Wide angle focus questions - front focusing -Schneider vs. Nikon

[schafphoto]

Hi I’m looking for some thoughts/help with an issue that has recently crept into my LF negatives. (Cambo 4x5) I photograph a lot of buildings, and I've noticed that some of my photos are focused in front of the selected focus plane chosen on the GG. For instance, I take a photo of a building across a parking lot that is just shy of infinity (80’) away. No, swings, tilts, shifts. GG aligned and leveled w/ lens board. Look through GG, choose focus point wide open, stop down to f22 (my standard go-to Fstop), expose 3 sheets of film at around 1/30th second (sometimes with polarizer). I get the film back and the building is in focus... but barely... just at the edge of focus (or the edge of the circle of confusion that makes it seem in focus based on DOF) The parking lot is razor sharp half way to the building at 40’ away.

It seems that If my calculations are right my GG in the springback is too close to the film back inner plane where the film back slides in. The GG probably needs to be shimmed backwards, away from the lens. I can do this through trial and error and film tests.

Here’s the thing I need help with. I use very wide lenses when I see this problem. It's MUCH more pronounced with my Schneider 72XL than my Rodenstock 55mm, or my Nikon SW 90mm f8, or my Nikon 120 SW. I know these lenses need tiny movements to go from in-focus to out-of-focus. So a millimeter is a BIG move and I will need to make smaller adjustments. I am also using the unusual wax-infused super-bright BOSSCREEN GG (which has the stock black tape shims in place and is custom made specifically for CAMBO), I have two Bossscreens, both have same focus plane if I swap them out. I am not using a Fresnel. While I have your attention, I want to confirm my understanding that depth of field moves away from the point-of-focus 1/3 closer and 2/3 farther away, which is what a hyperlocal focus scale seems to confirm. I’m not seeing this in practice, in fact I’m seeing no real depth increase behind the focus point. Maybe it is the design of ultra wide lenses? Maybe my GG is way off.

I will start a test of my lenses with a 1mm shim to see if the focused image on the negative jumps behind the spot chosen on the GG... Then I can fine tune +/- from there. Let me know if I’m on the right track of if I have it all backwards and save me some film. Thanks!

-Stephen Schafer

[B.S.Kumar]

I have two backs for my Cambo SC. The older reversible back has a fresnel mounted between the GG and the lens. The brand new revolving back doesn't have a fresnel. So, maybe if you have the reversible back it does need to be shimmed? Does everything lock down properly? When you insert a film holder, does the standard move? It might be helpful if someone can observe this while you are inserting the film holder, or you could use a video camera to record it. One way to help with making smaller adjustments is to use a larger focusing knob.

Kumar

[Doremus Scudder]

Stephen,

You describe a classic ground-glass positioning problem. It may have to do with the fact that you have after-market Bosscreens installed. If, for example, your Cambo originally came with a Fresnel/ground-glass sandwich with the Fresnel on the lens side of the assembly, then your Bosscreen will be too close to the lens (closer than the film) by 1/3 the thickness of the original Fresnel (this latter fact might give you a starting point for determining shim thickness). The fact that you see the error more with shorter-focal-length lenses is that the shorter the focal length, the less Depth of Focus you have at the film plane (as opposed to Depth of Field in front of the lens, which is greater with shorter-focal-length lenses). Lens design plays a role here too I would expect. The point here is that if you position your ground glass correctly using the most sensitive lens, i.e., your 72mm, it will be right on for the rest.

As an aside, the DoF does not divide neatly into 1/3 in front and 2/3 in back of the plane of sharp focus. It is a function of distance. At close distances, there is indeed a shorter in front/deeper in back relationship but this disappears to inconsequential at moderately long distances. The best way I've found to ensure adequate DoF is to use near and far focus points when focusing and to note the spread between these two focus positions on the monorail or the bed of the camera (having millimeter scales installed is really helpful). I then choose an optimum f-stop depending on the amount of focus spread as described here: http://www.largeformatphotography.info/fstop.html . IM-HO it's well worth reading and digesting and applying this. I never have a focus surprise anymore. At any rate, you should be seeing DoF well behind your plane of sharp focus at small apertures, so your ground glass is likely out-of-position.

