Bellows Extension versus Focal Length....

[Michael Graves]

Correct me if I'm wrong; but if you have a 210mm Componon lens and an enlarger with a 10.5" bellows extension (appx. 250mm), shouldn't that lens be able to focus the negative onto the baseboard? Or am I missing something here? I just acquired this lens and am trying to use it on an old Elwood 5x7 enlarger, and I don't enough bellows draw to bring the negative into focus.

[Dan Fromm]

See my comments re distances in this thread: http://www.largeformatphotography.info/forum/showthread.php?t=18393

Top of the second page.

[Michael Graves]

Well, that makes sense when shooting with a camera and dealing with macro situations. It also explains what I've been doing wrong when focusing in closeup situations. I'm going to try that approach next time. However, I'm dealing with an enlarger. The lens plane is a fixed point. Does bellows draw therefore change with magnification? Doesn't seem to matter with this setup. I can't focus on the negative.

[Leonard Evens]

I'm assuming that by "bellows extension", you mean that the lens is at the end of a bellows with the negative carrier at the other end. So the distance between the film and the lens at full extension would be 250 mm. According to the lens equation, if the focal length is 210 mm, and the negative is 250 mm from the lens, then the distance from the lens to the projected image is about 1.3 meters or 4.3 feet. I doubt that you can raise the enlarger head far enough to arrange that. It would appear that the enlarger is designed to be used with a much shorter focal length lens, but of course you would have to find one which covered. Alternately, if the head turns at right angles, you might be able to project on a wall. Finally, you can remove the baseboard, bolt the enlarger to something else and project on the floor.

[Dan Fromm]

Michael, its the same problem. You should be able to move the enlarger head up and down, also change the lens-to-negative distance.

Cheers,

Dan

[Leonard Evens]

Perhaps I don't understand the enlarger setup. With the bellows at full extension, what is the distance between the film and the lens? If it is 250 mm, you are clearly going to have a problem focusing on the baseboard. After all, if that distance were 210 mm, the projected image would be at infinity.

Is it possible that you can put the negative much further away from the lens. It doesn't really make sense for a 5 x 7 enlarger not to allow you to put the negative more than 250 mm from the lens. A normal enlarging lens for 5 x 7 would be about 210 mm, and we see that you are going to have problems unless the enlarger is truly enormous. If it were, it would be very awkward to use.

For the record

The reciprocals of the two distances, distance from lens to film and distance from lens to projected image, add up to the reciprocal of the focal length.

That is the lens equation, and you can use it to answer all questions of this nature.

[robc]

I may be off the mark here as I am not familiar with your enlarger. However, some modern enlargers have two means of setting magnification/focus. One is for fine adjust, i.e. the focus knob and the other is for coarse/approximate adjsutment. The fine adjustment needs to be in approx middle of its adjust range before adjusting the coarse adjustment so that you can then fine adjust up or down. If the fine adjustment was at one end of its adjustment when you adjust the coarse adjustment, then you will hit the end of its adjustment in one direction which is probably what has happened.

Also, some enlargers need an extension or recessed tube fitted to lens stage to allow them to focus some lenses. All depends on your enlarger and its adjustment mechanism.

[robc]

Well, that makes sense when shooting with a camera and dealing with macro situations. It also explains what I've been doing wrong when focusing in closeup situations. I'm going to try that approach next time. However, I'm dealing with an enlarger.

An enlarger is a dedicated Macro camera. Subject is negative. Object is print. Maginification is 1:to more than 1 i.e. just like using your camera for macro images where magnification is 1:1 or more. So a 5x7 neg to a 10x14 print is 1:2 magnification. That is Macro. Your enlarger IS a macro camera...

[Nick_3536]

X=(m+1)/m*f

X is distance from the negative to the lens point.
m is magnification
f focal length

So changing that to something more useful

x/f=m+1/m

250/210= m+1/m

If the table I have is right m is about 5.2

So with a 5x7 negative the smallest print you can make is over 25x35. Feel free to correct my math -)

[Leonard Evens]

Nick is right. More exactly

250/210 = (m + 1)/m = 1 + 1/m. But 250/210 = 25/21, and that means 1 + 1/m = 25/21, and 1/m = 25/21 - 1 = 4/21. That means m = 21/4 = 5.25. So with a 210 mm lens and a maximum extension distance from film to lens of 250 mm, the smallest print you can make from a 5 x 7 negative is about 26 x 36 inches. But to do so, as I've previously indicated, the lens would have to be 4.3 feet above the projected image.

There are several possibilities about how the Elwood 5 x 7 enlarger is supposed to be used. I don't have such an enlarger to look at, but some searching of the web suggests some possibilties. First it may in fact be that large, although I tend to doubt it. Second, it is intended to be used with a shorter focal length lens. I've seen a couple of ads for 5 x 7 Elwoods, one with a 7 1/2 inch lens and the other with a 6 1/2 inch lens. Third, there may be extension lensboards which allow you to increase the distance between lens and film. I saw one ad for a 12 inch extension, but I don't know what that might mean. Finally, it may be possible to put the film further up away from the lens.

If Michael or someone else were to fill us in on some more details about the enlarger, perhaps we say something more helpful.

Of course, the enlarger satisifies the same laws of optics as a camera at macro distances. But the previous discussion is not relevant, not because it doesn't apply in principle, but because quantitatively, it doesn't work out. With a 210 mm lens, you need significant extension from film to lens to make a small enlargement. If the enlarger doesn't allow it, you can't do it.

[Nick_3536]

I think somebody cut the bellows down. Mine is currently not assembled but I measured the bellows. I've basically got 380mm from the negative stage to the lens stage. I think I measured it right. It's certainly alot more then 250mm. Odds are somebody in the past was using it for a smaller format and decided to cut the bellows down for some reason. Maybe they had a hole in them.

[Leonard Evens]

Well, at long last, thanks to Nick, we have a solution. Get a new bellows of the proper length.

My guess is that the bellows was cut down to allow the use of much shorter focal length lenses with smaller formats. In that case a full bellows wouldn't have allowed a small enough minimal extension.

[Brian Ellis]

"I don't [have] enough bellows draw to bring the negative into focus."

If you're saying that your bellows isn't long enough to bring the negative into focus raise the easel by putting a book or something else under it so that the paper is closer to the lens (i.e. same effect as a longer bellows but from the opposite direction).

[Jim Noel]

Someone must have altered the bellows on your Elwood.
I have the same enlarger on which I use a 210mm lens with no problem. I have even used a 300 on a few occasions.

I can think of no way out of your situation other than a shorter lens. Since the lens is further removed from the negative in an enlarger than is true of a camera at infinity, a 165mm or 180 mm lens will cover the 5x7 negative.

Jim

[Turner Reich]

The head can't go high enough for the 210 bring the negative into focus. The head must go higher away from the baseboard or a shorter wider angle enlarging lens must be used. If you can put a spacer block to rase the head up you will have solved the problem without having to buy another lens.

Here is what you can do to see what the distance is for given print size:

Take the lens out of the enlarger.
Put a negative in front of a light, like a desk lamp.
Put the lens out from the negative and project it on a sheet of white cardboard.
In the dark you will be able to determine the length of bellows needed with a bellows setup.
This crude setup will be good enough to project a circle that you can see.
If the distance is greater than your enlarger setup then you will know exactly how much.
If you need more distance in the enlarger then you can make a wood lens extension box to hold the enlarging lens down from the enlarger. The most inexpensive and useful setup. It's a common solution to using taking lenses on bellows camera where the lens is of longer focal length than the camera bellows.

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