Triplet Lens Modification and resulting FL

[davido]

Hello

If someone where to remove the front two elements (in front of the shutter) and leave only the rear element ( plano-convex?) of a Triplet lens, what would the new Focal Length of the single element be?
Is there a way of knowing this without actually testing the lens?


David

 

[Ian Grant]

Try it, you can't calculate the rear FL without knowing the front FL and the other parameters.

Ian

 

[davido]

Try it, you can't calculate the rear FL without knowing the front FL and the other parameters.

Ian

I don't own the lens, but researching options.

I am looking for a TLR camera with a Triplet lens, which can adapted by only using the rear lens element. The result would be a TLR which shoots like a Holga but with much more control. There would also be the option of removing all lenses from the shutter and using a different meniscus or Plano-convex lens behind the shutter.

I know that others have utilized just the rear element for this effect but wondered if there was any way to find out if the FL would work other than by chance? If the lens is an 80mm, would the rear element be 80mm? I'm thinking probably not?

The lens I am looking at is a Yashikor 80mm Triplet 3.5 from a Yashikor D.

David

 

[Nodda Duma]

I can work you through the design reasoning.

Consider the Cooke. Between the positive-negative-positive you'll end up with the total focal length. The two positives are really a split of the positive element of what is essentially an achromat, and that original achromat has a long positive focal length element and shorter negative focal length element. A split positive has two shorter focal length elements, and roughly what you start with in the design is something like an +80 / -80 / +80 combination to get 80mm focal length. That's what the designer *starts* with...not what the design ends up at.

Balancing aberrations will shift the focal lengths a bit. So one positive will be longer and one will be shorter. I'd guess by no more than 30% but depends on what the designer did. What I couldn't say without knowing the details of the design is which one is shorter or longer. I *think* the longer is usually in the back for aberration correction.

If the front surface of the first element is strongly curved then the rear lens will have a longer focal length (i.e. farther from 80mm). So you'll need to be able to extend your focus travel to accommodate something maybe like 100mm. Keep in mind that's an educated guess on my part.

Don't know if this helps but maybe it will.

 

[pagonzales]

I don't have a yashikor but I have a spare 80mm triplet taking lens from a ricohflex TLR. The rear element forms an image on a ground glass around 30-40mm from the retaining ring of the rear element mounted in shutter. The image is pretty wide with lots of barrel distortion but yes, it does give you a nice holga-ish look.

 

[Denverdad]

That's what I was going to say - with the rear element behind the iris you should theoretically get barrel distortion, unlike the pincushion distortion of the Holga which results as I recall from having it's lens in front of the aperture. You may like the effect just as much or more though. If you're willing to experiment a little you might also want to see if you prefer the results better with the rear element installed as designed, or reversed.

Unfortunately I don't know any quick way to determine the focal length of the rear element without either having the prescription in hand, or else just measuring it. Maybe there is some rule of thumb or typical relationship among the lenses that will get you in the ballpark? Not sure.

 

[davido]

Thanks everyone for the great info!

Denverdad, I had forgotten about the difference in distortion from behind verses in front of the aperture. The thing is is I would really prefer to create a lens with shorter focal length (as I mainly shoot landscape type images) hence installing behind the shutter.

THanks for trying out your Ricoflex rear element Pagonzales. I had the feeling that the FL was going to be quite different with the separate elements. So it looks like installing a different lens might be the only way to go, especially since there really isn't much flexibility with focus movement with most of these TLRs (unless you have a Mamiya C series with bellows)

Nods Duma, thanks for the explanation of how a triplet works. I am able to grasp most of what you say. So if the rear element has a longer FL of around 100mm, perhaps I could use the front element moved to behind the lens, as it would be closer to 60mm? Not sure if those two front elements can be separated though?

It seems that the odds of any of the elements working on there own within the depth of specific camera body is doubtful. I more likely to find a single meniscus of the actual focal length (film plane to lens distance). Trying to find a simple lens of the right diameter and FL is going to be a challenge

In the end, I may end up building a TLR camera around a set of lenses. It would be a very shallow body of around 50-60mm with the ground glass on the back instead of top (thereby not needing a mirror). I would also use a film back with a winder on it. Down the road ...

David

 

[Nodda Duma]

davido check the lens catalogs like Edmunds or Thorlabs. You should be able to find a singlet close to 80mm

 

[bdial]

Surplushed is another possible source, they have a large stock of lenses from various manufacturers.