another light fall-off question

[djkloss]

I just noticed when making a 16x20 print from a 35mm negative more light fall off than is acceptable. I'm using a 50mm lens on an Omega D2 Superchromega with a 2 1/4 mixing chamber. I checked the light source with no negative in the carrier with my vintage Heathkit Ohm Meter and there is definitely fall off. I also checked the other lenses and mixing chamber and they all have some level of fall off.

Is there any way of correcting this? I'm guessing it increases when using high contrast filters. I tried burning in the side, but that takes more skill than what I have. Also, if I stop down, the exposure becomes extremely long.

Any suggestions?

Thanks!

Dorothy

 

[Claire Senft]

Pull out your mixing box and examine it to make sure that it appears appropriate and does not have say one edge that has darkened or yellowed etc. If you do then perhaps a new mixing box is in order. A decent enlarging lens stopped down 2 stops show not be causing a problem of the sort you mentioned in your post.

 

[fschifano]

May not be significant. Could be that the sensor you're using doesn't see the light the same way in the center of the field as it does at the edges. My enlarger is set up similar to yours, and I use a Jobo Comparator 2 enlarging meter. If I measure the light cast onto the baseboard with a blank piece of film in the negative carrier, the meter reads off by 1/3 stop at the edges. It doesn't print that way, and is very even if I print that blank negative to a medium grey. How does yours print? Is it noticeably darker in the center if you make a print as I described? If yes, then you really have a problem. If not, your Heathkit machine is not telling the truth.

 

[djkloss]

Thanks Frank,
I used the ohm meter because my large prints are so much darker in the center. (My dad always used this method in his darkroom) It seems more so when the enlargement is from negatives of 35mm to 16x20 print. I assumed it was from the inverse square law thing about light and the large print shows it more than a 4x5 would.

Thank you Claire, I wonder if it is possible to take the mixing box apart and clean the glass without destroying it.

something else I've noticed - the infrared filter on the mixing chamber is clear at the center - instead of solid. would that have any effect on light? I read that it's function is to keep heat out of the box.

Dorothy

 

[Nicholas Lindan]

How much light fall off are you measuring? If you are using a CdS photocell then they (very roughly) change resistance by 2x for every stop change in light intensity.

 

[djkloss]

How much light fall off are you measuring? If you are using a CdS photocell then they (very roughly) change resistance by 2x for every stop change in light intensity.

what's a CdS photocell?

 

[Claire Senft]

I am not an owner of a Super Chromega and I am not up on the construction of the mixing boxes used by them. It may be that what you are seeing on the IR filter is normal and part of its design. Is it possible to contact the distributor or manufacturer to get some advice?

 

[bobwysiwyg]

what's a CdS photocell?

http://en.wikipedia.org/wiki/Photoresistor

 

[Nicholas Lindan]

The small thing I imagine you have connected to the ohm-meter. It should have a serpentine or zig-zag pattern on the sensor face, though it may just have a line down the middle of it. If the sensor face is a dark-bluish square then it is a photo-diode. Or, it could be a photo-transistor ...

 

[djkloss]

I am not an owner of a Super Chromega and I am not up on the construction of the mixing boxes used by them. It may be that what you are seeing on the IR filter is normal and part of its design. Is it possible to contact the distributor or manufacturer to get some advice?

I'm guessing this enlarger is from the 1950's or 60's. However, there is a guy in Chicago at Glennview who sells parts and also Harry at Classic Enlargers. I was trying to save some money.

 

[djkloss]

The small thing I imagine you have connected to the ohm-meter. It should have a serpentine or zig-zag pattern on the sensor face, though it may just have a line down the middle of it. If the sensor face is a dark-bluish square then it is a photo-diode. Or, it could be a photo-transistor ...

Is that like Cadmium Sulphide? If so, yes it's a photocell like the kind found at Radio Shack. I don't know what strength.

 

[Nicholas Lindan]

What change in resistance are you seeing?

 

[ic-racer]

Dorothy,
Two things. First, most enlarging lenses have some falloff that is 'normal' and unavoidable. What lens are you using. I have the datasheets on my Schneiders and they clearly show the falloff. For this 'normal' lens falloff, I just burn the edges with a round paddle. This 'lens' falloff increases as you magnification increases. This may be why you are noticing it now with the 16x20s.

