The maths of how to achieve accurate focusing.

[Danny Treacy]

Hi,

I wondered if anyone out there could explain how to ensure accurate focusing. I am using a Cambo Legend 10" x 8" with a Rodenstock 210mm macro lens.

I'm shooting at F:22, which is as deep as I can go with the depth of field before losing clarity.

I am finding that when the image appears to be in focus at the point I want on the object i am shooting, the results are slightly out. I seem to recall something about not focusing on the plane that you want but focusing deeper and the results will mean that the original desired plane of focus is achieved.

Do I sound confused? I am!

If anyone out there knows an accurate solution to this that would be great, thanks!

Danny.

[Bruce Watson]

I am finding that when the image appears to be in focus at the point I want on the object i am shooting, the results are slightly out.

This shouldn't happen. The ground glass should never lie. This sounds like an alignment issue -- that the plane of the gg is not the same as the film plane, and they should be exactly the same.

This topic has been covered before and better than I can explain it. Search the archives for how to test for this and how to fix it.

An alternative explanation is user error. Focusing is not as easy as it seems. Human eyes in low light (large pupils) under the dark cloth have some of the same aberrations as camera lenses do. It helps to be using a good quality loupe of sufficient magnification (I use 6-8x magnification with my 5x4 camera).

Finally, don't be afraid of smaller apertures. Yes, you will start seeing some softening due to diffraction at f/22 and below. But unless you are making huge prints from your 10x8 film (remember, a 50x40 inch print is just a 5x enlargement of 10x8 film), you won't see this slight increase in softness in the print. What you will see is defocus so it's often better to have a image that's sufficiently in focus but somewhat soft -- the human visual system is much more forgiving of this. So if you need f/64, use it.

[Danny Treacy]

Thanks for your reply, I use a 6x schneider loop, it gives me enough magnification to be sure of focusing.

As far as I can tell the camera gg should be spot on, its been very well looked after and in the past when shooting on a standard lens was always pin. I thought that perhaps due to the great loss of depth of field at macro level there may have to be some extra adjustments.

Perhaps I'll just have to go that extra stop or so deeper, thanks again.

Danny.

[Paul Bujak]

Don't forget your own eyesight. I finally went to autofocus for 35mm when I found that I couldn't get a sharp photo without my glasses and got eyeball grease all over my glasses when I did use them.

Now with LF I just have to be careful where on my progressive lenses I look through.

[Bob Salomon]

Thanks for your reply, I use a 6x schneider loop, it gives me enough magnification to be sure of focusing.

Danny.

Have you focused your loupe so that it is focused on the grain side of the GG and not on the flat, top side of the gg?

To focus it place it on the gg, open the lens and point it at a light source and adjust the loupe's diopter until you sharply see the grain of the gg.

[john borrelli]

As Bruce mentioned, there are a lot of threads on this technical subject and those members can explain technical stuff much better than I could.

I would recommend trying to increase the light hitting the subject which should make things easier to focus, so if indoors use an artificial light, focus, turn off the light, recheck everything, then take the picture.

If outdoors,arrive a little earlier when the light is brighter, focus, then wait for the right light.

Your eyes will adjust to a darker viewfinder under a focusing cloth if you can stay under the cloth for a while. Incidently,if you don't have a focusing cloth you will need one, you could use a shirt or jacket or any practical, dark cloth you have around.

One question I do have is are you doing your focusing at the lens's widest aperture or with the lens closed down? You should use the largest aperture first, use movements if they help, then close down the lens to take a look for overall depth of field.

I am not sure if you are referring to the image on the groundglass or how a final print looked after you took the photo. The image on the groundglass will be dark and difficult to see at f22 but with an 8X10 camera and a decent lens, I would think the final print would like fine at a smaller aperture than F22 even with diffraction issues. As you get to macro ranges with this format, dof is tiny.

Hope this helps but let us know if it does not.

