Accurate focus through enlarger

[DREW WILEY]

I would never assume that a grain focus device is correct, esp a cheap one. I use the finest Peak model
they ever made, always with the correct thickness of film or paper in place, and nearly always just end
up at the same spot my reading glasses said was correct.

 

[cliveh]

Something that should be mentioned in this thread and I noticed someone mentioned full aperture (I do hope so). But for some of the newcomers to darkroom work, always focus at full aperture with the filtration switched off and with a darkroom illumination ratio to projection that allows you to see what you are doing.

 

[DREW WILEY]

Full aperture is fine for general focus, not for critical - some lenses have a bit of focus shift between
the widest aperture and the next. Just depends what you are doing too. When I'm doing critical work
like making precision internegs or interpositives with an enlarger, I use only apo process lenses and
check focus with my best grain magnifier, with film in place on a vacuum filmholder. That is often overkill in ordinary black and white printing, but I still tend to do it anyway. My old Scoponet was way
off for any critical work. You can't assume something is properly sized per focal distance just because it is new! Even the double-faced tape used to attach the mirror ruined any precision to the design. Was
fine for casual work, but I'm glad I eventually dropped it on the floor!

 

[Francis in VT]

Focusing without a magnifing glass.

It has been a long time since I last did any darkroom work but I read somewhere if you put a narrow piece of opaque tape across the front of the enlarging lens it would give you two images. When you focus the lens the two images will combine into one. At that point you will be in focus. I remember trying this and it worked.
The hassel was attaching the tape on and off.
I hope this is not my mind p[laying tricks on me but I vividly remember this working. I read through all 3 pages of this thread and am surprised no one else mentioned it.

Francis in VT

 

[pentaxuser]

A new one on me. I wonder how this one works? Anyone care to explain in simple optical terms?

pentaxuser

 

[cliveh]

Full aperture is fine for general focus, not for critical - some lenses have a bit of focus shift between

Are you suggesting that we focus with the enlarger stopped down?

 

[eddie]

Clive- I've seen what Drew's referring to. Years ago, a friend was having trouble getting sharp prints. She thought it was her magnifier, so I brought my Peak over, and we focused wide open. Then, closing down 2 stops, the print still wasn't sharp. Checking the Peak, at the chosen aperture, it wasn't sharp, and needed refocusing. It was a fairly cheap lens. She started focusing at the chosen aperture, without issue, even with her cheap magnifier. I haven't seen it happen with better lenses, though.

 

[Steve Smith]

A new one on me. I wonder how this one works? Anyone care to explain in simple optical terms?

In theory, it shouldn't work as any point on the negative should be projected onto the same place on the paper from all areas of the lens.

Something I did years ago when my father was having a slide show was to cover up half of the lens with my hand thinking it would block out half of the image. Instead you get the whole image at a reduced brightness.


Steve.

 

[Mr Bill]

A new one on me. [tape across center of lens makes double image if out-of-focus] I wonder how this one works? Anyone care to explain in simple optical terms?

pentaxuser

In theory, it shouldn't work as any point on the negative should be projected onto the same place on the paper from all areas of the lens.

Steve, you're right when the image is in good focus. Out of focus, different result. It's easy enough to explain the general principle.

Imagine a plane through the center of the lens (as in a cutaway showing the lens construction). Imagine pointing the lens directly toward a distant, small subject, such as a star (imagine something larger if that helps). We know that this subject illuminates the entire front of the lens, BUT... consider only the light on one side of our plane - that is, on one half of the lens. As this light converges to form an image, all light is coming from one side of the imaginary plane. As the light passes the ideal focus position, it crosses to the other side of the imaginary plane.

Now repeat this process from the OPPOSITE side of the lens. This light wants to also pass through the focus point and diverge on its opposite side of the imaginary plane. Thus, a double image, but only when out of focus.

I don't consider this as very useful for focusing an enlarger, but a lot of experienced photogs would probably bite on it as a barroom bet.

 

[DREW WILEY]

There are all kinds of hypothetical optical issues one could discuss. The main point, however, is that most enlarging lenses are not ideal wide open. You might need full apterture for general composition and approximate focus, but silver grain or dye clouds might not focus crisply at full aperture. So it's
a good idea to refocus a stop down (not necessarily at working aperture, which might be too dim to
focus well - just depends). But never assume a focus device is itself correctly aligned. You need to test.
And nothing will make senses if your enlarger is not perfectly aligned on every plane, or if your negs
are not truly flat in a glass carrier. I don't want to open a can of worms in the latter respect, but at a
minimum, you should have a glass carrier on hand with a focus target for the sake of general calibration. Some ten buck Home Cheapo level isn't the ticket.

 

[cliveh]

If I am using a reasonable quality enlarger lens I expect it to stay at that point of focus when stopped down and therefore fail to see the point of refocusing again at one stop down.

 

[DREW WILEY]

Modern enlarging lenses are optimized one or two stops down, and if any focus shift occurs it is likely
to be only one the first stop. But I don't know about old enlarging lenses or cheapo ones. And in my
own collection of enlarging and process lenses, only one lens exhibits any focus shift at all, at it's the
least expensive one of all, but still very usable a stop down. Some of the very best lenses are very
sharp wide open, but might have excess illumination falloff compared to stopped down. One just needs
to test the variable, which is quite simple to do in terms of basic characteristics.

 

[cliveh]

As you stop down you increase depth of focus, so how can you judge this is better or worse than when you focused at full aperture?

