Apertures on Enlarger Lens

[Christopher Walrath]

When and how should this be adjusted? My lens on my fujimoto enlarger goes from 3.5 - 16. Any pointers? Thank you.

 

[Ray Heath]

I'm not sure what you're asking, do you really never adjust the aperture?

 

[ann]

open to 3.5 when focusing the image

stopped down to f8 for printing. f8 is just a rule of thumb guide line,
run some test to determine which fstop will give you a time that is long enough for dodging.

 

[resummerfield]

I just finished testing some larger f5.6 El Nikkors for both resolution and light fall off. In every case, the best resolution at the center was at wide open, and the best corner resolution was between 1.5 and 2 stops from wide open. Light fall off improved progressively as the lens was closed down, and by f11 the corners displayed less than 1/3 stop loss, and usually less than 1/6 stop loss. So for me, with my system, the best compromise aperture was 2 stops from wide open, or f11.

 

[Jim Jones]

Ann and Eric have covered it well. Most quality 50mm enlarging lenses show a slight loss of sharpness when stopped down to f/11 and f/16 due to diffraction. This might not be noticable except in big enlargements. Enlarging lenses for larger formats can be stopped down a bit further with no loss of quality. Some of the less expensive lenses originally supplied on enlargers should be stopped down at least two or three stops for sharp prints. Stopping any enlarging lens down at least two stops helps cover any focusing or enlarger alignment problems.

 

[srs5694]

I've found that most of my 50mm lenses perform best at about f/8 (with maximum apertures that range from f/4 to f/2.8). The best lenses show much less improvement from being stopped down, though.

I disagree with the recommendation to focus wide open. I've forgotten the name of the effect, but lenses often suffer from a slight focus shift when changing the aperture. When I first heard of this I did a few tests to confirm it. This effect was almost unnoticeable with my best lenses but quite noticeable with others I've got. OTOH, my enlarger (a Philips PCS130) produces a very bright image, so I have no problem focusing at f/8. With an enlarger that produces a significantly dimmer image, it might be very hard to focus at anything less than full aperture.

 

[Ed Sukach]

That so called "focus shift" is due to curvature of field. The rays are focused at a different distance from the optical center of the lens as one progresses across the plane of focus to the edges of the field. When the lens is stopped down less of the edges of the ray trace "bundle" are used, so less will - can comprise the image area, and a greater fraction of them will focus on a flat plane.

The reason for focusing wide open is not primarily for brightness; it is to reduce the depth of field (actually, in a projection lens, it is the depth of focus) enabling the critical focusing distance to "snap into place".

If you notice something like a "focus shift", try focusing at the center of the paper... that is where the curvature of field will have the least effect when the lens is stopped down.

 

[Monophoto]

I prefer to leave my enlarger lenses set at about two stops from wide open. But if the resulting exposure time falls outside a reasonable band (minimum 10 sec, maximum 60 sec), then I may open or close by one stop.

 

[Jim Jones]

With used high quality enlarging lenses selling at such low prices, any lens with focus shift can easily be replaced. Anyone using fine lenses for image capture should also use fine lenses for enlarging.

 

[joeyk49]

Flash: Thanks for asking the question.

I was already aware of the f/8ish rule of thumb, but never really knew why. The responses to YOUR question may just have helped improve MY printing...Thanks everyone!

Joe

 

[photobum]

I was also a victim of focus shift. For years some were sharp and others not so sharp. Having a high end, big name lens did not eliminate focus shift. I found a top end 80mm Rodagon had shift and a 50 year old 75mm Wollensak showed none. I don't understand how that might work with Ed's answer, but it just is. When I started using a good Peak grain magnifier I had enough range and light to see the focus shift.

Yes, about two stops down is the sharpest for all my lenses. f/8 and be there.

 

[Ed Sukach]

I was also a victim of focus shift...
... I don't understand how that might work with Ed's answer, but it just is. When I started using a good Peak grain magnifier I had enough range and light to see the focus shift.

That is what I was afraid of. Serves me right for trying to condense an entire basic optics class into one message. It is so much easier to visualize, once you have gone through the ... now ancient ... exercise of ray tracing a lens from scratch.

Every lens is designed from a certain *desired set* of parameters, with "targets" in the center of compromises. Most enlarger lenses will be at their *best* performance in the centers of their aperture ranges, and in the center of their magnification ratios.... But any, worth their salt, will be absolutely *usable* at their published extremes.

Interesting that you should see *significant* focus curvature/ "shift" errors in Rodagons. I use both Rodenstock Rodagons and Schneider ... (name?) lenses, and I see *nothing* remotely resembling that ... and I've learned a long time ago that it was NOT wise to try to save pennies on grain focusers. Mine is a top-end Omega.

I might suggest that you contact Rodenstock. The latest Rodagons came with *lifetime* guarantees.

 

[firecracker]

My Nikkor 63mm lens looks good at F8, which is my standard setting. With the kind of negs I have, I don't really have to change the aperture.

 

[Christopher Walrath]

Good info everybody. Thank you very much. I love the Dark Side.