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Forum: Pentax Lens Articles
09-15-2014, 04:09 AM
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Live view will be the most consistent, because it uses the sensor not the viewfinder to measure exposure.
For the viewfinder/green button exposure, one thing to consider is to have the EV steps match the lens detects, I.e. 1/2 stops, because otherwise you will have some inconsistencies between the 1/2 stop indexing of the aperture and slight metering changes frame to frame in huge shot.
I have found the method I use to be quite consistent over the span of several years, where I can go back and repeat it, within some tolerances again and again.
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Forum: Pentax Lens Articles
09-11-2014, 02:04 PM
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For A lenses it is a problematic aperture because the line should be flat. But for manual aperture lenses, it is truly an issue I believe with the amount of scatter back off the focusing screen, as a function of aperture. I can't prove it of course but the results are consistent with all manual lenses
Shooting AV mode by backing a K mount off is not a solution as the error is there all the time, it is metering not aperture control that is the issue.
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Forum: Pentax Lens Articles
09-11-2014, 04:20 AM
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You have it backwards. It over exposes not under exposes. When you do a histogram, white is 255, black is zero, and normal exposure is about 128. Look at the A50/1.2 which is a flat line through the middle. What you see there is errors wide open and at minimum aperture but the remainder is perfectly flat.
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Forum: Pentax Lens Articles
09-05-2014, 08:06 AM
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after seeing yet another post about metering inaccuracies on K mount (non A) lenses, it occurred to me that i have never posted this chart here.
Metering with manual aperture lenses is not 100% linear, and while many times it is "good enough" it has its faults. The chart below was made using a K50/1.4 lens, and shots were taken with the different cameras and configurations i own.(plus a little help from other forum members). The process was to use manual mode and green button metering to set exposure, shooting a uniformly lit wall. I have used also paved roads, sidewalks and both interior artificially lit surfaces as well as outside concrete block walls, the result is not changed.
You shoot a shot at each manual aperture click, using green button for metering, and then in your photo editor, measure the greyscale value of the frame (i used a 10% centrally located selection but not all editors allow for greyscale value to be calculated on a selection) the result is below when plotted against aperture. All shots were done in JPEG with neutral contrast and saturation. Note each 45 greyscale is equal to 1 stop between the range of about 25-225.
I recommend serious legacy lens shooters map out each lens and body they own.
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