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04-20-2009, 02:26 AM   #1
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f half stops on manual lenses

Hi there!

I'm starting to extensively use a Super-Multi-Coated 50mm f1.4 Takumar as a route lens for my K10D. I really love this lens and I'm trying to master its use and know all its guts.

As I shoot, I have in my head all the DoF and hyperfocal numbers (or at least most of them) , so I know what is going on with the shot.

My main question is that this lens, apart of the range of f-stops (1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16) has half-stops, and I wonder if the relation of those is just half the difference between them (i.e: 1.7 - 2.4 - 3.4 - 4.8 - 6.8 - 9.5 - 13.5), or the relation is a non-linear one, as the normal f stops don't follow a linear progression as well.

I would like to know that so I can calculate the changes on the DoF and hyperfocal values for the half steps as well.

Thanks a lot for your help!

04-20-2009, 02:31 AM   #2
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I'm taking this from the Pentax "35mm interchangable lenses" book that I got when I purchased a lens.

The half stops are

1.2 - 1.7 - 2.4 - 3.4 - 4.8 - 6.7 - 9.5 - 13.5 - 19 - 26 - 38
04-20-2009, 03:47 AM   #3
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QuoteOriginally posted by Cosmo Quote
I'm taking this from the Pentax "35mm interchangable lenses" book that I got when I purchased a lens.

The half stops are

1.2 - 1.7 - 2.4 - 3.4 - 4.8 - 6.7 - 9.5 - 13.5 - 19 - 26 - 38
Is that specific for the 50/1.4 or for all Taks with half stops?

/Jens
04-20-2009, 03:55 AM   #4
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Surely half stops are the same for every lens?

In the book, it doesn't say if this is specific to a lens, or the Takumar range. I believe it's the universal half stops.

04-20-2009, 04:26 AM   #5
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If you are unsure of the 1/2 stops, set your camera to 1/2 stop increments in exposure, and with one "correct" reading, manually adjust the shutter and apature in 1/2 stop increments. then check the grey scale exposure of each combibation.

it should be the same, errors in apature will show up as a change in grey scale
04-20-2009, 04:27 AM   #6
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Nice! thanks a lot Cosmo.

It seems that all follow an average linear progression, except the 6.8 that it's a 6.7 probably because an average calculation.
04-20-2009, 05:57 AM   #7
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QuoteOriginally posted by Rafalas Quote
Nice! thanks a lot Cosmo.

It seems that all follow an average linear progression, except the 6.8 that it's a 6.7 probably because an average calculation.
It's actually a logrithmic progression, to base 2.
04-20-2009, 06:19 AM   #8
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QuoteOriginally posted by Cosmo Quote
Surely half stops are the same for every lens?

In the book, it doesn't say if this is specific to a lens, or the Takumar range. I believe it's the universal half stops.
The f stops are the same for every lens since it's a ratio, not a physical size that is indicated.

If they were different for every lens, the scale would be useless.

04-20-2009, 09:20 AM   #9
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You realize that depth of field isn't an absolute thing, right? It's all just slightly different amounts of barely distinguishable blurriness. The difference between aperture values of, say, f/6.7 and f/6.8 isn't going to translate into anything discernable on your image.
04-20-2009, 12:01 PM   #10
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The actual definition of what constitutes a a full or half f-stop is always the same across all lense, brands, an formats: multiply by the square root of two to get full stops, or by the square root of that (ie, the fourth root of two) to get half stops. The usual approximations are to use 1.4 as an approximation for the full stop factor and 1.2 for the half stop factor. However, different manufacturers label their half stops differently. Pentax tends to label them 1, 1.2, 1.4, 1.7, 2, 2.8, 3.5, 4, 4.5, 5.6, 6.7, 8... even though some of the half stop values are pretty wildly off (3.5 is much too high; 4.5 much too low). Of course, those half stop marking rarely appear on a the aperture ring, but that's how the progression goes when changing aperture using the camera controls.

Anyhow, as Sean says, DOF calculation are really rough estimates anyhow. The difference between f/3.5 and f/3.332 isn't going to matter, really - either way you're just taking a rough stab at approximating what will be more or less in focus.
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