More questions about f/4.5 75 mm Grandagon-M

[Leonard Evens]

I'm still trying to determine whether my problems with my new f/4.5 75 mm Grandagon-M are just a matter of learning how to use the lens or whether there is something wrong with it. My tests so far indicate that there may be some significant field curvature. But I have a couple of questions.

I've tried to locate information about the resolution I should expect from this lens. I've found some information at www.hevanet.com/cperez/testing.html#65mm_thru_125mm for the f/6.8 version of this lens. I don't know how applicable that would be. In addition, I wonder if one of the entries for that lens is a typo. The edge resolutions at f/11, 16, and 22 are given as 42, 24, and 38. I wonder if the 24 is actually supposed to be 42.

Is there any other source of information about lens tests for this lens?

I've also unscrewed the rear group to see if I could see anything. Badger had screwed it in finger tight, which is the way it should be. As I unscrewed I noticed some slight creaking as if it needed some oil on the thread. There was a small place on the last outer thread where it looked as if the paint was chipped off. But it was not hard to unscrew or screw, and there was no evidence of stripped thread. Is that normal? (Presumably one doesn't want to lubricate the threads in order to avoid buildup on the lens elements.)

Finally, several people had suggested there might be a mounting problem. That presumably wouldn't mean the mounting of the lens in the lens board. I suppose there could be a problem with crossed thread in the mounting of the front group in the shutter. Will there be any problem unscrewing the front group from the shutter to check?

I've lots of experience with disassembing and reassembling various things without problems, but I haven't fooled with my large format lenses yet. One thing I did manage to do successfully was to disassemble an old 180 mm Mamiya C3 lens to clean some of the internal surfaces, something I would never attempt with one of my large format lenses. But I do consider myself competent to do routine things of this nature. Still any additional advice would be helpful.

[Michael S. Briggs]

What you describe in unscrewing the rear cell sounds fully normal to me.

I think some of the people who were suggesting a mounting problem were thinking of mounting the lens to the board. There are two possible problems. One, if the lensboard is too thick on some lenses it will prevent the rear cell from being fully screwed in and cause the separation of the cells to be incorrect. The second is that many shutters comes with a small screw sticking out the back. On some lensboards this fits into a hole or slot to prevent the shutter from rotating. Most lensboards lack this hole or slot, and if someone makes the mistake of leaving the screw on the shutter, the shutter will be tilted with respect to the board. This would be clearly visible if you look at how the shutter is seated to the front of the board. If this mistake were made, when you focus in the center of the image, you will have focus problems to the sides because your lens will have some built in swing. You might also get light leaks from the mismounted shutter.

The front cell comes off just like the rear cell. The only difference is that many wide-angle lenses have a thin washer, so a little more care is needed when replacing the cell. Some manufacturers individually machine the shoulder of the cell to adjust the spacing instead of using a washer, so if your lens lacks the washer, it doesn't mean the washer was lost. The washer adjusts the spacing of the cells to be optimum for those particular cells, compensating for manufacturing variations. The element spacing of this type of lens is very critical. If someone lost the washer, it would degrade the optical performance. I think this is why the manufacturers place the washer in front of the shutter -- there is less reason to unscrew the front cell and a lower probability of the washer getting lost.

[Leonard Evens]

Michael,

Thanks for clarifying this for me. In my case, the lens is fully screwed in. Also, I believe that the screw in front is in place, although I would have to remove the retaining ring to be sure. I am using the Toho eccentric lens board, which does have the dimple and needs the screw to keep the lens from moving when the lens is rotated in the lens board. In any event, if it were causing a problem, it would be to produce a tilt. In that case, the fall off in resolution would not be symmetric, and I believe it is. However, it is something else to go back and check. In any case, I am going to remount the lens in a standard lens board since it turns out that the Toho has sufficient movement to accomodate the 195 mm image circle without any special arrangements. I will save the eccentric board for special circumstances, mainly with my 90 mm lens, which has a larger image circle.

I still would like to see some posted resolution test results for this lens.

[Holger Pfriem]

Leonard, I own the f/4.5 75 mm Grandagon-N and can tell you the performance of this lens is outstanding, in my test it was better then my Schneider 72XL. I dont't have the resolution test results any more, but with the F4.5 Grandagon-N it is possible to get 80lp/mm on Film. (center, F8, Flash, Tmax100 with Xtol 1:3,Bosscreen with 15x mag. loupe for focus) This is my most used lens and when ever possible I only stop down to F11-16. If you do your own lens tests you will soon recognize that all modern lens give the best resolution (in center) at wider Fstops, with F22 no lens is a good performer. So if you see lp/mm numbers increasing with F-stop then you know the tester is a jerk or there is a missaligned screen. The number you mentioned (42/24(or 42??)/38) are too low, if I find the time tonight I make a test with my lens at F16 and send you the results.

