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Spotmatic
Joined: 18 Aug 2008 Posts: 4045 Location: Netherlands
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Posted: Wed Aug 26, 2009 8:29 am Post subject: APS-C cameras potentially creating harsh bokeh? |
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Spotmatic wrote:
Most of us know the famous swirly bokeh of for instance the Helios-40 lens. The "cat-eyes" are caused by optical vignetting: with a wide open lens specular highlights in OOF areas are not round anymore.
Now, I just viewed this picture from a fellow Pentaxian. This was made with a Pentax 50/1.2 lens on a Pentax K10D (APS-C sensor):
As can be seen the brightest stars are rectangular instead of round. According to the maker it's a problem of this combination of camera and lens. A K10D has a small mirror box (smaller than that of a FF camera) and the 50/1.2 has a very big rear element. What happens now is that the pixels in the corners cannot see the whole back element, causing vignetting.
As can be seen here, the mirror box fits inside the bayonet mount completely:
The back element of the 50/1.2:
Knowing this, isn't it also possible that the bokeh of the given combination is affected, making the bokeh more harsh? And what does this mean for the speed of the mentioned f/1.2 lens as less light is able to reach the sensor? Wouldn't this defy the point of using a f/1.2 lens (at least on APS-C)?
I'm interested in your thoughts on this subject. I'm not an optics designer but I want to understand the whole concept and the possible drawbacks for bokeh lovers _________________ Peter - Moderator
Pentax K-5 + Pentax 645 + Canon 5D + Bessa RF 10,5cm Heliar, and a 'little' bag full of MF lenses. The lens list is * here *.
My fast 80s: Asahi-Kogaku Takumar 83mm f/1.9 - Super-Takumar 85mm f/1.9 - FA 77mm f/1.8 Limited - Cyclop 85/1.5 (Helios-40 innards) - Komura 80mm f/1.8 - Meyer Görlitz Primoplan 7,5cm 1:1.9 - Carl Zeiss Jena 80mm f/1.8 Pancolar - Canon 85mm f/1.8 S.S.C. - Canon 85mm f/1.2 S.S.C. Aspherical |
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poilu
Joined: 26 Aug 2007 Posts: 10472 Location: Greece
Expire: 2019-08-29
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Posted: Wed Aug 26, 2009 8:40 am Post subject: |
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poilu wrote:
APS-C is just a crop of the digital FF, bokeh should be the same
vignetting and bad border is certainly affected on FF
Now on film, bokeh is completely different; more smoother and nicer
the square stars are probably more a bayer demosaic algorithm problem |
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Spotmatic
Joined: 18 Aug 2008 Posts: 4045 Location: Netherlands
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Posted: Wed Aug 26, 2009 8:45 am Post subject: |
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Spotmatic wrote:
poilu wrote: |
APS-C is just a crop of the digital FF, bokeh should be the same
vignetting and bad border is certainly affected on FF
Now on film, bokeh is completely different; more smoother and nicer |
That's what I thought too. But there's more to it than just that the picture is cropped. I'm not speaking about different distances to the subject here based on the crop factor compared to a FF camera, but really the mechanical/optical problems that can be caused by a smaller mirror box.
It's also a common misconception that on a APS-C camera only the central part of a lens is used, this really isn't so. The light rays coming from a photographic subject never enter the front element of a given lens along the optical axis; instead light enters the front element from everywhere and at all angles. So the whole front element is always used... _________________ Peter - Moderator
Pentax K-5 + Pentax 645 + Canon 5D + Bessa RF 10,5cm Heliar, and a 'little' bag full of MF lenses. The lens list is * here *.
My fast 80s: Asahi-Kogaku Takumar 83mm f/1.9 - Super-Takumar 85mm f/1.9 - FA 77mm f/1.8 Limited - Cyclop 85/1.5 (Helios-40 innards) - Komura 80mm f/1.8 - Meyer Görlitz Primoplan 7,5cm 1:1.9 - Carl Zeiss Jena 80mm f/1.8 Pancolar - Canon 85mm f/1.8 S.S.C. - Canon 85mm f/1.2 S.S.C. Aspherical
Last edited by Spotmatic on Wed Aug 26, 2009 8:57 am; edited 2 times in total |
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martinsmith99
Joined: 31 Aug 2008 Posts: 6950 Location: S Glos, UK
Expire: 2013-11-18
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Posted: Wed Aug 26, 2009 8:50 am Post subject: |
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martinsmith99 wrote:
poilu wrote: |
a bayer demosaic algorithm problem |
That's just what I was thinking.
Whatever it means!
