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08-15-2010, 01:27 AM - 1 Like   #1
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Performance of M lenses, 28mm to 200mm, f-stops to f8

I was interested in the bokeh my M lenses produce. I decided to shoot a test, as I understand bokeh improves stopping down from max aperture as it gets rid of the ugly condom ring effect. Lenses run in the Picasa slideshow as follows 3.5/28, 2.8/40, 1.7/50, 2/85, 3.5/135, 4/200, 75-150.

What was interesting, was shooting a high contrast subject and keeping the subject the same size by always shooting at a distance equal to 20x focal length away from the subject (roughly, using a tape measure to the k-mount).

The quantity that really improves when stopping down is the longitudinal colour fringing. When I have more time, I will post crops. For now its the whole image, which is more representative of the details you would see in a photo in any case. I'm also getting to like the 200mm as a portrait lens.

Picasa Web Albums - hoojammyflip - lens testing

08-15-2010, 02:09 AM   #2
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Wow- that's quite through! I've found the same, but keep in mind that on film, the fringing wasn't as much of a problem as it is on digital. Perhaps you should also share some of the results here: Pentax Lens Review Database - Pentax M Series Prime Lens Reviews and Specifications

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08-15-2010, 02:49 AM   #3
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quantifying equivalent blur for different focal lengths

I'm pretty certain the fringing seen for the 85mm is unfair. With the 135 and 200, I had to use a black cloth to focus, as DOF was so narrow, but I didn't bother with the 85, as I thought I was in focus. As I understand it, longitudinal CA is worse when the focus is off. I still have not got round to putting a focus screen into my ist DS as well. [I manual focus by finding the range for the acceptable front and back focus, and rotating back to the middle of the range slowly. This would not be necessary with a split screen.]

Ultimately, these lenses are being used on 35mm BW film rather than digital, specifically because the slightly larger format produces nicer OOF effects. For portraits, in BW, the fringing starts becoming a little irrelevant, expecially when using filters. It might even be a creative asset?

I'm running through depth of field calculations to quantify the blur in the background. Just setting the circle of confusion to something like 1mm you can look at equivalent blur gained for different focal length lenses.

200mm at f5.6 equates to: 135mm at f4.5, 85mm at f3.2, 50mm at f2. Note that by 40mm, literally everything is coming into focus with the relaxed 1mm circle of confusion requirement.

Calcs were based on the same circle of confusion on the sensor, for background distance D=(20x focal length + 4m) so the 200mm has more detail still as its looking at a narrower field of view. I could probably pop the calc in a spreadsheet and bung it in the forum, but its just a manipulation of:

D=s(H-f)/(H-s), H=f(1+f/Nc)
D=far depth of field, in this case 4+20x f
f=focal length
s=focus distance
N=aperture eg 2.8 for f/2.8
c=circle of confusion, which is about 1mm for a 200mm lens at f5.6 and D=8

Last edited by whojammyflip; 08-15-2010 at 03:15 AM.
08-15-2010, 03:38 AM   #4
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Quite a lot of visual data there for demonstrating the concept, thanks for sharing it. You will certainly benefit from the exercise with an excellent understanding of how each of your lenses will perform in comparison to one another in a given situation. This would make a good teaching exercise to demonstrate the concepts possibly with just three common focal lengths.


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