MPP Mk8 not focusing right

[Ric Bower]

Hi- any advice would be appreciated on this subject. I have what I think is an MPP Mk 8 and it consistantly focuses 1.5" further away than it should if the subject distance is 6ft. I have tested using a newspaper pinned to a board at 45 degrees to the plane of focus and marked clearly the intended plane of focus on the paper- spotting the actual focus is easy with the finely serifed text. I only use 150mm symmars and usualy wide open for location portraiture so this is obviously a real problem. I use Toyo darkslides and focus with a powerful loupe.
Thanks in advance
Ric

[John_4185]

The problem could be the film holder. Toyo... well, my confidence is not with them. Try a different film holder, a Fidelity Elite for example.

[Steve Hamley]

jj,

What experience do you have with Toyos causing you to be "unconfidant"? Have you measured them? I'm curious since I have some 8x10s.

Steve

[John_4185]

Steve: I leave it up to you. I am skeptical of ALL holders. If you are in the US, then here's the simple test: stack three brand new US Quarters inside a film holder with the slide removed. Put a straightedge across the top. If the top of the stack is not even with the filmholder's side surfaces, then the holder is not good enough. IMHO! Leave it to the US Mint to make vast quantities of cheap thingies to greater precision than certain LF film holder makers.

[Michael Gudzinowicz]

Ric,

If you use the simple lens equation 1/focal_length = 1/image_dist + 1/object_dist,
you'll find that the film plane is 0.297 mm closer to the lens than the point of focus
using the loupe.

You might want to check the following:

1. Does your high power loupe actually focus on the ground glass surface, or is focus
displaced 0.3 mm back from the surface? Some GG panels have a clear or fine center
spot to view an aerial image which can lead to an error if loupe focus isn't perfect. If
this is the case, adjust the loupe focus 0.3 mm closer to the lens.

2. Check the film holders with a micrometer depth gauge for consistancy using a sheet
of film with the holder held vertically. If the holders are consistent, compare the distance
to depth of the GG in its frame, and check that the focusing panel makes even contact
with the registration surface of the back in contact with film holders.

If the holders are consistent and within the ANSI spec, consider moving the GG 0.3 mm
forward if possible. If someone has repainted the GG frame or put light-trap fabric on the
GG frame, remove it. If not, shim the GG by 0.3 mm if the GG is inserted from the front
the frame. If those approachs don't reveal the problem and the holders are out of the
ANSI spec by 0.3 mm, you might consider replacing the holders.

3. Stop the lens down to f/11 to f/16 which will give limited depth of field but will permit
sharp (front to rear) 16x20 portraits if the focus error persists.

Mike

[Colin Carron]

Ric,

Have a look on the MPP Users Club website for info about register distances :

http://www.mppusers.freeuk.com/registers.htm

I have an MPP Mk VII and use MPP, Fidelity and Toyo slides which all seem to be OK .

[robc]

everyone works to their own level of precision. If you want to work to the highest level of precision then its not safe to rely on someone elses level of precision. The moral being; Measure the depths yourself and don't rely on quoted numbers of what something is supposed to be. Often things turn out to be different than expected. For example; is your lens really 150mm? If its a schneider APO Symmar 150 then it may be 151.5mm focal length thereby altering numbers. OK the difference is thousandths of a mm in focus depth but all these little numbers add up and when they all happen to fall in the same direction then the perceived error can become large.
I agree with mikes numbers excepting I used 151.5mm focal length which makes it 0.3mm.
However, we are relying on your meausrements being 100% precise. i.e. is the object distance really 6ft or is that to within an inch or two. And is the error exactly1.5 inches. How did you measure these accurately?

TOYO Holders:
There is nothing wrong with them. It just happens that they may not conform to someone elses idea of what the standard should be. Who cares. If you have all TOYO holders and they are all the same depth and your GG is set for that depth then they will be just as good as any other film holder. (cheaper too)

[robc]

JJ,
Steves question related to 8x10 holders. I think your method relates to 4x5 holders.

