Understanding the Gossen Vari-Angle Tele attachment

[Toffle]

Hi folks,
It's been quite awhile since I've posted here, but I've still been busy shooting film on a daily basis. (I started the photo-a-day challenge on January 1, 2010, and have yet to miss a day)

Anyway, for my LF work, I've been using my trusty old Gossen Luna-Pro with consistent, reliable results. I've got the Vari-Angle 7.5°/15° attachment, but I tend not to use it because I can't seem to wrap my head around a discrepancy in the readings.

For example - metering a large, featureless, evenly-lit wall should return the same reading regardless of the metering angle. (shouldn't it? taking into account the EV compensations for the reduced light reaching the sensor?)
If I meter @Iso 50 without the attachment, I get a reading of 1/15" @f8
According to the literature, set to 15°, the attachment should require a compensation of +1 EV to give an accurate reading. So I should expect a native reading of 1/8", which, compensated +1 EV would once again give me 1/15" @f8. The problem is that the reading is 1/4", which would be +2 EV compensation.

Similarly, at 7.5° I should expect to compensate +3 EV to achieve the same reading, but the numbers don't add up. The native reading at f8 is 1", which would be +4 EV compensation.

Both readings are an additional +1 EV off the predicted results.

Obviously, I am doing something wrong here. Is it my assumption that an equally-lit scene should give the same (compensated) reading regardless of the metering angle? Is there something else I am not taking into account?

If I have this much difficulty metering a blank wall, how can I trust the readings for a more complex scene?

Cheers,
Tom

 

[ic-racer]

These results could be due to the optics of the adapter being hazy and dirty. Otherwise you assumptions are correct, all readings on a uniform surface should be the same.

 

[Toffle]

These results could be due to the optics of the adapter being hazy and dirty.

Thanks, ic. The optics appear bright and clear, but as the light for the viewfinder and the sensor does not take the same path that is no guarantee that there isn't some internal haze. All I can say for sure is that it looks clear. It strikes me as odd that my readings are consistently +1 EV off.

 

[BrianShaw]

Check out the manual. I think your assumption about the EV offsets may be incorrect.

http://www.butkus.org/chinon/flashes_meters/gossen_luna-pro/gossen_luna-pro.htm

Although I don't use mine much anymore, it was always close enough that I never worried about erroneous readings.

 

[Toffle]

Check out the manual. I think your assumption about the EV offsets may be incorrect.

Thanks, Brian. I'm not seeing those figures in the manual, but I could be looking in the wrong spot. Interesting that my manual is 19 pages, and your link is considerably longer. (but I still can't find it. )

At any rate, the compensations are printed on the back of the attachment, albeit without any detailed instructions.

I use my SBC almost daily sans attachments, with consistent results, and I should mention before anyone asks, yes, the batteries are fresh.

 

[BrianShaw]

I'd read it and interpret but busy with other things right now. Sorry. Look at the pic of the dial. The yellow diamond is for a straight reading. The other two circles are for the 7.5/15 attachment. I think they are 3 EV and 5 EV offset. That is my recollection also... but it is an older recollection. Also, My attachment is older than yours and does not have that sticker. I use the dial on a (regular) Luna Pro. Check it out to make sure I'm not inadvertently deceiving you.

 

[Sirius Glass]

I added red and green dots on the meter and the attachment to remind me about the EV offsets.

 

[BrianShaw]

Well, here's a possible confounding situation. I looked quickly at the manual for the LunaPro-S and the offsets seem to be 1 and 3 EV. Could they be different for the old LunaPro, Luna Pro S, and SBC models????

 

[Toffle]

This is getting more and more curious. My SBC does not have the same offset marks as the one in the manual you posted.

