At what point does aperture compensate for GG and holder errors?

[Shen45]

We all want perfect or near perfect focus but 5x4 film holders and ground glass focusing each have their own tolerances so does anyone know at what point smaller apertures cancel out these variations.

Steve

[Robert A. Zeichner]

I don't know that one could quantify that point so easily. Having experimented with this quite a bit, I would suggest that the only thing aperture (Depth of Field) might be counted upon to compensate for is our own error in focusing. If GG coincidence is off and the film plane is off in the direction that compounds the error, you'll never find an aperture small enough to compensate. 4x5 film holders were built to a nominal depth of .197" +/- .007". If the GG is aligned to .197" +/- as little as possible (maybe .001" is possible in a wooden camera, less in something like a Linhof or Sinar?) then, if the holders are within range of specified tolerance, your results should be fine. It's best to get the GG in the proper place to start with. The tolerance specified for film holders is to compensate for the fact they are made of plastic and subject to wear.

[BetterSense]

You can easily quantify this by calculating the depth-of-focus (note: this is distinct from depth of field) at the aperture of interest. As long as the depth-of-focus is larger than your GG/camera tolerances, you can say that your aperture is compensating for your camera tolerances.

[BrianShaw]

As said already, that kind of calculation can be done but it seems like an academic issue to me. Or, it would be specific to a particular camera and film holder. Even shooting at open aperture, which I do frequently, I have never suspected (meaning, "seen evidence") that manufacturing/assembly tolerances have been an issue.

[Brian C. Miller]

Old thread: Registration (http://www.largeformatphotography.info/forum/showthread.php?t=6952)

When I replaced the GG in my Super Graphic I made a lot of measurements, and no, it isn't academic. It is the difference between sharp and soft images. I had to shim my GG a bit, and it made a huge difference. The easiest place to find stable shim material is something most of us use a lot: film. Sheet film is 0.007-in, and 35mm is 0.003-in.

[Bob Salomon]

Aperture doesn't. Most people focus wide open and then stop down so unless your lens shifts focus (like an Imagon) you had best be dead on wide open. With a lens that shifts focus with aperture you have to focus at shooting aperture which can sure make bracketing a challange if you are changing aperture and have little time to re-focus.

Linhof has used a zeroed groundglass system for decades so there should be no reason why others can't be just as accurate, if desired. For many years Linhof actually put a serial number on their 8x10 camera backs which had to be supplied when buying an 810 to 45 reducing back to guarantee the same film plane for both 45 and 810 on the same camera! The reason for this was so 45 Polaroids and 45 test shots would have the same focus as 810 film. Since people stopped wanting that feature Linhof stopped doing this about 20 years ago.

[Struan Gray]

f# = tolerance / circle of confusion

Proof: similar triangles formed by tolerance/coc and aperture/focal length.

For a coc of 0.1 mm (very forgiving by most people's standards) and a film holder tolerance of 0.2 mm (8 thou, but who's counting) apertures of f2 and smaller will cover the slop.

Summary: most photographers don't need to worry about it.

[ic-racer]

At f16 your focus spread is about a millimeter, so that will cover a condition of combined intolerance of plus-or-minus 0.5 millimeters. That should be good even for home made cameras and film holders :)

What will kill you, though, is when the film shifts during an exposure. That won't be minimized by a small aperture. Only a rapid shutter duration will aleviate that one.

[Robert Fisher]

Guys, I just replaced the GG on my 5x8 Chamonix and notice that the new "boro" GG is just a fraction thicker than the "stock" Chamonix GG. To my feeble mind it should not matter because focus is made on the surface of the GG that is closest to the lens. However I am probably wrong so please advise.

Thanks

[BetterSense]

You can always test for gross errors by shooting an angled-away yardstick at close range...focus on the 1.5 foot mark at wide open aperture and then when you develop your film see if the 1.5 foot mark is in focus. To check the whole image area, you can do a similar test with a resolution chart. This will tell you if your camera back is on wrong, at least.

[BrianShaw]

...

When I replaced the GG in my Super Graphic I made a lot of measurements, and no, it isn't academic. It is the difference between sharp and soft images.

...

This may be a particularly problematic camera (as are some other Graphics), with an additional consideration. There is the additional variable of thickness of the fresnel.

