Vignetting with 135 Lens

I have been using a Beseler 45 with both a Schneider Krauz Compon0n 135 and Kodak Ektanon 135 for my 6 by 9 cm negs. No problems. But when I use try to enlarge a 4 x 5" negative I am getting vignetting at the corners. The condenser head cuts off the corners sharply compared to the cold light head. I am rolling up the negative tray as close to the light as possible to the light source. And when measuring the intensity of the cold light head's projection with a light meter, I am reading a full stop between the center and corners of projected light. Will a 150 lens narrow down the source so there is no vignetting?

All lenses project a circular image. This image is best at the center as it degrades with distance from the axis. Since the central portion is best, its bounties contain what is called “the circle of good definition”.

 

As a rule of thumb, we fit lenses to cameras and enlargers based on the corner-to-corner measure of the format. We are talking diagonal measure. For the 35mm format we choose a 50mm, for the 6cm by 6cm we choose a 75mm, for the 2 ¼ x 3 ¼ a 90mm and for the 4X5 a 150mm.

 

Let me add, special wide-coverage lenses are available mainly in use for camera. The enlarger suffers because such wide-coverage projection lenses are rare.

 

The problem is caused by the need to maintain a steep figure (curve of the lens) at its boundaries and what is called “cosign error”. The light from the projected image at the center is square but at the bounders of the projected image, the light hits the paper at a steep angle. Think about a flashlight beam shining on a wall, straight it makes a circle of light. At an angle, we get an ellipse of light. The ellipse has more surface area than the circle thus the light that plays on the ellipse is more feeble.

 

The negative / positive process, negative film projected on to paper has an advantage over the vignette as compared to positive film projected onto positive paper. The camera vignettes, the enlarger lens vignettes. These tend to somewhat cancel. The vignette of camera lens on negative film is countered by the vignette of the enlarge lens and the result is a more uniform print. Conversely, when printing slide film on a reversal paper, the effects of the vignette, both camera and projector are additive. This is the worst case scenario.

I have been using a Beseler 45 with both a Schneider Krauz Compon0n 135 and Kodak Ektanon 135 for my 6 by 9 cm negs. No problems. But when I use try to enlarge a 4 x 5" negative I am getting vignetting at the corners. The condenser head cuts off the corners sharply compared to the cold light head. I am rolling up the negative tray as close to the light as possible to the light source. And when measuring the intensity of the cold light head's projection with a light meter, I am reading a full stop between the center and corners of projected light. Will a 150 lens narrow down the source so there is no vignetting?

A good 150 should be helpful in limiting the vignetting. Remember that the diagonal of a 4x5 negative is 161 mm, so 150 is still a little short but works in my experience. For a comparison, the normal enlarging lens for 35 mm negatives is 50 mm while the diagonal is 43 mm. Alan is right that there were special wide angle enlarging lenses made for enlarging, but I have never used one so I can't recommend one from personal experience. You might also take a look at where the condensers are positioned as that can make a big difference in performance.

Will a 150 lens narrow down the source so there is no vignetting?

No. I'd say one stop is right. My 150mm lenses also show a considerable amount of vignetting. I don't have 135mm lenses so I can't tell what the vignetting difference is, but I think to avoid vignetting only the center of the sweet spot should be used ... so quite a long lens.

On 5x7" I use a "short" lens (180mm), and I`d say with a similar amount of vignetting. Not bad, I use to burn the borders a bit to "frame" the image on most prints.

Another issue is the field curvature... I find it to be somewhat insane on some lenses.

Reading Alan`s comment I remembered the old saying that "the best lens for enlarging is the one used for the take" ...

Edited November 25, 2020 by jose_angel

Alan, I had never even considered that exposure is cancelled out once printed.

 

I have always referred to the Fuji 69 as a great landscape camera because it “assists” this cosign error, allowing smooth lessening of exposure from center to borders. The Pentax 67 less so.

I will soon be using a 150 mm!

What degree of enlargement are you trying to use?

 

The extension of any lens - enlarging or taking - is 1/m. Where m is the magnification.