[Ric Bower]

Thank you all for the quality and speed of your replies- Here is what i have wound up doing as a stop gap since i have a job with camera and lens combo on Monday. I made a mark on the measuring newspaper where it was crisp 1.5 inches back from the point where had focused previously, the actual plane of focus established by my test prints, I then focused carefully on this point. Next I shimmed the ground glass screen using strips of 4x5 film until the previous focal mark 1.5" further back came into focus. I have just shot some test sheets to see if it has worked!
regards
Ric

[Ric Bower]

I got the direction wrong, shimming made the problem worse meaning the ground glass needs to be sunk into the frame by a fraction of a mil- is grinding paste a viable option here? would a fresnel installed lens side have the desired effect, if so is there any particular type and how would i best go about it? I think I will stop down for the job on Monday!
Thanks again
Ric

[Michael Gudzinowicz]

Ric,

I found a MPP users site that has some interesting information on the cameras.
If you aren't certain of the camera model, check the following page for
descriptions and photos: http://www.mppusers.freeuk.com/microtec.htm

Apparently, the GG was recessed 0.200" (5.08 mm) in older models, which is
more than most manufacturers would use. The ANSI standard for holder specifies
the frame surface to septum distance as 0.197". That does not include film. Kodak
Estar base films have a thickness of 0.008", so the "correct" offset is 0.189",
or 4.80 mm. That value is used on later models of MPP cameras. The registration
details are at: http://www.mppusers.freeuk.com/registers.htm

You might have an earlier model or the GG frame or back of your MPP may have
been switched with an older camera.

The difference between 5.08 mm and 4.8 mm would give a focus plane shift of
0.28 mm, which is close to the estimate of 0.3 mm (0.297 mm) based on your
observations and the equation.

If the plane of sharp focus on film is 1.5" further away from the camera than
the 6 foot distance upon which it was focused, the ground glass should be
moved 0.28 to 0.3 mm closer to the lens in the frame. If the GG is placed
into the frame from the "outside" of the frame (away from the lens), then
it can't be shimmed.

If that is the case, you might consider measuring the depth of the GG from
the surface of the frame that contacts the camera back where the holders
slide in to verify that it is the source of the error (0.200") or that
the holders aren't up to ANSI specs (if the depth is 0.189").

Toyo holders supposedly are made to meet ANSI specs. However, the ANSI spec
permits a variation of +/-0.007". Under normal circumstances, the shift
in focus when the lens is stopped down to typical operating apertures
of f/22 to f/45 is masked by the depth of field. In your application,
it is not.

Assuming the problem is due the "wrong" back resulting in a registration
error, you have a few options. If it's an older model of the MPP, consider
upgrading to a MK VIII, but measure the GG depth of the "new" camera. If
the camera's a MK VIII with an "old" back, you might be able to find
a replacement. Another option is lapping the front surface of the frame
where it contacts the back. If you do that, you need to accurately measure
the depth over the entire GG. A piece of plate glass may be used as a flat
bar or rule from which you can measure depth with a dial gauge. Plate glass
may be used as a flat for grinding with valve compound, or 400-600 grit
emory paper over the glass.

In any case, consider using smaller apertures than f/5.6, since that choice
will not provide optimal sharpness from the front to rear of a typical
portrait. If you want to limit DOF to that degree, then you'll have to
accurately set the GG, and measure every film holder selecting only those
that correspond to the GG depth when they are loaded with your chosen film
(measure the ->film<- surface to frame distance... approx 0.189 "). That
should be more accurate than the ANSI spec frame to septum distance (no film)
of 0.197". Anyway, measure EVERYTHING before implementing a strategy to
bring the camera and holder up to the level of accuracy required for your
work.

Good luck,

Mike

[robc]

if the GG is fitted from the lens side of back then you should note that the MPP web site says that the thickness of the GG should be 1.1 to 1.2mm. If your GG has been replaced with thicker glass at some point, then that could also be the source the of the problem(if fitted from the lens side of back).

www.mppusers.freeuk.com/ground.htm (http://www.mppusers.freeuk.com/ground.htm)

measuring the depth of both your film holders and the GG should confirm which way the glass needs to be adjusted and therefore which method of correction is required.

[Ric Bower]

Thank you Mike and Rob for your replies- The GG is not on the lens side of the glass so I think some delicate grinding is in order! I will then bench rest all my holders to check they are up to scratch as I do not posess an accurate depth measuring device. For my job tomorrow I will focus a couple of inches in front of the subject's eyes and stop down to f11 or f16.
Thank you for your advice
regards
Ric

[Michael Gudzinowicz]

Ric,

You can get an inexpensive dial gauge ($7) or digital caliper with a depth
probe on the handle end ($15 & $16) from Harbor Freight. The comments I've
seen on the digital caliper (more useful) are favorable.

http://www.harborfreight.com/cpi/ctaf/Category.taf?CategoryID=551&pricetype=

To get the actual film plane and GG to coincide exactly for each holder
will require a bit of work. For the approach outlined, the reproducibility
of the instrument is more important than the accuracy specs.

One approach you might want to consider is adjusting the rangefinder
focus at 6' to focus exactly on the object distance determined by the
film plane. That may be more convenient and accurate than rebuilding
the camera. You'll still need to check the holders for consistency,

Mike