relationship between shooting very wide open (2.8) and film not being held flat?

[joshdaskew]

Hi, I have recently purchased a 150mm Xenotar 2.8 for my Chamonix 45N-1 and was just wondering what the relationship is between the film being held perfectly flat in the dark slide and shooting wider open than f5.6? So after some tests, I have had great difficulty in getting really sharp pictures ( portraits ) at f2.8, f3.5 and even f4, as the depth of field is really limited. This is basically the whole reason I bought this lens, to have a very sharp drop of focus. The original screen that was provided with the Chamonix ( a fresnel type arrangement ) was absolutely hopeless and would pull focus a half foot/foot in front of the what was focused on the groundglass. When I contacted Hugo about this, he said that was a problem with those fresnel type screens and shooting really wide open. He then sent me a "normal" groundglass which really seemed to fix the problem... That is with stationary objects mind you. I recently had a friend enquire about a Maxwell screen for this camera and Bill Maxwell wrote back sayingthat his screen would help with this problem but there was still the concern of the piece of film in the darkslide moving around and there being a tolerance of plus/minus 0.25 mm mm for it to affect sharpness. Apparently his screens are meant to be excellent ( although they cost about half as much as the whole camera ) but I was just wondering if someone could explain how stopping down helps this problem? I would have thought that if the film wasout of the "plus/minus 0.25 mm" range , then it would be affected regardless of what aperture was used to take the shot. Any insights would be greatly appreciated. Kind Regards Josh

[Joanna Carter]

... I was just wondering if someone could explain how stopping down helps this problem? I would have thought that if the film wasout of the "plus/minus 0.25 mm" range , then it would be affected regardless of what aperture was used to take the shot
Depth of field is increased with smaller apertures and this translates to greater tolerance in the accuracy of the film plane.

[Bruce Watson]

The concept you are looking for is called depth of focus. (http://en.wikipedia.org/wiki/Depth_of_focus) Depth of field occurs on the subject side of the lens, while depth of focus occurs on the film side.

Both are effected by f/stop. The bigger your aperture the less depth of field you have with your subject, and the less depth of focus you have with the film plane.

[Bob Salomon]

Also the shorter the focal length the more critical the depth of focus is.

[JohnGC]

Hi Josh,

Film doesn't usually move or warp within a 4x5 holder. This is more of a concern for larger films.

Are you using a very thin, off brand film?
Is the holder old, and worn down wood? Could this holder be warped, or does the problem exist with more than one holder?

Incorrect gg thickness can also cause focus differences between film and gg, but this should show up at all but the smallest apertures.

Film plain problems could be noticed by knowing you have good focus on the gg, then having the lack of focus on the developed film. Good clean focus at such wide apertures is real difficult. Are you getting a clean focus at the point on the subject that you want? I'm not trying to question your skills, but you mention having trouble focusing, and film movement really is not a very likely probability.

[Nathan Potter]

You can just compute the depth of focus for a given focal length lens using the relationship: D=2CNi/f where D = depth of field, C = circle of confusion diameter (choose how critical you want it), N = f no. chosen, i = lens to film distance, f = lens focal length. For your most critical application, for example, a 150mm lens at f/2.8 and requiring a COC of 100um (0.1mm) we have D (in mm.) = (2)(0.1)(2.8)(150)/150. Since i = f (no magnification) the 150s' cancel and we have D = .56 mm. That's not much DOF ( about 8 human hair thicknesses). You can see from the formula as C and N increases your DOF will increase linearly. Note that a 100 um COC is equivalent to only 5 lp/mm. and your lens can do much better than that.

Nate Potter, Chicago IL.

[Bob Salomon]

"Incorrect gg thickness can also cause focus differences between film and gg, but this should show up at all but the smallest apertures."

That would not necessarily be so. Most 45 cameras have the posts, frame or support for the gg made so that the gg is positioned ground side down so the thickness of the gg is of no concern as long as it fits under the gg hold down clips. An exception would be the Boss type screen which is two pieces of glass with a wax layer between them. The image plane on this screen is the wax layer so the wax layer must be positioned where the ground side of a gg would normally be placed. Otherwise you would be out of critical focus by the thickness of the bottom layer of glass.

Should a gg be accidently positioned ground side up (closest to the eye) then you could have focus problems due to the thickness of the gg.

One exception, where the wrong thickness of a gg will be a problem is with 6x9cm Linhof Technika and TK backs. George Tice broke the gg on his 69 TK while shooting in Maine. A shop up there said that they could replace the gg for him so he bought a generic gg from him and had him install it. The installer had no idea how to correctly adjust the gg in a 69cm Linhof back so installed it improperly. Every shot George took on that trip was out of focus. When he discovered that George decided to play safe and rather then run the risk of having this happen he had Linhof Service replace and readjust the gg and then bought a spare Linhof 69 gg back.

[JohnGC]

I stand corrected, thanks, Bob! Your right, I should have said incorrectly installed gg...

[goodfood]

Another point is your model move forward or backward after you focus and shoot.

[Ivan J. Eberle]

If you're using a loupe on the GG and getting the sharpness across the field that you're not getting on film--and you know for certain that you've got an accurate GG depth, gate and film holder-- then you've got a problem with film flatness and not depth of focus.

Thinking out loud here, could the problem be related to warm August weather and the film "popping" as a 35mm slide often does in a projector?

There are vacuum back regimes that ranging from very inexpensive DIY approaches (like http://www.deadbread.com/crumbs/vac.html) to very expensive. Also, Grafmatics are purported to hold the film flatter than standard double darkslide holders.

[joshdaskew]

Thanks so much to everyone for your responses! Greatly appreciated!!Good to learn about all these things. I think that one of the main problems I have is just being somewhat overambitious with what I am trying to achieve. While I like the look of shooting wide open, it doesn't produce the sharpest subjects ( a combination of very limited depth of field, subject movement and the lens not being so sharp wide open ) so it may be a matter of stopping down a little bit... Also, I have noticed when testing this lens ( and probably all lenses ) that the area slightly in front of my focus point has greater depth than the equivalent area behind my focus point.. Does anyone ever focus slightly in front of their subject ( if shooting people ) to get better focus? Or is that just crazy talk?

In response to Nathan, I didn't quite understand your last point when you talk about 5lp/mm and the lens being able to do better than that... Is that purely referring to the Xenotar achieving better results when stopped down from 2.8?

Ok, thanks again for all your responses. Anyone had any experience with the Maxwell screen for helping overcome such a problem.

Thanks again. Much appreciated.