[pure-silver] Re: holga focus

 
On Monday, November 05, 2007, at 07:37PM, "Richard Knoppow" 
<dickburk@xxxxxxxxxxxxx> wrote:
>
Don't knock plastic, if your making a really cheap lens the dispersion of 
plastic would favor plastic over glass for both a single or a double meniscus 
design. Plastic is easier to mold homogeneously than optical glass, thus when 
making modern "super-cool" (= high pose factor)  aspherical lenses by casting a 
layer of plastic onto a glass substrate, its the material of choice. I hesitate 
to say this, but even people like le-ica or cannon or nikon have been known to 
do this.
The down side of plastic is poor thermal characteristics, the advantage is low 
weight elements, which mean less mass for an auto focus system to throw around 
when looking to compete on focusing speed.
My 3c. (adjusting for the current $ euro exchange rate.) 
All the best
Larry Cuffe
 
>----- Original Message ----- 
>From: "Justin F. Knotzke" <jknotzke@xxxxxxxxxx>
>To: <pure-silver@xxxxxxxxxxxxx>
>Sent: Monday, November 05, 2007 4:07 PM
>Subject: [pure-silver] Re: holga focus
>
>
>> On 05/11/2007, Shannon Stoney 
>> <shannonstoney@xxxxxxxxxxxxxxx> wrote:
>>>
>>> He said all the holgas did this.  (It's true that holgas 
>>> are not
>>> uniform.)
>>>
>>> I think he meant very, very soft, although I have not 
>>> seen the pictures.
>>
>>
>>   Ok.. that's pretty normal. They really aren't sharp. I 
>> mean, the lens is
>> plastic!
>>
>>   J
>>
>    I think the two words in the subject line don't really 
>go together.
>    There are a lot of 120 box cameras around, mostly very 
>cheap because few of them are collector's items in any 
>respect. Kodak made cameras mostly for 620 but 120 will fit 
>most of them with a little coaxing. The lenses in these 
>guys, although simple meniscus lenses like the Holga, may 
>actually be better. For one thing they are glass, not 
>plastic. Although some very good lenses have been made from 
>plastics in general plastics have too narrow a range of 
>optical properties to make really high quality lenses from 
>them.
>    The best arrangement for a meniscus lens is to have the 
>concave side facing the world with the stop in front. This 
>arrangement can be set up to minimise spherical aberration 
>and coma and to minimise geometric distortion. This 
>arrangement is typical of older box cameras but Kodak and 
>others found that putting the lens in with the convex 
>surface facing front and the shutter behind the lens 
>minimised accumulated dirt and damage to the shutter. If 
>designed correctly this arrangement will still result in an 
>acceptable image but not as good as the reverse.
>    According to Kingslake Kodak experimented with using an 
>achromatized lens in box cameras but found that few people 
>noticed the difference and gave up on it. Achromatizing 
>requires two cemented elements, a much more expensive lens 
>to make than a simple meniscus type or even an air-spaced 
>doublet.
>    On a front stop lens the f/stop can be found by simply 
>measuring the diameter of the stop and figuring the focal 
>length is the distance from the lens to the film. Most box 
>cameras are around f/11 somewhere.
>    The actual focus distance of a box camera lens is 
>probably in some middle distance to provide a compromise 
>between close and distant subjects. The large amount of 
>uncorrected spherical aberration results in a sort of 
>extended depth of field. The nature of spherical is that it 
>mimics the image of a lens with many focal lengths all at 
>the same time. While there is some blur in the overall image 
>the concentration of light from an object at any distance 
>tends to make the focus look sharper, that is, the in-focus 
>part of the image will be brighter than the out of focus 
>light that surrounds it. As a result box cameras make 
>sharper looking images than one would expect.
>    Typically, a good box camera will _not_ have the 
>distortion and vignetting of the Holga so, if you want these 
>effects the box camera will not do, most of them are too 
>good.
>    Box cameras were designed for orthochromatic film or 
>even non-color sensitized film. The lenses are not 
>achromatized, so they may not be quite as sharp with modern 
>panchromatic film as they were with the stuff they burned 
>when they were new. A blue or green filter will help. Most 
>modern medium speed film is enough faster than the stuff the 
>camera was designed for to compensate for the filter factor.
>    BTW, cover the red window with something, a bit of 
>opaque tape will do, except when actually winding the film. 
>While the backing paper does pretty well in eliminating 
>light leakage box cameras are not exactly the most precise 
>things on earth so its best to minimise the exposure.
>
>---
>Richard Knoppow
>Los Angeles, CA, USA
>dickburk@xxxxxxxxxxxxx
>
>
>
>
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