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Optical experts please help (focusing problem)
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PostPosted: Fri Sep 25, 2009 12:41 pm    Post subject: Optical experts please help (focusing problem) Reply with quote
This is a "call to arms" for all the optical experts in the group.
I am facing a very strange focusing problem that I am not able to figure out.
I changed the focusing screen of my 5DMarkII with a focusing screen of the Contax RTS.
Tried it with Vario-Sonnar 80-200: focuses almost perfectly (just a little tiny bit front focusing, negligible in real world photography)
Tried it with Macro-Elmarit-R 60: focuses 100% perfectly
Tried it with Summilux-R 50: it front focuses a lot

How is that possible?
I am speaking of totally manual focus - no AF chip assistance. Therefore there is nothing that can go wrong with the machine. It' my eyes, only.
How is it possible that when I mount one lens, what I see sharp in the viewfinder is also sharp in the picture, and when I mount another lens, what I see sharp in the viewfinder is out of focus?

Is there a voodoo going on, or something?


PostPosted: Fri Sep 25, 2009 1:10 pm    Post subject: Reply with quote
It's almost a relief to see someone else with this problem, which in my own case I thought was down to my ageing eyesight. I find the same thing happens with my 50mm f2 Pentax SMCA lens on both my K100D and K10D - viewfinder looks absolutely crisp but image is distinctly mis-focused, even at close range where it should be easiest to detect focus errors. With my 90mm there's no problem at all.

That's no answer for you, Orio, but at least you know you're not on your own!


PostPosted: Fri Sep 25, 2009 1:18 pm    Post subject: Reply with quote
I just made a revealing experiment.

I mounted the Summilux-R again, but this time, stopped it down to f/2.8 (the same aperture of the Macro 60mm lens wide open), and focused at that aperture.
After focusing, I open the iris to wide open (f/1.4) and shot.
Result? Perfect focus.

So, for whatever reason, that I can not imagine, is the f/1.4 aperture that screws the focusing.
Why?
May I boil forever in the cauldrons of hell if I have even a faint idea.


PostPosted: Fri Sep 25, 2009 3:26 pm    Post subject: Reply with quote
Correction to what I wrote above:

- if I focus wide open and then stop down to at least f/2.8, it works (correct focus is recovered);

- if I focus stopped down (f/2.8 or f/4), then set to wide open and shoot, it works only with focus to at least 5 meters. Close up (around 1-2 meters focus), it does not work Sad

- and of course, focusing wide open (f/1.4) and shooting wide open, never works Sad It seems that for this last task I have to rely only on live view. Sad

Something is going on, but what?


PostPosted: Fri Sep 25, 2009 7:02 pm    Post subject: Reply with quote
Same happens with my Nikkor 1.4/50 and split screen! Front focuses a lot. With 2.8/50 macro, perfect. Maybe this screens designed at least f2.8.


PostPosted: Fri Sep 25, 2009 7:16 pm    Post subject: Reply with quote
horvlas wrote:
Maybe this screens designed at least f2.8.


That's the point.

Matte screens are not at all reliable when using apertures "larger" than f2.8, not even the ones for manual cams.
The DoF is so narrow that even if it seems to be in focus, it isn't.

Orio, try to do the following:

- Focus at f1.4, if possible on a monopod, shoot
- the move the cam a tiny little bit, just as much as it still seems to be in focus, shoot again
- and again
- and again...

It might easily be that one of these shots is in focus and the othes aren't, although on the screen they all seemed to be in focus.

Can you try another very fast lens and see if it has the same problem like the Summilux?
If it doesn't, forget what I just wrote. Wink Then I cannot explain what is going on.


PostPosted: Fri Sep 25, 2009 7:48 pm    Post subject: Reply with quote
LucisPictor wrote:
horvlas wrote:
Maybe this screens designed at least f2.8.


That's the point.

Matte screens are not at all reliable when using apertures "larger" than f2.8, not even the ones for manual cams.
The DoF is so narrow that even if it seems to be in focus, it isn't.


Split-image rangefinders are rangefinders indeed. Their angle, the flange distance of the camera system and the aperture of the lens together define their effective base length, and for fast lenses, that must match the lens pretty exactly. Too narrow, and you lack accuracy, too wide and you'll meter at the astigmatic margins of the lens.


PostPosted: Fri Sep 25, 2009 8:45 pm    Post subject: Reply with quote
Thanks for the suggestion. I am sure it's not the Summilux. It is something related to the 5D Mark II. Somehow the focusing screen becomes unreliable if there is not enough stopping down. F/2.8 seems the point from where things improve. Best focus seems to be around f/5.6.
f/1.4 and f/2 are unreliable.

Again no idea why...


PostPosted: Fri Sep 25, 2009 11:39 pm    Post subject: Reply with quote
Focus screen can only work with lenses with physical aperture size not too big.

When lens physical aperture is too big for focus screen, light from edge of lens hits screen at too big angle, misses going through viewfinder. That missing light is exact light needed to focus the bigger physical aperture.

physical aperture is calculated f/F, focal length of lens divided by lens aperture.

50/1.4=36mm physical aperture; too big for focus screen.

60/2.8=21.4mm physical aperture; not too big for focus screen.

Canon Eg-S focus screen works okay with 36mm aperture because it has smaller and greater number of grooves in fresnel lens; greater number of groves transmit more of the light from edges of lens to permit better focus on that light.


PostPosted: Sat Sep 26, 2009 7:31 am    Post subject: Reply with quote
Isn't that even worse on a crop camera? That would explain a lot of focusing problems.


PostPosted: Sat Sep 26, 2009 9:49 am    Post subject: Reply with quote
LucisPictor wrote:
horvlas wrote:
Maybe this screens designed at least f2.8.


That's the point.

Matte screens are not at all reliable when using apertures "larger" than f2.8, not even the ones for manual cams.
The DoF is so narrow that even if it seems to be in focus, it isn't.



This seems a little puzzling. What about all those cameras with standard f1.8 or f1.4 lenses that are designed to be focussed wide open, only stopping down when you push the shuter?

Surely they'd make sure the screens show the correct focus?


PostPosted: Sat Sep 26, 2009 2:45 pm    Post subject: Reply with quote
kansalliskala wrote:
Isn't that even worse on a crop camera? That would explain a lot of focusing problems.


Yes! Also screen area is reduced; more difficult to focus smaller view.

Xpres wrote:
This seems a little puzzling. What about all those cameras with standard f1.8 or f1.4 lenses that are designed to be focussed wide open, only stopping down when you push the shuter?

Surely they'd make sure the screens show the correct focus?


Ah,yes, thank you, then the problem on 5DII must be focus screen needs shimming so it shows focus at f/1.4.

60/2.8 lens will still show in-focus after shim adjustment is applied.
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