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06-09-2020, 07:42 PM   #31
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QuoteOriginally posted by JensE Quote
I measured a central area (376x432 - just for the simple math) of the raw files using rawshack and compared the Mean values (for Green_2, others are proportional) and also checked the visual results. In all four cases, f/2.8 came out visually darker than the other two (f/4 and f/5.6), which were very consistent (0.1%, 0.1%, 2.5% and 1.6% difference) to each other in each series. The f/2.8 exposures had 29% less (or -0.5EV) at infinity and 24% (-0.4EV) less at MFD, relative to the f/5.6 exposure - very consistent between life view and viewfinder.
Question...Same set EV* for each exposure? (E.g. f/2.8@1/500s, f/4.0@1/250s, f/5.6@1/125s)


Steve

* EV in the original sense of being the actual exposure (x aperture for y time)

06-10-2020, 02:53 AM   #32
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QuoteOriginally posted by JensE Quote
I shot 4 different short tests series against a reasonably evenly illuminated white door, all in center-weighted average measurement mode, at f/2.8, f/4.0, f/5.6 respectively:

1. infinity, live view
2. infinity, viewfinder
3. MFD, live view
4. MFD, viewfinder

I measured a central area (376x432 - just for the simple math) of the raw files using rawshack and compared the Mean values (for Green_2, others are proportional) and also checked the visual results. In all four cases, f/2.8 came out visually darker than the other two (f/4 and f/5.6), which were very consistent (0.1%, 0.1%, 2.5% and 1.6% difference) to each other in each series. The f/2.8 exposures had 29% less (or -0.5EV) at infinity and 24% (-0.4EV) less at MFD, relative to the f/5.6 exposure - very consistent between life view and viewfinder.

For a comparison, I did the very same test with my D-FA 100 WR. At infinity, both the f/2.8 and f/4 exposures came out darker - by 10% (-0.16EV) and ~7% for both LV and viewfinder than the f/5.6. At MFD, only f/2.8 was darker, by 29% (ringing a bell?), and f/4 was actually brighter (~4.5%) than the respective f/5.6 exposures. Overall, the MFD exposures were significantly brighter than the infinity ones.

Somewhat surprised, I double checked the results visually using geeqie's 4-way view, zoomed in to the center at 1:1 to rule out vignetting effects. Visually, the appearance matched what I have calculated. If I find the time, I may put together some scripting to automate evaluation of the results and generate graphs for some of my lenses and maybe compare K-1 and KP.


In conclusion, there doesn't seem to be anythng out of the ordinary with my Irix 150mm.
Watching inside at the aperture diaphragm blades of your irix lens have you noticed any difference between f2.8 and f3.2?
As I said, on my copy, blades activate at F3.2, but so little that they are not visibile, and in manual mode the exposure is identical with f2. 8 or f3.2
My 100wr macro doesn't act like this f3.2 is darker than f2.8 and I can see the aperture blades.
06-10-2020, 03:34 AM   #33
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QuoteOriginally posted by Chosa Quote
Watching inside at the aperture diaphragm blades of your irix lens have you noticed any difference between f2.8 and f3.2?
They are engaging and get visible - as backed by the measurements. The initial step does look smaller than the following ones, though.

QuoteOriginally posted by stevebrot Quote
Question...Same set EV* for each exposure? (E.g. f/2.8@1/500s, f/4.0@1/250s, f/5.6@1/125s)
No, this was just a quick test using aperture priority - relevant when you want to know how the camera exposes with a particular lens at a given aperture. Checking the Light Value in the EXIFs, it does vary. I'll redo with constant EV tonight, when it is dark enough for controlled constant light.
06-10-2020, 03:39 PM   #34
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I redid the above test using more F-Stops, but only for minimum focusing distance and viewfinder, keeping exposure constant but closing up aperture in 1/3EV steps and increasing exposure time in 1/3EV each step. Results are displayed in a suitable log-log plot below.
I did a single test run only, but it gives you an idea about consistency of exposure once closed far enough, about 0.1EV, whereas f/2.8 is about 1/3EV below average and f/3.2 is almost in the error margin already.
(Just to make this clear - the vertical scale has an arbitrary starting point, '0' is not special. I could have put it from 5.0 to 6.0 - it's just meant to show the relative exposures in EV scale - i.e.log2-based.)

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Last edited by JensE; 06-10-2020 at 03:52 PM.
06-10-2020, 04:22 PM   #35
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What is the light source and shutter speed in case AC cycles can be an impact. At shutter of 1/120 and 60 cycles it only gets half a cycle.
06-10-2020, 04:40 PM   #36
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Should be OK - high quality electronic ballasts, T-5 tubes, shutter 1/6s and slower. With electronic shutter, I can see the >20kHz flicker, but no slower changing component.
06-11-2020, 01:59 PM   #37
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Maybe I should add that the above 1/3 EV is nothing to worry about for me at all. For critical JPG use, I would need to check the histogram anyway and for everything else, a +1/3EV more of "exposure" (raw value multiplier) doesn't matter at all, so I can use the Irix 150mm with the same exposure settings at f/2.8 as for all other settings.
06-25-2020, 07:39 AM   #38
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I've send it back and take another copy of the lens...
which behave similarly to the first one (the serial n. is different)

It could be the case that I'm particularly unlucky, but I think that this lens have some generic error in the aperture calibration.

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