_________________ Casual attendance these days |
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Spotmatic
Joined: 18 Aug 2008 Posts: 4045 Location: Netherlands
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Posted: Wed Aug 26, 2009 8:52 am Post subject: |
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Spotmatic wrote:
poilu wrote: |
the square stars are probably more a bayer demosaic algorithm problem |
I think such a problem only exists on a pixel level, not in the case of large groups of pixels (like a star). _________________ Peter - Moderator
Pentax K-5 + Pentax 645 + Canon 5D + Bessa RF 10,5cm Heliar, and a 'little' bag full of MF lenses. The lens list is * here *.
My fast 80s: Asahi-Kogaku Takumar 83mm f/1.9 - Super-Takumar 85mm f/1.9 - FA 77mm f/1.8 Limited - Cyclop 85/1.5 (Helios-40 innards) - Komura 80mm f/1.8 - Meyer Görlitz Primoplan 7,5cm 1:1.9 - Carl Zeiss Jena 80mm f/1.8 Pancolar - Canon 85mm f/1.8 S.S.C. - Canon 85mm f/1.2 S.S.C. Aspherical |
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Dco
Joined: 17 Aug 2009 Posts: 19 Location: Vigo, Spain
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Posted: Wed Aug 26, 2009 9:29 am Post subject: |
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Dco wrote:
Interesting, I've never seen such a thing when I used the 50mm f1.2...
I'm not so sure about the results. Isn't the photo focused at infinity? In that case, OOF areas shouldn't exist, I believe that the stars should be in focus...
Another thing, that photo is a long exposure (30 sec.) so it's normal the stars appear a bit blurry because of their movements.
I'd like to see a 100% crop of one of the biggest stars, and also another sample, with a faster shutter speed, to compare...
One last question. Was it a raw o jpeg image? _________________ Pentax K100d super, *IstD, K5
SMC-M 50mm f1.7, Sigma 24mm f2.8, Kiron 24mm f2, Samyang 85mm f1.4 Aspherical IF, Sigma 500mm f7.2
Some "evil" AF lenses |
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no-X
Joined: 19 Jul 2008 Posts: 2495 Location: Budejky, Czech Republic
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Posted: Wed Aug 26, 2009 10:02 am Post subject: |
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no-X wrote:
Spotmatic wrote: |
poilu wrote: |
the square stars are probably more a bayer demosaic algorithm problem |
I think such a problem only exists on a pixel level, not in the case of large groups of pixels (like a star). |
Unfortunately not. Low-pass optical filter transforms pixel level details to cover areas of 3x3 (9!) pixels. In combination with debayering, sharpening and other algorithms, it can cause surprising artifacts.
If a camera doesn't have low-pass filter, color artifacts may appear.
I'm not sure, if these artifacts could be caused by bayer/LPOF related issues, but I wouldn't say it can be created by the mirror box. in the area behind the rear optical element the image is already "completed", so nothing like optical vigneting or anything similar could affect the image, esp. in center and for a such tiny objects. _________________ (almost) complete list of Helios lenses |
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Spotmatic
Joined: 18 Aug 2008 Posts: 4045 Location: Netherlands
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Posted: Wed Aug 26, 2009 12:30 pm Post subject: |
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Spotmatic wrote:
Jiri,
Photographing stars is a quite demanding application. So lens abberations and things like vignetting will show up very quickly. Regarding the vignetting of the mirror box the following link (provided to me by the maker of the star photo in my first post) is a quite interesting read. This example (in the link) is on a Pentax LX so it's understandable that on APS-C the vignetting is even more severe.
http://plg.komkon.org/k50_12/wo1.html
So my question still remains: will the bokeh on APS-C be affected too? I'm inclined to think it will.
Edit: the maker also wanted to add the following. Astrophotography is indeed quite demanding for the quality of a lens. Some stars are much brighter than others. The brighter ones only need a second to be exposed properly on the sensor/film; the less bright ones need an exposure of many seconds/minutes but this causes the brightest stars to be overexposed.
If a good lens is able to focus 99% of the star's light in one dot, and the other 1% in a larger disc, then a 1000-fold overexposure makes the larger disc to show up. Normal photography is of course less demanding.
Note from Spotmatic: the above explanation is correct for objects in focus. But what about OOF objects, that's what interests me? _________________ Peter - Moderator
Pentax K-5 + Pentax 645 + Canon 5D + Bessa RF 10,5cm Heliar, and a 'little' bag full of MF lenses. The lens list is * here *.