 

[Bob Carnie]

I have always focused wide open with the paper I am using under the peak device corrected for my eyes.
I want to see grain, sharp grain. I then close down two stops and work, I have never seen the reason for stopping down.
I have always used APO lenses and my film is always in glass, we check alignment periodically.

Seems to work for me over the years.

 

[cliveh]

I have always focused wide open with the paper I am using under the peak device corrected for my eyes.
I want to see grain, sharp grain. I then close down two stops and work, I have never seen the reason for stopping down.
I have always used APO lenses and my film is always in glass, we check alignment periodically.

Seems to work for me over the years.

Please don't cloud the water here, as I appreciate the value of stopping down, but not checking focus at one stop down.

 

[DREW WILEY]

Some types of film grain just seems to bounce more light at larger apertures. Then there are cases where one wants to be certain what they are focussing upon is the emulsion side of the film itself and
not the grain of an attached mask. Then different kinds of Newton ring glass bounce incident rays a
little differently with different lenses and diffusers. Sounds complicated in theory, but is pretty simple
if you just examine the image at more than one f-stop. I like to print color with a very shallow depth of field for all the above reasons, so want the widest aperture which bags the result. B&W printing is less
fussy unless an unsharp mask is also employed. And like I already indicated, one simply can't assume
a lens is devoid of focus shift - there are a lot of cheap lenses in use out there, and I sure as heck don't
intend to purchase and test a bunch of them. Gimme Apo Nikkors and Apo Rodagon N's and Rodagons
and Nikkor EL, whatever... just nuthin old or cheap (I do sometimes use a wretched old Carl Meyer process lens because it wonderfully complements certain soft portrait images, but that's a deliberate
tweak into oldie land).

 

[cliveh]

Cliveh, Bob is not clouding the water. He's saying he focuses wide open and then closes down two stops to make the exposure.

It depends on the lens. Most modern high quality enlarging lenses from Nikon, Rodenstock and Schneider should be free of focus shifting. Stopping down a stop (usually recommended for highly corrected top line lenses) or two, is really only to help minimize aberrations and falloff. It also slightly increases depth of focus (at the film plane) and depth of field (at the paper plane) which can help reduce the effects of incorrect focus, enlarger misalignment, and lack of negative flatness (assuming you don't use a glass carrier).

With older and/or lower quality lenses there can indeed be focus shifts when stopping down. You can sometimes see this in the grain focuser.

Try this: Focus wide open. Then stop down to the working aperture. Make slight/fine adjustments to focus and see if you can get the grain any sharper than it was. Once it is as good as you can get it, open up to maximum aperture again and check the grain focuser. You may or may not see fuzzier grain than when you originally focused wide open.

Perhaps you don't understand my point, as I am all for stopping down to print, but not to focus.

 

[cliveh]

Your point was you don't want to focus at the working aperture. I'm simply explaining why it might make a difference, and giving you a quick way to check to see if it matters with your lens.

Can someone help me out here?

 

[Bob Carnie]

You are joking right.

Please don't cloud the water here, as I appreciate the value of stopping down, but not checking focus at one stop down.

 

[MattKing]

Can someone help me out here?

Some lenses do suffer from focus shift, but most don't.

So Michael described a method of checking whether your lens is one of the problem ones.

Perform the suggested test. If you observe focus shift, you will need to build a "focus at working aperture" step into your workflow.

If the test doesn't reveal any shift, you can confidently focus at maximum aperture for all your work.

 

[cliveh]

Some lenses do suffer from focus shift, but most don't.

So Michael described a method of checking whether your lens is one of the problem ones.

Perform the suggested test. If you observe focus shift, you will need to build a "focus at working aperture" step into your workflow.

If the test doesn't reveal any shift, you can confidently focus at maximum aperture for all your work.

But how can you judge when you are viewing with an increase in depth of focus?

 

[DREW WILEY]

Relevant question, but generally a non-issue in practice. This is because enlarging lenses are typically fast and used fairly wide open. So adjusting down just one stop won't make a radical difference in depth of field as far as viewing grain is concerned. But even with slower process lenses any such issues can be detected using a high-quality magnifier.

 

[cliveh]

Relevant question, but generally a non-issue in practice. This is because enlarging lenses are typically fast and used fairly wide open. So adjusting down just one stop won't make a radical difference in depth of field as far as viewing grain is concerned. But even with slower process lenses any such issues can be detected using a high-quality magnifier.

I take it you mean depth of focus and not depth of field, but it still means you are still trying to judge focus in a wider depth of focus.

 

[DREW WILEY]

Yes. But the change in depth from one stop to the next at this point is generally not enough to cause
a viewing issue under the magnifier, at least if you have a reasonably bright light source. Sometimes it
can be too bright and you might need a ND filter somewhere in the lightpath! I work with quite a few
different enlarging lenses on several different enlargers, both in color and b&w printing, sometimes fairly large, so am functionally aware of these kinds of issues. But that doesn't mean everyone has to
be nitpicky about them. Once you're comfortable with the characteristics of a specific lens, it gets pretty
routine or second-nature to check such things in a matter of seconds.

 

[Newt_on_Swings]

I think that if your working distance and brightness of your lamp at the working aperture is adequate to judge focus either on the paper or through the grain finder/magnifier, you might as well focus at that aperture and leave it there. It will save you a step of having to open all the way up and then stopping down to working. If you cant or are not sure of the focus, then do it wide open, or at whatever aperture allows you to see the image clearly, and then stop down.