Advice on Telephoto for K-3 iii

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JoeOptics314 Forum Member • Posts: 62
Advice on Telephoto for K-3 iii
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I am really enjoying my K-3 iii that arrived 4 days ago. Today I took pictures of day old cygnets at a local pond with my old Pentax 55-300 ED lens.  Here is a cropped example:

Cygnets and mother

I used the lens pretty consistently at 300 mm at around F/8 to F/10 in mid-day sunlight. I am wondering how much improvement I would see if I splurged and bought the Pentax 150-450? The ephotozine.com site seems to present MTF inconsistently between the 2011 review of the 55-300mm (in 2011 at: https://www.ephotozine.com/article/smc-pentax-da-55-300mm-f-4-5-8-ed-lens-review-16547) and the 150-450 review (in 2015 at: https://www.ephotozine.com/article/pentax-hd-pentax-d-fa-150-450mm-f-4-5-5-6-ed-dc-aw-review-27359). Oddly the 150-450 mm review uses qualitative values on the vertical Axis for MTF instead of line pairs/mm sort of thing.

Alternatives to the 150-450 at the long focal length side. I could live with a smaller zoom range or a prime, but want something at least 300 mm with better image quality than my 55-300 zoom at 300mm

So I am looking for:
1. A proper MTF comparison at the same contrast level.
2. Peoples impressions of the 2 lenses.
3. Suggestions for alternatives to the 150-450. Either prime or zoom.

This camera is such a pleasure to use.  I have a lot to learn about its use and options, but it is very comfortable coming from my old K-70.

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David Lees

 JoeOptics314's gear list:JoeOptics314's gear list
Pentax K-70 Pentax K-3 Mark III Pentax smc DA 55-300mm F4.0-5.8 ED Pentax smc DA* 300mm F4.0 ED (IF) SDM Tamron AF 18-200mm F/3.5-6.3 XR Di II LD Aspherical (IF) Macro +6 more
Pentax K-3
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