sorry for my english

[Holger Pfriem]

Sorry,

me again. I just recognized that the values you mentioned (42/24(or 42??)/38) are at the edge for different f-stops and not (center/"middle"/edge). At f/11 67 60 42 is not a bad result if you compare it to the other lenses tested on this side. And keep in mind that the F6.8/75mm version is a much cheaper design then the not tested F4.5/75mm version you own.

sorry for my english

[Michael S. Briggs]

Holger, unless the only part of your photos that you care about is the center, measuring lp/mm resolution only at the center may lead you to a mistaken conclusion that modern lenses have their best resolutions at wider apertures. On many modern lenses, stopping down some will bring major improvements in the resolution at the edges of the image at the expense of only modest declines in the center, for an overall best result that is not at a really wide f-stop. Someone who finds lp/mm rising with f-stop may not be a "jerk", but might instead be someone who is measuring not just the center resolution.

[Kerry L. Thalmann]

So if you see lp/mm numbers increasing with F-stop then you know the tester is a jerk

Holger,

I am one of the two principle parties involved in the lens tests cited at Chris Perez' web site. Believe me, Chris is not a "jerk". He has spent a lot of his own time and money conducting these tests and generously shares the results with the large format community and asks nothing in return. Before you resort to insulting the man and calling him names in public, you might want to do a better job of understanding what the results mean and how the tests were performed.

As Michael mentioned, and you seem to (hopefully) now realize, while most modern large format lenses are sharpest in the center of their fields at f8 or f11, the best corner resolution is not usually realized until f16 or f22. This is consistant with the manufacturers' specifications that usually state that the lenses are optimized for best overall performance at f22 for most lenses; f16 for a few of the more recent designs; and f11 for some of the really short focal lengths that do not fully cover 4x5 and are intended for roll film use.

with F22 no lens is a good performer

This statement is highly inaccurate and could not be further from the truth. While the best modern lenses do start to bump up against the theoretical diffraction limits at f22, in most cases this is still the "best" overall aperture for many (most) modern lenses. I'd hardly call a lens that can resolve 60 lp/mm corner-to-corner at f22 a poor performer. In fact, unless you regularly make prints larger than 30" x 40" from a 4x5 original a lens that can resolve 48 lp/mm corner-to-corner will still yield prints of critical sharpness. In many cases, the added depth of field and improved corner resolution resulting from stopping down to f22 far outweigh the slight loss of resolution at the center of the field due to diffraction. Heck, I've even been known to stop down to f32 or f32½ to gain added depth of field when necessary. The "best" aperture is really a function of the lens, the subject and the desired result. For me personally, that means I shoot over 95% of my images at f22 plus/minus one stop.

Kerry

[Holger Pfriem]

@Kerry @Michael

First of all I never said Chris Perez is jerk simply because he isn't. All I wanted to say is that you get the best performence in the center at wider F-stops and not at F22(for modern lenses). When you look at the chart for the Rodenstock 6.8/75mm you see that Chris came to the same conclusion. Very often he gets very good resolution at F11 for the lenses in his test, so you can see I dont meant him to be a jerk.

Kerry wrote: "As Michael mentioned, and you seem to (hopefully) now realize, while most modern large format lenses are sharpest in the center of their fields at f8 or f11, the best corner resolution is not usually realized until f16 or f22."

Never said anything other, we are talking about lens testing not taking photos. When taking photos you have to know the lens you are using and the decide what F-stop is the best for a good corner performence.

Kerry wrote: ""with F22 no lens is a good performer " This statement is highly inaccurate and could not be further from the truth. ......I'd hardly call a lens that can resolve 60 lp/mm corner-to-corner at f22 a poor performer. "

Again I am talking about lens testing not taking photos. Sorry perhaps it is my fault but I am not able to get 60 lp/mm at F22 on film. A theoretical max. center value for F22 is 55lp/mm at 20% contrast but this is without film. With perfect processing and if you are very picky you might be able to distinguish some lines under the microscope but my problem with this is the term "some". At the corner at F22 the effective F-stop is at least F32 for a wideangle lens, you get a max. (38lp/mm at 20% contrast) of course tangential and radial values are different. Again when taking photos F22 is of course absolutly ok, or even F45 if there is a need for max. DOF. When I look at the corner of a photo taken with the 4.5/75 at F22 then there is always some fuzzyness simply because the effective F-stop is lower there then in the center, this is why I often choose F11-16 with this lens. Rodenstock say the best aperture for this lens is F16-22 but my findings differ by a stop. ...works for me



I am sorry if my previous posting cause some missunderstandings and my apologize to Chris, I never wanted to pi.. on your parade I really like your side.

Holger

PS: There are longer lenses performing very good at small F-stops, but I am not able to explain why with this lenses even at F32 quality is good and with shorter focal length everything became fuzzy. In theory diffraction at a given F-stop is not a function of focal length.