Focus help / fresnel installation

[Larry H-L]

Hi, I need a little help.

I have a Shen 4x5 XPO. I just installed an "inside the camera" fresnel lens. The Shen has a lip under the ground glass for holding the fresnel.

So, I pulled out the ground glass and put down some layers of tape to shim the ground glass out approximately 1/3 the thickness of the ground glass. Several threads suggested that this was the procedure, to shim "out" the ground glass a bit.

Loaded up some paper negatives and did a focus test. Used a 210mm f4.5 Fujinar.

What do you think? I this close enough? Looks slightly behind the "6" inch mark where I focused. I refocused between each shot to eliminate user error (60-year-old eyes and progressive lenses).

Should I shim a bit more, or is this OK considering that I normally don't shoot this close?

Thanks,

L

[Steve Barber]

Hard to tell. This test target might work better for you than the ruler.

http://www.ragarecords.com/photo/ima...testtarget.tif

[Doremus Scudder]

It's hard to tell from the images you posted, but it looks pretty close.

I would be inspecting the negative directly with extreme magnification. If you don't have a microscope, use the most powerful loupe you have.

Don't settle for second best, if you think you need another shim, install one, take a few test shots and examine them too. Maybe you'll decide to remove it, or "split the difference," but you'll never know you've got it right till you've gone too far.

Remember, DoF is greater behind the focused plane than in front, so expect a smaller area of the ruler to be in focus in front of the mark you focused on than behind.

FWIW, when I do these kind of tests, I usually do it while set up for printing. The negs then just go into the Dektol for 3-4 minutes, into the stop, then the fix and I examine them when wet. Then they hit the trash.

Another very good check to see if your Fresnel is properly installed is to shoot a distant, detailed horizon wide open. You'll be focused at infinity, and any "overfocusing" is readily apparent.

Best,

Doremus

[Larry H-L]

Thanks for the replies, the infinity idea is a good one.

But now I am a bit confused. I'm thinking that what I need is LESS shim?

The resulting images show that the true focus point is about 1/2 - 3/4 inch behind where I focused. (Shot wide open 3 ft. from still life set up).

So, I am thinking that the ground glass needs to go closer to the lens?

My thinking: the camera is truly focused on a spot at 6.75 inches on the ruler, the prints show this. The ground glass is showing focus at 6 inches.

So, I need to move the ground glass into a position that SHOWS focus at 6.75 inches (matching the true focus point), which would mean moving the ground glass closer to the front standard (similar effect to racking in, moving focus slightly toward infinity).

Is this correct thinking? Wow, I need more coffee.

[Doremus Scudder]

Thanks for the replies, the infinity idea is a good one.

But now I am a bit confused. I'm thinking that what I need is LESS shim?

The resulting images show that the true focus point is about 1/2 - 3/4 inch behind where I focused. (Shot wide open 3 ft. from still life set up).

So, I am thinking that the ground glass needs to go closer to the lens?

My thinking: the camera is truly focused on a spot at 6.75 inches on the ruler, the prints show this. The ground glass is showing focus at 6 inches.

So, I need to move the ground glass into a position that SHOWS focus at 6.75 inches (matching the true focus point), which would mean moving the ground glass closer to the front standard (similar effect to racking in, moving focus slightly toward infinity).

Is this correct thinking? Wow, I need more coffee.

Let's think this through... First, your gg focuses closer than your film, so that means the camera is longer with the gg than with the film. Which means that to get the two together (and since you can only move the gg) you need to move the gg closer to the lens. Which means... Less shim.

So, take some out and test again. (Getting this exactly right can be time-consuming, but it's worth it in the long run).

And, do check the infinity focus wide open.

To help your focusing, focus with a rather powerful loupe. You should be able to use a 10x (I do).

Best,

Doremus

[Steve Smith]

So, I am thinking that the ground glass needs to go closer to the lens?

My thinking: the camera is truly focused on a spot at 6.75 inches on the ruler, the prints show this. The ground glass is showing focus at 6 inches.

This means that the ground glass is further away from the lens than the film is so I think you are right that you have shimmed it out a little too far.


Steve.

[Larry H-L]

Good to know that I am thinking somewhat clearly...

This should be easier than adding more shim, I just need to pull out one layer of tape and try again.

Many, many, thanks guys!

[Steve Barber]

I still think it is easier when using the target I linked to, earlier. It eliminates almost all of the possible variables and any subjective judgments and problems with near vision. Also, it does not require any special equipment other than the loupe you normally focus with.

Shown is a section of the target verifying the registration of an 8x10 camera using a 480mm APO-Ronar lens. Looking at this scanned image on a monitor does not compare to what you see when looking at the negative, but you can get the idea with it and you can quickly tell if the registration is off or not.