Len and Shutter combination

[SMBooth]

What the effect of putting and lens in the wrong shutter, eg a 90mm f8 in a f5.6 shutter (assume normal copal shutter), does that mean at f5.6 marked on the shutter is really f8 or does the aperture not go past f8 to f5.6 because the len won't close down?

 

[nick mulder]

There is no '5.6 shutter' - the fastest aperture given to a lens is a calculation of the largest aperture the particular shutter design in question can give and the focal length of the lens put on it...

As to the effect - well, if its the same shutter design but with a different aperture scale printed on it (they were originally for different lenses) then the effect will be zilch - nothing - but an incorrect aperture scale ...

The other way round - different shutters - well, I ask you exactly how you are going to go about putting a lens in a 'wrong' shutter ... the threads wont be the same

 

[SMBooth]

OK yes, its a shutter with an aperture scale of f5.6....
So your saying that the aperture scale set to f5.6 on a f8 lens screwed into that shutter with the f5.6 scale will be offset by 1/2 stop.

 

[nick mulder]

OK yes, its a shutter with an aperture scale of f5.6....
So your saying that the aperture scale set to f5.6 on a f8 lens screwed into that shutter with the f5.6 scale will be offset by 1/2 stop.

I don't have quite enough info (or knowledge) to say with absolute authority anyway here

is it a Copal or ?!

If you had two lens sets (front and back elements) that were designed to fit say a copal 0 and one when in a copal 0 was defined (by that calculation above) to be 5.6 and the other was 8 then all the stops would be offset, but by 1 stop...

A copal 0 or a copal 3 or whatever are all the same, the aperture scales often 'stuck' on them (removable for just this reason) are dependent on the lens focal length that they came with...

But where I confuse myself now is how you have for instance f4.5 90mm lenses in a copal 0 and f8 90mm in what I thought were copal 0's also - that contradicts me... so I'm wrong somewhere or I'm not wrong but there's more to it...

help !

 

[SMBooth]

Ok my thinking of the mechanics was wrong anyway. There is no moving parts in a lens, for some stupid reason I was thinking the aperture blades were in the lens (doh). The lens (lens set) screws into a shutter, the scale is designed to suit a particular focal lens if you put a different focal length in you need to calculate the max f stop using your n=f/d by measuring the max (wide open) aperture. Eg: if the wide open diameter was 16mm, then with a 90mm len the smallest scale reading would equal f5.6 no matter what the scale said.

Therefore re your last paragraph if you put a f4.5 lens into the same f5.6 shutter you would not physically be able to open the aperture to f4.5 because it just won't open up that far due the the mechanics.

I think...

 

[nick mulder]

uh huh - I'm thinking the apertures will open up fully but the back face of the front element (and/or the front of the rear) will be the 'largest hole' and after a certain point of opening the iris in the shutter there is no more light to be gained... or they use smaller shutters...

Or we're both wrong

If you keep thinking about it and put yourself in the place of a lens designer you really are restricted in some ways by the common shutter size apertures - your lens is either capable of faster speeds but restricted by the shutter or you are forced (?) to use a larger shutter than you need ...

 

[Barrie B.]

Greetings Shane;
All shutters are similar in as much as what they can do : eg open and close for the time engraved on the shutter, shutters come in various sizes to suit lens sizes; eg Copal OO , Copal O , Copal 1 , Copal 3.

Lenses too come in different sizes: eg a 100mm lens ( for any format ) has its own f Stop scale eg. f 4 , the actual diameter of the iris blades will be 1/4 of the focal length= 25mm AND @ f 8 the actual hole will be 1/8th. of the 100mm = 12.5mm. the Aperure scales engraved on the 'shutter plate'willONLY suit that " 100mm" f/length.

TO KEEP THE MATHS SIMPLE ; same shutter , but a 50mm lens ( if there was a l/f lens this size ) =
f 4 aperture , the diameter of the aperture blades would measure 12.5 mm , and f 8 the hole = 6.25mm.
..........Therefor ANY or every two part l/f lens, when fitted into the correct size copal shutter will require an engraved "aperture scale" to suit.

Copal # 3 shutters are required for larger lenses, often with large max. apertures eg. a 360mm f 5.6 Nikkor, for 8X10, then f 8 on this lens will be a 45mm diameter hole in the aperture blades , far too large for a copal O , or Copal 1 shutter , both of which have much smaller ' holes '.

At the other end of the lens sizes , most compact 'Di***al' cameras have very large ' Max. apertures and are difraction limited often to f 8 . It is very difficult to get OUT-OF -Focus Backgrounds with these small toy DIG***L cameras
.........................Cheers Barrie B. ,, FILM FOR EVER .

 

[SMBooth]

Its a mute point now anyway I refuse to pay to much money for a lens in the wrong shutter, when I can buy one for less in the right shutter. But i understand now.