Ft. on lens and not matching reality

C

ClayM

Member
I just recently got a Brevis to use with a HVX. I setup the adapter, set the back focus by focusing on the grain just as in the FreshDV tutorial, and mounted a 50mm Nikon lens. I was using an external monitor so I could see the grain clear as day. I then measured out 10 feet to my subject with a tape measurer, set the Nikon lens to 10 feet, and my subject was completely out of focus. I tried different distances, same result. Switched to a 28mm Nikon, same results. I'm able to get focus, just not at the correct distances as indicated on the lens.

I believe the GG was frozen at this time during the testing, so I'm not sure if that had something to do with it. I'm going to be testing again shortly but any insight into what may be causing the disjunct would be awesome.
 
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You may have been confused. A functioning adapter requires two focusing adjustments.

1. Back Focus: Also known as flange focal distance. The distance between the back of your 35mm lens and the ground glass. This distance must be perfect in order for the distances on the lenses to focus correctly, and judging by your description, your back focus must be off quite a bit.

2. Ground Glass Focus: Focusing the camera on the ground glass. You have to focus your camera such that the ground glass which is a few inches in front of the lens element appears in focus perfectly. I think this is the focus that you adjusted, as opposed to back focus.
 
There are two backfocuses. From the back of the lens to the GG, and from the GG to the camcorder. The first is off and you can adjust this by setting your lens to infinity and adjusting the Brevis micro-collimation ring until the image appears sharpest.
 
Chris Knight said:
You may have been confused. A functioning adapter requires two focusing adjustments.

1. Back Focus: Also known as flange focal distance. The distance between the back of your 35mm lens and the ground glass. This distance must be perfect in order for the distances on the lenses to focus correctly, and judging by your description, your back focus must be off quite a bit.

2. Ground Glass Focus: Focusing the camera on the ground glass. You have to focus your camera such that the ground glass which is a few inches in front of the lens element appears in focus perfectly. I think this is the focus that you adjusted, as opposed to back focus.
Click to expand...

Thanks for clearing that up. I only set the ground glass focus then.
 
Verify that your lens is properly focusing when set at infinity (perhaps the easiest way is to shoot the moon). If you focus to infinity and an object is in sharp focus an extremely far way away, you've probably got your lens-to-adapter distance set properly. If you can't get proper infinity focus then the distance between your lens and the ground glass is not set properly; if that's the case you can't trust any of the distance marks.
 
marco0782 said:
There are two backfocuses. From the back of the lens to the GG, and from the GG to the camcorder. The first is off and you can adjust this by setting your lens to infinity and adjusting the Brevis micro-collimation ring until the image appears sharpest.
Click to expand...


So when adjusting the lens to the GG, do i just rotate the micro-collimation ring, or do I actually adjust it more in an in and out fashion if the adapter is oriented horizontally?
 
Rotating the micro-collimation ring will let the lens mount sit further in or out by very small increments. Keep adjusting until you have a sharp image at infinity.


Marco
 
marco0782 said:
Rotating the micro-collimation ring will let the lens mount sit further in or out by very small increments. Keep adjusting until you have a sharp image at infinity.


Marco
Click to expand...


Are you supposed to do this for every lens, or just once to dial it in?
 
just once.
Otherwise one would have to adjust the film plane in still cameras for each individual lens. Once the Flange distance is set, its set.

The best way to test this is as stated, set your focus to infinity. Use the most telephoto lens oyu haev, and tighten the screws so much, so that they hold the mount in place, but you can still wiggle it to adjust the distance. twist it ever so gently until you find the sharpest image with your lens set to infinity. Then for more tests, i check the macro end, i.e. i grab my widest lens, set it to 1 foot, and make sure it matches up.
 
Can you adjust the backfocus on the SGpro this way? I've never touched mine, only done the ground glass part of the adjustment. I don't use the foot scales on my lens when focusing, so I wouldn't have noticed if backfocus was off. I guess I'll have to try focusing at infinity on the moon and see what comes out.
 
We're about to start shooting (Monday) an eight-part series on adapter setup, imaging tuning, rails configuration, troubleshooting etc. Micro-collimation is the process that we're referring to here, and it was a very important addition to the Brevis in terms of rapid setup/fine tuning. What's missing from our user materials is a formal section on this topic..and we'll show you how to get it perfect in 2-3 minutes :-)
 
Dennis Wood said:
We're about to start shooting (Monday) an eight-part series on adapter setup, imaging tuning, rails configuration, troubleshooting etc. Micro-collimation is the process that we're referring to here, and it was a very important addition to the Brevis in terms of rapid setup/fine tuning. What's missing from our user materials is a formal section on this topic..and we'll show you how to get it perfect in 2-3 minutes :-)
Click to expand...

I don't have a Brevis but that is pretty sweet.
 
Back focus problem!!

Back focus problem!!

Which back-focus should i do first the lens to GG or the GG to camera?
Can someone take little of their valuable time and explain once and all
to correctly setup the backfocus for my LetusFE and Canon A1?

I did and read every possible thing to get the sharpest possible image on my camera, hence it always looked soft to me!:embarasse
 
Take the 35mm/Nikon lens off your adapter. Point the adapter at a light source, and put your A1 in autofocus mode. It should sharply focus on the ground glass, you should see a textured grainy image in very sharp focus. Switch to manual focus mode now, and you should have perfect focus on the ground glass.

Now put the Nikon/35mm lens back on, and point the camera at something that's an extremely, extremely far way away -- like the moon. Adjust for proper exposure, and set the focus ring to "infinity". Is it in focus? If so, you're done. If it's not in focus, adjust the lens mount in or out until the moon is in perfect focus, and lock it down.
 
Back focus problem!!

Back focus problem!!

Thanks infinity to you Barry!! As i told you before i did move back to my home country to fulfill my dream!! My first short-feature is going to happen soon. Wish me good luck!! I will never forget the amount of help and learning resourses i have recieved from this site!! Thanks again.:smile:
 
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