I have seen a few examples of constant aperture zooms (eg a 28-100mm f/4 Ricoh.) in the secondhand markets where the aperture appears to be not fully open (visibly non-round hole) at some zoom settings. Did some designs actually do that intentionally to enforce the constant aperture, or are these just cases of mechanical defects where unrelated mechanical parts interfere with each other due to friction or bad justage?
I am not sure about other manufacturers, but Canon uses a "secondary diaphragm" in some of their constant aperture wide and standard zooms.
Here is a lens grouping Block Diagram depicting the secondary diaphragm in front of the primary diaphragm.
Here is a video that shows it in operation as you zoom the lens: EF 24-105mm f/4 Secondary Diaphragm
Of the four Canon zooms I have within arms reach at the moment: I had already noticed it in the EF 24-105mm f/4 L IS a long time ago. I do not see it at all in the EF 70-200mm f/2.8 L IS II nor the EF 24-70mm f/2.8 L. There may be just a bit of it happening in the EF 17-40mm f/4 L. The strong negative magnification between the front element and diaphragm make it hard to tell for sure.
According to the block diagrams published by Canon, the EF 17-40mm f/4 L and EF 24-70mm f/2.8 L do have a secondary aperture, but when examining the movement of the unmounted lens as the zoom ring is turned their action is far less noticeable than that of the EF 24-105mm f/4 L IS.
Perusing block diagrams at the Canon Camera Museum, it seems that telephoto zooms don't have a secondary aperture while upper tier constant aperture wide to normal zooms (those that use a retrofocus design) usually do.
Unfortunately, the entries at the Canon Camera Museum for very few of Canon's earliest EF zoom lenses, particularly the "L" series, include published block diagrams.
EF Zoom lenses with published block diagrams that do not show a secondary aperture:
- EF 35-135mm f/4-5.6 USM (1990)
- EF 100-300mm f/4.5-5.6 USM (1990)
- EF 28-105mm f/3.5-4.5 USM (1992)
- EF 20-35mm f/3.5-45 USM (1993)
- EF 28-80mm f/3.5-5.6 II USM (1993)
- EF 35-80mm f/4-5.6 III (1995)
- EF 70-200mm f/2.8L USM (1995)
- EF 75-300mm f/4-5.6 IS USM (1995)
- EF 28-135mm f/3.5-5.6 IS USM (1998)
- EF 55-200mm f/4.5-5.6 USM (1998)
- EF 22-55mm f/4-5.6 USM (1998)
- EF 100-400mm f/45-5.6L IS USM (1998)
- EF 75-300mm f/4-5.6 III (1999)
- EF 75-300mm f/4-5.6 III USM (1999)
- EF 70-200mm f/4L USM (1999)
- EF 28-90mm f/4-5.6 (2000)
- EF 28-90mm f/4-5.6 USM (2000)
- EF 28-200mm f/3.5-5.6 (2000)
- EF 28-200mm f/3.5-5.6 USM (2000)
- EF 28-105mm f/3.5-4.5 II USM (2000)
- EF 70-200mm f/2.8L IS (2001)
- EF 28-105mm f/4-5.6 (2002)
- EF 28-105mm f/4-5.6 USM (2002)
- EF 28-90mm f/4-5.6 II USM (2002)
- EF 90-300mm f/4.5-5.6 USM (2002)
- EF 28-90mm f/4-5.6 II (2003)
- EF 55-200mm f/4.5-5.6 II USM (2003)
- EF 90-300mm f/4.5-5.6 (2003)
- EF 28-300mm f/3.5-5.6L IS USM (2004)
- EF 70-300mm f/4.5-5.6 DO IS USM (2004)
- EF-S 17-85mm f/4-5.6 IS USM (2004)
- EF-S 18-55mm f/3.5-5.6 (2004)
- EF-S 18-55mm f/3.5-5.6 USM (2004)
- EF 28-90mm f/4-5.6 III (2004)
- EF-S 18-55mm f/3.5-5.6 II (2005)
- EF-S 18-55mm f/3.5-5.6 II USM (2005)
- EF 70-300mm f/4-5.6 IS USM (2005)
- EF 70-200mm f/4L IS USM (2006)
- EF-S 18-55mm f/3.5-5.6 IS (2007)
- EF-S 55-250mm f/4-5.6 IS (2007)
- EF-S 18-200mm f/3.5-5.6 IS (2008)
- EF-S 15-85mm f/3.5-5.6 IS USM (2009)
- EF-S 18-135mm f/3.5-5.6 IS (2009)
- EF 70-200mm f/2.8L IS II USM (2010)
- EF 70-300mm f/4-5.6L IS USM (2010)
- EF-S 18-55mm f/3.5-5.6 III (2011)
- EF-S 18-55mm f/3.5-5.6 IS II (2011)
- EF-S 55-250mm f/4-5.6 IS II (2011)
- EF 200-400mm f/4L IS USM Extender 1.4x (2013)
- EF-S 55-250mm f/4-5.6 IS STM (2013)
- EF 24-105mm f/3.5-5.6 IS STM (2014)
- EF 100-400mm f/4.5-5.6L IS II USM (2014)
- EF 70-300mm f/4-5.6 IS II USM (2016)
EF Zoom lenses that do show a secondary aperture:
- EF 28-70mm f/2.8L USM (November 1993) This is the oldest lens with a published block diagram at the Canon Camera Museum that includes a secondary aperture.
