RB67 photos sft at the edges?

[stradibarrius]

I did a comparison test with my RB67 w/127mm and my Nikon F100 w/28-105.
I had set up a still life shot using two lights with silver umbrellas.
Both cameras were loaded with Acros 100. Same tripod and ballhead.
I used the F100 meter and a hand held meter. They agreed on the exposure so I set both camera with the same f/stop and shutter speed.
took the shot processed the film and printed the negatives on Ilford VC RC paper with #3 filter.
The F100 shot was sharp edge to edge. the RB67 was tack sharp in the center but got soft out around the edges.

This really surprised me!! I expected the RB67 to be at least as sharp as a 35mm negative.

I went back and took a second look at the negatives on the light table. The 35mm negative was sharper than the RB67 negative.
But keep in mind that the RB67 negative was tack sharp in the center of the shot.

Any ideas?

 

[archphoto]

What f-stops did you use and what lens-length ?

Bear in mind that on 6x7 you will need to add 1 f-stop compared to 35mm in order to get the same quality results.
This has to do with deph of field allso.

Peter

 

[paul ron]

That negative should be sharp edge to edge. Something is wrong.

Either your lens is dirty, shutter is not opening properly, film is not sitting flat.

Put a ground glass on your film plane n see how it focuses? Use a lens hood? If it is sharp edge to edge then it's your back, not the lens. Take the lens off the camera, keep it open in T mode to see if the glass is clean at teh edges? Any fungus? Try talking a few pics with another lens? Has anyone been screwing around in your lens before you got it? Does it look like screws were tampered with by a novice... slots damged, mared or scratched name ring, paint cover ups?

Fire the shutter at different speeds n make sure it is opening properly, iris not sticking or slugish?

Using a test roll, run it through the back while off the camera n see if it is laying flat... maybe the mask screws are loose? Try another back?

Not much else can go wrong causing your focus problem.

 

[nickandre]

What f-stops did you use and what lens-length ?

Bear in mind that on 6x7 you will need to add 1 f-stop compared to 35mm in order to get the same quality results.
This has to do with deph of field allso.

Peter

Wait, what? F stop is a ratio. It needn't be changed. They should match. The DOF will be the same when comparing a 90mm Sekor with a 90mm Nikon, just with a wider field of view.

Do you have the sekor or the C version? The Sekor(without C) isn't as "good."

Nikon makes VERY good glass.

Just the same any lens will perform best at F8 or so. This varies. Some ($$$) lenses peak in sharpness at F2.8. Your lens probably peaks at F8. If you used it wide open the images are likely to be soft.

 

[paul ron]

Good call Peter. Actually that is correct n may be the problem in a nut shell.

DOF is flatter with MF vs 35mm. Using the same F stop as the 35mm will show much less DOF.

 

[stradibarrius]

for some reason I had a feeling that it might be something to do with the DOF. some one else suggested that it might be that the shutter not opening all the way but would that not really being stopping the lens down?

So with the same scene the 6x7 will need an extra stop to give the same DOF?

 

[archphoto]

What it boils down to is: a 127mm RB ~ 55mm on 35, but has the DOF of a 127mm, and the 55 has the DOF of a 55 at the same aperture.
So to get the same DOF on both lenses you will have to stop-down the 127mm more.

There is allso something like the best aperture for a lens, the aperture at which the lens performs best.
For 35mm lenses that is in general f:8
For 4x5 inch lenses that is in general f:22 and 6x7 cm is somewhere in the middle, so have a look at f:11 or inbetween f:11 and f:16.

Quote: so with the same scene......End Quote YES, there about.

Peter

 

[keithwms]

First of all, show us the results. One cannot diagnose that which one has not carefully examined! You may well have an inferior lens copy on your hands, or it may be any number of other issues. Or maybe there's no issue at all! I see no point in throwing speculation at you. *Iff* you show us the negs then we may be able to say whether it is a DOF issue.

Second, to test a lens properly, you need to remove DOF and vibrations and such from the equation. You need to test a subject that has details that are within the DOF, and you need to use a tripod, and you need to use MLU etc. You need to be sure that the same type of film sees the same exposure and development too.

Third, compare apples to apples: enlarge the 35mm negative to the point that the print is the same size you'd get from the medium format print. Indeed, if you loupe a 35mm neg and a medium format neg side by side, you may well see slightly less edge sharpness on the medium format neg, especially toward the corners. This is no surprise; there are few medium format lenses that can compete with good small format lenses right across the frame (if you don't believe me, then shoot 35mm through your medium format lens. In fact I have personally used only one medium format lens family that pulls off a 35mm crop well, namely the mamiya 6 and 7/7ii RF lens family). Overall, you need to bear in mind that over the same field of view, the medium format neg is capturing waaay more detail than the 35mm neg. The format advantage is in the size of the negative; not in the glass

So ... compare an rb 127mm shot to a ~60mm shot on 35mm. Take a shot of something fairly flat so that DOF assuredly isn't an issue... artwork on a wall or something. Shoot the rb lens at f/8 and the 35mm lens at f/8 or f/5.6. Then print the medium format and the 35mm neg to 12" or whatever. Then we'll talk

 

[stradibarrius]

Unfortunately at this point I don't have a scanner so I can't share the shots. I would have done that straight away if I had been able to post them.
I did think about "shutter slap" on the RB. In my little studio area they way I had the subject lit, required a slow shutter speed and did not lock up the mirror. But if it were vibration would the whole shot not been out of focus??

 

[keithwms]

That's true, shutter slap should affect everything. I am just saying, to test lenses...

 

[Alexander Ghaffari]

Try it with a different film back, so we can eliminate a misaligned pressure plate. My bet is that the pressure plate is at fault.

 

[stradibarrius]

I am going to take some test shots today.

 

[MattKing]

If it is sharp in the centre, it is not a problem with camera vibration (shutter slap).

Matt

 

[JRJacobs]

I have found that Mamiya RB lenses vary quite a bit in optical quality from one sample to the next. I owned three different 90mm lenses and two were soft and one was noticably sharper. They are so inexpensive you might want to try getting another lens to compare.

 

[paul ron]

Don't waste your film.

Slap a ground glass on the film plane (insert removed, GG over the mask) n focus off a tripod like a view camera. Check the GG with a loupe to see if it sharp from edge to edge using a well lit news paper as your test target?

If it is sharp, the back is your problem. If not, then the lens needs to be instected for fungus n dirt or faulty shutter operation.

 

[stradibarrius]

I did go back and look at some negatives of some color 220 film I shot. Everything was the same except it was a 220 back rahter than the 120 that I had B&W loaded in. The shots from the 220 were sharpe edge to edge so that takes the lens out of the equation. My gut is telling me that it is a DOF thing. I have some more film that I did test with in the refrigerator waiting to be processed.