[pure-silver] Re: 4x5 ground glass on Cambo
- From: Bob Younger <ryounger@xxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Fri, 7 Sep 2012 16:53:01 -0700
Richard brings up a good point, but I'm afraid did not make enough of it.
If you put the fresnel on the inside then you have changed the distance of
the GG to lens distance. You then have to use shims and careful measurement
to ensure that the film plane (once you insert the holder) still is in the
same place as the now moved GG. Also, my experience with fresnel was that
wide angle lenses (165-210) were much more difficult to focus than normal
(300) to long (600) lenses.
Bob Younger
On Fri, Sep 7, 2012 at 4:35 PM, Richard Knoppow <dickburk@xxxxxxxxxxxxx>wrote:
>
> ----- Original Message ----- From: "Bogdan Karasek" <BKarasek@xxxxxxxxxxxx
> >
> To: <pure-silver@xxxxxxxxxxxxx>
> Sent: Friday, September 07, 2012 2:57 PM
> Subject: [pure-silver] 4x5 ground glass on Cambo
>
>
>
> Hi, and my follow up question.
>>
>> I have several backs for my 4x5 Cambos and they come with the Cambo
>> labelled gg with the grid and a 6x9 outline. Using an 8x loupe, I find that
>> the corners are a tad, a lot?, darker.
>>
>> Can I do better (brighter) than the Cambo gg? I have heard praise for
>> the Beattie Intenscreen; is the hype justified? I believe that the
>> Beattie uses a Fresnel lens in front of the gg like the gg/fresnel on my
>> Crown Graphic. But it's hard to judge when I have 26" of bellows extension
>> and light coming through on a 210/5.6. And the Beattie's are expensive :(
>>
>> I'll listen to all recommendations. Thanks!
>>
>> Cheers.
>> Bogdan
>>
>> The caveat when using a field lens for the GG is that it works
> provided the focal length of field lens is right for the camera lens and
> that no tilts or swings are used. The field lens is designed to gather the
> divergent cone of light from the camera lens and make it somewhat
> convergent. Although the ground surface of the GG should scatter the light
> striking in all directions equally it does not so that light which strikes
> it in a direction toward the viewer's eye continues that way resulting in a
> hot spot. Usually, the coarser the grind the hotter the spot. The field
> lens will focus the light from the GG (or striking the GG) so that its
> approximately parallel. If the light from the lens strikes the field lens
> at an angle the lens will focus it at an angle so that the ground glass may
> appear very dark unless the eye is moved so that its on the optical axis of
> the lens. This effect is considerably less for a plain ground glass than
> when the field lens is used. The focal length of the field lens is ideally
> about equal to the distance from the camera lens to the focal plane. That
> results in the light emitted from the field lens + ground glass being
> approximately collimated. If any camera movements are contemplated an
> easily removable field lens will be appreciated.
> Some "bright" screens are not field lenses but simply very highly
> diffusive material, such as a layer of fine wax between two thin sheets of
> glass. The image from such a panel may seem bright but will not be very
> sharp. Most field lenses are Fresnel lenses molded in plastic sheets.
> These can be used on either side of the ground glass but the ideal location
> is on the inside with the ground and ridged surfaces facing each other in
> order to minimize reflections. This is usually not a practical location
> unless the focal distance can be adjusted to compensate. The results with
> the field lens on the viewing side is usually not much different than the
> ideal location. The Ektalite field lens found in the Graflok back for
> Speed Graphics is mounted inside and the back made to compensate its
> thickness.
>
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk@xxxxxxxxxxxxx
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