35mm DOF w. this attachment

[Sam Wells]

The flange focal distance for a PL mount camera (this is from specs for the Arri 35BL series) is 51.97 -51.98mm. One reason for this depth is to clear the mirror shutter of the camera. I don't know the flange focal depth of a Canon XL-1, but it's clearly much less than 51.97mm, so to acheive proper focus with a PL mount lens on that camera, you need to move it away from the camera body 51.97mm minus Canon's specified flange focal depth.

<{POST_SNAPBACK}>

 

You need to match the "in air" rear distance when using PL mount film lenses on a prism optics camera; which will be ~ 1.4 x the physical depth of the prism.

 

I too am unsure as to exactly what these xl1 folks are talking about.

 

They claim superiority for "film lenses" - and then cite an Angenieux 15-150 zoom as an example of film lens on their adaptor, but the AFAIK the 15-150 was based on one of Angeniuex's video lens designs !

 

 

-Sam

[Patrick Neary]

thanks for the clarification, Sam-

 

As further proof that I am officially spending way too much time on this- Here's something I found at: http://www.theiabm.org/pdffiles/canonoptics.pdf (a very informative primer on video optics from Shigeru Oshima from Canon)

 

chew on this:

 

(2) F-number

To maintain the same brightness, the smaller the image size of the lens is then the smaller (brighter) its Fnumber

needs to be. In other words, the F-number should be proportional to the image size.

In a television camera the number of raster lines is the same regardless of the image size, so reducing the

image size reduces the area of the pixel (the diameter of the electron beam). The amount of light received by

the pixel therefore decreases as the image size decreases. This decrease hasto be compensated for by reducing

the F-number in the same proportion. A film camera is different; the same F-number can be used with any

image size. The pixel on film is the grain of silver chloride, the diameter of which is the same for all film sizes.

As the image size is reduced, however, resolving power is lost, even though the F-number is not affected.

 

If I'm reading this right, then reducing the imager size DOES require compensation in terms of light. But is this, as David mentioned, simply a matter of the camera being rated at a lower "ASA", or are lenses designed specifically for certain video cameras (and their optical blocks) endowed with F-stop scales that relate only to those specific cameras?

 

My head hurts.

 

That pdf is fabulous reading though, I heartily recommend it!

[Patrick Neary]

...so i did write to the company, asking what causes the 1-2 stop light loss.

 

the answer:

 

"Hi open up the film stop...1 or 2 stops. Thanks"

 

Ok then, mystery solved!

[Laurent Andrieux]

"Hi open up the film stop...1 or 2 stops. Thanks"

 

:blink: Quite clear, as an explanation...

 

From their site :

 

When using the adaptor with either 16MM or 35MM lenses, the (DOF) depth of field is preserved, and it effectively increases the focal length of the PL mounted lens by 2x.

 

The fact that they say it "increases" it by 2, whatever 16 mm or 35 mm makes think it's only the viewing angle that is different.

 

(2) F-number

To maintain the same brightness, the smaller the image size of the lens is then the smaller (brighter) its Fnumber

needs to be. In other words, the F-number should be proportional to the image size.

In a television camera the number of raster lines is the same regardless of the image size, so reducing the

image size reduces the area of the pixel (the diameter of the electron beam). The amount of light received by

the pixel therefore decreases as the image size decreases. This decrease hasto be compensated for by reducing

the F-number in the same proportion. A film camera is different; the same F-number can be used with any

image size. The pixel on film is the grain of silver chloride, the diameter of which is the same for all film sizes.

As the image size is reduced, however, resolving power is lost, even though the F-number is not affected.

 

This thread is about the comparison of the sensitivity of different ccd sensors sizes.

 

as David Mullen said, the F stop of a lens doesn't depend on the sensitive surface.

 

With this pl mount, we are comparing a movie lens beetween its normal use and a video use. There is no reason why its brightness should change. Again, the fact that the sensor only uses a part of the image that the lens creates only reduces the viewing angle, just like mounting a 35 mm lens on 16 mm camera redusces its viewing angle.

 

After all and the interesting posts that were written here, I am not so shure about the flange focal distance of this system, but this loss of light is still a mystery to me.

 

As to write tio the manufacturers and reading this answer you've got, I don't know if it would be worth byt these are the questions I would ask them :

 

- What is the focal flange distance of the system ?

- are the focus marks of the mounted less still working when mounted on such a system ?

- Can you focus at the infinite ?

[Patrick Neary]

Hello again Laurent!

 

yes, a lens' brightness or transmission doesn't change, no matter where you mount it.

 

Looking more carefully at the pdf I mentioned earlier, it seems issues of video lens design are more about correcting chromatic abberations than light transmission, so that was a purely bizarre tangent of mine.

 

I was working toward a guess at what the company's rationale is with their claim of a 2 stop light loss with that adaptor...I'm beginning to think it's something they just pulled out of their shorts.

 

Or there's a very simple explanation, and I'm going to feel like an idiot-

 

Perhaps it's time to post this question as it's own thread to see if anyone here knows either why there would be a light loss, or if in fact there would be a light loss with just a simple metal adaptor ring.

 

this is starting to feel unhealthy!

[Laurent Andrieux]

I was working toward a guess at what the company's rationale is with their claim of a 2 stop light loss with that adaptor...I'm beginning to think it's something they just pulled out of their shorts.

 

There must be a reason, it's not too commercial to say that their system looses light ! :huh:

 

Or there's a very simple explanation, and I'm going to feel like an idiot-

 

Don't worry, so am I ! :D

 

 

Perhaps it's time to post this question as it's own thread to see if anyone here knows either why there would be a light loss, or if in fact there would be a light loss with just a simple metal adaptor ring.

 

Why not... Or write to this company again, saying something like, look, we are a community of operators, newbies ask us what to think about your system, it's your interest to give us valuable explanations, have a technician giving us good specifications on this item, because at this point, we cannot say anything good about your product... or something so... :(

 

This is starting to feel unhealthy!

 

And very funny at the same time ! :D