Originally posted by jpzk
Edit: " ... but with open aperture the IQ noticeable suffers to the point the resulting image is only acceptable for birding purposes ("record shots")"
Do you mean with or without the TC at wide open aperture ?
With the converter. Without, it's pretty nice at open aperture.
I should probably wait with posting photos until I've had a chance to shoot in better light and take shots that are relevant to other people than those familiar with taking bird shots... for that latter subgroup it's a bit easier to recognise the specific value of a lens under adverse conditions. We tend to be happy with shots
Actual Photographers would completely snub, simply because we know how the shot would normally look. When you're used to birds moving through dense shrubbery when it's severelly overcast, your standards tend to be different than when you're used to, say, conditions at which you'd be able to use the sensor shift capabilities of the K-3 ml II

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Just a teaser from tonight; click for the full-sized image on Flickr.
For birding purposes, you'd end up with a crop like this:
Common Buzzard - Buteo buteo 2015-05-01 | Vogelwerkgroep Amsterdam
In this one, you can see the value of the IBIS.
Flare test; strong backlight. Crop to about 35% of original frame; unfortunately I just missed the focus (it's on the flowers behind the heron instead of on the heron).