Letus ultimate and ex1 focus issue

[Koravik Rakpetchmanee]

Hi,

I just got the letus ultimate for my ex1.

I can not acheive the sharp focus across entire the ground glass.

I set the iris to 4, zoom at 77 and i used peaking to help focusing but the peak color only show at the centre of the GG.

I checked on big screen and i could see that the centre of GG was sharp bit the right and left edges are soft.

If i tried adjust focusing to make edges sharper then the centre would not be sharp anymore.


Is this normal?

Thank you

[Bob Hart]

Koravik.


Did you buy your Letus Ultimate with the special EX1 achromatic dioptre fitted ( lens in back of the Letus ).

The Fujinon zoom lens on the EX1 was a problem. A special achromatic dioptre was made to fix this problem. After fitting the special achromatic dioptre, a tiny amount of soft corners remains if you look hard enough with the camcorder iris wide-open but it is usually not enough to worry about with ideal camcorder iris settings.

It seems to me you might have an adaptor which does not have the special achromatic dioptre fitted. The achromatic dioptre can be seen at 00:35 where I am removing it in this clip :-

http://exposureroom.com/members/DARA...b07bb39e69a48/


If you go to the bottom of page 1 on this thread :-

http://www.dvinfo.net/forum/alternat...g-extreme.html

you will find a Lemac chart shot with a Letus Extreme and EX1. That is about as good as it gets. There is still some corner softness. Some is related to the Nikon lens I used. Some is related to the wider view of the groundglass which I was framing at 38mm to 42mm on the zoom ring. With the Ultimate, I think you will be framing a smaller area of the groundglass so corner softness will not be as apparent with the correct achromatic dioptre. Your Ultimate's frame at zoom "77" will be tighter, approximately from the edges of the siemens rings on the Lemac chart. There, image softness is very much less, even with both camcorder and Nikon Lens irises wide-open.


This clip is a test I did with the Letus Extreme/EX1. :-

http://exposureroom.com/members/DARA...d6b3f40a2cadd/


The movie "Monsters" was shot with a Letus Ultimate on a EX3 and shows just how good the combination can be. If you rent the DVD the "extras" the director discusses it.


Maybe post a frame grab for us to look at in case there another issue as well.

[Koravik Rakpetchmanee]

hello,

i got it from ebay so i am not sure the one i got is the special achromat or not.

i compare the achromat with letus website wiki:letus-adapters - Letus Direct Wiki

and it look like the optimized achromat one.

however when it arrived the achromat lens was very loose, i even can push it out off its frame.

so far the frame of achromat looks like optimized one but the achromatic lens alone might not be the right one.

I will email the guy who sold it to me.

thank you for your response.

[Bob Hart]

If the achromat is the correct one and has come loose in the rim and fallen right out, there may be a chance that the glass element has been replaced back to front. If you are able to move the glass element out of the rim, maybe try reversing the glass element in the rim.

Be very careful doing this. The achromatic dioptre is made from optical glass which is very brittle. The glass elements appear to be retained in their rims with an optical adhesive. When they come loose, probably in violent shipping, there seem to be chips come off the glass and remain stuck to the rim.

If you leave those chips and glue there and shove the glass element back against them, the glass element could be damaged more. Broken crumbs of glass and glue should be thoroughly cleaned out of the rim. Wear eye prortection when doing this.

If you can push the glass element out and there are chips missing off it, there may be matching pieces of glass still glued to the rim.

If you study them and any chips missing from the glass element closely, you might be able to work out which way in the rim the glass element should be.

If there are chips in the glass element, paint them with flat black paint or black marker ink so that flares do not appear on the image.

To secure the glass element in the rim, do not use a hard adhesive. Instead, use a non-hardening adhesive. Use only a few small drops. I used non-silicone water cleanup white bathroom sealer.

[Koravik Rakpetchmanee]

Hi Bob,

Thank you for your help.

I followed your steps and I found that when I reversed the lens, I cannot focus on the ground glass at all.

so the right orientation should be the one that i can focus on ground glass?

cheers

[Bob Hart]

Koravik.


The right orientation is the one where you can focus on the groundglass.

As for your dioptre, it seems to me that it may not be the special dioptre for the EX1 if the corner softness is excessive. As I mentioned and I think Dennis Woods backs this up, the Fujinon lens on the EX cameras offers unique challenges. There seems to remain, a little corner softness when the centre is sharp.

With the lens on front iris closed to f16 and camcorder iris wide-open, this is the worst optical condition for the camcorder's lens. When viewing an even background through the Letus, with my EX1 in this state, I would expect to see the red peaking "grain" texture to cover an area of about one third of the image at centre.

As you pull camcorder focus through the relay focus point, this red zone should broaden, then fade in the centre and become a spreading ring of red until it runs out of the corners when you finally lose relay focus.

With the camcorder lens iris set at a more favourable f5.6 to f8 as Dennis suggests, the red peaking area should be much broader. It may still be disappeared in the extreme corners.

I will measure the special achromat that I have with a vernier and post those results. I cannot remove the glass element without risk as I have reset it with sealer in its rim.

[Koravik Rakpetchmanee]

Thank you Bob

I look forward to seeing your results.

[Bob Hart]

EX special achromat dimensions measured :-


Glass element front-to-back = 15.075mm thickness at centre.

Rim front-to-back = 14.5mm thickness.

Rim inner diameter for glass element = 56.07mm.

Depth to glass at outer edge inner face = 1.3mm.

Depth to glass at outer edge outer face = 3mm.

Therefore glass element front-to-back thickness at the outer diameter will be in the ballpark of 11mm (15.075mm - 4.3mm = 10.775mm).


There may be some very minor differences in the depth measurements due to thickness of the tail of the vernier used.

Note that the glass element centres bulge out very slightly past the rim borders on my example, therefore you should not place the achromat on any hard surface but instead on a soft cloth or the centres may become scratched.

A flat bar should not be used as a lens spanner unless a clearance is cut in the centre to avoid contact with the glass.

A chiselling technique with a screwdriver being struck or tapped on the handle, should NOT be used to loosen the achromat rim in the Letus body. The ideal tool for removing and fastening optics is a "Rolyn" spanner. They are not cheap.

"Chiselling" may damage the achromat by distorting the rim and fracturing the adhesive bond or the glass it attaches to.

The compound prism within the adaptor housing is very heavy. It is retained in the assembly by snug casework fitment and a small rubber wedge. The prism may become damaged by shockloading when the screwdriver handle is struck.

This will appear as scratching or chipping of main and sub-prisms at the points of contact on the centre spine or by major flaking at the toes of the sub-prisms where they contact the casework inside the front and rear. You will observe in my "Roiding The Extreme" clip on Exposureroom, that I added a thin plastic sleeve over the centre spine and black felt strips to protect the compound prism because my x-y adjustment modifications allow the prism to move. The Ultimate I understand has x-y axis adjustments built-in.

Additionally, shockloadings may damage the spinning groundglass in the Ultimate.

I hope the dimensions help you confirm if the glass is the special achromat for the EX. The metal rim dimensions may be different.

It is unlikely that the rectangular shaped biconvex condensor element in the front of the prism housing has become reversed. There is a donkytrap in the form of contoured faces in the retaining collar which would make a misfit immmediately obvious. I would avoid going there to have a look as there are some serious difficulties involved including dust and specks getting in and the risk of damaging the groundglass disk.

[Koravik Rakpetchmanee]

Hi,

I have not had time to measure my achromate yet.

By the way thank you for your help Bob.

Have a nice weekend!