Enhanced Focusing Screens (ex: Boss Screen) for removing hot spot on GG

[marshallarts]

Happy New Year,
I have a specific need to take pictures of the GG on my Graflex (either Speed Graphic or Crown). Unfortunately, as you know, this is impossible with a stock GG as there is a terrible hotspot off-axis.

After finding THIS (http://www.largeformatphotography.info/gg.html) article here I am intrigued by the Boss Screen but don't have any way to test it out. Does anyone have experience with them? I would like it without grid lines so I can take the picture of the GG and have an effective "preview". Does anyone have an example of this so I can see what quality to expect.

Thank you so very much

[Patrick13]

Based solely on one picture I have of my GG (which has a fresnel in front), a longer focal length lens (I was using a 10" tele-optar) doesn't suffer the hotspot you're talking about. Now I didn't take this shot off-axis, either, so YMMV...

If you don't mind a little photoshop work, you can take one on-axis shot to capture your GG image as best as possible and then your regular shot. Post-process and mask the better GG image into the final picture.

In VFX that would be called "fix it in post" :p

Alternately, if you have a color printer and extended time to set up your shot: take a quick picture of what the GG is supposed to show, print it on any color printer with filters applied to make it look GG, then stick the image onto your camera so you can take your final shot.

In VFX that would be called "a practical shot."

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[marshallarts]

Thanks Patrick13. I'm not sure why I stated a concern off-axis, all my shots would be front on to capture just the GG cleanly so technically anything but center will be off-axis. Don't think your post & practical suggestions would work in my situation, I really need the digital preview.

Thanks for the sample of your fresnel. Your sample shows decent corner brightness but it's hard to see the image quality. I've heard Boss Screens are even better (in that article I linked they stated fresnels have "concentric bright rings which look like double images"). Do you see any artifcats.

More importantly - Your sample shows decent corner brightness but it's hard to see the image quality. With some levels adjustments do you feel such a digital capture would yield a decent image? (nothing comparable to an actual LF print, but maybe a decent digital picture?)

[Patrick13]

Yes, in person and in high res digital photos you know that the fresnel rings are behind the image. I will run off a high resolution picture of a properly lit and focused subject for you tomorrow night, not a problem, and you can see if the effect is objectionable. I'll use the normal 135mm instead of the tele so everything is as standard as possible for your comparisons.

[Jason Greenberg Motamedi]

BosScreen does a great job reducing hot spots and I love mine, but I don't think they are in business.

[marshallarts]

Wow, that would be wonderful, Patrick. A normal or wider lens would be perfect! I hope to not put you out too much. You must have a good workflow for such experiments, it takes a lot for me to do something like your offering, much appreciative!

I imagine the BosScreen would at least not have the fresnel rings but this would be a great starting point.

I believe the BosScreen is still available, in that this website is still up - http://www.stabitech.nl/Bosscreen.htm . But it does seem like a real pain to get one. So, Jason, could you shed some light on your experience with one vs a fresnel? Any samples?

[cowanw]

I believe the Boss screens are camera specific; perhaps they are difficult to grind down or resize. I have a cambo 4x5 Boss that I have never used, for no good or bad reason.

[marshallarts]

I would really love to see a digital shot of the image on a Bosscreen to see if it would be up to standards for my use. Does anyone have the ability to do this? It will be very difficult to source one of these along with a matching camera only to learn it won't do what I need.

[Nathan Potter]

I'm guessing about this a bit but I think that as one goes to shorter focal lengths in 4X5 the off axis rays will transmit a diminishing intensity of light as one goes from center to corner of the GG. The effect will be dominated by the lens focal length more than the type of GG that is used.
Some GG are measurably brighter than others but the light scattering phenomena is relatively consistent unless the GG itself changes the scattering angle radially from center to edge. I know of no such GG that does this.

I'd accept the fall off and take the digi picture, then correct the fall off in PS and print the result.

A Fresnel would clearly help the fall off situation especially if the Fresnel focal length matches the focal length of the taking lens. But at sufficient resolving power of the digi camera the Fresnel lines would become apparent and it sounds as if this is undesirable for you.

Nate Potter, Austin TX.

[marshallarts]

Hi Nathan, you are correct about the fall off being worse at wide angles. But I hear that's where the Bosscreen performs better than others (at least without fresnels rings-plus the need to pair with the lens). Stated in the article I linked above "I use one on my 4x5 with lenses from 75mm to 600mm and I am very pleased with it. With the 75mm there is a hot spot in the center, but the corners of the format are still clearly visible without fresnel. They have screens fitting most view cameras and you can also choose between a model with clear corners (similar to cut-away corners) and the standard rectangular shape.".

I would never need to go that wide, but it's good to know you have some visibility in those corners. It is precisely this that I want to see and judge for myself, if I may convince someone to do this.
(thanks to Patrick for offerring to show me what his fresnel can do - I would love to see what anything can do!)

[Patrick13]

Since you're trying to image something that doesn't exist in real life you have to cheat. But you knew that already :D

[Patrick13]

Sorry about the delay, I'm tied up at work with a new project kicking off. I'll grab that picture for you soon!

[marshallarts]

Thanks Patrick! Look forward to seeing it!

[flavioruiz]

If the problem is your GG or you'd like to experiment with, there are some articles around on how to make one yourself (ie, http://www.apug.org/forums/forum216/78392-making-ground-glass-focus-screen.html). Hope it helps, good luck!

[Patrick13]

Finally. This is a crop of approx 1/9th of a 4x5 GG with a (standard on graflex) 135mm wide open and maybe representative of what a loupe would see. I chose a textured background, at this sampling you can really make out the fresnel intruding upon the image. At other resolutions and samplings the rings come and go, this is just the way digital works when a regular pattern is found in the source. If you really want to know why then ask about Nyquist and sampling theory, but I wouldn't recommend it :cool: In any case, while a fresnel does help with brightness I doubt that it's what you're looking for.

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[marshallarts]

Thanks so much for this! I tried thanking you earlier but the forum was down?

How does the overall image look? Decent? Maybe it'll give me an idea what kind of quality to expect from a Boscreen. Aside from the fresnel circles is the image quality decent?

[Patrick13]

Very much a "grab something white, fast" effort =) Overall shot resized, I also boosted the contrast to bring out any lighting flaws.
Full spec for the shot: Graflex Optar 1/4.7 135mm distance to subject appx. 3feet, aperture wide open.
Picture taken hand held and slightly off axis behind with a 105mm macro on a 35mm digital, the focus going out is just my camera being not parallel and not the GG.
Corners look fine, not using any movements. With a mismatched lens like a 250mm the fresnel doesn't compensate as well, but still serviceable. I don't have any wider lens right now. It's a sloppy picture, but gets you in the ballpark of what I see in person.

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[marshallarts]

That is perfect! Thank you so much! Those specs are exactly what I was looking for! Very much appreciated.