Now, if the light box is not even, or if it is a coldlight, a simple circular paddle may not work. With my coldlight I wound up making a special 'center filter' that was an exact inverse of the uneven light pattern.

http://www.largeformatphotography.info/forum/showthread.php?t=40384

 

[Claire Senft]

Noticable fall of with conventional photo materials when a good quality enlarging lens stopped down 2 stops on a proper enlarger should not be a problem.

 

[djkloss]

What change in resistance are you seeing?

I just went to check the resistance. Somehow, I lost my light source. Checked 3 bulbs and no light. So I'm up a creek with a broken paddle. Could be a fuse, spent light bulbs or everything just dies all at once.

I opened up the mixing chamber, doesn't look like anything is wrong with it, it doesn't look dirty.

I agree, I think the size of the enlargement does have something to do with it if there is light fall off when wide open, and also about stopping down. However, I didn't stop down because of the exposure time being so long at grade 5. A few minutes anyway. I was going to try stopping down, but then the light wouldn't come on. It's just not my day

The lens is 50mm EL Nikor with a 2 1/4 x 2 3/4 mixing chamber and a flat lens board (not a cone).

Thanks for your responses everyone.

As for the light bulbs. Can someone tell me if I can test them with a multimeter? I have one, but don't know anything about them. They're 24V 250W ELC bulbs by GE. I hope it's the bulb and not something else. you can pm me about that

Thanks again.

 

[Lee L]

Is your lens an f:4 or f:2.8?

Ctein evaluated the El-Nikkor 50mm f:2.8 N in Post Exposure. He found the optimal aperture to be f:4, and performance to start dropping smaller than f:7. At f:4 falloff at the corners was 0.3 stops. If you're printing at f:2.8, you'll see more falloff. You might try f:5.6 or f:8 to see if it evens out more there.

Sorry about your electrical problems. Hope it's just a fuse.

As for your printing times, there is a lever on the right side of the Super Chromega D that controls the light output. When it's horizontal, all filters are taken out of the light path. When vertical, it inserts a diffusion grid that cuts the output 2 stops. When at 45 degrees, the color filters are inline, but the little diffuser grid isn't inline and the output is two stops greater than with the lever vertical. For maximum light output, put the lever at 45 degrees.

Lee

 

[Nicholas Lindan]

If you are enlarging with the lens wide open then fall-off is going to be horrid - and that is going to be true of any enlarging lens. Stopping down one stop will usually fix things and bring the fall-off down to an acceptable level.

 

[ic-racer]

Here is a graph showing the falloff of a Schneider 50mm, you lens is probably similar. As pointed out, the falloff is worse when wide open and worse at bigger enlargements.

The two solid lines show the falloff at a high magnification. The lower line is wide open and the upper solid line is f8.

It shows that wide open the edges will be a little over one stop darker.

 

[djkloss]

thank you for the graph and the info. It's good to know there are so many knowledgeable people out there willing to help.

the lens is an f/4. I also replaced the bulb, so I know it's not the switch or fuse.

thanks again!

 

[jp80874]

Dorothy,

You made a lot of friends at the 2008 Sept. Northern OH gathering. This thread looks like our attendance form, except for Claire. Come join us next year Claire.

Dorothy, stay away from "guy in Chicago at Glennview". That is Glennview. If you search here you will find lots of negative feedback including mine.

The University of Akron darkrooms have 35 Omega enlargers. I don't remember what model. I do remember critique after critique where the professor told the students to burn in the sides on 11x14 or larger enlargements from 35mm. Think how many times that 35mm negative has to be magnified. 16x20 and larger can be done, but every little problem is magnified many times.

It is such a common problem that the man who built my 8x10 enlarger used a 12x12 cold light head instead of the 10x10 you would expect to limit this problem by not using the outer area of light.

Hope you get it working soon.

John Powers

 

[djkloss]

Dorothy,

You made a lot of friends at the 2008 Sept. Northern OH gathering. This thread looks like our attendance form, except for Claire. Come join us next year Claire.

You're making me homesick! If I thought I could sneak away I would. Anyone ever do winter camping? It's a blast! (I mean in a tent). I should have posted this question over at the Northern OH thread.

Dorothy, stay away from "guy in Chicago at Glennview". That is Glennview. If you search here you will find lots of negative feedback including mine.

