Brightness of 8x10" GG image with 210mm G-Claron

[Tim V]

Hi all,

Just trying to ascertain if my ground glass is useless–it's a a very thick chunk of plastic made by Gibellini for it's early cameras–or if my 210mm G-Claron lens is simply very dark on the edges wide open due to smaller image circle wide open at f9. I can accept the latter, but I'm looking for an excuse to upgrade the ground glass with something with finer grain and the possibility of better brightness anyway. I'm used to high end focusing screens on my Linhof Techno, so not sure if my expectations are out of whack for this format or not...

For others using the 210mm G-Claron, how do you find the image on the ground glass? With my camera, the only way I can see anything outside the central portion of the image is to position my eye very close to the ground glass, right at a corner, and even then I can only see that corner as the rest of the images goes very dark. If I stand back and try to take in the entire image on the ground glass, I estimate I can only really see about 50% of the image relatively clearly, being only in the centre part, as the corners and edges are too dark to assess.

Anyway, just looking to hear other peoples experiences. The g-claron is a great little lens especially as I only use it stopped down to f45 where the image circle is more than ample, but perhaps the pay off for it's tiny size and price tag is simply a very dim ground glass image? Maybe this is compounded by a really crappy focusing screen?

Thanks,

Tim

[ic-racer]

If the specs show 226mm circle at f9 then I suspect it will look dark 150mm out from center at f9.

The 210mm I use has a documented image circle around 300mm at f5.6 and with those specs and a fresnel, I can report my view under the cloth is pretty bright all around. In fact I do my initial composition without the cloth, and only use that for the final checks and fine-tuning before exposure.

[Tim V]

Thanks for your reply.

I guess I ask mainly because it's widely reported that the IC specs for the g-claron range of lenses is drastically underrated, as it was marketed (if not designed) for the reproduction market. For such a purpose, very conservative IC specs were given due to the higher demands or expectations of the application, i.e. the need to stay within the area of coverage of the lens where resolution is at its best. Now, I have no idea what the 'real world' specs are for the lens used in the field, but I do expect there to be a large falloff wide open. I suppose I am not sure if at such an aperture–as opposed to a typical working aperture of f22-45–if there would be a drastic amount of both light and resolution. In other words, I wondered if the area of good illumination might be okay for the purposes of image composition, but just the outer edges would lose definition.

In any event, it's not impossible to use the lens for for field work, I'm probably looking for an excuse to replace my crap ground glass...

[G Benaim]

I've used it on my 810 and though the corners are darker than the middle I didn't experience the extreme vignetting you're describing, and it certainly covers the negative without vignetting. So I would look for a brighter gg if I were you.

[Tim V]

My feeling is you're right, and the ground glass (actually thick plastic) does seem really bad... That'll be my next job, I think, right after calibrating the position of the GG as I'm also experiencing a bit of back focus...

[cdavis324]

You might want to try a Fresnel first... I cut down a full sheet magnifier(from an office supply store) to fit. Not sure a different ground glass will be much brighter - saying this as a gibellini owner. A Fresnel is really essential with wide lenses on 8x10.

Sent from my LM-V350 using Tapatalk

[Dan Fromm]

Tim, about y'r vignetting. I just did the calculation, also took a quick look at my 210 G-Claron. With a 210 focused to infinity and assuming no mechanical vignetting, 8x10's corners should be ~ 1.2 stops down from the center. I saw a little mechanical vignetting with the lens wide open, perhaps enough to cause the effect you saw. To check, mount and focus the lens wide open, remove the GG, and look at the lens from the corner of the frame. If the exit pupil is invisible or even much occluded, bingo!

About coverage increasing on stopping down. Stopping down can reduce mechanical vignetting -- whether it does depends on the lens' mechanical design -- and expand the circle illuminated. Stopping down reduces the effects of some off-axis aberrations, improving image quality towards the edge of the field. This has nothing to do with illumination.

[Drew Wiley]

I never use fresnels. The secret is a properly ground glass. I have a Satin Snow glass on my 8x10. My shortest G-Claron is the 240, which is no problem in use except for the mechanical vignetting wide open at strong front tilts. Best to use rear tilts as much as possible. But I have played around with my Fuji A 180 just for fun on the 8x10. It is a very similar optical design to the G-Claron. The 180 isn't sufficient for good corner sharpness on full 8x10 film, but the circle of illumination covers, and I have no problem focusing with it, at least in the usable part of the image. All my 8x10 lenses are f/9 or even dimmer. So a 210 G should present no serious problems. There will be some illumination falloff, but not like that of true wide-angles lenses. If you need movements, you'll most likely be working in the f/45 to f/64 neighborhood. I generally compose the image and set movements with the lens wide open, then fine tune things viewing thru a loupe with the aperture set halfway down, then finally close the aperture clear down to the working stop.

[Leigh]

The GG brightness should be the same with ANY lens set for the SAME aperture.

That's the only reason photographic exposures work.

If using a fresnel, there may be a slight difference in observed brightness with angle of incidence, so a wide-angle and a telephoto may produce slightly different views.
But this is only slight, and does not affect exposure since the fresnel is on the GG.

- Leigh

[Dan Fromm]

The GG brightness should be the same with ANY lens set for the SAME aperture.

t/stops, anyone?