Vignetting on Imagon?

Greetings,

 

I am trying to get the hang of using an Imagon, and am about ready to admit

defeat in one thing--I can't figure out when the lens will vignette.

 

 

I am using the 300mm f/5.8 (circa 1960) on both 5x7 and 4x5, and checking

vignetting in my normal way; stop down--in this case use the chosen 'filter'--

look though cut corners, and make sure I can see all sides of the aperture (or

filter).

 

 

For example, I did this yesterday, with a shot out my back window. I used

significant drop, and according to what I saw in the cut corners, it should have

significant vignetting--however today, when I picked up the film, the corners

were perfect, no vignetting at all. While I have not yet had unintentional

vignetting, I am a bit concerned that I have no control over this.

 

How does one check vignetting on this lens? Should I make sure that all the

peripheral holes are visible, or should I check with these holes open or shut?

 

Any advice would be appreciated, thanks in advance

I have a 360mm Imagon which I use on 8x10, so it is not quite the same as your situation. However, what I would say is that although the disc in use may give a notional f-stop of, say, f/11, that clearly doesn't mean that the light passage through the lens is restricted to the same narrow channel that a conventional iris would give.

 

In short, as soon as even one of the outer holes begins to be covered by the lens barrel you will have some light fall off. Whether you feel that is significant is up to you. Because the disc fits right at the front of the barrel, when it is open you will get some light passing through right up to the point where the last hole is covered by the rear rim. However if the disc is closed the cut off will happen much sooner.

 

So, the absolute limits will be different for each disc, and according to whether it is open or closed - with the worst case being the smallest disc in the closed position.

 

I can certainly use my 360mm and not notice any undesirable uneven illumination, and that with any of the discs, so it may just be less of a problem than you fear.

Actually I don't think I made things quite as clear as I should have:

<br><br>

With the <i>largest disc</i> the cut off begins sooner, but ends later. With the smallest disc it begins later, but is completed sooner. I know that is somewhat counter-intuitive, but that is how it happens.

What shutter is the lens in?

 

A current 300mm in Copal 3 comes with 2 disks rather then 3 as the opening

in the shutter is not large enough to allow the lens to be used at an aperture

larger then 6.8 without vignetting. That is why the 200 and 250mm in Copal 3

have 3 disks while the 300 only has one. If the opening of your shutter is the

same size, or smaller, then a current Copal 3 then your lens is not a 5.8 it is a

6.8. It is also a 6.8 in NF mount or Prontor Professional shutter.

Bob,

 

this is the older 300mm f/5.8 version with three discs, it comes in a massive

Compound shutter, about the same size as an Ilex 5.

Are you leaving the aperture lever wide open?

 

Also do you understand how the disks control the image?

 

The values are not f stops. They are H stops.

 

The central part of the lens creates a soft center image while the outer holes

cast a sharper image. This results in a spreading of the highlights into the

shadows.

 

It does not behave as a normal optic.

Bob, what is an "H stop"? Is there an instruction book available on this interesting lens? It is a coated optic?

Bob,

I really think you have your answer the wrong way round. The central portion of the lens produces a corrected sharp image while the out portion adds varying ammounts (depending on the number of holes in the filter) uncorrected image to produce the dreamy effect.

Bob

"The central portion of the lens produces a corrected sharp image while the

out portion adds varying ammounts"

 

Oops, correct. Sorry

My Imagon is single coated, although I am not really sure why it needs to be

so, since it is a very simple lens (two elements, one group). My older lens It

came with a three page instruction book, "Introducing the Imagon" by J.

Foldes. I have also seen, on eBay, another book, written by A. Scholz, about

the Imagon (if anyone has the Scholz book I would gladly pay for a photocopy

or scan!).

I have posted a copy of my older instruction manual for the imagon at: <a

href="http://homepage.mac.com/sultawi/Imagon.pdf">http://

homepage.mac.com/sultawi/Imagon.pdf.

Jason,

 

Thanks for posting the "Foldes" manual. I have a 250mm Imagon with a Compur 3 shutter (mid 1970's). It has three disk, plus a hood and ND filter. The manual references a yellow filter included...mine must be missing? Thanks again.

 

Best regards,

 

J. P. Mose

"The manual references a yellow filter included...mine must be missing?

Thanks again. "

 

Not missing. Has not been available for well over a decade.

 

The filters are made by Heliopan.

Even my Imagon, which was manufactured 40 years ago, came with a ND

filter. I did however see a Imagon for sale with a green filter rather than a ND

or a yellow filter.

All Imagons come with the requisite 2 or 3 disks, an ND filter and a lens hood.

 

Depending on how old it is it could also come with the no longer available

yeoow or green filters.

 

In an Imagon catalog page printed in August 1975 only the ND filter is listed

as being supplied with the lens and no mention is made of any other filter.

That would mean that an Imagon with the colored filter/s was made prior to 8/

75.

My Imagon 250mm w/Compund shutter came with a yellow filter but no ND filter. I'm going to have one cut from a Conkin ND, unless anyone has a better idea. And BTW, why would you particularly want a yellow filter with an Imagon??

The currently available Imagon ND filter fits Arthur's old Imagon. Just order a

replacement. Not difficult.

 

Disks, ND filter and lens hood fit all 200, 250 and 300mm Imagons. Even

those in Compound.