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03-17-2019, 12:37 PM   #1
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An interesting fact about 31mm limited!

Hello! I've observed an intriguing fact about my 31mm limited: at f1.8 and f2 it gives the same exposure value. So, if at 1.8 it's 1/50 it will be the same at f2! Even if they have the same shutter speed, the image it's different, more contrast at f2 and not so rounded bokeh... This is good especially when you shoot in low light... Your copy of this lens behaves the same?

03-17-2019, 12:48 PM   #2
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QuoteOriginally posted by zburatoru Quote
Hello! I've observed an intriguing fact about my 31mm limited: at f1.8 and f2 it gives the same exposure value. So, if at 1.8 it's 1/50 it will be the same at f2! Even if they have the same shutter speed, the image it's different, more contrast at f2 and not so rounded bokeh... This is good especially when you shoot in low light... Your copy of this lens behaves the same?
About this lens i don't know my friend but the same happens to me with the Pentax FA 50mm f/1.7. and with the Sigma 35mm f1.4 Art. I really don't understand. With the sigma it's really strange. With the aperture at f1.4 the shutter speed is slower than f1.8 or f2.0
03-17-2019, 12:51 PM   #3
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QuoteOriginally posted by zburatoru Quote
Hello! I've observed an intriguing fact about my 31mm limited: at f1.8 and f2 it gives the same exposure value. So, if at 1.8 it's 1/50 it will be the same at f2! Even if they have the same shutter speed, the image it's different, more contrast at f2 and not so rounded bokeh... This is good especially when you shoot in low light... Your copy of this lens behaves the same?
Are you using one of the modes that automatically alters ISO setting, perhaps - such as Auto (Green) or TAv?

If the diaphragm is moving when you stop down from f/1.8 to f/2 - and the fact that you're seeing better contrast but less-rounded out-of-focus highlights suggest that it is - then it will be letting in less light at f/2. As such, one of three things must change along with the aperture... ISO setting, shutter speed or the amount of light on the subject.

Check your ISO, aperture and shutter speed in the EXIF data of your files. If ISO and shutter speed are the same at aperture settings of f/1.8 and f/2, the light level changed.

Last edited by BigMackCam; 03-17-2019 at 01:10 PM.
03-17-2019, 01:03 PM   #4
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QuoteOriginally posted by zburatoru Quote
Hello! I've observed an intriguing fact about my 31mm limited: at f1.8 and f2 it gives the same exposure value. So, if at 1.8 it's 1/50 it will be the same at f2! Even if they have the same shutter speed, the image it's different, more contrast at f2 and not so rounded bokeh... This is good especially when you shoot in low light... Your copy of this lens behaves the same?
You didn't say which camera and which shooting mode. Av? Also, do you have the EV adjustment step set to 1/3 or 1/2 EV steps (first item in the C menus on most Pentax cameras)? Is ISO set or on Auto?

Since the difference between f/1.8 and f/2 is only 0.3 of a stop, your EV adjustment setting may not be fine enough to register it.


Last edited by Paul the Sunman; 03-17-2019 at 01:15 PM.
03-17-2019, 01:12 PM   #5
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QuoteOriginally posted by BigMackCam Quote
Are you using one of the modes that automatically alters ISO setting, perhaps - such as Auto (Green) or TAv?

If the diaphragm is moving when you stop down from f/1.8 to f/2 - and the fact that you're seeing better contrast but less-rounded out-of-focus highlights suggest that it is - then it will be letting in less light at f/2. As such, one of three things must change along with the aperture... ISO setting, shutter speed or the amount of light on the subject.

Check your ISO, aperture and shutter speed in the EXIF data of your files. If ISO and shutter speed are the same at aperture settings of f/1.8 and f/2, the light level changed.
I'm using av mode... More interesting: in m mode with green button: f1.8 1/50, f2 1/30, iso 800... I think it's not the lens maybe the camera not measuring properly...

---------- Post added 03-17-2019 at 01:16 PM ----------

QuoteOriginally posted by PedroCosta Quote
About this lens i don't know my friend but the same happens to me with the Pentax FA 50mm f/1.7. and with the Sigma 35mm f1.4 Art. I really don't understand. With the sigma it's really strange. With the aperture at f1.4 the shutter speed is slower than f1.8 or f2.0
That's quite strange... Could be the camera... Try the fa 50mm f1.7 in manual mode with green button!

Last edited by zburatoru; 03-17-2019 at 01:17 PM.
03-17-2019, 01:32 PM   #6
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I have seen this too but if you look at the image carefully you will see the borders lighten up quite a bit at f2 (due to less vignetting) and this affects auto exposure level.
So because the overall image (averaged across the frame) looks brighter this balances, somewhat, for the change of aperture. If you look carefully at the centre of the image you will see its exposure level is lower at f2 than f1.8 (at least thats case on my test shots) - its just that the rest of the image looks the same / brighter.

