180mm enlarging lens recommendation needed (5x7")

[jose angel]

Hi all,
My Componon-S 180/5.6 (the one with the blue rim) show what I consider an excessive amount of field curvature at small magnifications. I need to stop it down beyond f11 to get a barely acceptable flat field (to my taste). The lens is very sharp and perfectly centered, so I think it is its normal behavior.

I wonder if it is normal with other lenses in this range. Maybe the working aperture of this lens is f16-22... I bet neither of my other lenses are that bizarre (Apo-Rodagons, from 50 to 150mm).

Could you advice me a proven, better lens? I`d prefer to stay in 180mm; a 210mm lens imply the need of building a new cone, something I`d like to avoid -unless there is no other option- (It is really hard to find parts for my 5x7" enlarger).
Thanks.

[djdister]

Sounds pretty odd, and it should not be considered "normal." I could recommend a 180mm Kodak Anastigmat lens, like the one that was on my Kodak 5x7 Autofocus enlarger.

[Drew Wiley]

I use a 180 Rodagon frequently, but for 4x5 format, and it's been excellent overall. But I prefer Apo Nikkors (graphics lenses) for critical small scale enlarging.
A 240 would be my choice for 5x7.

[Sal Santamaura]

...curvature at small magnifications...

Please specify the magnification range you're working in.

[ic-racer]

Although I'd probably not use a 180 at less than f11, At small magnification you should be using just the center of the field and f8 should give acceptable results. For me that means I suspect something may be wrong with your lens. Any one that has been in the darkroom long enough has probably seen a lens with an element missing or reversed. I even had a 75mm Rodenstock in which someone glued the rear element with epoxy (crooked). The good thing is that you can detect what is wrong, so you can tell if you have fixed the problem with a new lens.

If you still have a focus shift from corner to center, you can use DOF and the large format lens focus equation to get it all in focus. Focus with the column to a point geometrically between the focal point from the center of the negative to the corner (ie. don't use the focus bellows). Then use this equation to determine the aperture needed (or just stop down and watch with the grain magnifier until it looks ok).

N = 20/(1+M) * square root of 'dv'

N = Aperture number
20 = constant (equated to circle of confusion 0.15mm for me)
M = magnification
'dv' = millimeters of focal depth on the baseboard. That is the linear distance on you enlarger column between clear focus of the corners of the negative and the center. This is the same as the focal spread when focusing a view camera.

[jose angel]

Djdister, yes, I find it odd if compared to my other (smaller formats) lenses. I even have two Componons (50 and 80 non "S") which are great performers at its optimal apertures.

Drew, I was thinking on it... to try a process lens instead. Although I have (all 150mm) an Apo-Gerogon and a G-Claron that I`d say are not better performers than the Apo-Rodagon-N I have on 4x5". I`ll have a look at the auction site. BTW, why 240mm? Is it specially good, or maybe to minimize the curvature?

Sal, near 2X. I`m aware that field curvature may vary with focus distance, maybe this one is optimized for higher magnifications, I don`t know. I have been searching for the original Schneider literature, no success (the Schneider site directs to a unavailable german page).

Ic, this is what I`m currently doing (not based on maths, but focusing at a middle point between the center and the side). Thank you for the equation, I`ll use it to be more accurate.
My lens looks to be in pristine condition, I bought it long ago, actually the curvature is quite perfect and the sharpness is very high. What makes me to be disappointed is that huge difference between center and sides. If I focus in the center I need to stop down at least to f16 to have "acceptable" corner sharpness under the loupe. I`d say even at f16 there is still some room for a small focus improvement at the corners of the frame. I`ll check it later.
"Although I'd probably not use a 180 at less than f11... " As you say the "usual" rule is to close two stops, some "older" lenses are better at three stops (that is, f16) So maybe it is normal to work at f16-22 in 5x7" with this lens; I just wonder if others have experienced a similar issue.

[jose angel]

Ic-racer, if I got it right, the column height difference between center and side of the frame focusing is 17mm (actually a bit more because I used a Microsight that doesn`t allow corner check). I understand I`m just checking the film to print distance difference, by using the enlarger`s head lifter instead of the focus knob.

Let`s use the equation;
N=[20/(1+2)]*SQR17, so N=27.2 (that is, near f22+1/2, pretty close to my experience... )

BTW, may I ask how could I calculate the constant for a different CoC? Thanks.

[ic-racer]

The background on the equation and determination of minimum CoC size is here: http://www.largeformatphotography.info/fstop.html Using a view camera at a distance one can forget magnification, but in our case, we need to include that in the equation.

The enlarger uses the same physics laws as view camera. The bellows is in the wrong place, however. So imagine the bellows going from the lensboard of the enlarger to the paper. Now you have your view camera orientation and can see that focusing with the rear standard of a view camera is the same as moving an enlarger head up and down.

The main place I used that technique was enlarging 35mm to 16x20" without a glass carrier. The curvature of the negative could be totally accounted for by focusing in just the right place and stopping to f16. The prints were excellent and much better than the ones with the blurry corners I had been making. Now, however, I have obtained or made glass carriers for all my enlargers and all formats.

[jose angel]

Much appreciated, Ic. I`ll work on it. I need some kind of DIY ruler on the focusing stage to set focus with higher accuracy.

Anyway, if there is someone who knows about a good (flatter!) enlarging lens for 5x7", I`ll be still around. Thank you very much for your comments.

[Luis-F-S]

Do you see this in the prints or only when looking at the projected image on the baseboard. The only Componon-S I own is a 100mm which I bought new ages ago. Even though your lens may look pristine, it may just be a bad lens. The 180 Rodagon and 180 Apo-Rodagon I use for 5x7 are both excellent. I also have a 210 Rodagon, but seldom use it for 5x7, and use it more for 8x10 at small magnifications. Hate to suggest this but you might want to try a Rodagon or a different Componon-S (or both) to see if you have the same issues. Perhaps you can "borrow" one nearby to try. Luis