[Nathan Potter]

As Bruce mentioned above the GG and film plane must be in exactly the same plane.
Another factor alluded to above is the ability of your eye to see the GG fritted side by having your loupe focused correctly. The focusing helicals on some of these loupes can slip while in use.

I find it very useful to have a focusing screen (GG) with actual black rulings at center and corners (like Linhof makes). The first thing I do is make sure the loupe focuses sharply and critically on the ruling. Then keeping my eye focused on the ruling I focus the image simultaneously often switching my vision between ruling and image to ensure a parfocal condition. This is an infallible focus technique regardless or your particular vision correction.

Nate Potter, Winter Harbor Me.

[Bill_1856]

Don't worry about f:22 -- remember that the best know photography group (Weston, Adams, et el) called itself f:64, for good reason.

[mortuus]

Don't worry about f:22 -- remember that the best know photography group (Weston, Adams, et el) called itself f:64, for good reason.

But weren't they also known as The Dukes of Diffraction ? :D :D :D

[Bob Salomon]

Exactly what reproduction ratio are you shooting at?

The 210 and 300mm Rodenstock Makro Sironars were desiged that the front and rear groups must be reversed at different reproduction ratios. Do you have the groups installed in the shutter properly for the ratio you are using?

From 1:3 to 1:1 the front group is in front and the rear group is in the back of the shutter. From 1:1 to 3:1 the rear group (smaller diameter) is in front and the front group is screwed into the back of the shutter.

Is this what you are doing?

[Brian Ellis]

"I seem to recall something about not focusing on the plane that you want but focusing deeper and the results will mean that the original desired plane of focus is achieved."

I think you're confusing depth of field and plane of focus. The "something" you recall probably is the suggestion that to maximize depth of field you can focus somewhat behind the subject. The idea is that the circles of confusion in front of where you're focusing will be small enough to cause the subject appear "sharp" even though technically it isn't in focus, and you will have gained depth of field behind the subject because you focused behind it. The usual, but usually wrong, suggestion is to focus a third of the way into the scene. That has nothing to do with the plane of focus however, it's just a question of what you focus on once you've chosen the appropriate plane of focus (or are stuck with the plane of focus the camera gives you when using a camera that doesn't have movements). Hope this is clear, it's hard to explain in a brief paragraph.

There's something wrong somewhere if you can't use an aperture smaller than f22 with 8x10 film without losing "clarity" (by losing "clarity" I assume you mean the print looks soft). It would help to know what you're doing - contact printing, enlarging in a darkroom, scanning and then enlarging - and what size prints you're making if you're enlarging. But with 8x10, f22 is about the widest aperture that can be used and still get adequate depth of field for most subjects, given the focal length of the lenses. In other words, it's usually the starting point and apertures actually used tend to get smaller from there. I'm not sure I ever used an aperture as wide as f22 when I shot 8x10.

I think Bob Salomon's point is excellent. Not only are you going to get poorer results if you aren't using this lens within the range for which it's optimized (3x reduction to 3x enlargement), you also have to switch the front and back of the lens around as he says. So it could just be that you aren't using the right lens for what you're doing.

[Danny Treacy]

Thanks for all of the advice, I have readjusted the diopter on my loupe a few mm, hopefully that will help.

Also going to fit a bright halogen light to the tripod for focusing.

If that doesn't work, I may even try stopping down!

Thanks.

Danny.

[David Karp]

Assuming your equipment is in good shape, the Fresnel (if any) is properly located for your camera type, etc., try this: http://www.largeformatphotography.info/fstop.html or maybe this will help: http://www.largeformatphotography.info/how-to-focus.html

[Bob Salomon]

Let's not overlook some faults that can take place behind the camera.
Are the film holders loaded properly (film is under the rails)?
Is the film emulsion facing the lens or is the back side of the film facing the lens?

Either of these can give the results that you are complaining about.

Does the back of your camera move when you insert the film holder?

Is the film holder fully inserted in the back?