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1899 4" f/16 Cooke Photovisual Apo refractor

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#201 clamchip

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Posted 30 June 2020 - 04:22 PM

Here's something you may or may not have seen, 'The Cooke Triplet and Tessar lenses',

https://www.willbell.../ChapterB.3.pdf

 

Robert


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#202 Peter Ceravolo

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Posted 01 July 2020 - 12:32 PM

Here's something you may or may not have seen, 'The Cooke Triplet and Tessar lenses',

https://www.willbell.../ChapterB.3.pdf

 

Robert

Robert thanks for the posting about the Cooke Triplet, but the system in the text is actually a photographic lens and not a telescope objective. Unlike the telescope objective the Cooke triplet’s lenses are widely spaced and the center negative element is smaller than outer positive lenses so it can cover a 20 degree or so field with minimal vignetting.

 

Cooketriplet.JPG

 

It’s confusing since both lens systems were designed by Dennis Taylor at T. Cooke and Sons. From what I’ve learned the telescope objective is referred to as the Taylor triplet. In the optics world the photographic lens has always been referred to as the Cooke triplet.

 

 

Taylortriplet.JPG


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#203 macdonjh

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Posted 01 July 2020 - 12:44 PM

Unlike the telescope objective the Cooke triplet’s lenses are widely spaced and the center negative element is smaller than outer positive lenses so it can cover a 20 degree or so field with minimal vignetting.

 

So what are the consequences of these design choices, field curvature?  Inability to focus at infinity?  Something else? 



#204 Peter Ceravolo

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Posted 01 July 2020 - 01:33 PM

So what are the consequences of these design choices, field curvature?  Inability to focus at infinity?  Something else? 

Books have been written about that...

 

The bottom line is that the two optical systems do different things. The Taylor triplet forms a visually color free, diffraction limited image over a narrow, slightly curved field of view. The Cooke triplet forms acceptable spots of light across a very wide, flat field of view.



#205 ccwemyss

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Posted 01 July 2020 - 03:06 PM

Responding to macdonjh,

 

It's important to keep in mind that photo lenses of that era were used for metal or glass plates and then sheet film. The plates could use a direct positive, printing out, process or a negative process. Sheet film was more typically negative. But the negatives would be contact printed. We're talking about negatives typically ranging in size from 4x5 to 16x20 inches. So the spot size, which was not going to be subject to enlargement, could be quite a bit bigger than what would be acceptable when magnified through an eyepiece. It just needed to be not significantly larger than the grain of the emulsion.

 

In addition, the lenses were expected to cover significantly more area than the plate or film, because the cameras had rise/fall/shift movements to compensate for perspective distortion by moving the image circle around over the area of the film. In some cases, tilts were also used to change the angle of the focal plane so that it wouldn't be parallel to the film (for example, in a landscape, enabling focus to run along the ground to the horizon, and letting an empty sky be out of focus). So a lens for a 16x20 camera would need about a 36" image circle with minimal falloff.

 

I still shoot 4x5, 8x10, and 12x20 film, with modern variations of these lenses. It should also be noted that they don't necessarily give a flat field for an equidistant plane in front of the camera. I was recently shooting a socially-distanced group portrait on 12x20 with a 450mm Nikon lens, and found that the focal plane corresponded to a significant arc of curvature in front of the camera. The subjects, spaced across 30', and about 36' away from the camera had to be positioned so that the people in the middle were about 8' closer than the ones at the sides.

 

Once you work with these simple lenses, you gain an even greater appreciation for the early masters of photography, and for the early opticians, like Cooke. 

 

Chip W. 


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#206 macdonjh

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Posted Yesterday, 07:56 AM

Books have been written about that...

 

The bottom line is that the two optical systems do different things. The Taylor triplet forms a visually color free, diffraction limited image over a narrow, slightly curved field of view. The Cooke triplet forms acceptable spots of light across a very wide, flat field of view.

 

 

Responding to macdonjh,

 

It's important to keep in mind that photo lenses of that era were used for metal or glass plates and then sheet film. The plates could use a direct positive, printing out, process or a negative process. Sheet film was more typically negative. But the negatives would be contact printed. We're talking about negatives typically ranging in size from 4x5 to 16x20 inches. So the spot size, which was not going to be subject to enlargement, could be quite a bit bigger than what would be acceptable when magnified through an eyepiece. It just needed to be not significantly larger than the grain of the emulsion.

 

In addition, the lenses were expected to cover significantly more area than the plate or film, because the cameras had rise/fall/shift movements to compensate for perspective distortion by moving the image circle around over the area of the film. In some cases, tilts were also used to change the angle of the focal plane so that it wouldn't be parallel to the film (for example, in a landscape, enabling focus to run along the ground to the horizon, and letting an empty sky be out of focus). So a lens for a 16x20 camera would need about a 36" image circle with minimal falloff.

 

I still shoot 4x5, 8x10, and 12x20 film, with modern variations of these lenses. It should also be noted that they don't necessarily give a flat field for an equidistant plane in front of the camera. I was recently shooting a socially-distanced group portrait on 12x20 with a 450mm Nikon lens, and found that the focal plane corresponded to a significant arc of curvature in front of the camera. The subjects, spaced across 30', and about 36' away from the camera had to be positioned so that the people in the middle were about 8' closer than the ones at the sides.

 

Once you work with these simple lenses, you gain an even greater appreciation for the early masters of photography, and for the early opticians, like Cooke. 

 

Chip W. 

Thank you, both.




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