[Bob Salomon]

Guys, I just replaced the GG on my 5x8 Chamonix and notice that the new "boro" GG is just a fraction thicker than the "stock" Chamonix GG. To my feeble mind it should not matter because focus is made on the surface of the GG that is closest to the lens. However I am probably wrong so please advise.

Thanks

If the ground surface of the original ground glass and the ground surface of the new gg are resting on the same shims then there is no focus shift. If you reset the positioned the position of the shims then you have shifted focus. If you had a Frenel screen under the gg and you replaced it with a thicker fresnel or a thinner one then you may have shifted focus.

If all you did was remove the gg hold down clips, removed the original gg and placed the new gg in the same position and replaced the clips then you did not shift the focus.

What did you do when you did the replacement?

[Bob Salomon]

[dupe

[Robert Fisher]

Thanks Bob Solomon! I simply replaced the Chamonix GG with the Steve Hopf "boro" glass - no shims were involved. OK, now back to shooting!

[Ivan J. Eberle]

In the situation with the Super Graphic (referenced above) it could/would change the focus, if the original focusing panel was an Ektalite with the fresnel sandwiched in front of the ground glass (i.e. toward the lens)-- and the new replacement GG was simply a piece of ground glass mounted in the same plane. (Very common error).

[Armin Seeholzer]

At f16 your focus spread is about a millimeter, so that will cover a condition of combined intolerance of plus-or-minus 0.5 millimeters. That should be good even for home made cameras and film holders

Sorry it is not thad easy, because at a longer lens you get more deep of focus behind the lens then with a 47 XL it is very criticaly even at f16 or f22!
I really know it from the past!

Cheers Armin

[Brian C. Miller]

Yes, Ivan, you are right about the GG. When I bought the camera it had a replacement GG, which had no fresnel, and the focus was quite definitely off. Then I replaced it with a fresnel GG, and checked focus again. The focus was better, but it still needed adjustment. I spent lots of Polaroid shots on a ruler, and time measuring and shimming the GG. Now the focus is spot-on! :)

[Bob Salomon]

Yes, Ivan, you are right about the GG. When I bought the camera it had a replacement GG, which had no fresnel, and the focus was quite definitely off. Then I replaced it with a fresnel GG, and checked focus again. The focus was better, but it still needed adjustment. I spent lots of Polaroid shots on a ruler, and time measuring and shimming the GG. Now the focus is spot-on! :)

Polaroid's film plane is not the best thing to rely on unless you will only shoot Polaroids. And that is becoming ever more doubtful today.

[BrianShaw]

...

When I bought the camera it had a replacement GG, which had no fresnel, and the focus was quite definitely off. Then I replaced it with a fresnel GG, and checked focus again. The focus was better, but it still needed adjustment.

...

Had you replaced with a gen-u-ine Ektalite it would have worked as a drop-in, no matter which GG was used. The OEM Ektalite, however, is getting VERY hard to find... hence the need for measuring, shimming, etc.

At least that was my experience and the wisdom imparted by WD Service, Inc., which was the Western Division Graflex repair depot.

[Brian C. Miller]

Hi Bob! If you read the original thread, you'll see all of the measuring I did between all of my film holders. Both Fidelity and Polaroid holders have the same depth, and Graflex is 0.005in shallower. Now, I would agree with you that it would be an issue if I had been using the 550 pack holder instead of the 545, which is what I used during most of the calibration.

The final shots were made using Fidelity holders and Techpan. Yeah, it is something special to see bicycle spokes at two blocks away!

[Thalmees]

