Optical Glasses used at Taylor Taylor Hobson Series XV Triple Convertible Anastigmat

[Mustafa Umut Sarac]

I had been contacted with Cooke Ltd and chief designer sent me cooke ltd series xv triple convertible anastigmat patent and than i asked again and he sent me optical glasses dispersion etc details in that lens , if you search the these lens threads in this forum ,you can find it.
Find the attachment.

Mustafa Umut Sarac
Istanbul

[Mark J]

Thanks. I'm intrigued by the TF 530512.
It looks to be one of the old titanium flints. Might have odd dispersion properties. I'll see what I can find in our OD archives.

[Mustafa Umut Sarac]

I cant remember his name but I asked several questions to cooke head designer may be 15 or more years ago to get his comment on XV and he said cooke lenses have a sweet spot on colors and they make green spectrum area most important - I dont know what is the technical term but I think you can figure out.. He had been generously sent me the patent and ordered his assistant to prepare glass details. Around two years ago , I wrote him again and president of cooke said we lost him. RIP.

[Mustafa Umut Sarac]

Thanks. I'm intrigued by the TF 530512.
It looks to be one of the old titanium flints. Might have odd dispersion properties. I'll see what I can find in our OD archives.

Can you tell me what happens if the lens have odd dispersion chracteristics ? How this effect a portrait at daylight and artificial light ?

[Mark J]

I know Jon Maxwell quite well. Do you mean he retired from the company ( finally ! ) or it worse than that ?
Did you speak to Steve Pope - head of engineering recently ?
Steve worked at our company many years ago.

Jon was head of the Optical department at Imperial College when I was a student. Excellent lecturer.

[Mark J]

Can you tell me what happens if the lens have odd dispersion chracteristics ? How this effect a portrait at daylight and artificial light ?

Oh, I mean it might reduce secondary spectrum. Not any sort of colour balance effect.

[Mustafa Umut Sarac]

Oh, I mean it might reduce secondary spectrum. Not any sort of colour balance effect.

I found his post , he went to the god.

Hello again Mustafa,

Thanks for this very interesting mail. You are clearly into the interesting minutiae of really creative photography...Good for You!

(1) I did not hear of tiffdruck screening. Maybe this is a German propriatory process. I would be interested to hear from you about this. And you mention a software trick used by NG that sounds interesting.

(2) (a)Your question about designing lenses for different colour temperatures is not so easy to answer simply. Frankly we do not really address the issue of photographing in different colour temperature light because any lens may be used with any colour temperature by different photographers at different times in the day, and at different seasons etc etc
(b) But (and this is not really answering your question, but just trying to give you some background), at Cooke we design our lenses to be somewhat "warm". This is not so much about the colour of different glass types or anti-reflection coatings (although it sometimes is!), as you might think, it is more about the correction of the chromatic aberration. When you correct chromatic aberration, in effect you "fold" the visible spectrum so that red and blue light focus at the same focal plane, or, off axis, at the same image height. But because of what is known as the irrationality (a better word would be cantankerousness) of the spectral dispersion charactaristics of most optical glasses you get left with what is called "secondary spectum". On axis (that is, at the centre of the image) this gives a green focal plane closer to the lens than the red+blue focal plane. Somewhere between those two focal planes there is an achromatic focal region where all the colours mix nicely to give a neutral image. At Cooke we play with exactly which red and which blue wavelengths are superimposed, to try (in principle at least) to maximise the range of the achromatic focal depth....This results in a rather warm image, which some cinematographers say they like, but of course once the images get to the lab (whether it is film or digital) anything can happen.....
(c) In the case of TV and Video, where there are usually three separate colour chips this secondary spectrum can to some extent be removed by placing the chips at carefully selected focal depths from the lens, but that all gets messed up when different focal length lenses are used. This is because the magnitude of secondary spectrum is more or less proportional to focal length on axis and more of less inversly proportional to focal length off axis.

Best wishes

Jonathan (Jon) Maxwell

[Mark J]

2 (b) is very interesting. Not something I had come across before. I wonder if this affects how light bleeds from bright to dark areas in contrasty boundaries, and if this affects the rendition ?

[Mark Sawyer]

The pdf noted "The optical glasses used in the original TTH Ser XV Triple Convertible photographic lens". I wonder if they kept the same (or as close to the same as they could) in the reissued Series XV?

[Mustafa Umut Sarac]

The pdf noted "The optical glasses used in the original TTH Ser XV Triple Convertible photographic lens". I wonder if they kept the same (or as close to the same as they could) in the reissued Series XV?

Hello Mark, I asked for attached patent and its glasses from cooke, they were very old , Chance brothers isnt in optical glass business for a long time and chemical compositions and their production is illegal from dated 1970s. I am sure old glass and new glass could not be the same. And I contacted with Chance brothers and they said they have not even the glass catalogs from that era. I think Britain lost its glass and chemistry business to america and germany after ww2.