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04-26-2012, 04:08 PM   #1
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Lens modification

Hello,

I need a little help to solve a problem....

I will use a Pentax K1000 camera and a Vivitar 28mm objective to build a project.

For this project I will mount the objective 1" away from the camera with an extension tube.

When doing this, as expected, I lose the ability to focus beyond 6".

I need to solve this and restore the lens ability to focus to infinity.

Do you have any ideas?

One possibility is to dissasemble the lens and reposition the glass housing 1" back. Is this possible?

Second possibility is to add a special glass between the film and the rear element. In this case, what kind of lens should I buy?

And third possibility is to add a special glass lens on the front element. Again: what kind of lens should I buy?

I would love to hear your opinions and suggestions.

04-26-2012, 05:32 PM   #2
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https://www.pentaxforums.com/forums/pentax-lens-articles/152336-cheap-macro-b...lose-work.html

QuoteOriginally posted by gkyetes75 Quote
I will use a Pentax K1000 camera and a Vivitar 28mm objective to build a project.

For this project I will mount the objective 1" away from the camera with an extension tube.
What is the purpose of this project?

QuoteQuote:
One possibility is to dissasemble the lens and reposition the glass housing 1" back. Is this possible?
Almost anything is *possible*, but this might not be *practical*.

QuoteQuote:
Second possibility is to add a special glass between the film and the rear element. In this case, what kind of lens should I buy?
I'll give the same answer -- this probably isn't practical.

QuoteQuote:
And third possibility is to add a special glass lens on the front element. Again: what kind of lens should I buy?
If you just want to shoot macros+closeups with the lens, and also to be able to shoot normally, there are better ways than modifying the lens. See my article above for details. Here are the simplest options:

* Get a set of inexpensive close-up optics. These are usually sold as sets of +1+2+3 or +1+2+4 dioptres. Screw one of those onto your Vivitar and you can shoot close. Unscrew it, and the lens is as normal.

* Use extension (macro) tubes. The cheapest do not support any aperture automation. Your camera is what we call M-type, with a mechanical aperture linkage. I haven't seen macro tubes with M-type linkage, but they must exist. An inexpensive alternative is to find an M- or A-type teleconverter, and remove the glass. 2x TCs are usually about 25mm thick, so one of those on the Vivitar 28mm will give nearly 1:1 magnification.

With both of these options, you need not modify nor damage the lens. Just add or remove the adapters or extension to switch between macro and normal work.

Another alternative, which I mention in the article, is to get a cheap SMC-A 35-80/4-5.6 zoom, and a PK-49mm mount-reversal adapter, very inexpensive. The lens may be the worst Pentax ever made when used normally. But reversed, it is quite decent. It can be used for very close macro work, but will also focus beyond infinity. You should also get a cheap PK macro tube set; use a section of tube as a lens hood. Good luck!
04-26-2012, 06:01 PM   #3
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Hi, gk
You can study ray optics.
And the practical experiments are fun:
get:
a book-end (2 pieces of flat wood at 90 degrees)
a stainless steel rule, mm scaled
a roll of 3M Magic Tape

Tape the rule parallel to the long side of the book-end, and tape some white paper to the short side.
Dis-assemble the lens to the "lens groups" (If it is an old lens of no value)
In a dark room with a window, point the book-end to the window, with the rear group near 40~ 50 mm from the paper.
focus the window sharply on the paper by moving the lens.
Now add the front lens, etc.. and try to answer your questions.
04-26-2012, 06:05 PM   #4
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Quite a bit depends on exactly what your constraints are; here's some possibilities given what we know of your constraints.

A 1.4x or 1.5x teleconverter is usually about 1" thick, but increases the 28mm lens focal length by 1.4x or 1.5x. .so if the 28mm lens can transform to a 36mm lens that'd work to give you the inch extension you seek

If the actual focal length must be around 28mm when you finish, you might use a 1.4x teleconverter with a 20mm lens.

If the 1" extension can be increased a bit I think there are a variety of "2x macro focusing" adapters that will focus at infinity....But they have a field of view when focused at infinity like a lens of 2x the original focal length, so would turn the 28mm lens to 56mm.

Dave

04-26-2012, 06:14 PM   #5
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QuoteOriginally posted by gkyetes75 Quote
Hello,
Second possibility is to add a special glass between the film and the rear element. In this case, what kind of lens should I buy?

And third possibility is to add a special glass lens on the front element. Again: what kind of lens should I buy?

I would love to hear your opinions and suggestions.
A 1.5X teleconverter on the back and a .67 wide angle converter on the front would focus to infinty with the same field of view as a 28mm lens and be 1" more distant from the camera body

II can't vouch for how good the image quality might be - not so bad near the center I'll guess.

I think I have the necessary lenses to try it - I'll get back to you all with the results. The wide angle adapter on front is not small.

Dave
04-26-2012, 06:41 PM   #6
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GK, to my above post, one picture is worth 1000 words

https://www.box.com/s/c4d83644587ba57339d4
04-26-2012, 07:32 PM   #7
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First of all: thank you all of you for your (very quick) replies!!

About the project...

I need to place the 28mm objective 1" away from the camera body using an extension tube (there must be an empty space between camera and objective). Also, I must be able to focus at infinity (if I also end up being able to focus at close distances, even better).
When I place the extension tube between camera body and objective, it can't focus to infinity anymore. And that's my problem: to restore the ability to focus at infinity.
If I lose a little bit of angle it's ok.