To test this try using spot AE metering in the centre of the image and I think you will different shutter speeds then.
03-17-2019, 01:41 PM   #7
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QuoteOriginally posted by kiwi_jono Quote
I have seen this too but if you look at the image carefully you will see the borders lighten up quite a bit at f2 (due to less vignetting) and this affects auto exposure level.
So because the overall image (averaged across the frame) looks brighter this balances, somewhat, for the change of aperture. If you look carefully at the centre of the image you will see its exposure level is lower at f2 than f1.8 (at least thats case on my test shots) - its just that the rest of the image looks the same / brighter.

To test this try using spot AE metering in the centre of the image and I think you will different shutter speeds then.
No matter what metering I use and I've tried all three, same story, same shutter speed! For me this behavior ain't bothering me...
03-17-2019, 03:17 PM   #8
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I've seen similar behavior with several fast lenses, too.


My explanation is that vignetting causes this effect. Fast lenses tend to have significant vignetting wide open and less vignetting when the aperture gets more closed.


You'd expect that the meter in the camera gets more light at f=1.8 than at f=2.0, since geometrically the lens is wider open . Hoewever, due to the stronger vignetting at f=1.8 the amount of light the meter collects may be nearly the same as at f=2.0, resulting in same shutter speed with same ISO.


With my Samyang 2.8/10mm the camera has even a tendency to overexpose wide open by as much as one full stop, which becomes insignificant when the lens is stepped down to f=8. That behavior caused me to search for the root cause. At first I was wondering if the aperture mechanism of the lens was working correctly. But then I noticed the same effect (to a lesser extend) with another fast lens.


As long as the deviation is only 1/3 EV I wouldn't worry about it.

03-17-2019, 03:49 PM   #9
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As another alluded: what is your ev setting for ISO and shutter speed? If this is 1/2 then some % of the time this 1/3 ev stop won't change the matching values but will alter the actual capture. Compare the histograms.
03-17-2019, 05:09 PM   #10
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There is inevitably going to be some rounding in the exposure values that are displayed in an autoexposure mode. Given the 1/3 stop difference between f1.8 and f2.0, it's conceivable that the displayed shutter speed doesn't change even if it was in fact adjusted accordingly (i.e. steplessly) under the hood.


If you compare f1.8 and f2.0 under various ambient EV conditions you should eventually see a discernible difference in the reported shutter speed, however.

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03-17-2019, 11:02 PM   #11
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QuoteOriginally posted by Adam Quote
There is inevitably going to be some rounding in the exposure values that are displayed in an autoexposure mode. Given the 1/3 stop difference between f1.8 and f2.0, it's conceivable that the displayed shutter speed doesn't change even if it was in fact adjusted accordingly (i.e. steplessly) under the hood.


If you compare f1.8 and f2.0 under various ambient EV conditions you should eventually see a discernible difference in the reported shutter speed, however.
Thank you all for the answers! For me this ain't a problem just mentioned it for curiosity... The aperture it's working as it should so no issue at all.
03-18-2019, 12:09 PM   #12
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Have you confirmed this by examining the histograms and EVs using the info tab in say Photoshop?

As Adam points out there well be rounding errors. My old Minolta X-700 had a continuously stepping shutter - even though the viewfinder indicator only showed 1/60 the actual speed could have been between 1/45 and 1/75.
03-18-2019, 01:36 PM   #13
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QuoteOriginally posted by Not a Number Quote
Have you confirmed this by examining the histograms and EVs using the info tab in say Photoshop?

As Adam points out there well be rounding errors. My old Minolta X-700 had a continuously stepping shutter - even though the viewfinder indicator only showed 1/60 the actual speed could have been between 1/45 and 1/75.
No, I don't use Photoshop, but I can check the histogram...
03-26-2019, 10:23 AM   #14
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From my own experiences, I find that the EV can vary within a 1/2 stop on almost any shot if the camera is set in any auto exposure mode (i.e. not M mode).

The only way to probably really see what the differences may be is to actually shoot in M mode and fix all three exposure parameters and then adjust only the aperture and/or shutter speed to see how the histogram and image change. ISO is a wildcard in this, and it is best to make such comparisons with the ISO fixed. As ISO increases, the dynamic range generally goes down and you get less contrast. Of course other settings will impact contrast too, but it is definitely worth not having ISO be a variable (if you haven't already).

By the way, I don't usually worry about the difference by 1/2 or 1/3 of a stop while shooting. I do use the histogram to confirm photos are ok, but I generally find that photos that lightly clip on the histogram are not clipped when brought into my PP software; RAW files have more dynamic range than the JPG files that are shown on the camera's screen, and those JPG files are subject to JPG parameters that I rarely touch because I am shooting RAW to begin with.
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