- EF 17-35mm f/2.8L USM (1996)
- EF 24-85mm f/3.5-4.5 USM (1996)
- EF 28-80mm f/3.5-5.6 II (1999)
- EF 28-80mm f/3.5-5.6 V USM (1999)
- EF 16-35mm f/2.8L USM (2001)
- EF 24-70mm f/2.8L USM (2002)
- EF 17-40mm f/4L USM (2003)
- EF-S 10-22mm f/3.5-4.5 USM (2004)
- EF 24-105mm f/4L IS USM (2005)
- EF-S 17-55mm f/2.8 IS USM (2006)
- EF 16-35mm f/2.8L II USM (2007)
- EF 8-15mm f/4L Fisheye USM (2011)
- EF 24-70mm f/2.8L II USM (2012)
- EF 24-70mm f/4L IS USM (2012)
- EF 16-35mm f/4L IS USM (2014)
- EF 24-105mm f/4L IS II USM (2016)
- EF 16-35mm f/2.8L III USM (2016) - shows three aperture positions in the block diagram
Looking at these lists, we can see some tendencies:
Lenses with secondary apertures are "L" grade premium lenses or mid-to-upper tier consumer lenses. (The singular exception in terms of optics is listed next)
The lower tier "kit" zoom lenses and other "entry level" zoom lenses produced during the same eras, even those that also use retrofocus designs, usually did not include a secondary aperture in their design. A pair of 28-80mm kit lenses from 1999, which share the same optical formula and only differ with regard to the type of focus motor each uses, are the only exceptions.
Some lenses with secondary apertures are constant aperture while others are variable aperture lenses.
All lenses with secondary apertures use retrofocus designs.
The vast majority of retrofocus entry level lenses are not given a secondary aperture.
There are no Canon lenses of any grade which do not use a retrofocus design that include secondary apertures in their design.
In summary, the thing Canon lenses with secondary apertures have in common is they are retrofocus designs incorporated into zoom lenses well above entry level (with the noted exception of one lens design that was offered in USM/non-USM versions).
EF zoom lenses that do not show any aperture positions in the published block diagram:
- EF 35-350mm f/3.5-5.6 L USM (1993)
- EF-S 18-135mm f/3.5-5.6 IS STM (2012)
- EF-S 18-55mm f/3.5-5.6 IS STM (2013)
- EF-S 10-18mm f/4.5-5.6 IS STM (2014)
- EF 11-24mm f/4L USM (2015)
- EF-S 18-135mm f/3.5-5.6 IS USM (2016)
As you know, when the camera shutter clicks, light plays for a time on the surface of film or the digital sensor. This action establishes the exposure. We can regulate the volume of the exposing light energy in several ways -- topmost is the aperture adjustment. We are talking about how we set the working diameter of the lens. It is the surface area of the aperture entrance that controls the amount of light that the lens will pass. However the strength of the exposure is also delimited by focal length. You need to know that changing the focal length as we zoom has an enormous influence over exposure. A twofold change, say from 25mm to 50mm in focal length, results in a 4X (2 f-stop) change to the exposing energy.
This zooming in changes the magnification 2X and this induces a 4X change in the area of the projected image. In other words, a 2X change in focal length results in a magnified image that covers more surface, so the light intensity at any given point is reduced by a factor of 4 (2 f-stops reduction). Conversely, zooming out (shorter focal lengths) results in an upsurge of the exposing energy.
Thus both the working diameter of aperture and the focal length intertwine to influence the amount of exposing light energy. This would result in chaos if it weren’t for the f-number system. We divide the focal length by the working diameter and calculate an f-number. This is a value used to express the relative amount of light passed by a lens. The f-number system is a ratio that is universal. If various lenses are set to the same f-number, they will all will pass the same amount of exposing light energy.
From you question, I deduce that you think the zoom lens maintains a constant exposure with the zoom by changing the working diameter of the aperture. And you are correct, but there is no mechanical linkage that does this trick. The zoom lens uses a clever regulating method. The front group of lenses, those ahead of the iris opening, magnifies the apparent size of the aperture opening. As you zoom, the spacing between these lens elements and the iris is altered. In other words, as you zoom, the apparent diameter of the iris changes with the zoom. This is the stuff of constant aperture.
This works because the outside world sees a working diameter that varies in proportion to the zoom. If this optical trick works out, the exposing light energy remains constant throughout the zoom.
Such an optical arrangement is costly to make so the price of a constant exposure zoom is high. Less costly zooms give up the ghost as the zoom increases. Thus these fail to maintain a constant exposure.
All that being said, what you are seeing could well be old lenses with mechanical failure. The iris blades (leaves) are thin and thus fragile. Failure of the iris leaves is common especially due to age as they are lubricated. Often this oil evaporates or clumps and a malfunction results.