View Full Version : 150mm LF lens

Ho Pei Jiun
4-Feb-2005, 08:59
Dear Sirs/Mdms,

I've another question:

If i want to focus the Rodenstock Rodagon Apo-Sinaron-N 150mm large format lens at infinity, do i need to use the detachable rear element of this 150mm LF lens?

Please kindly advise. Many thanks.

Ho Pei Jiun

Ted Harris
4-Feb-2005, 09:50
Yes, absolutely you need to use the rear element. The front and rear elements of modern large format lenses are detachable to allow mounting the lenses in the shutter.

You can use the rear element by itself. That is you can remove the front element. The lens i snot designed for this use bu can be used this way to give you a longer focal length if you want to play around. If you decide to do this remember that your aperature scale will no longer be accurate, it will be off by about 2 stops.

Chris Gittins
4-Feb-2005, 15:56
Remove both the front and rear elements and stop down as much as possible. The aperture should be no larger than 1 mm in diameter. That combination will provide good focus at infinity.

Dan Fromm
4-Feb-2005, 16:24
Ho Pei, in my dialect of english the first two replies told you that you're an idiot.

Less insultingly, you need the whole lens. The front cell and rear cell were made to be used together.

Good luck, think more and ask less,


Andrew O'Neill
4-Feb-2005, 22:41
Gee Dan, are you ever rude! You are obviously a rascist too. So when you saw your first LF lens you knew exactly what it was and how to use it??? YOU are the idiot.

Witold Grabiec
4-Feb-2005, 23:06
It's a wonder what kind of brain thinks MORE and asks LESS.

Ho Pei - don't get discouraged, ask at least as much as you think. It's the only way to PROGRESS.

Nature Photo
4-Feb-2005, 23:26
Ho Pei:
I am by far not an expert on this subject, but I know that there are lenses (called 'convertibles') which have different focal lengths if you selectively uses the front or back element or combinations thereof. For example, the Cooke Triple Convertible (http://www.cookeoptics.com/cooke.nsf/0/7fe9d6d60bdcf96a85256e8600546ed2?OpenDocument) or the Wisner Plasmat (http://www.wisner.com/Page15.html) convertibles.
The lens you mention is not a convertible, thus you'll need both elements and the shutter between them. To focus at infinity, you extend the camera, so that the distance between the film plane and flange (=lens) is equal to the focal length, that is 150mm or thereabout.

Dan Fromm
5-Feb-2005, 07:33
Andrew, I translated. I didn't call Ho Pei names, the first two posters did. Excoriate them, not me.

Now tell me, Andrew, are modern 6/4 plasmat type lenses such as Ho Pei's Apo-Sinaron-N separable? Mine aren't and his isn't. That's why I read the first two replies as thinly disguised insults.

If you'd been paying attention, you'd have noticed that Ho Pei has been asking essentially the same question for some time now. He seems not to have understood the answers or, perhaps, not got one that told him what he wanted to hear.

Racist? Where did you get that tripe? This is more anglophone, possibly white man's, 'expletive'.

As for being an idiot, of course I'm an idiot. Its a bad day when I haven't heard "Don't be such a 'expletive' idiot" from my colleagues at least three times. But unlike you I know I'm an idiot. And unlike you, I've learned to read and to seek information from books, not bulletin boards. Bulletin boards are sometimes good ways to find obscure facts. The bulletin board is a poor medium for learning technique, which includes "how things work."



Andre Noble
5-Feb-2005, 09:12
That's a modern lens. Who would think of removing the rear element to shoot with it?

Reminds me of the really basic questions my younger brother used to ask as a kid just to get attention.

Andrew O'Neill
5-Feb-2005, 10:07
Dan, Ho Pei is obviously someone who knows very little about modern 6/4 plasmat lenses or LF for that matter. Still no need to call him an idiot. OKay, maybe you are not a rascist...you came across as one, though. The point of this forum is to help.

Michael S. Briggs
5-Feb-2005, 10:18
One problem with the internet is that from written words it can be hard to detect humor or sarcasm. An answer that is meant as a either a good-natured joke or an insult can easily be misunderstood as incorrect advice, and another answer explaining why a previous answer is an insult can be misread as an insult itself.

Getting to the optical problems: Ho Pei Jiun, I suspect that is one basic aspect of the use of this lens that you are not understanding.

Is your lens in the standard configuration of having the two lens cells screwed into a Copal shutter? Do you know how to operate the shutter? To focus you should move the black triangle so that the shutter blades are open. You should move another level to point to 5.6 so that the aperture blades are fully open.

Here are some simple experiments to see how the lens operates: Take the lens to a room with one window and the lights off so that the room is darker than the outdoors. Hold the lens in one hand with the front facing the window. With the other hand, hold a white card behind the lens. Change the distance between the card and the lens until you see an image of the view through the window on the card. The reason for doing this in a room with dim light and a bright window is so that the room light doesn't overwhelm the light from the lens that makes an image on the card. This is a simple example of the focusing procedure that Jim described in answer to your previous question (www.largeformatphotography.info/lfforum/topic/500885.html). As I explained, the card should end up close to 142 mm behind the back of the Copal shutter.

A second experiment: go to a room without windows and a single light. Hold the lens with the front facing the light and at a distance of one or more meters. Now hold a white card behind the lens and adjust the spacing until you see an image of the light. This time the distance from the white card to the lens will be larger, according to the basic focusing law of optics. You can read about this equation at the Lens Tutorial: http://www.photo.net/learn/optics/lensTutorial (http://www.photo.net/learn/optics/lensTutorial).

These experiments should show you how your lens works. Your next step is to replicate this spacing in the camera that you are building, along with some method of adjusting the focus. If you want a fixed focus camera without any means of adjusting the focus, you will need to make accurate measurements of the actual focal length of this lens, which will be slightly different from the design value, and you will also need to know the distance between the principle points or planes. A simple but tedious method of adjusting the focusing is to use spacer shims that you add or remove between the lens and the camera.

Reading some books or showing your setup to an experienced photographer are good ideas. Perhaps you have access to a library with books about optics?

Tom Diekwisch
5-Feb-2005, 15:56
Gosh, there are no stupid questions. Only stupid answers.

justin mueller
21-Feb-2005, 14:38
Wow! How many smart replies! My Rodenstock sironar has two aperture scales on it (maybe Ho Pei's also has) so before you guys start cooke-ing on plasmats check out what Rodenstock lens you have. The answer is : you can focus on infinity with both elements on (the lens focal length will be 150mm), and you can focus on infinity with the rear element and the focal length will be 450mm (at least on my lens that is) - providing you camera can pull 450mm bellows draw at least.