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Lens starburst at fully open aperture

Started Feb 17, 2022 | Discussions thread
Sittatunga Veteran Member • Posts: 5,413
Re: Lens starburst at fully open aperture

Atis photo wrote:

J A C S wrote:

Atis photo wrote:

Sittatunga wrote:

I don't have that RF zoom, but other constant aperture zooms I do have achieve their constant aperture by stopping down the iris towards the wider end of the zoom.

WOW, that's a new one for me! That might be the issue! So in reality, at wider end of the zoom, the lens might be brighter than f4, say f3? And it is stopped down to f4 just so that it matches fixed aperture of f4?

This lens opens fully at the long end only, indeed. There is no conspiracy here. Take a look here. The off-corner mechanical vignetting is much stronger than what the physical diaphragm does. If you open the aperture somehow more, a large off-center part will get no extra light; you will get a hot spot in the central part only. In other words, the vignetting would be horrible.

And no, it is not a crippled 24/0.95 lens...

Thank you! This answers all my questions! Love the https://www.photonstophotos.net/ and didn't know that they had these cool graphs!

As I shoot mainly night photos and strive to achieve starburst-free photos it is a bit disappointing that L series zoom lens does not provide that. Seems like RF24-105mm f4-7.1 is stm is more suited for me, BUT it is not weather sealed (and probably optically less sharp).

No constant aperture zoom of any quality will provide that at anything other than the long end of the zoom.  The STM lens might, and you can probably arrange the composition so that the dodgy corners are black anyway.  Night scenes generally look horrible if it's raining or dusty anyway.

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