 

[BrianShaw]

I seem to recall that there are two versions of the vari-angle attachment: gray and black. Mine is gray and has no offset sticker. The other is black and had an offset sticker. I can't recall if they are the same or different. I'd guess the same, but that is just a guess. There seem to be several different dial configurations with different offsets, depending on the vintage of the meter. My recollection is that the gray vari-angle went with the old gray luna pro (the gear I use). I also have a vague recollection that the black vari-angle went with the SBC... but I could be all wrong on that. I think the trick is to get the two pieces of equipment to be from the same vintage. But if your SBC doesn't match the SBC manual... then I'd be totally befuddled!

 

[Toffle]

Mine are black and black. As a matter of fact, I think I have three different black SBCs and two black vari-spots. One vari-spot sounds like a cheap set of maracas, so I never even tried it. And one of the SBCs survived a tumble over a waterfall and still works perfectly.

 

[Sirius Glass]

There are several models of black and gray. The manuals give different offsets. Once one has aligned their equipment with the proper manual, not an easy task, understanding what is written versus what was really meant is not easy since all the manuals were badly written.

 

[RobC]

if you tested it find out what the actual offsets are, then what is the problem with using the offsets your tests give. This is no different than a lot of us testing for personal film speed/EI which takes into account any meter inaccuracies/offsets that we have in our equipment and/or procedures.

 

[Toffle]

if you tested it find out what the actual offsets are, then what is the problem with using the offsets your tests give. This is no different than a lot of us testing for personal film speed/EI which takes into account any meter inaccuracies/offsets that we have in our equipment and/or procedures.

Thanks, Rob. That's very insightful.

 

[nanthor]

Here is my original Luna Pro meter with the spot attachment on it. You will notice on the bottom of the setting dial the arrow with the two circles next to it, the circles colored red and green. The arrow is used for setting the dial when using the meter alone in reflected light use. The circles are where you set the dial when using the spot attachment. The spot attachment has matching colors both on the circles that you view through and the writing of the angles used (7.5 and 15 degrees) to remind you which colors to set the dial to. I think that the settings give a full compensation so that the meter gives the same readings but I haven't tested it to prove it. Something I will do in the next few days. Bob.

 

[Toffle]

Here is my original Luna Pro meter with the spot attachment on it. You will notice on the bottom of the setting dial the arrow with the two circles next to it, the circles colored red and green. The arrow is used for setting the dial when using the meter alone in reflected light use. The circles are where you set the dial when using the spot attachment. The spot attachment has matching colors both on the circles that you view through and the writing of the angles used (7.5 and 15 degrees) to remind you which colors to set the dial to. I think that the settings give a full compensation so that the meter gives the same readings but I haven't tested it to prove it. Something I will do in the next few days. Bob.

Thanks, Bob. I've gotten a number of responses cueing on the the coloured dots. (which, to paraphrase Henry Reed's poem, The Naming of Parts, "in this case, I do not have") It seems clear that the attachments are calibrated across certain models in the SBC family, but less clear that it is universal to all models. As RobC suggests, I have done done enough work to fit the spot attachment into my process, regardless of what the published numbers may be. What I had been hoping for was some explanation of why my readings are consistently +1 EV off. I was convinced that for some reason, my observations were flawed, and that I was missing some underlying logic in setting, reading or interpreting my devices.

Cheers,
Tom

 

[MattKing]

Which Luna Pro do you have?

Luna Pro (Lunasix 3)
Luna Pro ? (Lunasix 3s - the black one)
Luna Pro SBC (Profisix)
Luna Lux SBC (Luba Lite)
Luna Pro F (Profisix F)

If yours is the Luna Pro SBC (Profisix), then page 43 of the manual has the answer:

 

[Toffle]

Thanks, Matt. I believe that is the correct model (Luna-Pro SBC Profi-System) and the setting on page 43 appears to be what I've been using, but the reading is still off by +1 EV. Given that that meter works fine without the attachment, it may well be that the Tele unit is itself out of kilter. I'm going to do some further testing in the next few days to see if I can hammer out some results that will match the meter readings to properly exposed and processed negatives.