What is "focusing in"?

[nicholas_f._jones]

To complement the recent thread on depth of field, maybe someone can

explain the meaning of the phrase "focusing in" which has occurred

from time to time on this board. And, whatever its meaning,

does "focusing in" have any application to use of enlarging lenses in

the darkroom? Thanks, Nicholas.

[glenn_kroeger]

Nicholas:

 

Wide angle lenses were traditionally nearly symmetrical which is great for 1:1 but not so great at infinity. If you focus one of these lenses at a great distance in the center of the field, the field curvature can cause the edges to be out of focus. For example, if you focus the center at 50 meters, the edges might be in focus at only 20 meters. So you have to focus closer (focus-in) to flatten the field and then carry infinity with depth of field.

 

Newer lenses have reduced this effect, but it isn't completely gone, and when shooting with a 90mm or 75mm or 65mm (in 4x5) its always good to remember not to focus on too distant an object in landscape work.

[pvp]

<B><I>"Newer lenses have reduced this effect, but it isn't completely gone, and when shooting with a 90mm or 75mm or 65mm (in 4x5) its always good to remember not to focus on too distant an object in landscape work."</B></I><P>

Being relatively new to the forum, I had not seen the previous references to this (although I'll admit I'm somewhat selective about which threads I read completely.) Thanks for the wakeup.<P>

However, <B>isn't this pretty much academic if one is using the focus spread method of determining final focus?</B> Inquiring minds want to know!

[nicholas_f._jones]

No, Alan, I don't it's necessarily "academic" if by that you mean of no practical consequence. What I was hoping someone would point out (and maybe someone will yet) is the difference, if any, between "focusing in" and the procedure we all know of focusing at the hyperfocal distance. The case I had in mind is where you've focused at the hyperfocal point, bringing everything from one half that distance to infinity *at the center* into acceptable sharpness, but where (at least with a wide angle lens) for the reasons Glenn has explained, the curvature of field results in the edges *not* being sharp all the way to infinity. In this scenario, is "focusing in" still required?

[pvp]

Nicholas, it sounds to me like we are on slightly different pages here; I'm not convinced that hyperfocal technique has much application in LF, where we are able to manipulate the focal plane and adjust the focus <I>by inspection on the ground glass.</I> Once the lens is tilted or swung, adjusting the lens-to-film distance has the effect of swinging the plane of focus about the Scheimplug point. While hyperfocal calculations can still be made, they are not trivial and will certainly not correspond to tables published for non-tilted setups. The alternative, then, is to use the focus spread; the distance you must move the lens (or film) on the rail in order to bring the extremes of the subject into sharp focus. You then set the camera midway between those extremes and calculate the optimum f/stop for the given spread.<P>

If you check the edges and corners of the GG while measuring the spread, you will take in and compensate for any focus differences caused by field curvature.

[pvp]

That's "Scheimpflug." Can't tipe.

[nicholas_f._jones]

Yes, Alan, we are talking about different things. My original question concerned only the meaning of the phrase "focusing in" with reference to optics/photography in general. I'm still not sure I understand the phenomenon of curvature of the field of focus as it applies to taking or even enlarging lenses. My initial (and subsequent) questions seem to remain unanswered. Guess I'll have to run some dry tests with loupe (camera) and grain-enlarger (enlarger).

 

BTW, two of my taking lenses barely cover my format (8x10), so I do take quite a few zeroed-out LF pictures, for which of course hyperfocal technique is very relevant.

[glenn_kroeger]

This isn't an issue with enlarging lenses (normally!). Enlarging lenses are not very wide angle, they are designed for specific reproduction ranges, and when used in those ranges, have very flat fields (because the designers optimize them for flat fields!).

 

Focusing in is specifically a problem with symmetrical wide angle taking lenses. When focused at or near infinity, some of these lenses exhibit a strongly curved field (concave toward the camera).

 

I guess careful evaluation of the gg at the center and edges helps, and if you are averaging near and far focus points you will minimize the problem. But the "focus-in" approach is mearly a "reminder" if you will, that it is best not to focus on very distant objects, or infinity, but to carry them with DOF.

 

This curvature of field is different from focus shift. Many (even the latest apo) wide angles show some focus shift stopping down. This effect is very small, and I have only seen it doing lens tests with flat targets at 50x focal length. Should be totally unimportant with 3 dimensional subjects.