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Nokton 58/1.4, behavior at f/1.4
#1
Hi there,



I recently bought a (new) Voigtländer Nokton 58/1.4 from Hong Kong (Pentax mount). It's a great lens, but I am puzzled by its behavior at large apertures. Maybe someone here can help me understand this.



The problem: Images shot at f/1.4, f/1.6 and f/1.8 look identical. I noticed this because (1) the size of background highlights doesn't change anymore when you stop further down from f/1.8, and (2) f/1.4 gives noticeably underexposed results. I first thought this might be a defect of my copy, but if you look at the bokeh test images in the [url="http://www.opticallimits.com/nikon--nikkor-aps-c-lens-tests/360-voigtlander-nokton-58mm-f14-sl-ii?start=1"]Photozone review[/url], they show the same thing concerning highlights. Even the difference between f/1.4 and f/2 is negligible in those test shots.



I went on and shot a series of images with only the aperture varying between shots. f/1.4 through f/1.8 expose identically, for all practical purposes. Yet visual inspection (i. e. looking into the lens while shooting) shows that the aperture opens up completely at f/1.4, a little less at f/1.6 and less again at f/1.8. So both a mechanical defect and a problem with the electronic aperture control are unlikely. (I can't reproduce the effect manually since the aperture ring does not allow a value between f/1.4 and f/2.)



Does anyone have an explanation for this? I was already about to send the lens in for service, but if this is "normal" behavior that probably wouldn't do much good.



Best regards,

Sebastian
#2
[quote name='Ballonseide' timestamp='1343313676' post='19602']

Hi there,



I recently bought a (new) Voigtländer Nokton 58/1.4 from Hong Kong (Pentax mount). It's a great lens, but I am puzzled by its behavior at large apertures. Maybe someone here can help me understand this.



The problem: Images shot at f/1.4, f/1.6 and f/1.8 look identical. I noticed this because (1) the size of background highlights doesn't change anymore when you stop further down from f/1.8, and (2) f/1.4 gives noticeably underexposed results. I first thought this might be a defect of my copy, but if you look at the bokeh test images in the [url="http://www.opticallimits.com/nikon--nikkor-aps-c-lens-tests/360-voigtlander-nokton-58mm-f14-sl-ii?start=1"]Photozone review[/url], they show the same thing concerning highlights. Even the difference between f/1.4 and f/2 is negligible in those test shots.



I went on and shot a series of images with only the aperture varying between shots. f/1.4 through f/1.8 expose identically, for all practical purposes. Yet visual inspection (i. e. looking into the lens while shooting) shows that the aperture opens up completely at f/1.4, a little less at f/1.6 and less again at f/1.8. So both a mechanical defect and a problem with the electronic aperture control are unlikely. (I can't reproduce the effect manually since the aperture does not allow a value between f/1.4 and f/2.)



Does anyone have an explanation for this? I was already about to send the lens in for service, but if this is "normal" behavior that probably wouldn't do much good.



Best regards,

Sebastian

[/quote]



What body? edit, I see it must be a pentax? So, it's an APS-C?

I have seen reports where people say that it's hard to take advantage of very fast lenses, since basically with very fast lenses, a lot of the peripheral rays are cut off by the microlenses of the sensor.

Still, it seems this couldn't explain the out of focus highlights.

Another alternative is perhaps that the out of focus highlights get vignetted. You get cat's eyes at the borders due to vignetting. Is it possible that you get round vignetting with very large aperture lenses in the center? (lens hood?)
#3
That is not normal. You can choose in between clickstops between 1,4 and 1,8.
#4
Thanks for your answers.



[quote name='photonius' timestamp='1343329019' post='19611']What body? edit, I see it must be a pentax? So, it's an APS-C?[/quote]

Yup, K-5.



Quote:I have seen reports where people say that it's hard to take advantage of very fast lenses, since basically with very fast lenses, a lot of the peripheral rays are cut off by the microlenses of the sensor.

That could at least explain the underexposure at f/1.4 and f/1.6. I understand this needn't happen with any fast lens but depends on the optical construction, too?



Quote:Another alternative is perhaps that the out of focus highlights get vignetted. You get cat's eyes at the borders due to vignetting. Is it possible that you get round vignetting with very large aperture lenses in the center? (lens hood?)

I don't even get cat's eyes in the corners, so unlikely. Also I don't think it's physically possible to get any kind of vignetting in the image center with the aperture wide open.



[quote name='karlmera' timestamp='1343407087' post='19620']That is not normal. You can choose in between clickstops between 1,4 and 1,8.[/quote]

Do you have the lens? Mine has half-EV clickstops between all apertures, except for the step from f/2 to f/1.4 which is a full stop. According to [url="http://www.lenstip.com/265.3-Lens_review-Voigtlander_Nokton_58_mm_f_1.4_SL_II_Build_quality.html"]LensTip[/url], the older version didn't even have half-EV clickstops.
#5
[quote name='Ballonseide' timestamp='1343469889' post='19623']

.





Do you have the lens? Mine has half-EV clickstops between all apertures, except for the step from f/2 to f/1.4 which is a full stop. According to [url="http://www.lenstip.com/265.3-Lens_review-Voigtlander_Nokton_58_mm_f_1.4_SL_II_Build_quality.html"]LensTip[/url], the older version didn't even have half-EV clickstops.

[/quote]



Yes I have the lens for Nikon. It has 8 full clickstops from 16 to 1,4. But you can choose all fine between positions.
#6
[quote name='karlmera' timestamp='1343495134' post='19624']

Yes I have the lens for Nikon. It has 8 full clickstops from 16 to 1,4. But you can choose all fine between positions.

[/quote]

Interesting. Is it the SLII version, too? I get exactly 12 clicks between f/16 and f/1.4: 1 EV ("large") clicks from f/1,4 to f/2 and from f/11 to f/16; and 1/2 EV ("small") clicks between all other positions.



As regards my bokeh problem, I did both some more testing and some more browsing through the Photozone reviews and I now believe the issue is just a depth of field effect, although one I did not know existed. All 50/1.4 lenses reviewed show no difference in background blur strength between f/1.4 and f/2 in the test shots; on the other hand, the Canon 50/1.2 does and most 85/1.4 lenses do, both of which should have smaller DOF at full aperture in the given scene. So I tried to provoke a smaller DOF myself by using a very close focus distance, and indeed, I saw increasing blur down to f/1.4. (The shots from f/1.4 to f/2 are attached if someone is interested.) The difference is still small; it would probably be more pronounced if the background were farther away (like close-up portrait with distant trees as background). Quite useful to know that there will be no difference in the amount of background blur in many scenes under f/2; it seems wise then to default to f/2 and use the better resolution (and reduced outlining) it provides.
  


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