Which F-Stop for prints

There are many f stops on the enlarger lens for the same reason there are many on a camera lens: so that you can pick one which gives you the desired exposure time. You want a time that is long enough to do whatever manipulation you need to do, but short enough to minimize reciprocity failure and possible shaking of the enlarger. Optical quality of any lens is not at its absolute best at either extreme of the f stop range, but optical quality at either extreme will still be more than good enough for pretty much any application as long as you have a decent quality lens. I have made perfectly sharp prints wide open and stopped all the way down using a nice lens, so I would not worry about the "ideal aperture" aspect of it unless doing very critical work (whatever that means anyhow).

 

From my own tests using a EL Nikkor 150 and 135mm lenses, f/11 was the sharpest apeture. Most enlarger lenses perform best stopped down about 2 stops from wide open.

 

When I print 35mm I print at f/8 (3 stops from maximum). The 80/5.6 seems to be optimal at about f/11.

Do some tests - but generally, 2-3 stops from maximum is the sweet spot where you get the best quality.

If you need to adjust the exposure time, by all means change aperture, but an exposure time of between 5 and 60 seconds is fine by my books. That's what I have a timer for.

 

What 2F/2F wrote. Yes, it's true that every lens, be it an enlarging lens or a taking lens will have a "sweet spot" somewhere in the F stop range. With a decent enlarging lens, the difference will be so subtle that you'd need to make an extreme enlargement to notice it. That said, I tend to avoid the extremes of the aperture range. Wide open with my 50 mm F/2.8 Nikkor or Rodenstock is just too fast to time accurately. With the lens stopped all the way down, the image becomes very dim, the exposure times become too long, and I worry about any spill light from the enlarger fogging the paper. Any aperture between f/4 and f/11 with either of these lenses produces virtually identical results, provided of course, that the exposure time is appropriately adjusted.

 

My 80mm Rodagon works best at two stops down. My 50mm Componon S works best 3 stops down (at least to my eye).

 

This is something I have thought about too, and I think I have the stupidest answer...Just pick one and work with it.

I have a broken blade on the aperture of my favorite enlarger lens, and I have had it set at the same spot for months, somewhere between 8 and 11, just short of the detent at 11 where the blades give the best perfect circle.

I think 3 answers here are perhaps the best and make the most practical sense.

A--how long of an exposure do you need to do any burn/dodge work?
B--is it short enough to prevent a reciprocity failure?
C--is it short enough to prevent shaking of the enlarger?

 

I also did some testing by making moderate enlargements of a very detailed subject at different apertures. The difference between the middle range and the extremes was startling. 2 stops down was best, and seemed very slightly better than 3 stops down. So those are the apertures that I use. If enlarger shake is a concern, consider anchoring the top to the wall. And be sure that your enlarger is properly aligned. Barry Thornton's "Edge of Darkness" covers these issues in great detail and is a good darkroom reference.

 

most papers, but not all (as you have discovered ) default to a grade 2 paper. Using filters will allow you to make contrast changes.

I do have a question, you mentioned your taking a class, why isn't your insturctor addressing these issues?

 

I second Ann's opinion on the classroom instruction. This should all have been discussed before you even got into the darkroom... but at least you can come here and ask these questions!

I haven't done any real enlarging in years, but my method was generally to focus wide open, stop down to f/8 for exposure, and if I then needed more time for dodging/burning, to try f/11. I don't think I ever had exposures longer than about 60 seconds.

 

MTF curves from the lens manufacturer will tell you the optimum aperture for enlarging flat negatives. If the negatives are not flat, then the optimum aperture to balance diffraction with depth of field is similar to the equation used for focusing a view camera:

N_max ~ 20 / (1 + m) sqrt(dv)

N_max = maximum tolerable F number
m = magnification
dv = distance on the enlarger column between good focus on the highest and lowest portions of the curved negative.

(thanks to Jeff Conrad for the equation).

 

I think that A above is often a crucial element for me, particularly when a print requires more than a couple of print controls. A smaller aperture/longer exposure also offers a larger margin of error. A 1/3 stop burn on an 8 second exposure is much more critical than for a 16 second exposure. This is especially true for those of us using analogue timers. (good luck estimating a 1/4 second increment on a Gralab 300 )

I think it is necessary to add this to our list:

D-- is it long enough to fully expose your blacks beyond the toe of your paper's exposure curve?

I tend to avoid overly short exposures at any rate, but for some papers this seems more critical.

I suppose this would qualify as reciprocity on the short end of the scale. I find on medium to long exposures Ilford MGIV and Kentmere LC Deluxe are quite similar. On very short exposures as with a very thin negative the Kentmere reacts very quickly whereas the Ilford seems much more gradual. This is only my perception, and is not backed up with hard testing. I only noticed this because I accidentally switched paper in the middle of a sesson and totally blocked my shadows on a very short exposure.

Cheers,

 

My approach is completely unscientific. I don't care about the optical quality of my Nikkor and Schneider enlarging lenses at the extremes compared to the 'optimum'. I really don't. I just use the aperture to get the exposure I need. Same math as when you make the exposure in the camera to get shutter speeds you want. I'd rather use that approach than risk running into reciprocity failure with the paper, because then if you actually DO switch to a different aperture, there will be no logic to your results. (For instance, if you go from f/11 to f/8, half the time will not result in the same print tones anymore).

I do a lot of lith printing, which is like silver gelatin printing with a twist. I often blast my prints at f/2.8 for a minute or more with a 250W bulb in a condenser enlarger to get the results I want. If I was at f/8 I would have to go for 8 minutes + whatever reciprocity compensation that would entail, something like half an hour or so.

