Diffusion in Velostigmats and other Tessars (part II of my confusion))

[Fotoguy20d]

To step back a bit, what's the difference exactly between soft focus (and/or diffusion) and out of focus? And, is it something that will only become apparent once I start shooting as opposed to peeping at the ground glass?

I understand the implementation of diffusion on a Velostigmat Series II, which I understand to be a garden variety Tessar formulation. If I'm correct, the rear glass and back glass of the front group are kept stationary, and the front glass moves in and out.

Is anyone familiar with the Kodak Anastigmat Special 127mm f4.7, as used on the Graflex 45 (or 4x5 Combat Graphic)? I understand that to also be a Tessar.

I have one sitting on my shelf, and noticed some very weird stuff about it. It's worth noting that it's got a 100 something serial number so its one of the first ones. The one on my Combat graphic has a number in the mid 1000s (out of about 2500 made I think). In disassembling that lens, I saw that the rear glass from the front group is in a separate sleeve, threaded into the shutter. The frontmost glass then threads into that one, and focus is achieved by moving that glass forward and back (with infinity focus when the glass is close to bottomed on the shutter). It seems to me that this is the same effect as what is done with a series II Velo. If I put this lens on my Speed and focus on the GG (by eye, no loupe for brevity), by rotating the front, focus goes out. Lacking the loupe I'm not sure how sharp it is depending on glass position, maybe its never really crisp unless all the way back against the shutter. What should I try and look for?

Thanks,
Dan

[Mark Sawyer]

To step back a bit, what's the difference exactly between soft focus (and/or diffusion) and out of focus? And, is it something that will only become apparent once I start shooting as opposed to peeping at the ground glass?

Out-of-focus is just that, everything fuzzy, nothing in focus. Soft focus is acheived tyhrough aberrations in the lens, usually spherical, but sometimes chromatic. It causes the light to focus on different planes, so there is a sharply-detailed image with an accompanying soft image. Highlights especially show the soft image as a "glow". Diffusion is a softening of the image by putting some sort of interference in front of the lens, like a fuzzy filter, a layer of very sheer fabric, or a smear of something greasy. It softens the image, but not like a true sf lens would. Diffusion scatters light all over; aberrations keep them near the focal point with different spreads, intensities, and other characteristicsw that define the signature of each soft focus lens.

If you have a Velostigmat with the diffusion option, you might want to check out this thread:

http://www.largeformatphotography.info/forum/showthread.php?t=57385

[Paul Fitzgerald]

"It seems to me that this is the same effect as what is done with a series II Velo. If I put this lens on my Speed and focus on the GG (by eye, no loupe for brevity), by rotating the front, focus goes out. Lacking the loupe I'm not sure how sharp it is depending on glass position, maybe its never really crisp unless all the way back against the shutter. What should I try and look for?"

EXACTLY the same effect as the Velo II, now just focus the front lens of the Kodak to 3" and re-focus on the GG to see the effect.