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View Full Version : How to picture an enlarging lens in practice?



John D Gerndt
29-Dec-2003, 20:51
I see in my mind two cones stacked tip-to-tip, the larger cone on the bottom, with the lens at the meeting of the tips. How does one go about determining the size relationship of these two cones? It seems like they should exist in some sort of limited and shifting relationship bounded by how the lens was formulated.

Can one get the details? I am specifically interested in the El Nikkor 210mm. Many thanks.

Jay DeFehr
30-Dec-2003, 00:51
Hi John. I'm not 100% sure I understand what you want to know, but I'll give it a try. When the distance from the negative to the lens is double the focal length of the lens, the projected image will be 1:1, or the same size as the negative. As the lens moves closer to the negative, the projected image is magnified, or enlarged. When the distance from the lens to the negative is 1/10 of the distance from the lens to the baseboard, the image is magnified 10x. So, for a 210mm lens, 1:1 magnification occurs when the lens is 420mm from the negative, and 420mm from the baseboard, or easel. A 4x5 negative will project a 4x5 image 420mm from the lens to the easel. To double the magnification the lens must be 1/2 the distance from the lens to the negative as the lens to the easel. When the projected image of your 4x5 negative is 8x10 on your easel, the lens will be 1/2 the distance from the negative to the lens as the lens to the easel. For a 210mm lens, the lens will be 315mm from the negative and 630mm from the easel. The formula is u = F x ( 1 + 1/m) F = focal length of lens m = image magnification u = lens to negative distance

I hope this helps.

John D Gerndt
30-Dec-2003, 17:23
Thanks Jay, the formula helps. The thing I wanted to understand is how much coverage a given focal length gives, thus the cone image in my mind. I know it depends a great deal on the lens design. I wonder if at low magnification one might use a “size under” focal length and thus reduce enlarger height and the associated errors. It has never been recommended and I wonder why.

Cheers,

Jay DeFehr
30-Dec-2003, 17:57
The size under approach will work given enough coverage, and I've done it, but it probably isn't recommended because of the drop off in definition towards the edges of a lens' coverage. I shoot portraits and can get away with that sort of thing more than the corner to corner sharpness guys can. If you can find your lens' angle of view, you can use the formula I posted to detrmine its coverage at a given magnification. Alternatively, you could just put your negative in your enlarger and check sharpness with a grain focusser. Good luck.

John D Gerndt
31-Dec-2003, 20:40
Jay, if you’re still interested,



My enlargements of 8x10 negs to 20x24 using that 210mm all look fine to the naked eye but I don’t own a first class lens for 8x10 – I am using a Protar V, f18, ment for 6.5x8.5 so my corners are not so hot even at f45. I should think of myself as a 6.5x8.5 shooter, I have got that covered, but how can I let all that film go to waste?



I tried testing the 210mm El Nikkor for true 8x10 coverage at 2.5 magnification using 5, 35mm negatives, one at each corner and one in the center of my 8x10 carrier. They all printed/looked fine to the naked eye but all looked like crap under an 8X lupe. The 35mm negs may be crap (I didn't think so when I shot them); maybe everything looks like crap at 20x! I will need to dig deeper. Maybe the negative carrier is not what it should be. Maybe I just shouldn’t look so hard.



I know the parallelism is in order because I used a laser to build and test the design I am using. I don’t know if I will actually see the gains that may be had, but I am going to rebuild anyway; I will be using that formula to design a new negative stage anticipating the day I can buy the proper lenses. Thanks for the input; it is much appreciated.

Jay DeFehr
1-Jan-2004, 08:37
Hi John.

If your prints look fine to the naked eye, I think you've succeeded, despite the technical obstacles. The only time I look at prints with a loupe is when examining rollfilm proof sheets. Your standards are clearly much higher than mine. I am happy to have helped in some small way. I wish you continued success in all of your endeavours.

CP Goerz
2-Jan-2004, 11:52
Perhaps a better taking lens would also help you get better enlargements. A 141mm Series V isn't the best source to enlarge from.

CP Goerz