Part I: Exakta lens to Pentax mount adapter
I've mentioned this around here a couple times (buried within other threads) but here it is now for all to see:
ANNOUNCING: Easy cheap non-destructive Exakta-Pentax adapter
What it boils down to is this: A cheap M39-M42 adapter ring fits easily around an Exakta bayonet; use glue to hold it there. Exakta register (flange focal distance) is ~0.75mm shorter than Pentax Register, so grind about that much from the flange on a Bower-type M42-PK adapter. How to grind it? On 100-grit sandpaper. See the link for details.
Note: I'm leaving the instructions and pictures posted at that link because posting pictures is easier there and I don't want to have to do that again.
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Part II: DOF (Depth of Field) Index
During
a discussion of fast lenses on another forum, a member mentioned that you can get a relative ranking of the DOF of various lenses by dividing the focal length by the maximum aperture of each lens. I'll call that number the DOF INDEX. The higher the number, the thinner the DOF. My response:
This is VERY interesting. We can easily build spreadsheet tables that calculate this DOF INDEX for any focal length and aperture. The 50/1.7 and 58/2 are equivalent, and not far from a 90/2.8 or 180/5.5. The 50/1.2 and 85/2 are close. The 135/2.5 handily beats all of those.
So I built such a spreadsheet table. Here it is, with some lenses I have and many I can only dream of. It's sorted with the highest DOF index ( = thinnest DOF) on top.
107.1= 300/2.8
100.0= 400/4.0
75.0 = 300/4.0
71.4 = 200/2.8
70.6 =
127/1.8 (this is a cheap projector lens, on bellows)
64.3 = 180/2.8
63.5 =
400/6.3
57.1 =
200/3.5
56.7 = 85/1.5
54.0 =
135/2.5
53.6 =
300/5.6
53.3 =
240/4.5
50.0 =
200/4.0
50.0 = 50/1.0
48.3 = 58/1.2
45.8 = 55/1.2
42.8 = 77/1.8
42.5 =
85/2
41.7 = 50/1.2
39.3 =
55/1.4
38.6 =
135/3.5
37.5 =
105/2.8
36.0 =
162/4.5
35.7 =
50/1.4
32.7 =
180/5.5
32.4 =
55/1.7
32.1 =
90/2.8
29.4 =
50/1.7
29.2 = 70/2.4
17.5 = 35/2.0
12.0 =
24/2
11.2 =
28/2.5 Boldface entries are lenses I have now - I can compare them without fantasizing nor bloviating. That entry with the comment is my TDC VIVID Anastigmat 127/1.8 that cost me US$1.15 (ONE BUCK!!) a couple weeks ago and is arguably my 'fastest' lens now. Note that you get thinner DOF with a 135/2.5 than with a legendary 50/1.0.
That
other-forum discussion was/is about getting minimal DOF with wide-open lenses for portraits with greatest OOF (out-of-focus) zones) for various opticals. No claims are made about sharpness at any aperture, or size or weight or any of that. Just notice that cheaper, longer lenses give thinner DOF than faster, shorter, expensive lenses. Maybe I can stop lusting after a 55/1.2 after all, eh?