My advice is to use the aperture scale as such. To get a reasonable exposure time that, as previously discussed, gives you all the time you need to do the dodging/burning/diffusion/whatever you need to do to make your image come alive and sing like Pavarotti.

- Thomas

 

I expose and develop my film so that I get reasonable printing times with minimum exposure for max black at F/11. All my enlarging lenses are six element 5.6s, so that means two stops down. Never tested for sharpness, etc, though. I always expose at F11 because it keeps one thing a constant. KISS.

 

Out of curiosity, just what brand/ focal length/ aperture lens exhibited such a "startling" differemce?

 

What's missing here is what seems ALWAYS to be missing. We talk a lot about stuff the eye can't see, like differences in resolution that can only be revealed by controlled tests. That stuff is IRRELEVANT! Let's deal with stuff we, with our imperfect eyes, can actually SEE!

With no negative in your carrier, focus the edges of the frame, and stop the lens down rapidly from wide open. You will see that the center darkens rapidly, but the outer part of the field doesn't begin to darken until the lens is stopped down considerably, at least one stop, more likely two. This will look very strange. If you plucked one of your eyeballs out and laid it in the corner of the easel, looking up at the lens, it would NOT see a circle of light. However, were it to be placed in the middle of the field, looking up, it would see a round circle of light. What we are talking about here is "cutoff". From the corner, seen wide open, the light is cut off by the front of the lensmount and on the other side, the rear. Lenses are mounted in a tube. If you want to have an even field with the print not getting overly light in the edges and corners, it is necessary to stop the aperture down to where the open area is entirely enclosed in the part of the lens that is NOT cut off. This usually means at least f/8. That is where I instruct MY students to start, and to go down from there, not up. If this isn't clear, hold a toilet paper roll tube up to your eye, and while looking through it, rotate it a bit off axis. You will see what I mean. So the center of the print gets LOTS of exposure, the corners, edges, much less. Very uneven print. UGLY.

Of course, you could burn the edges like mad and probably not achieve a really graceful result; it won't be rational like the "falloff" - the effect of the cosine. This requires a small amount of edge burning (which most people wouldn't even notice) because the light source (here the lens) is farther away from the paper at the corners than it is in the center (that, too, means lost light but it is MUCH easier to deal with). Usually about 20% on each edge, giving twice that on the corners, is adequate for that. It won't change much, but your images will have a subliminal authority that would be missing if you don't do it.

I learned all this stuff the hard way in commercial custom labs with exposures that sometimes required hours. In one case, I thought I'd save time by printing wide open. I gave a 15 second main exposure and had to burn for two hours. Had I given the whole thing a 2 hour exposure, I'd hardly have had to burn at all. A very painful lesson, not to be forgotten.

 

Just recently I was printing and needed very long times so I opened up wide ( both Rodenstock and Schneider @ 2.8)and discovered exactly what bowzart has said. Bang on! f8 and look great

 

That sounds to me as though there was a collimation error in the optical system of a condenser system. What *might* work to detect an error is to remove the enlarging lens, and attempt to re-focus on the baseboard. At some point, you should be able to see a relatively clear image of the lamp filament, CENTERED in the field.

Ouch!!! I would suggest removing the enlarging lens - same idea - a LOT less pain.

 

I don't think so. Try it with ANY enlarger. You will see exactly the same thing.

In fact, the experience I mentioned with the 2 hour exposure was not with a condenser enlarger at all; it was a 10x10 Saltzman Chromega, a very even diffusion system.

So, if you follow your own advice and remove the lens, then tilt its axis in relation to your eye first wide open, then repeat the same tilt stopped down, the reason for it will be perfectly obvious.

 

So what do you do if/when you need more light?

I always use my enlarging lens at f/2.8, f/4, f/5.6, and sometimes f/8. Normally with a 150W bulb. There must be something wrong with me, because I can't tell a difference. All of my lith prints are f/2.8 and f/4. I use Rodenstock Rodagon, Schneider Componon, and Nikkor lenses if that matters.

- Thomas

 

More time or brighter bulb.

I don't know much about lith printing, but I understand that a great deal of exposure must be given. I suspect that there may be something in the process that masks differences of a stop or two.

Try this, NOT with lith, but with conventional silver paper. Use a normal grade unless you want a dramatic result. Higher contrast will give you that.

With no negative at all in the enlarger, expose a piece of paper at say, f/11 or f/16 to get a mid region gray.

Repeat the same wide open, changing to a lower wattage bulb if necessary to give a practical exposure time.

The images I've seen from you are really great. I have never seen your actual prints, though. I know also, from prior conversations, that you do a good deal of burning/dodging. I suspect that some of that local control is needed to simply correct problems that just stopping down to begin with would have eliminated.

Also, you can minimize this falloff effect by using a longer than normal focal length. Then you are using mostly just the center of the field. The longer the focal length, the wider the aperture you can use. Of course, there is a diminishing advantage as your elevation must be higher.

 

You may be right that the fall-off doesn't affect lith prints as much. The process masks a lot of other problems too, like excessive contrast, which can be controlled much more than with regular printing by just exposing the hell out of the paper, and get details in highlights that are otherwise blocked with regular printing.

But, while I do a lot of burning/dodging, I never do it excessively. And I do some edge burning, but only to weigh the print a certain direction. I'm sorry, I just don't see it in the prints what you're describing. Perhaps I'm not critical enough. Even standard prints from dense negatives where I've been at f/2.8 or f/4. Doing the test seems like a moot point for me, since I'm happy without knowing about it, but somebody else might benefit from it.

Like you suggest, I routinely use my 80mm Rodi for 35mm negatives, and my 135mm Rodi for MF negatives. It's more due to laziness than anything, but sometimes I have to switch lenses anyway to compensate for the height the column is at.

Thanks for taking time to explain.

- Thomas