: We all chose medium format because we want higher quality in our images than 35 mm could give us. Perfect film flatness and registration is an important prerequisite here. It could be achieved best with vacuum backs. One is on the market already (for Contax 645, and don't overlook that the Contax RTS III 35 mm SLR comes with one, built-in!), others are under development. But all these vacuum backs can only work with film that has no paper backing, like 220 and 70 mm!!! So, let us all, for the sake of medium format perfection, pave the way for 220 and push its use!