Lens Rentals reviews the 35-100 / 2.8

Started Nov 2, 2012 | Discussions thread
Great Bustard Forum Pro • Posts: 39,710
Not quite.

sigala1 wrote:

Horshack wrote:

In terms of getting an acceptably sharp image, at least at the long end where it's likely to be used the most. Disappointing for a $1500 lens that is supposed to compete with the big boys from Canon/Nikon.

Actually, the lens is probably an f/2.0 to f/2.8 lens, which is why it looks sharper "wide open" at 35mm, because it's actually stopped down at that aperture and focal length.

This is the laws of physics... as the focal length increases, you need a physically larger aperture to arrive at the same f/stop ratio.

I don't understand why Panasonic didn't make it an f/2.0 to f/2.8 lens.

The following applies equally to the 35-100 / 2.8 zoom:

http://www.josephjamesphotography.com/equivalence/index.htm#aperture

The term "aperture", by itself, is vague -- we need a qualifying adjective to be clear.  There are three different terms using "aperture":

1.  The physical aperture (iris) is the smallest opening within a lens.

2.  The virtual aperture (entrance pupil) is the image of the physical aperture when looking through the FE (front element).

3.  The relative aperture (f-ratio) is the quotient of the focal length and the virtual aperture.

For example, a 50mm lens whose  entrance pupil (virtual aperture) has a diameter of 25mm will have an f-ratio (relative aperture) of 50mm / 25mm = f/2.  Alternatively, a 50mm lens at f/2 has an entrance pupil (virtual aperture) diameter of 50mm / 2 = 25mm.

Interestingly, a "constant aperture" zoom is a zoom lens where physical aperture (iris) remains constant, but the virtual aperture (entrance pupil) scales with the focal length, thus keeping the relative aperture (f-ratio) constant as well.  Let's consider a 70-200 / 2.8 zoom.  Assuming the diagram in the link is accurate, then the proportions of the diagram results in the diameter of the physical aperture (iris) to be 38.5mm since the diameter of the FE (front element) is 77mm.

The virtual aperture (entrance pupil) is the image of the physical aperture (iris), that is, it is how large the physical aperture appears when viewed through the front element.  The diameter of virtual aperture is 25mm at 70mm f/2.8 (70mm / 2.8 = 25mm) and 71mm at 200mm f/2.8 (200m / 2.8 = 71mm).  So, as the lens zooms, neither the physical aperture (iris) nor the relative aperture (f-ratio) change, but the virtual aperture (entrance pupil) changes in direct proportion to the magnification.  Basically, we can think of a 70-200 / 2.8 zoom as a 70 / 2.8 prime, with additional elements that magnify the image as we zoom.

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