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12-07-2010, 01:49 PM - 2 Likes   #1
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EXAKTA TO PENTAX ADAPTER - and - DOF INDEX

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.
________________________________________________________________

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?

12-07-2010, 01:56 PM   #2
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Isn't the dof exclusively based on the focal length? You'd think that larger apertures only allow for smaller dofs when used.
12-07-2010, 07:56 PM   #3
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As we know from the omnicient Wikipedia entry, DOF is a complex function of photography, presentation, and perception. But the factors under a photographer's control are focal length, the aperture used, distance from subject, and frame size. The table I presented doesn't address all the variables, nor does it quantify DOF for specific lenses. It just says that you can expect a similar DOF from lenses having similar calculated index numbers.
12-08-2010, 04:21 AM   #4
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QuoteOriginally posted by RioRico Quote
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.
________________________________________________________________

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?
162MM?

And what happened? You sold your 85 1.9?

12-08-2010, 07:45 AM   #5
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To Part1 - T2 adapter mount

With some exacta lenses you can remove the whole mount - it is just screwed on. I successfully replaced it with a T2 to Pentax K adapter. You will need a certain make of T2 adapter where the inner T2 thread is rotatable and held in place by 3 screws. Open the 3 screw remove the T2 thread and put the adapter on the lens, you can fix the adapter on the lens by fastening the 3 screws.
12-08-2010, 01:58 PM   #6
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QuoteOriginally posted by Ira Quote
162MM?
That's the Wollensak Enlarging Raptar 162/4.5 stuck on bellows.

QuoteQuote:
And what happened? You sold your 85 1.9?
Never had a 1.9, just the Nikkor 85/2. But I'm accepting donations.

QuoteOriginally posted by chse Quote
With some exacta lenses you can remove the whole mount - it is just screwed on. I successfully replaced it with a T2 to Pentax K adapter.
I just got a pile of those adapters, 5 for US$30 shipped from China, and I still have some unused. Do you happen to know which Exakta lenses can be adapted that way? I don't think my big Meyer Primagon 35 nor pancake Helioplan 40 are built for that. Maybe I should gargle for EXAKTA LENS T2 and see what pops up.
12-08-2010, 04:44 PM   #7
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I have a hard time wrapping my head around this. What does these numbers really mean? From what I understand it is a measure of perceived DOF. However I can't seem to figure out if this is a linear scale or some other kind. Have you done any practical tests? Say have a large board leaning at an appropriate angle and use some tape to make a frame on it, and then shoot different lenses with the same framing and measure how much DOF is in the picture and compare this to the values? If this works out as a linear scale it is indeed interesting.
12-09-2010, 04:47 AM   #8
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I have done no rigorous testing. It does support some intuitions, like that an 80/2.8, 58/2, and 50/1.7 have similar DOF, or that my 127/1.8 has the thinnest DOF of all my lenses. For what it *means*, or whether it's linear or logarithmic or whatever, I'd have to ask someone with more optics knowledge than I. Is there a lens designer in the house? Until I know more, I'll just use it as a rough comparative index.

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