Hi,
A problem related purely to depth of focus. As I understand, the question can be rephrased as:
Is the film plane exactly set/located at the Focal Plane(GG plane in LF cameras) ?
To test for it, I think, you have to choose a scenario that gives the smallest possible depth of focus.
Smallest possible depth of focus is attainable by using:
1. Wider aperture,
2. Far distance subject,
3. Shorter Focal length; both 2 & 3 require shortest possible bellows extension; and,
4. Smaller COC.
So, your shortest focal length lens, fully opened aperture and focused at infinity, will do the job.
Any other scenario will be lesser critical, I think, and will not end in a better magnified results.
Generally, If a carefully focused image on GG by a short focal length lens for a subject at infinity, resulted in a sharp photo on the negative or whatever material, I think I can conclude that the film plane as well as the GG plane are located in the Focal Plane.
In this scenario, the focus plane(on the subject area) should be set/located, at, before or beyond infinity. Other scenarios, and in case of wrongly set film plane, the focus plane will be only displaced within the subject area, and will result in a situation that can be masked by a wider depth of focus regardless of the size of aperture used. With most subjects, this situation, can be corrected by using narrower aperture.
Somebody may correct me please, if I'm wrong here.
If, on the other hand, the test scenario was enough to reveal a wrongly placed film plane(unsharp photo), a corrective measures may be taken to correct position/location of the GG or its frame to coincide with the film plane.
Somebody may correct me please, if I'm wrong here.

[Bob Salomon]

But then you have to take into consideration the amount of sag in a sheet of large format film. Set the plane of focus exactly at the 0 point and the sag may make you have less then perfect focus on the point focused on.

[BetterSense]

Somebody may correct me please, if I'm wrong here.

You are quite wrong. DOF is a function of magnification, aperture, and CoC. To obtain minimum DOF you would use a LONG lens, focused CLOSELY, with a LARGE aperture, and a SMALL CoC.

[Thalmees]

You are quite wrong. DOF is a function of magnification, aperture, and CoC. To obtain minimum DOF you would use a LONG lens, focused CLOSELY, with a LARGE aperture, and a SMALL CoC.
Hi BetterSense,
Appreciate your valuable comment, though doubt its validity.
With just little more better sense, may you reconsider what I've written under DEPTH of FOCUS not DEPTH of FIELD ?
Thanks too much.

[BetterSense]

My comment is valid whether you are talking about depth of field or depth of focus. When one is minimized, the other is also minimized.

[Thalmees]

My comment is valid whether you are talking about depth of field or depth of focus. When one is minimized, the other is also minimized.
Hi BetterSense,
I did not get exactly what do you mean ? and think a point is missed by either one of us.
Really do not wish to insist on something that will show lastly as a genuine mistake. But it looks to me that you may have to reconsider similarities and differences between depth of focus & depth of field.
Factors that increases DOField:
1. Smaller aperture,
2. Lower Magnification(whether attained by wide angle lens or far subject), and
3. Larger COC.
Factors that increases DOFocus:
1. Smaller aperture,
2. Near distance subject,
3. Longer Focal length; both 2 & 3 require longer bellows extension; and,
4. Larger COC.
Now, I do not know really where is the mistake in my post ? Appreciate your(and others) help. Thanks in advance.
Again sorry(to all readers here) for any mistake I may have made in my post, if there is any.

[BetterSense]

Factors that increases DOField:
1. Smaller aperture,
2. Lower Magnification(whether attained by wide angle lens or far subject), and
3. Larger COC.

I agree completely.



Factors that increases DOFocus:
1. Smaller aperture,
2. Near distance subject,
3. Longer Focal length; both 2 & 3 require longer bellows extension; and,
4. Larger COC.

I agree on 1 and 4, but I'm curious as to why you think 2 and 3. Why would the conditions for minimizing depth of field and depth of focus be different? They are the same thing, just on opposite sides of the lens. When your depth of field is deep, your depth of focus is also deep. When your depth of field is shallow, your depth of focus is shallow. I don't understand why you think something that increases depth of field decreases depth of focus. That doesn't make any physical sense.

[ic-racer]

Sorry it is not thad easy, because at a longer lens you get more deep of focus behind the lens then with a 47 XL it is very criticaly even at f16 or f22!
I really know it from the past!

