Another focus question (from a LF newbie)

[Jehu]

I'm having trouble focusing in dim lighting situations.

I open the aperture all the way and focus/tilt. Then I stop down to f22-f32 where I can't see the image well enough to focus. As near as I can figure, that keeps the central focus at the same distance and expands the depth. Or does the center move with aperture changes?

[Ole Tjugen]

With most modern(ish) lenses, there is NO focus shift on stopping down. There are some that do have focus shift, especially soft-focus lenses, but even then I have found it easier to learn how to focus correctly than to refocus at f:32.

[BrianShaw]

... Or does the center move with aperture changes?

If you mean point-of-focus by "center", then this shouldn't be happening. Your first guess was right.

[Jehu]

If you mean point-of-focus by "center", then this shouldn't be happening. Your first guess was right.

I'm still a little weak on terminology. Thanks.

[BrianShaw]

Don't worry... you'll get it. Have fun!

[Vaughn]

What will help when closing down the lens is not looking at the ground glass at an angle perpendicular to the glass (except when looking at the center of the GG). Instead, when looking at the GG, look at an angle that puts your eye along the same path as the light coming from the lens. Usually this just means that if you are looking at the image near a corner (or edge) of the GG, look towards the center of the lens.

Hope that made sense!

Vaughn

[Leonard Evens]

You normally focus wide open. If you have a modern lens, there shouldn't be any significant shift in the exact subject plane when you stop down to the taking aperture. If you want to test to see if there is a shift, it is pretty hopeless to do it by stopping down and examining the ground glass. First, as you noticed the image gets pretty dim. Second, as you also noted, what you will see is the depth of field, and everything within that depth of field will look in focus. That is the point of depth of field.

You would have better luck by focusing carefully, stopping down to take a picture and then examining the negative or a print carefully. But even doing that, you have to know what to look for. This is a bit complicated, so let me try to explain it.

First consider the case where the lens is not tilted. Generally the exact subject plane will not be centered between the limits of the depth of field, but if the focus point is reasonably close, say at most 15 times the focal length, then it will be pretty close to being centered. So take a picture of something with regular marks like a tape measure proceeding away from the camera. When composing focus as best you can, stop down, and make the exposure. Carefully examine the negative or print to see if the exact focus point is centered in the depth of field.

If you tilt, it gets both harder and easier. Keep the back vertical, and select a target plane which is vertical and contains sufficient detail both above and below where you want to put the exact subject plane. Tilt the lens to get the exact subject plane where you want it, stop down and take a picture. Look just at detail in that single vertical plane. The point for the exact subject plane should be centered precisely within the limits of depth of field. If it isn't, then it is possible that stopping down caused a focus shift.

Unfortunately, there may be other reasons why tests like this may mislead, the most obvious being that the exact subject plane is not actually precisely where you think it should is. The reason for that, is that you can't really focus better than the depth of field you have wide open. You can improve this by using a loupe, but there will always be some uncertainty in the focus point. Another possibly explanation for an apparent shift is a displacement between the ground glass surface where the focusing image is formed and the position of the film when taking a picture. You can test for that by the method suggested earlier but without stopping down when you take the picture.

Another difficulty is that, if there is a shift on stopping down, it may depend on just where you are focusing. So you really have to test at different distances, different tilts, etc.

As I said, it is complicated. Probably you are best off just taking a lot of pictures and if you then notice some consistent shift from what you expect, try to compensate for it when focusing to see if that improves the situation.

[Jehu]

That's a lot to process but it makes sense. Thanks guys.