Bellows Length Relative to the Adjustment of f-stops

[Ed Sukach]

Ed, were you serious when you wrote the following?

"The distance from the aperture to the film plane divided by the aperture diameter = f/stop.

Example: 150mm to film plane / 75mm diameter aperture = f/2"

This statement, copied direct from an earlier post of yours in this thread, is what I commented on. It is flatly wrong. Now, what did you mean?

"... flatly wrong ... *, huh?! Because you say so?

No. The statement is correct.

The only way to resolve this - or at least one way - would be to cite a reliable source ... otherwise it will be a classic "urinating up a tree" contest - one I will not continue until I find - or that citation is presented.

I've already pored through "old relaible" - "The Encyclopedia of Photography" searching everything I could find about exposure, "f/stops", exposure modification as a result of inordinately close focusing ... with nothing really concrete - carved in stone. I've gone through some of my notes from the preparatory Optics Classes - all that information is far above anything as basic as "f/stops".

Possibly you can help. What is your source of information regarding entrance/ exit puplis --- etc.?


Cheers,

Any guess about that camera manufacturer?
- And who was "Waterhouse"?

 

[JBrunner]

Jason, it works if you mean that the lens' focal length is 150 mm and the aperture (or entrance pupil) is 75 mm. But that's not what Ed wrote.

Sorry,

Dan

I see, yes. you are correct, but I'm pretty sure think Ed was trying to communicate the same thing.

http://www.youtube.com/watch?v=CIFNdfjem18

The above is an unreliable source.

 

[Dan Fromm]

Jason, the problem with Ed is that he won't come clean about what he meant or even about what he wrote. He's being very weaselish, and this worries me. Its not like him.

By the way, I did the arithmetic exercise and although Ed denies it the formula you and I wrangled politely about is a way of calculating effective aperture. Its much harder to use in the field than the conventional way, which has been posted by several of us. And it has nothing to do with T/stops. It has to do with effective f/stop.

To say it again, this time with an example, at T/2 a lens transmits as much light as a lossless lens at f/2. I once had an Angenieux 8x8B zoom (8-64/1.9) that T stopped, based on tests with K40, around T/3.3 wide open. But at f/1.9 it gave the depth of field expected at f/1.9, not the DoF expected at f/3.3. DoF is controlled by magnification and geometric aperture.

Ed's recent posts make him seem awfully addled. This isn't like him. I hope you'll join me in being concerned about him.

Cheers,

Dan

 

[Ole]

Ed's method is correct, and happens to be related to the way I calculate effective apertures when focusse closer than infinity. But what is simple and intuitive to me is not always simple and intuitive to others, as I have learned...

The f in f/ is the focal length. Dividing the focal length by the diameter of the entrance pupil gives a dimensionless number commonly known as the f-stop.

Since this is physics, the "focal length" is really "the focal length of a theoretical perfect lens with zero thickness which gives the same image size (only better) as the lens system under investigation". Thus when focussed at infinity such a "perfect lens" would be exactly one focal length from the image plane. Since "infinity" is the only easily defined fixed point, that is what defines "focal length".

Note that back when lens makers were opticians and mathematicians, many lenses were marked with "EFL" for "Effective Focal Length" instead of just "FL" or "F". These people knew that the definition is for nonexistent perfect lenses, and combined lens systems have at best a measurable "Effective Focal Length".

 

[Ed Sukach]

Jason, the problem with Ed is that he won't come clean about what he meant or even about what he wrote. He's being very weaselish, and this worries me. Its not like him.

This has gone beyond discussion of the subject. I have corrected an error in the original statement -- (where I had reversed the division) but have NEVER AVOIDED OR RECANTED WHAT I HAVE SAID HERE.

Anyone who doubts this can easily read back through all the messages.
YOU ARE WORRIED ... ABOUT WHAT? YOU DO NOT KNOW WHAT IS OR IS NOT "LIKE ME"!!!

I dislike "ad hominem" attacks. They are a mark of cowardice, and indicate someone who cannot coherently reason out support for their position.
[SARCASM KEY] ON --- but I am glad no one around here attempts to use them.

Apparently, you are not accustomed to anyone disagreeing with you.


You claim that the focal length of a lens system *MUST* be considered when determining "f/stop". If so, then a "pinhole" system could not be properly described as having a "f/stop" - and this description enjoys widespread use... although that particular characteristic is not the best parameter for use in designing pinhole systems.

One big problem here is determining just WHERE the diaphragm is located in the system. occasionally, it isn't in the body of the lens at all. "Telephoto" lenses have, as a rule, their optical centers (the point from where the distance to the film plane when focused at infinity establishes "Focal Length" .. and the ideal location for an 'iris diaphragm) beyond the physical limits of the lens itself. From there on, everything is a "compromise" of "ideal" desigm."

Now ... I recall the "stadard" useages to be: "F" (capital) - indicating "Focal length when focused at infinity"; and "f'" (lower case), to indicate "distance from aperture to film plane." Given these useages, the equation, f/d (distance to film plane divided by aperture diameter) is self explanatory.

By the way, I did the arithmetic exercise and although Ed denies it the formula you and I wrangled politely about is a way of calculating effective aperture.

Not true!!!! Again you are talking about something other than raw "f/stop" - read what you just written, and note the addition of the word "effective" - not the same attribute!!!

Ed's recent posts make him seem awfully addled. This isn't like him. I hope you'll join me in being concerned about him.

Hmmm... I haven't done this for a while, but I will direct your attention to the sprig of mistletoe I have just installed at the center rear belt loop of my trousers. Take appropriate action!!

Cheers,