When you mention the use of a teleconverter, where should I place it?
Camera > ExtensionTube(1" long) > Teleconverter > Lens
Or...
Camera > Teleconverter > ExtensionTube(1" long) > Lens

Will this allow me to focus to infinity? Sorry for double and triple checking but it seems too easy to be truth

Thanks!!
04-26-2012, 08:57 PM   #8
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If you use an extension tube behind your lens, you will not be able to focus to infinity, period.

The only way a lens can focus to infinity is if its optical center is located at its focal length from the frame (film or sensor). A 28mm lens uses a retrofocus element, the opposite of a teleconverter, to change the apparent focal length. Moving the lens elements further away loses infinity focus.

If you wanted to add an extension tube AND modify the lens to retain infinity focus, you would need to move all the lens elements into the tube. But then you would not be able to change focus at all, because the lens body contains a focusing helicoid and the tube doesn't.

If you add a teleconverter behind the lens, the lens body will indeed be further from the camera body. The TC makes the lens appear longer than it is. Close focus remains the same; you retain infinity focus; but the lens becomes longer and slower. If your Vivitar is a 28mm f/2.8, a 2x TC would turn it into a 56mm f/5.6 optic.

As I said, most 2x TCs are about 25mm / 1in thick. Teleconverters other than the very common 2x exist, and are likely of different thicknesses. We still don't know WHY you want the lens pushed 1in away, but if that's your main goal (along with infinity focus), then a 2x TC will do the trick, and extension tubes won't.

04-26-2012, 09:06 PM   #9
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QuoteOriginally posted by gkyetes75 Quote
First of all: thank you all of you for your (very quick) replies!!

About the project...

I need to place the 28mm objective 1" away from the camera body using an extension tube (there must be an empty space between camera and objective). Also, I must be able to focus at infinity (if I also end up being able to focus at close distances, even better).
When I place the extension tube between camera body and objective, it can't focus to infinity anymore. And that's my problem: to restore the ability to focus at infinity.
If I lose a little bit of angle it's ok.

When you mention the use of a teleconverter, where should I place it?
Camera > ExtensionTube(1" long) > Teleconverter > Lens
Or...
Camera > Teleconverter > ExtensionTube(1" long) > Lens

Will this allow me to focus to infinity? Sorry for double and triple checking but it seems too easy to be truth

Thanks!!
Camera > 1.4x teleconverter > lens ..... by objective do you mean camera lens with focusing ring, f-stops, etc?

The teleconverter will be about 1" thick so it acts as the extension tube as well. But the lens now acts like a 28mm x 1.4 = 40 mm lens - it will focus from as close as it used to as well as focus to infinity.

If the system must behave as if it were a 28mm lens in image size for a particular distance, one can start with a smaller focal length lens and/or add a "wide angle adapter" to the front of the 28m lens.

It turns out that my Kenko 1.5x teleconverter is about 0.71" thick, so perhaps a 2x teleconverter might be closer to the 1" you need?

If this concept works for you, a search for the right thickness teleconverter will be needed.

Image quality hasn't been mentioned - how good does it need to be? what will finally happen to the images? 4x6" prints or computer display?
04-26-2012, 09:37 PM   #10
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I have no idea if this will work for you and it might be difficult/hard/expensive finding parts and experimenting to try it. Some teleconverters have the optical elements in a single unit that can be threaded forward or backward with a lock ring holding it in place. Threading it forward will return some infinity focus if the flange focal distance is too short (I learned this trying to make canon fd-pk adapters out of teleconverters). The first problem is finding a teleconverter that can be changed like that (some can, some can not). Further, I was trying to correct for a few mm. At one point it was able to correct for the few mm plus allowed the lens to focus way past infinity, but I have no idea if you could get enough adjustment to compensate for that much distance. I kind of doubt it.
Its something you could look into anyway.
04-27-2012, 03:05 PM   #11
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What if I disassemble the teleconverter and find a way to fix the glass to the rear element of my 28mm? Will this work?
I mean, I will end up with a 56mm focal lenght, but I will also be able to focus to infinity and also the lens will be mounted in my extension tube... right?

And it's cheap. I see TC for Pentax for $10 in ebay.
04-27-2012, 03:25 PM   #12
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I don't get what you are up to

Why do you need to extend the lens but retain infinity focus? Essentially you are looking for a retro focus group like what is at the back of the lens, which allows a focal length shorter than the registration distance of the body
04-27-2012, 04:45 PM   #13
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QuoteOriginally posted by gkyetes75 Quote
What if I disassemble the teleconverter and find a way to fix the glass to the rear element of my 28mm? Will this work?
I mean, I will end up with a 56mm focal lenght, but I will also be able to focus to infinity and also the lens will be mounted in my extension tube... right?

And it's cheap. I see TC for Pentax for $10 in ebay.
Why the need for an extension tube? I guess I don't really understand what you are fundamentally trying to do. Must there be a free space gap between the lens and camera?

What is wrong with camera||1" Teleconverter||lens

It is exactly the same as camera||1" extension tube with extra lens group||lens

Or do you really mean camera|| 1" open gap|| lens that'll focus to infinity||?

I've little doubt that your problem might be solved, but we simply don't understand why the solutions offered so far are insufficient.

A teleconverter is simply an extension tube with a lens in the right place. If you removed the lens from the TC and put it in the same location inside another extension tube you will have simply recreated the teleconverter you took apart to get its lens.

In fact one way people make 1" extension tubes is by removing the lens from a 2X TC.

If for some reason you simply MUST have a stand-alone, hollow extension tube or empty gap you could use a lens made for a medium format camera like a Pentax 645 lens - which needs to be about exactly 1" in front of a K1000 body.

I'd really like to hear more about what you are trying to do.
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