I bought Sigma 150-600mm f5-6.3 Contemporary DG OS HSM Lens for my Crop Sensor Canon 1300D.
What I've noticed is that it gives me max aperture of f5.6 at 600MM instead of f6.3 and f4.5 at 150mm on my Canon 1300D.
Please provide your views. Here's the link to the pic with Metadata–
Most/some third-party lenses report better apertures than they actually have - e.g. with f/6.3, the Sigma 150-600mm f5-6.3 Contemporary DG OS HSM would no longer work with phase-detection-AF (or, to be more exact, the 1300D, as most entry- and mid-range-models, would deactivate PDAF) :
[If] the maximum aperture becomes higher than f/5.6, AF shooting will not be possible (except in FlexiZone - Single and Live mode during Live View shooting).
See p. 100 in your camera's manual
As you can see, contrast-detection-AF (as used in Live View) would still work - and it would not stop, ever, as long as it has enough light to work.
As a workaround, such lenses often report 5.6 as maximum aperture - so AF will still work. Most of the time, it is somewhat slower than the AF of a real f/5.6-lens.
This does not explain the f/4.5 at 150mm, though. My guess would be that it is easier to shift the whole max-aperture-value down than to just "correct" it for values above f/5.6.
On a side-note: If the Sigma lens really was f/4.5-5.6, they would definitely sell it as such, as larger apertures usually sell better (and at a higher price).
Sigma 150-600mm f5-6.3 Contemporary DG OS HSM I see no mentioning of anything else in this whole question. If you refer to why Sigma doesn't claim it to be f/4.5-5.6, then your comment is just my side-note, inverted.
\$\endgroup\$
Most older Canon digital SLR cameras are rated to focus at f/5.6 or wider and, unlike newer bodies that may be firmware limited to turn off AF above f/5.6, will still attempt to focus when an f/6.3-f/8 lens or combination is mounted. Sometimes they even succeed.
For example, an EF 24-105mm f/4 L IS + Kenko 2X PRO 300 + Canon 5DII (which gives a 48-210mm f/8 combination) will Auto Focus in bright conditions. The combination focuses noticeably slow, but does work. The IQ from that combination is also very good in conditions where the primary source of light is behind the camera. The same combination mounted on a 7D (APS-C camera, thus smaller mirror and narrower focus array) will hunt for focus but not generally succeed unless it is pointed at an area of very bright/dark contrast such as a bare light bulb, and then only certain focus points will acquire focus lock. Note that this is only the case with third party lenses or lens + third party converters. The Canon extenders appear to use firmware to disable AF with f/8 combinations and AF won't even attempt to find focus with an f/8 combination mounted.
Apparently when the EOS system was developed back in the 1980s, the EF mount was designed to allow the provision for a lens with a manually set aperture ring on the lens. Thus the lens reports two different numbers to the camera: one that is the current aperture setting and another that is the maximum aperture of the lens.
Third party teleconverters and even lenses can fool the camera by reporting the lens as an f/5.6 lens set at f/6.3 (In the case of several third party zoom lenses that are f/6.3 when at maximum focal length) or even f/8.
That is probably what is going on with your Sigma 150-600mm f/5-6.3 C lens.
