Exakta Focus error: screen not seated ?

[Luddite Frank]

Was out to a huge antique car meet last weekend (Macungie, PA), shooting my Canon G-1 digi cam, and took along my Exakta with Kodachrome-64, in case something really special turned-up.

On my first shot with the Exakta, I focused, then took the camera away from the eye to check the aperture, in the process noting the reading on the focusing scale: scale read darn near Infinity, thought the subject was only about 20 ft away. Lens is a CZJ Flektogon 35 /2.8.

I decided to set the focus by the scale, and since my exposure was f:8 @ 1/50, I figured that DOF would cover any errors.

The next time I went to use the Exakta, I tried focusing through the finder, and again, found that my subject 20 ft away, was "focused" at Infinity.

At this point, I decided to shoot hyperfocal for the rest of the day.

The more I thought about this, the more I'm wondering if there could be focusing error caused by the plastic focusing screen not being correctly seated in the prism assembly.

When I first got this camera, about five years ago, I was shooting the 50 mm CZJ Tessar 2.8 , and was dissapointed by the "fuzzy pictures", which I eventually blamed on the supermarket processing, once I got my LTM and Summicron and was still getting fuzzy pics. Changed processors and pics improved, but I also retired the Exakta in favor of the RF at the same time.

So, my VX II came with both a waist-level finder(never used it), and a prism.

Eventually I acquired a meter-prism, which had a split-image screen. Since the prism meter is dead, I decided to swap the screen into the other prism.

I've never felt as though the screen "seats" correctly, or "locks" into place...

If this is indeed the case, would it create an error when focusing ?


( Sorry for the rambling question ! )

Thanks,

LF

 

[Ranchu]

Could the screen be upside down, or a different thickness?

 

[Dwig]

out-of-focus images with SLRs are the result of:

1. user error, misfocuesd. (lets ignore this one for now)

2. screen in the wrong position

3. mirror at the wrong angle.

With a camera like the Exakta, #2 is the likely issue if #1 isn't. A mispositoned screen can be the result of the finder not being seated correctly or the screen not attached to the finder properly. Ranchu's suggestion that it may be upside down should be tested; this could also be the reason it doesn't seem to attach properly.

Try inverting the screen to see if it fits inverted and whether focus is better or worse if it does. Also try using the waistlevel finder.

The objects being closer than your focusing indicates means that the screen is further away from the lens than it should be. The error in the screen's position would be exactly the same as the amount the lens moves when the focusing ring is turned from the correct in-focus-on-film position and the in-focus-on-screen position.

 

[Luddite Frank]

Thanks for the replies...

I will try the WL finder with the camera on a tripod and see if there's variance against the prism finders.

The focusing screen has a flattish side and a more curved side; I've been putting the screen in so that the flat-side faces the mirror... there's little "claws" that grab the screen by the edge...

Will investigate further...

( Makes one appreciate fixed-prism cameras...0

 

[Luddite Frank]

An update...

I pulled out my user VX that I suspect of having focus issues, and a "new" VX IIa, which came with its own Ihagee meter prism and WL finder, as well as a fine selection of Schneider lenses.

Started-out comparing finders and focusing screens:

The WL finders have a ground-glass screen, very thick, with the flat side facing the reflex mirror. The screen seats in spring loaded clips, with some" give" to them.

The prism finders use a plastic screen, which may or may not have a split-image center or micro-prism rings. Again, flat side towards mirror, in springy clips.

In either case, the flat, mirror-side of the screen bears against a flat boss inside the top of the camera body, insuring proper mirror-to-screen distance. To get the finder to lock in place, you have to push down, compressing the screen spring clips, until the finder latch catches the lugs on the side of the finder body.

So that's that.

I tried my 35 Flektogon on the new camera & prism, focusing on near and far objects, noting the scale reading, then swapping the prism from my user VX; VF focus image stayed consistent.

Then tried the same Flektogon on my user VX, with both prism finders, focusing on same near & far targets. Scale readings were consistent with either prism, and similar to readings on the other body.

I did notice that the lens rocked a little bit on the mounting on the user camera; tried a couple of other lenses, and they also rock slightly, so I think there is some wear in the lens mount of the VX; the VX IIa has a revised mount with a sort of "compression spring" built into the bayonet mount .

So far my conclusion are:

1) I should probably be checking this out with a lenes longer than 35mm, as the DOF of the WA lens probably minimizes/masks focus error (mechanical or user). It is a very short turn (5 to 10 deg) from "10 ft" to "Inf", whereas it is nearly 1/2 revolution of the barrel from "10ft" down to "1.5ft"(minimum focus).

2) The lens mount on my user VX needs attention. ( Sadly, the curtains on the VX-IIa are like Swiss Cheese, so I can't just swap camera bodies )

Will try the comparison thing again with my Schneider 135 and see if I learn anything more.

Regards,

Luddite Frank

 

[ferider]

Mis-aligned screens on SLRs causing focusing errors are very frequent, more so than mis-aligned RF rangefinders, IMO.

I found that like with a rangefinder, using infinity alignment (moon, horizon or star) works well as a first order check.

Counter-intuitively, on-film focusing error will be worse with wide angles, not with teles, due to shorter Depth of Focus (in contrast to longer Depth of Field).

Roland.

 

[Luddite Frank]

"Counter-intuitively, on-film focusing error will be worse with wide angles, not with teles, due to shorter Depth of Focus (in contrast to longer Depth of Field)."

Ow... now my brain is starting to hurt... 🙄

DOFocus vs DOField... back to the books for me ! 😉


😎

LF

 

[ferider]

Sorry, lots of words to say that your 35 Flektogon is better than your Schneider 135 for testing.

 

[Luddite Frank]

Sorry, lots of words to say that your 35 Flektogon is better than your Schneider 135 for testing.

Okay - thanks for the clarifcation on which lens is better for testing.


I still need to eventually understand the difference between the two DOFs !

😎

 

[lacavol]

I had a problem like that with my little 4 speed Exa. It came with a scratched screen so I replaced it with a screen that I bought with no scratches. It was always out of focus. In checking I found out that the groove on the screen was closer to the bottom of the screen and the screen was being held above the little shelf by the spring clips. I took the screen out of the spring clips and placed it on the shelf with tweezers and then pushed the prism into place. It worked fine then. Only problem is you have to 'catch' the screen if you take the prism off. Of course YMMV.