The guy at Glennview... I bought a 16x20 easle for 300 + shipping. It's not square. I don't want to spend the money to send it back. Unfortunately he has most of the Omega parts.

The University of Akron darkrooms have 35 Omega enlargers. I don't remember what model. I do remember critique after critique where the professor told the students to burn in the sides on 11x14 or larger enlargements from 35mm. Think how many times that 35mm negative has to be magnified. 16x20 and larger can be done, but every little problem is magnified many times.

I could always move up to 8x10 - or at least 5x7 - maybe just do 4x5...

I'm learning so much, thanks... Now I have to try it out at f/8. But now I have good lights with a CRI of 90 - thank you Lee! and you can't see the spots even in selenium spot tone

Thanks all!

Dorothy

 

[jp80874]

"Unfortunately he has most of the Omega parts"
Try eBay. They have most of the parts for everything.
I just got parts for Dolly's 30 year old sewing machine and our old Electrolux vac.

"I could always move up to 8x10 - or at least 5x7 " You should have come to the rematch a month later. Under Jeff's watchful eye I took Eva to a grave yard and then the covered bridge to shoot my 8x10. She liked it. Jeff later said that the pics came out well except for the sheet of film I had double loaded in the film holder. I said no more small stuff. She has to shoot 7x17 next time. That means the 8x10 is available.

John

 

[djkloss]

"Unfortunately he has most of the Omega parts"
Try eBay. They have most of the parts for everything.
I just got parts for Dolly's 30 year old sewing machine and our old Electrolux vac.

"I could always move up to 8x10 - or at least 5x7 " You should have come to the rematch a month later. Under Jeff's watchful eye I took Eva to a grave yard and then the covered bridge to shoot my 8x10. She liked it. Jeff later said that the pics came out well except for the sheet of film I had double loaded in the film holder. I said no more small stuff. She has to shoot 7x17 next time. That means the 8x10 is available.

John

I wish I'd seen this sooner... must be that AADDS kicking in...

Anyway... too late, I bought my friends 5x7 since no one else seemed to want it. Maybe next year I'll be into it.

p.s., wish I could be there on Wednesday at your gig, but I have another artiste's obligation/Christmas Party.

I'll have to go back through the thread to see when the dates are for your show, unless you want to remind me

cheers.......

 

[youngrichard]

Stopping down to reduce light fall-off

I just noticed when making a 16x20 print from a 35mm negative more light fall off than is acceptable. I'm using a 50mm lens on an Omega D2 Superchromega with a 2 1/4 mixing chamber. I checked the light source with no negative in the carrier with my vintage Heathkit Ohm Meter and there is definitely fall off. I also checked the other lenses and mixing chamber and they all have some level of fall off.

Is there any way of correcting this? I'm guessing it increases when using high contrast filters. I tried burning in the side, but that takes more skill than what I have. Also, if I stop down, the exposure becomes extremely long.

Any suggestions?

I have just run into this problem which I had not had before, printing 20x16 from a 35mm neg on a Durst DA900 with Ilford 500 head. I have limited headroom in the cellar so to get 20x16 I swapped from my usual El-Nikkor 50mm f2.8 to an Apo-Componon f4/45mm lens and printed with lens wide open. Light fall-off at edges was like a vignette. Looked in all my technical books - nothing on the subject. Investigation with meter showed fall-off at edge was 1 1/2 stops at f4, but this reduced to 1/3 stop at f8, which when printed is barely discernible, or at least OK so far as I am concerned.
I've got "Basic Photography" and "Advanced Photography" by M J Langford; Ansel Adams "The Print"; Tim Rudman's "Master Printing Course"; and manuals for various enlargers. Advice to stop down a couple of stops is frequent but relates to edge sharpness; no one seems to mention reducing edge light fall-off. Or have I missed it? It seems important to me; but perhaps it is just a problem when using a wider angle lens than recommended to avoid swivelling the head and printing on the floor.
Richard

 

[djkloss]

I have limited headroom in the cellar so to get 20x16 I swapped from my usual El-Nikkor 50mm f2.8 to an Apo-Componon f4/45mm lens and printed with lens wide open. Light fall-off at edges was like a vignette.
Richard

My Omega has a recessed lens board. Maybe you could try something like that?