Therefore printing a 4x print (= 20" x 16") from 5x4 needs a 135mm enlarging lens to be extended to nearly 169mm, and that means the lens will cover as much as a 169mm lens. So lens vignetting should be no problem printing up to that size. Certainly with the Componon lens. Maybe not so much with the Ektanon.

 

If you want to print bigger, then maybe a 150mm lens will give a slight improvement in vignetting. But you'll need a distance of well over 900mm between negative and baseboard for the same 20" x 16" print.

 

Personally, I would look at the cold-light head and check that the tube is fully working, and that the vignetting isn't already present at its diffuser panel.

 

P.S. the theoretical Cos^4 falloff is going to be close to 1 stop in the corners regardless of which lens is used.

Edited November 25, 2020 by rodeo_joe|1

Remember that the diagonal of a 4x5 negative is 161 mm

No it's not. The actual image area is close to 90 x 120mm, with a diagonal of almost exactly 150mm.

 

P.P.S. You'll get more even illumination of the negative by increasing the spacing from the cold-light diffuser, not decreasing it.

Edited November 25, 2020 by rodeo_joe|1

I am making mural prints as large as 24 x 30, sometimes larger. Until I get a 150 will I begin with 4x5s.

 

My particular enlarger has the negative slightly offset from the center. AJG, if I am understanding you correctly, you say that the soft light head will give more even illumination the further it is above negative. But I have found the opposite: in trying to solve this very problem I put in another circle of plexiglass 1 inch above the first level of plexiglass, and found quite a bit more of a change in exposure from center to border, and less overall light, and thus ability to stop down, of course. I removed the 1” spacer, began with a new plexiglass. And found much better ratio of light from center to border. (This is always a good idea for any enthusiast in that you see dust to clean out.)

 

Rodeo, tube is bright but doesn’t create a ghost image through plexiglass. Very even illumination. There is a heater unit plugged in, presumably to keep each and every exposure of fluorescent even. ( I assume this is standard for a cold light heads).

I am making mural prints as large as 24 x 30, sometimes larger. Until I get a 150 will I begin with 4x5s.

 

My particular enlarger has the negative slightly offset from the center. AJG, if I am understanding you correctly, you say that the soft light head will give more even illumination the further it is above negative. But I have found the opposite: in trying to solve this very problem I put in another circle of plexiglass 1 inch above the first level of plexiglass, and found quite a bit more of a change in exposure from center to border, and less overall light, and thus ability to stop down, of course. I removed the 1” spacer, began with a new plexiglass. And found much better ratio of light from center to border. (This is always a good idea for any enthusiast in that you see dust to clean out.)

 

Rodeo, tube is bright but doesn’t create a ghost image through plexiglass. Very even illumination. There is a heater unit plugged in, presumably to keep each and every exposure of fluorescent even. ( I assume this is standard for a cold light heads).

I was referring to condensor light sources, not cold lights or other diffuse light sources. Some condenser heads have a third condensor, others can change the spacing between condensors to cover different sizes of film. My cold lights have always been directly above the negative carrier.

There is a heater unit plugged in, presumably to keep each and every exposure of fluorescent even. ( I assume this is standard for a cold light heads).

Not standard. Quite unusual for cold-cathode heads. But irrelevant to any vignetting I suspect.

 

Even with a 6x enlargement, the effective focal length of a 135mm lens is over 157mm. At the same magnification a 150mm lens would have an EFL of 175mm.

 

Using those figures, the theoretical Cos^4 corner falloff is about 0.66x for the 135mm lens and just over 0.71x for the 150mm lens. That's just over 1/10th of a stop less corner falloff with the 150mm lens. Barely noticeable IMO.

 

The bottom line is: That vignetting is inevitable, regardless of any practical focal length used. And the improvement got from changing lenses might not even be noticeable. Certainly not any more noticeable than, say, printing at f/11 as opposed to f/8.

Although I have never done something for that task, I think in a similar same way center filters are made for LF wide lenses, some kind of mask can be made (placed over the negative) to avoid vignetting.

Anyway, it has never been a problem for most.

I assume vignetting has been measured with a (clear) negative on the carrier ... the way it should be checked.

Edited November 27, 2020 by jose_angel