SP is the standard aperture diaphragm. But consider – f/number is focal length divided by aperture diameter. So, a 200mm f/2.8 lens needs a max aperture of ~71mm (200/2.8). Zoom a 70-200mm lens to 70mm, that lens could be f/1.0 (although that's an oversimplification, and even so it would entail all sorts of other optical problems). Regardless, the intent is a constant zoom. A 70mm f/2.8 lens needs a 25mm diameter max aperture, and focal lengths between 200mm and 70mm need max apertures between 71mm and 25mm. The SSP aperture stop creates that effective max aperture to maintain a constant f/2.8 at all focal lengths.
F-number is focal length divided by entrance pupil (often mislabeled as the *effective aperture*), which is affected by magnification between the physical aperture diaphragm and the front of the lens. The SSP in Canon constant aperture zoom lenses only compensates for the slight difference in the ratio between changes in magnification and changes in entrance pupil diameter as the lens is zoomed in and out. There are also many variable aperture Canon EF lenses that us SSPs.
Including an SSP in a non-retrofocus lens, such as a 70-200mm f/2.8 lens for a 20mm registration distance, is new territory for Canon EOS lenses.
With 44mm registration distance EF lenses, all the zoom designs thus far that incorporate an SSP are also retrofocus designs, at least when zoomed at their widest focal lengths.
This is true of every lens with block diagrams included at the Canon Camera Museum site. Many very early EF lenses do not have block diagrams there, but most mid-grade and higher lenses introduced since about 1990 do.
Looking at the lists below, 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 EF zoom 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 with an SSP shown in their block diagrams at
The Canon Camera Museum:
- 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
EF zoom lenses that do show the SP position but no SSP in their block diagrams at
TCCM:
- 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 lenses with block diagrams at
TCCM that show no aperture positions:
- 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)
Every lens that includes a block diagram but does not show aperture position(s) is a retrofocus design.
I find the exclusion of aperture position(s) for the 11-24mm f/4's block diagram particularly curious. Other than the 1993 35-350mm, all the rest are relatively new EF-S variable aperture lenses.