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Forum: Pentax SLR Lens Discussion 03-13-2018, 01:10 PM  
Effects of 2x converter
Posted By normhead
Replies: 21
Views: 1,817
Putting a 1.4 TC on the 100 2.8 makes it a 140 ƒ4...

Pentax K-30
According to DoF Master
Online Depth of Field Calculator

100mm , ƒ2.8, @ 10 feet, total DoF .17 feet.
100mm, ƒ4 ---,@ 10 feet, total DoF .47 feet
140mm , ƒ4... , @ 10 feet, total DoF .24 feet

So wide open, more DoF wide open on the 140 ƒ4. From the same position you actually get more DoF wide open with the TC.
Diffraction tends to become an issue at ƒ8 and goes downhill from there.

100mm 15 feet DoF = total DoG 2.16 feet
Back up to 21 feet to keep the framing roughly the same

140mm, 21 feet, ƒ8 - total DoF = 4.28 feet.

You can actually double your DoF without changing your framing by using the TC and backing up, at the same ƒ-stop.
These kinds of things are usually a lot more complex than most people expect.
Forum: Pentax SLR Lens Discussion 03-13-2018, 07:36 AM  
Effects of 2x converter
Posted By normhead
Replies: 21
Views: 1,817
MY A-400 works with the 1.4 TC not with the 1.7 at distance in direct sun. With the 1.7x I turn off focus confirmation and auto-focus and just focus manually, and get better results.

But the 1.7x makes a purple fringing lens even worse. It's designed for * quality 2.8 lenses. That would be my big concern with putting a TC on older glass. If made before Low Dispersion glass and other corrective elements, like aspherical optical plastics were common, those lenses are likely to purple fringe badly on digital sensors. A TC makes it worse.
Forum: Pentax SLR Lens Discussion 03-13-2018, 07:06 AM  
Effects of 2x converter
Posted By normhead
Replies: 21
Views: 1,817
With the F 1.7x checking in at ƒ6.3 on an ƒ4 lens that's probably not an issue. When I put the 1.7 on an ƒ5.6 lens, I definitely have issues, so ƒ6.3 is close to the line but not over the line.
Forum: Pentax SLR Lens Discussion 03-13-2018, 06:57 AM  
Effects of 2x converter
Posted By normhead
Replies: 21
Views: 1,817
Better to listen too someone who has a similar combo, and you didn't say which 2x converter. From what I can tell, people's satisfaction with their teleconverters depends a lot on what brand they are. Off brands can be really crappy.

The lens looks sharp enough...
SMC Pentax-M 200mm F4 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database

The ideal would be the F 1.7x AF adapter... that would give you a little more light to focus with and limited AF.
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