If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Shutters and apertures
I've noticed that many older lenses have nonlinear aperture
scales. By nonlinear I mean that the distance between successive indicated f-stops varies as the lens is stopped down. In contrast, many modern lenses have linear scales, i.e. a constant angular spacing between f-stops. I'm assuming there was some advance in iris design, either in the shape of the aperture blades or in some mechanism inside the shutter that controls them. When did this happen? Can anyone shed some light on the subject? Always curious. steve |
#2
|
|||
|
|||
Shutters and apertures
"Stephan Goldstein" wrote in message ... I've noticed that many older lenses have nonlinear aperture scales. By nonlinear I mean that the distance between successive indicated f-stops varies as the lens is stopped down. In contrast, many modern lenses have linear scales, i.e. a constant angular spacing between f-stops. I'm assuming there was some advance in iris design, either in the shape of the aperture blades or in some mechanism inside the shutter that controls them. When did this happen? Can anyone shed some light on the subject? Always curious. steve It is a change in the way the iris leaves are hinged. Modern irises originated with the Compur EVS shutter in the late 1950's. In order for the exposure to remain constant the distance the stop and speed controls move must be the same for an equivalent change in exposure. Its fairly easy to make a shutter so that the speeds are evenly spaced on the speed scale, but to match it with stops the iris must follow an inverse square law since the stop is a square law function of the diameter of the stop. Linear stop irises usually have fewer leaves than the older variety. The old type iris is composed of a series of crescent shaped sections which are hinged on a fixed pin on one end and have a pin running in a slot on the other. Sometimes the relative positions of pins and slots are reversed but the idea is the same, A rotary motion of a ring connected to the moving end of the crescent shaped segments moves them together or apart. The uniform spacing type iris has blades simiar to a shutter. These have two pins at one end and are brought together by tilting them. Its hard to describe but a diagram makes it plain. The low number of blades on newer type irises results in stops which are not very good circles. Some think that this results in an unpleasant effect on the out of focus part of the image, what the Japanese call bokeh. Ghost images of bright objects take on the shape of the stop. Typically they are pentagonal on newer lenses. Some old lenses, particularly process lenses, like the Apochromatic Artar, may have as many as twenty leaves or blades making up the iris diaphragm. Most modern lenses have only five. -- --- Richard Knoppow Los Angeles, CA, USA |
#3
|
|||
|
|||
Shutters and apertures
"Richard Knoppow" wrote in message nk.net...
"Stephan Goldstein" wrote in message ... I've noticed that many older lenses have nonlinear aperture scales. By nonlinear I mean that the distance between successive indicated f-stops varies as the lens is stopped down. In contrast, many modern lenses have linear scales, i.e. a constant angular spacing between f-stops. I'm assuming there was some advance in iris design, either in the shape of the aperture blades or in some mechanism inside the shutter that controls them. When did this happen? Can anyone shed some light on the subject? Always curious. steve It is a change in the way the iris leaves are hinged. Modern irises originated with the Compur EVS shutter in the late 1950's. In order for the exposure to remain constant the distance the stop and speed controls move must be the same for an equivalent change in exposure. Its fairly easy to make a shutter so that the speeds are evenly spaced on the speed scale, but to match it with stops the iris must follow an inverse square law since the stop is a square law function of the diameter of the stop. Linear stop irises usually have fewer leaves than the older variety. The old type iris is composed of a series of crescent shaped sections which are hinged on a fixed pin on one end and have a pin running in a slot on the other. Sometimes the relative positions of pins and slots are reversed but the idea is the same, A rotary motion of a ring connected to the moving end of the crescent shaped segments moves them together or apart. The uniform spacing type iris has blades simiar to a shutter. These have two pins at one end and are brought together by tilting them. Its hard to describe but a diagram makes it plain. The low number of blades on newer type irises results in stops which are not very good circles. Some think that this results in an unpleasant effect on the out of focus part of the image, what the Japanese call bokeh. Ghost images of bright objects take on the shape of the stop. Typically they are pentagonal on newer lenses. Some old lenses, particularly process lenses, like the Apochromatic Artar, may have as many as twenty leaves or blades making up the iris diaphragm. Most modern lenses have only five. Richard, are you sure that having a linear aperture scale entails having few blades? I ask because my 600/9 Apo Ronar L (for linear aperture scale) has more blades than are easily counted. Cheers, Dan |
#5
|
|||
|
|||
Shutters and apertures
(Richard Knoppow) wrote in message . com...
snip (Dan Fromm) wrote in message Richard, are you sure that having a linear aperture scale entails having few blades? I ask because my 600/9 Apo Ronar L (for linear aperture scale) has more blades than are easily counted. Cheers, Dan It isn't necessary but I think the diameter of the shutter body must be made larger. I have to look at the drawings of how the aperture blades are set in the late version Compur to be sure. Richard Knoppow Los Angeles, CA, USA Richard, I've looked at my 600/9 Apo-Ronar L again. I was mistaken, it has only 8 blades. And it is in barrel, so any constraints that may affect Compur shutters should have no affect on it. Cheers, Dan |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
zone system test with filter on lens? | Phil Lamerton | In The Darkroom | 35 | June 4th 04 02:40 AM |
Apertures and focal length | Stephan Goldstein | Large Format Photography Equipment | 12 | February 29th 04 03:28 AM |