Cheers Armin

Actually it is that easy ;)

On the LF Home Page. Article: how to select the f-stop.
Read down to the paragraph as follows:


To summarize, if D is the focus spread expressed in millimeters, then the optimal f-stop which yields the sharpest possible image at the depth of field limits is N = sqrt(375 D). This works regardless of focal lengths, formats, and movements. The resulting resolution at the limits of depth of field (ie for your far and near points) cannot be improved in anyway and determine the maximum possible enlargment. Here are some tabulated values by whole fstops:

[Thalmees]

Hi BetterSense,
Now things made clear.
My 1st post is not wrong, as you've said.
My later points has the correct physical sense, not as you've mentioned.
Its your understanding of DOFocus Vs DOField that should be corrected.
The explanation of points 2&3 is made clear in my 1st post, to have at least a deserved level of physical sense.
The longer bellows extension(associated with longer focal length and near subject distance) means longer focused cones of light that remains in focus with a longer displacement(example: forward or backward) of the focal plane, compared with shorter focused cones of light(that associated with shorter focal length and far distance subjects) that change more abruptly at even short displacement of the focal plane.
Now, Its your duty to review the subject before adding any more comments.
Thanks too much.

You are quite wrong. DOF is a function of magnification, aperture, and CoC. To obtain minimum DOF you would use a LONG lens, focused CLOSELY, with a LARGE aperture, and a SMALL CoC.

I agree completely.
I agree on 1 and 4, but I'm curious as to why you think 2 and 3. Why would the conditions for minimizing depth of field and depth of focus be different? They are the same thing, just on opposite sides of the lens. When your depth of field is deep, your depth of focus is also deep. When your depth of field is shallow, your depth of focus is shallow. I don't understand why you think something that increases depth of field decreases depth of focus. That doesn't make any physical sense.

[BetterSense]

Thanks for your most recent explanation. Now I really do have to go study this some more.

[engl]

Im wondering the same thing, as I have had a couple of 4x5 images with a 125mm lens at F11 and F16 that are not as sharp as I had hoped. I also have some that are extremely sharp at these apertures, so it is not a lens issue. The camera used to take those shots had so many loose variables, and shot by an inexperienced me, so Im not going to worry too much about those shots. Still, Id like to be in control of final sharpness in the future (with another camera).

Operator focus error, camera movement, insufficient camera rigidity when inserting the holder, film shift during exposure, GG placement error, holder distance error, film bulge in the holder could all compromise control of the plane of focus, most probably affecting bigger apertures and faster lenses more. Getting consistent sharpness requires minimizing all those factors, so knowing the sharpness impact of say 0.2mm difference between GG and film distance would make it easier to prioritize which area to improve. Spending an eternity obsessing over finding holders within 0.01mm of spec might just not be worthwhile, I dont know.

Oh, and finding something worth photographing is important, too... :)

[engl]

most probably affecting bigger apertures and faster lenses more.

I meant bigger apertures and wider lenses.

[rdenney]

My comment is valid whether you are talking about depth of field or depth of focus. When one is minimized, the other is also minimized.

In that case, why is the movement of a 12" lens from, say, 1:1 to infinity focus 12 inches, while the movement of a 2" lens from 1:1 to infinity is only two inches?

Seems to me that for a given change in focus distance on the front side the lens, there is a smaller change of distance from lens to film with shorter lenses. Short lenses therefore have a greater demand for precision in the focus distance, and that affects the position of the film plane as much as the lens.

Rick "thinking this explains why short lenses used for digital backs on view cameras need those helical focus mounts" Denney

[Thalmees]

But then you have to take into consideration the amount of sag in a sheet of large format film. Set the plane of focus exactly at the 0 point and the sag may make you have less then perfect focus on the point focused on.
Agree Bob.
Sorry, did not notice your post.
I just imagined my self in the OP place and start thinking. Thanks for your valid point.
The scenario I suggest will result in two outcomes initially: Whither the carefully focused(on the GG) image at infinity, resulted in a sharp photo or NOT.
In case a sharp photo was the result, no need to consider any sag in the film sheet(at least in this stage).
In case un-sharp photo was the result(depends where is the focus in the resulted photo ?), sag in the film sheet should be accounted for and appropriate measures should be taken for continuation of the test on accurate/consistent bases.
Depending on how critical depth of focus on the reality, film sag should not produce a clear focus before infinity in the field area of the resulted photo. Sag is always toward the lens.
To abolish the effect of film sag, I think, simple measures like that adopted for long exposure/ fluctuant climate, should be taken(sticking small piece of thin double adhesive tape at the middle of the surface of film holder at which sheet film suppose to settle on).
In the reality, things should differ plus or minus from the theoretical view.
Thanks again Bob.