Fall off and long exposures

[ryanmills]

I have a new to me 8x10 and a 240mm Rodenstock Sinaron-n that im testing with Direct positive paper, because well its cheap. However the sun refuses to shine here right now and my exposures are longer, 3+ seconds. I am seeing significant vignetteing both close up and farther back. I'm guess this is just because of the longer expsure times and shooting a 5.6? Shooting a shorter time the vignette would be quite a bit less?

[Oren Grad]

Are you shooting wide open? Falloff with the 240 Sironar-N at f/5.6 and distant focus is pretty severe on 8x10 - per Rodenstock's spec sheet it's going to be something like 4 stops in the corners.

[Dan Fromm]

At infinity the corners should be ~ 1 stop down from the center because of optical vignetting (good ol' cos^4). Closer up they should be better illuminated. Exposure time has nothing to do with it.

Is the subject lightest in the center? That would do it too.

One last thought. The lens should reach full coverage at f/22, but that will affect image quality in the corners, not illumination.

Oren, you misread the chart. It shows -4 stops @ f/5.6 at 184 mm off-axis. 8x10's corners are ~ 150 mm off axis, there the chart shows a bit more than -2 stops @f/5.6, a stop worse than good ol' cos^4.

[Oren Grad]

Dan, the Rodenstock graph shows 1 - cos^4 as a reference, but illumination at f/5.6 falls far short of it. By comparison, the curve at f/16, which is as far as they go, tracks the theoretical ideal most of the way to the corner.

[Oren Grad]

Oren, you misread the chart. It shows -4 stops @ f/5.6 at 184 mm off-axis. 8x10's corners are ~ 150 mm off axis, there the chart shows a bit more than -2 stops @f/5.6, a stop worse than good ol' cos^4.

You're right, my bad. Assuming a diagonal of 310mm, It looks somewhere between 2 and 3 stops at the corner with no movements.

[ryanmills]

Interesting I wonder if my understanding is backwards. I assumed at f/5.6 I would have the best illumination and as I stopped down it would get worse due to diffraction? I know the 240mm does not have much in the way of movements on an 8x10 but wide open at f/5.6 is there not enough coverage unless I stop down to say f/16?

[Dan Fromm]

Diffraction affects resolution, not illumination.

Re stopping down, reread posts 3 and 4. Download the brochure, it is at http://1drv.ms/12OFqc2 After you've downloaded it, thank Bob Salomon for lending it to me and me for scanning and posting it.

[Oren Grad]

I know the 240mm does not have much in the way of movements on an 8x10 but wide open at f/5.6 is there not enough coverage unless I stop down to say f/16?

You need to stop down to get adequate coverage for 8x10; useful coverage at open aperture is substantially smaller than the specified image circle at f/22. The lens is really intended to be used at f/22 and beyond; balancing optical correction, depth-of-field and diffraction, Rodenstock generally considers f/32-f/45 as "best aperture" for 8x10. Open aperture is intended for focusing, though of course there's no reason you can't use it for pictures if the image character suits your purposes.

FWIW, I had the opportunity to test the 240 Apo-Sironar-N before settling on the Apo-Sironar-S for my own 8x10 kit. Stopped down, it's a fine lens within its specified image circle, and if I couldn't have the S I'd be very happy with it.

[ic-racer]

d shooting a 5.6? Shooting a shorter time the vignette would be quite a bit less?

You will need a LONGER exposure time once you stop down.

[ryanmills]

Thats interesting my understanding was backwards. Would focus distance change the light fall off in any way? I know a lot of people here shoot 8x10 for landscape and I could see with the focus point being so close how that might affect coverage. In my case I shoot portraits and while I know its not a ideal lens its a working distance im quite happy with for my own work and im often shooting full body shots, not close ups with distortion becomes an issue. Also I have seen some masters like Jock Sturges use this lenght quite effectively. I guess its a matter of knowing how much to stop down?

[djdister]

In order to get less vignetting - a wider cone of illumination, you could:

1. Stop down (f22 or smaller)
2. Move closer to the subject (as you approach 1:1 reproduction, the circle of illumination gets much larger)

both options will require longer exposure times, so you will also need to factor in exposure reciprocity failure.

[MAubrey]

both options will require longer exposure times, so you will also need to factor in exposure reciprocity failure.
The good news on that front is that Harman Direct Positive Paper reverses most characteristics of a normal negative and reciprocity makes it faster rather than slower once you go passed 4 seconds or so.

I'm still testing its effects...

[Oren Grad]

Would focus distance change the light fall off in any way?

You do gain coverage, with reduced falloff, as you increase the bellows extension to achieve closer focus.

Despite all this discussion, we still don't know for sure exactly what caused your particular problem as you're perceiving it. There could be other factors that are contributing as much or more than the aperture you chose. Could you share a scan of one of your direct positive captures?