Help! Weird Light Leak on the Baseboard - LPL 7451/ Apo Rodagon-n 80mm

etzet · Sep 7, 2020

I just acquired a LPL 7451 (or known as Sanders 4500) with a apo rodagon-n 80mm. It has this weird cycle light leak as the picture shown. I couldn’t figure out what the problem was or where the problem was, like it is the lens’ problem or the enlarger’s problem. I try to pinpoint the problem by switching the nikkor 50mm lens I have on the enlarger. The problem was gone, but when i installed the rodagon lense on my durst m370, the problem was also gone. So I am a bit confused. Does any one had similar problem or does anyone know how to resolve this?

bernard_L · Sep 7, 2020

Looks like flare inside the Rodagon, IMO. When you mount it on the M370 (24x36mm enlarger) the extent of the light source (the film gate) is less, the cone of light entering the lens is more restricted, and presumably this avoids having some light hitting a flare-producing part; maybe(??) an element's edge or a metal ring, improperly blackened.
Grab the Rodagon, peer into the front element, especially at oblique angles, while pointing the rear of the lens at an extended light source: clouds, LED flat panel, whatever. This might allow you to discover the origin of the flare.

MattKing · Sep 7, 2020

Try it with a focused negative. You may find that the flare disappears, or is reduced enough to not matter.
Welcome to Photrio.

ic-racer · Sep 8, 2020

Most lense do this to some extent, have you tried you other lenses to see? Occasionally it can be an issue if you easel blades are not wide enough, otherwise you can ignore it.

etzet · Sep 9, 2020

bernard_L said:
Looks like flare inside the Rodagon, IMO. When you mount it on the M370 (24x36mm enlarger) the extent of the light source (the film gate) is less, the cone of light entering the lens is more restricted, and presumably this avoids having some light hitting a flare-producing part; maybe(??) an element's edge or a metal ring, improperly blackened.
Grab the Rodagon, peer into the front element, especially at oblique angles, while pointing the rear of the lens at an extended light source: clouds, LED flat panel, whatever. This might allow you to discover the origin of the flare.

Thanks, Bernard.

I have looked into my lens. It indeed have something weird. The lens is not in a very good condition. I will probably consider to invest in another good lens and sell the old one who may find this tolerable.

etzet · Sep 9, 2020

MattKing said:
Try it with a focused negative. You may find that the flare disappears, or is reduced enough to not matter.
Welcome to Photrio.

Thanks MattKing!

I tried it with a negative on, and the ring light leak was invisible by eyes. I am planning to use my incident light meter to test how much light leak was still there, but Im not sure if this will work

etzet · Sep 9, 2020

ic-racer said:
Most lense do this to some extent, have you tried you other lenses to see? Occasionally it can be an issue if you easel blades are not wide enough, otherwise you can ignore it.

Thanks, ic-racer.

Good to know that it was not an uncommon problem for the lens. I will test whether this really affect the print outcome next time.

bernard_L · Sep 9, 2020

Enting Zhou said:
I will probably consider to invest in another good lens and sell the old one who may find this tolerable.

Before you do that, maybe you should take into account the advice of MattKing and ic-racer.

Proper test. as follows. Use soft paper (or soft multigrade filter #0); that is worst-case.

No negative in carrier (as in your picture in post #1, use same diaphragm opening).
Paper inside bright area on easel, find minimum time t1 for maximum black.
Another piece of paper in the circular ring outside the image rectangle. Expose for t1, with half of paper masked under black paper. Any signs of ring (light gray)?
Repeat with 2x t1, 4x t1. If still no trace of the ring on the developed paper, I'd say the effect that you show, can be neglected in practice.

A possibility also is that the effect is (in proportion to the illumination of the nominal image rectangle) less important when the diaphragm is closed down, because some peripheral rays are excluded. Normally you should perform the exposure closing down 2 stops from max aperture.

etzet · Sep 24, 2020

bernard_L said:
Before you do that, maybe you should take into account the advice of MattKing and ic-racer.

Proper test. as follows. Use soft paper (or soft multigrade filter #0); that is worst-case.

No negative in carrier (as in your picture in post #1, use same diaphragm opening).

Paper inside bright area on easel, find minimum time t1 for maximum black.

Another piece of paper in the circular ring outside the image rectangle. Expose for t1, with half of paper masked under black paper. Any signs of ring (light gray)?

Repeat with 2x t1, 4x t1. If still no trace of the ring on the developed paper, I'd say the effect that you show, can be neglected in practice.

A possibility also is that the effect is (in proportion to the illumination of the nominal image rectangle) less important when the diaphragm is closed down, because some peripheral rays are excluded. Normally you should perform the exposure closing down 2 stops from max aperture.

Wow thank you for this detailed instruction on how to do this test. I haven't set foot in my darkroom or this website for about a month now, cause school started lol. I will try conduct this experiment when I have time. Thanks!

Help! Weird Light Leak on the Baseboard - LPL 7451/ Apo Rodagon-n 80mm

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