Rb67 150mm SF exposure compensation?

[Alfred Fickensher]

Folks, help. I'm going nuts overthinking this I think.

(intro: I'm in my eighties, a lifelong technical person, am intimately comfortable with ASA/aperture/shutter/EV relationships, and have tried doing my homework about this on these forums and the WEB in general)

I have a 150mm Soft Focus lens and the three disks plus the Mamiya instruction sheet for my RB67, and keep thinking that some sort of additional exposure compensation ought be added to account for the loss of light when one uses one of the disks in the lens. The Mamiya instruction sheet makes no mention of so doing.

Please, am I inventing a dilemma where none really exists?
Thanks,
alf in Iowa

 

[Don_ih]

Always set this lens at full aperture (f/4) when using one of the Softness Control
Discs. The f-value of this lens changes according to the Disc used. When the No. 1
Disc is used the effective lens aperture is approximately f/5; for the No. 2 Disc this
value is approximately f/5.6; and for the No. 3 Disc it is approximately f/6.3

From here.

 

[xkaes]

You're not "losing it". I'm sure that lens has a special instruction sheet -- like all of the SF lenses I've ever met. Typically, it's the DISCS that are missing -- not the manual -- so you're lucky!

 

[Alfred Fickensher]

Folks, help. I'm going nuts overthinking this I think.

(intro: I'm in my eighties, a lifelong technical person, am intimately comfortable with ASA/aperture/shutter/EV relationships, and have tried doing my homework about this on these forums and the WEB in general)

I have a 150mm Soft Focus lens and the three disks plus the Mamiya instruction sheet for my RB67, and keep thinking that some sort of additional exposure compensation ought be added to account for the loss of light when one uses one of the disks in the lens. The Mamiya instruction sheet makes no mention of so doing.

Please, am I inventing a dilemma where none really exists?
Thanks,
alf in Iowa

Aahh, of course . . . Overthought!
Both of you, thanks. I do have that instruction booklet and read that but utterly misinterpreted it. So even tho my lens is physically set at f4, the film will be seeing the light at the value etched onto the disks. I'll make my calculations based on those values.

Thanks greatly and to anyone who might add to this later,
alf in Iowa

 

[xkaes]

PLUS -- from what I've heard, it's going to be a BUMPER corn crop!!!

 

[Alfred Fickensher]

PLUS -- from what I've heard, it's going to be a BUMPER corn crop!!!

We've certainly had plenty of nicely spaced rains this year. I'm a totally metropolitan boy and pay no attention to ag matters but I can comment that lawns and grasses are still lusciously green this for being this late in August. Makes for good photography in our public gardens and parks.
alf in Iowa

 

[xkaes]

A bumper crop ain't necessarily good for farmers -- the old supply and demand factors.

Out here in Colorado the corn crop has been hit HARD by a corn worm. The problem isn't just all the worms, it's that the farmers have to shuck each and every cob to see if it's been attacked by the worms. They can't simply box up the raw cobs like they normally do. The problem is that the growers simply don't have enough workers to shuck each and every ear of corn -- so much of the crop is being left in the field.

This is SWEET corn -- not the corn that COWS eat -- but that's where it might end up.

 

[SMD]

Unfortunately the booklet says nothing about how to calculate bellows extension. Simply go with the apperture value of the inserted disc and calculate normally?

 

[BrianShaw]

Unfortunately the booklet says nothing about how to calculate bellows extension. Simply go with the apperture value of the inserted disc and calculate normally?

bellows extennsion would be in the close-up section of the camera user manual.

 

[MattKing]

Unfortunately the booklet says nothing about how to calculate bellows extension. Simply go with the apperture value of the inserted disc and calculate normally?

bellows extennsion would be in the close-up section of the camera user manual.

There is a guide built into the side of the focusing mechanism - provided your camera has an entry for the 150mm focal length. Even without such an entry, interpolating between the 127mm and 180mm markings will probably work. Here is the page from the Pro-S manual:

 

[Alfred Fickensher]

Unfortunately the booklet says nothing about how to calculate bellows extension. Simply go with the apperture value of the inserted disc and calculate normally?

As the original overthinking poster here, my sense is that the insertion of these discs into the light stream at the lens shares no relationship with any effects caused by the distance between lens and film. So my guess would be that the extended lens compensation recommendations remain as they would be with any of the other lenses. Use the scale as you normally would. (Thanks too to the later contributors to this thread)
alf in Iowa