GG Focusing Tip: For Flipped Users

K

Kholi

Section Moderator
I'm seein'/Hearin' this a lot: I zoomed in and focused on the ground glass and then zoomed out.

DO NOT DO THIS!!

You need to find your frame with the camera, then focus on the grain. A great way to do this is to use Auto Focus on the ground glass til it finds it properly, then turn your auto focus off.

For Example: HVX200

-Zoom into 27.3mm (Z82 about)
-Gaff down your rings (Focus mainly)
-Point your Camera toward a White wall or until you see visible grain with the GG off.
-Auto Focus by switching to AUTO FOCUS then, once the camera finds it's tack sharp mark ( for 27.3mm it's 5.25/38 borderline or 5.12/25 borederline, can't remember), turn Auto Focus off.

Now you're focused on the ground glass. With the flip units, the zoom will change critical GG focus. At about 50mm It's 5.12, at 27.3 it's 5.25/.38

Do NOT zoom in and focus. That is the improper fashion of doing so. If I'm wrong, someone correct me.
 
You are correct. Focus with varies with the zoom amount so you need to correct the focus point if you change your zoom position. An adapter achromat is not like a zoom through wide angle adapter!

However, auto focus on the grain wil probably only get you best center focus (leaving soft corners) and the best center to corner compromise is probably a little past that. Test and get used to the correction, then chose your method for best focus.

Lenny Levy
 
Lenilenapi said:
You are correct. Focus with varies with the zoom amount so you need to correct the focus point if you change your zoom position. An adapter achromat is not like a zoom through wide angle adapter!

However, auto focus on the grain wil probably only get you best center focus (leaving soft corners) and the best center to corner compromise is probably a little past that. Test and get used to the correction, then chose your method for best focus.

Lenny Levy
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Lenny you should do a test of this. Cause I finally got my Letus back and I'll be damned if you're not right about this.

Auto Focusing will just focus the center, I nudged the focus a little while using a Filled DVD shelf and 37" HD LCD and in doing so I found a COMPLETELY acceptable value (for me) the focus area on my Letus Extreme.

Good info, man. Seriously. Do a video test showing the difference between Auto Focus and manually finding the sharpest point.

The best method I've found so far was to auto focus so that I can get the center as best as possible, then compensate very very carefully.

Thanks for the tip!
 
Another tip, point your camera at a bright light and stop down on the taking lens to f/16 (or as far as you can). That will give you the clearest grain on the GG with which to check your backfocus.


Marco

P.S. I agree about zooming. Since the minimum focus distance changes depending on how far you're zoomed in, I have noticed the HVX change focus when zooming in and out and focused at close distances.
 
Are you guys positive about this non zoom-thru achromat business? Lately I would reframe slightly (zoom say betw 75-82) without noticing any focus shift. On the Brevis.

EDIT: using the non-flip version
 
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nudge said:
ah, thanks kholi, thats the end to that i guess.
Click to expand...

Actually, let me back up here. That's how it was on the M2. I've heard other users say the same for the Brevis.

The only exception seemed to be the SGpro? That's because it had an extension tube.

SO... if you TUBE the Brevis then you can do it I believe.
 
Another tip:

Make sure your camera aperture is fully open, so that you have the shortest DoF when selecting the MF value.

Another tip:

Then make a plug in focus adapter, so that this MF value is always there and un-nudgeable. Very easy - find the thread. It's a must.

Lucas
 
Just tested the non-flip brevis to see if I could zoom through the achromat without focus shifts. Used a Nikon AIS 85mm at f4 and at about 90cm (nearest focus) while the iris dial was wide open on the HVX200. Also used a back focus plug so that's a constant.

Here's stills of what I got:

Z72
Z74
Z76
Z78
Z80

The range isnt that wide, but i was just testing to see if while shooting i could zoom in or out to reframe. Anything below Z72 would reveal the GG and anything above 80 would be out of the focal range. Didnt notice shifts in focus afterall (in the centre at least).

EDIT: Edges seem soft but that's also because the back focus was set in the centre and the iris on the HVX was open full. From what i saw centre focus was right at both extremes. Though the test was for relative focus, maybe it's best to stay as wide on the GG as possible when fully open on the HVX.
 
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One thing I do is to point the camera at a TV or computer screen. Then using a combination of focusing on the GG and lens focusing you see a real sharp image due to TV's horizontal lines or a computers screen pixels.

If you are completely flat to the screen and the screen fills your full frame then you can see the edge to egde sharpness.

Fully open of course on the 35mm lens.

Thanks
Richard
 
Backfocus plug after you find your numbers, of course. Bob, can you make Back Focus plugs for each measure?

Say I wanted better center sharpness for one shot, then needed edge to edge for another? Silly that we would have to do that but eh, you know.

Is that possible?
 
kholi the backfocus is adjusted my turning the screw on the potentiometer. 15 turns on the pot correspond to about 6 MF values on the HVX so you got plenty of range. Between edge and centre the value stays on the same MF number. Either keep a screwdriver handy or you could build two and preset the values. I had built one and still testing it cause i found it suddenly lost the mark while on a shoot. Bob did you ever get that?
 
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