Therefore, it seems you need to shim your gg. I'd proceed as follows:

Before doing anything examine your camera back and filmholder seat area to make sure everything is in order and that the filmholder is seating properly. If you've replaced felt or anything like that that could change the position of the seated filmholder, double-check to make sure this isn't your problem (DAMHIKT). If you're confident that everything is seating mechanically correctly then..

Assemble an assortment of shim material in different thicknesses from very thick to very thin. When I have done this, I made final adjustments using pieces of transparent tape = not very thick...

Since you have shots with the error to refer to, if you've kept notes about which lens you used you can recreate the shot and then focus between where you wanted to originally focus and where the focus eventually ended up. Noting the difference here will give you an idea of a shim thickness to start with. If you can't do that, you'll just have to guesstimate...

Install a shim. If you can remove and re-install easily, you might be able to check focus position against one of your earlier shots: First focus without the shim on your original desired focus position. Add the shim without moving anything and see if focus now corresponds to the second (erroneous) focus position. If you can do this, you're way in the ball park. In lieu of this, you'll have to use tests to find the focus position (and to refine what you've just done anyway...). So...

Test 1: Using your most sensitive lens wide open shoot a ruler close-up at an oblique angle to the lens (~45-90°). Your shortest lens (or in your case the 72mm) is what you want to use here. Focus on a well-defined spot, say the 15mm mark (or 6" if you're not metric...). Take a shot, develop the negative and see how you did. If actual focus is closer to the lens, you need to add more shims, if farther from the lens, you need to remove shims. Repeat this until you are satisfied with focus. Yes, I know it's a PITA, but necessary and better than measuring methods unless you are a trained machinist. I usually do these tests when printing and just toss my neg in the print developer for 4 minutes, evaluate it wet and then toss it. To check and cross-reference you should also do the following:

Test 2: Using your most sensitive lens wide open shoot a distant horizon. You want to use your longest focal length this time as it will have the shallowest DoF at a distance. A lens with a wide max. aperture (e.g., f/5.6 or greater) will be better than one with a smaller max. aperture. Develop and check the negative. If the horizon is sharp, you're in business, if not, and you're lucky, the plane of sharp focus will be somewhere in the scene and you can add shims as needed. If not, repeat Test 1 to see if you need to add or remove shims.

Once you get the focus you want, test using both methods from time to time (when printing so you can use the print developer as described above is easy for me).

It's well worth doing this. It's simple and it will give you confidence about your equipment and abilities, so don't be apprehensive about it. You can't damage anything and you're not doing anything irreversible.

Good luck,

Doremus

[schafphoto]

Thanks for the long and thoughtful replies. I am going to check my spring backs (i have a few) and see if they have different gaps/spacing. Maybe I inadvertently swapped a back that is set up for a fresnel sandwich.

[ic-racer]

How is your side-to-side focus. Once you have critical focus established between the GG and the film plane, you may want to ensure the front standard is parallel to the film plane, this is also an essential parameter when using short focal length lenses.

[Huub]

In addition to the extended and excelent answer of Doremus: you describe a situation where the plane of focus seems closer to the point then where you have actually tried to put it. This makes think that your ground glass probably is further from the ground glass then it should be. A fresnell in front of your ground glass or something alike could be the cause...

[Neal Chaves]

Many modern wide field lenses do gain more depth of field toward the camera from the point of focus when stopped down than behind the point of focus. I make it a practice to focus deep into the scene, but focus shift at infinity should be negligible.

[schafphoto]

This makes think that your ground glass probably is further from the ground glass then it should be. A fresnell in front of your ground glass or something alike could be the cause...

I'm confused by this? I don't have a fresnel in between the GG.

[Huub]

I just rethought my conclusion and i think i made a mistake. The plane of focus of the film is obviously a bit further form the shutter then the plane of focus of the ground glass you are using, as focussing closer means moving the lens from the film. Might be caused by a missing fresnell or some missing shims of whatever sort.

[brucetaylor]

I was unsure about the plane of focus on an 8x10 camera. I wanted to confirm the gg position. I took an old holder and a hole saw, I drilled 5, 1" holes in a pattern, 4 corners and the center. I put a developed sheet of film in the holder. I removed the gg and put the holder in place. Wide open, I focused on a focus chart and with a loupe confirmed the targets were in focus. I then removed the holder and replaced the gg. Again with the the loupe I confirmed focus. In my case no adjustment was needed, but I think it would have been relatively easy to make gg shim adjustments to match my true plane of focus as determined by the film at the actual film plane.