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I am an experienced amateur photographer and videographer using a Sony a7sII with a Sigma 28-300mm f3.5-6.3 hyperzoom lens for a music video shot. I notice that when I test the shot zooming out all the way from 300mm to 28mm on this lens, the aperture changes from f11 to f6.3 by the time it reaches 28mm. I have ensured that I am in manual exposure mode.

Is this normal? I understand that zoom lenses have variable apertures but I did not think that this meant you cannot have a constant aperture when zooming in if it above the minimum aperture for any given focal length across the available range.

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  • \$\begingroup\$ I have a 120-400 Sigma zoom and the aperture remains constant when I zoom in and out, provided it is within the aperture range of the focal length (so f/11 the aperture doesn't change over the hole zoom range). Have you checked if Sigma states the compatibility of uour camera model with your lens? \$\endgroup\$
    – xenoid
    Commented Jun 19 at 20:43
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    \$\begingroup\$ What specific Sigma 28-300mm lens is this? I don't think has made any 28-300mm f/35-6.3 in Sony E-Mount. So what specific adapter are you also using with it? \$\endgroup\$
    – Michael C
    Commented Jun 20 at 1:54
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    \$\begingroup\$ This is likely the Sigma 28-300mm f/3.5-6.3 DG introduced in 2002 in Nikon F, Canon EF, Minolta A, Pentax, and Sigma mounts. To use it with a Sony E-Mount camera some sort of adapter is involved, whether mechanical only or mechanical + electronic. This question is impossible to properly answer until we know the specific lens model, the specific mount, and the specific adapter being used. \$\endgroup\$
    – Michael C
    Commented Jun 20 at 2:11

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That does not seem normal to me...

What it is doing is holding the physical aperture size constant as you change the zoom... rather than the f#. That's what will normally happen if you start at 28mm f/3.5 and change the zoom; because it can't maintain the f#... but in manual or aperture priority it should maintain the f# if possible.

Maybe try aperture priority to see if it behaves the same.

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  • \$\begingroup\$ This is a film era lens and has never been made in Sony E-mount. We need to know what specific lens this is, in what specific mount, being used with what specific adapter before we can do anything besides shooting in the dark. \$\endgroup\$
    – Michael C
    Commented Jun 20 at 2:03
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Try setting your ISO to float (auto) and manually set your aperture and speed. When you zoom, your ISO will course correct in one-third stops... and is not noticeable.

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