f stop of Kowa B&H 2x anamorphic ?

[buggz]

What is the f stop of the Kowa for Bell & Howell 2x anamorphic.

Anyone work this out?  Or, is this a known spec listed somewhere?

[Caleb Genheimer]

I don’t know that it has an f-stop in a conventional sense, as it is not a complete optical system. I suppose you could still technically define one based on the size of the front element versus the size of the smallest opening within the system, but it doesn’t really lose much light. Your question is similar to asking what the f-stop of a diopter is. The Kowa is an optical device, but it’s not an entire lens on its own. 

Since it is compressing by 2X, it may even technically be adding almost a stop to whatever lens you put it in front of, but I’m not a technical expert so I am not sure about that.

[buggz]

Just wondering if I am limiting my taking lens.

I have been using the Minolta Rokkor 58/1.2 as a taking lens.

Wondering if my Minolta Rokkor 50/1.4 would be just as good?

I recently just purchased a beautiful Canon 50/0.95 RF modded for Leica M mount.

I thought there is an fstop for any lens, I remember my old 4x5 Sinar F2 system lenses, there is a mathematical formula, holding the lens to strong light, and displaying on paper for focus, measuring the length, then the lens diameters, etc., etc, etc. 

I've done this before, I will have to find this old information and post it.

[Caleb Genheimer]

I am fairly confident that you are not limiting your taking lens if what you are talking about is depth of field. If you are talking about actual light transmission, AKA T-Stop, that may be a different matter.

If you have both lenses, you’re in a good position to check yourself in a controlled light environment.... locked off tripod shot comparing  lens A to lens B, with and without the anamorphic mounted. You may even be able to test it by stopping down your same lens and comparing... but this would only work if your taking lens front element is bigger than your anamorphic front element. f-stop is a ratio of front element diameter versus aperture diameter. 

[leslie]

i found this its, not this website i was looking for but maybe it may help  https://www.diyphotography.net/build-your-own-lenses/

i'll keep looking

edit :  ok i found it its a flicker group on lens making it may help, at least its interesting reading

https://www.flickr.com/groups/homemadelens/discuss/72157622612617105/

[leslie]

ok found the page that related to fstops  and measuring focal length

https://www.flickr.com/groups/homemadelens/discuss/72157623828975961/

 

[Caleb Genheimer]

Yes, my bad. F-stop is focal length versus aperture diameter.

So to be most accurate, your 50mm f1.4 remains a 50mm f1.4 vertically. The math is pretty simple actually:

50mm/1.4=35.7mm

That is, at f1.4, your aperture is a 35.7mm diameter opening. 

It gets interesting when considering that your lens is horizontally a 25mm lens. A 25mm f0.7 lens has an aperture of 35.7mm if doing the math in reverse from the known aperture diameter.

However, I’m no optics expert, and it seems from my experience that anamorphics maintain an equivalent f-stop while increasing width. That at least would seem to explain the oval bokeh. So it is actually a f1.4 horizontally and vertically, with two different focal lengths. A 25mm at f1.4 has bokeh half the size of a 50mm at f1.4, and that is why it is oval.