My fast 80s: Asahi-Kogaku Takumar 83mm f/1.9 - Super-Takumar 85mm f/1.9 - FA 77mm f/1.8 Limited - Cyclop 85/1.5 (Helios-40 innards) - Komura 80mm f/1.8 - Meyer Görlitz Primoplan 7,5cm 1:1.9 - Carl Zeiss Jena 80mm f/1.8 Pancolar - Canon 85mm f/1.8 S.S.C. - Canon 85mm f/1.2 S.S.C. Aspherical |
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no-X
Joined: 19 Jul 2008 Posts: 2495 Location: Budejky, Czech Republic
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Posted: Wed Aug 26, 2009 1:22 pm Post subject: |
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no-X wrote:
Yes, vignetting can be present, but it will never change shape of in focus objects. Some forms of optical vignetiting can change shape of OOF objects - most known exaple are ovoid OOF lights, but it will never affect the shape of in focus objects.
Imagine, how would it affect usage of P6 lenses on M4/3 cameras. Or old large format lenses on APS-C cameras, where the crop factor isn't 1.5x, but 4x or more. But these combinations don't embody these (or similar) artifacts, even with such significant differences. _________________ (almost) complete list of Helios lenses |
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Nesster
Joined: 24 Apr 2008 Posts: 5883 Location: NJ, USA
Expire: 2014-02-20
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Posted: Wed Aug 26, 2009 1:31 pm Post subject: |
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Nesster wrote:
Perhaps rather than the size of the mirror box - after all, the rear of the lens fits in there - it's a matter of the angle of light hitting the individual sensor elements. My understanding is that a digital sensor does not gracefully accept light except at specific angles... and now that I think of it, perhaps there's some veiling flare possible from light that hits at a too low angle? _________________ -Jussi
Camera photos
Print Photographica
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greg
Joined: 21 Mar 2009 Posts: 683
Expire: 2012-12-03
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Posted: Wed Aug 26, 2009 1:38 pm Post subject: |
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greg wrote:
I believe there was a thread earlier regarding the use of a choke - similar to a fixed aperture - placed between the rear of the lens and the mirror to prevent reflection problems. It cuts down on the size of the image circle hitting the inside of the camera body and was rectangular in shape. I seem to remember it was from a Russian based forum.
In the above picture it shows some really regular shaped squares. Are they the same in other images from the same camera? If so, they could simply be dead (frozen) pixels - a common thing. Take a picture with the body cover/lens cap in place, this will make the dead pixels stand out, or at least it was the method used with the old Fuji 6800z. My copy of that camera had over 50 dead pixels and was considered a low number at the time. Actually, it isn't a 'dead' pixel, some are colored, it depended in which state the pixel was frozen. |
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naplam
Joined: 22 Mar 2007 Posts: 469 Location: Spain
Expire: 2013-11-30
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Posted: Wed Aug 26, 2009 1:43 pm Post subject: |
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naplam wrote:
I'd say the mirror box is vignetting; the rear element is so big there isn't a clear path from the extreme borders of the lens to the sensor. |
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sichko
Joined: 20 Jun 2008 Posts: 2475 Location: South West UK
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Posted: Wed Aug 26, 2009 1:51 pm Post subject: |
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sichko wrote:
greg wrote: |
In the above picture it shows some really regular shaped squares. Are they the same in other images from the same camera? If so, they could simply be dead (frozen) pixels - a common thing. Take a picture with the body cover/lens cap in place, this will make the dead pixels stand out, or at least it was the method used with the old Fuji 6800z. My copy of that camera had over 50 dead pixels and was considered a low number at the time. Actually, it isn't a 'dead' pixel, some are colored, it depended in which state the pixel was frozen. |
I don't understand the phenomenon but the "regular shaped squares" are the bright stars of the constellation Cassiopeia. _________________ John |
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Spotmatic
Joined: 18 Aug 2008 Posts: 4045 Location: Netherlands
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Posted: Wed Aug 26, 2009 1:59 pm Post subject: |
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Spotmatic wrote:
sichko wrote: |
I don't understand the phenomenon but the "regular shaped squares" are the bright stars of the constellation Cassiopeia. |
It's Cassiopeia indeed. You know your stars
But gentlemen, let's not talk about sensor defects because that's not the cause of the problem.
@Jiri: I'm not talking about the size of the image circle that the lens projects - I want to know what happens if the pixels in the corners of the sensor cannot see the whole back element of the attached lens - be it a LF lens or a 35mm lens. If the pixels cannot see the whole back element then it's vignetting. _________________ Peter - Moderator
Pentax K-5 + Pentax 645 + Canon 5D + Bessa RF 10,5cm Heliar, and a 'little' bag full of MF lenses. The lens list is * here *.
My fast 80s: Asahi-Kogaku Takumar 83mm f/1.9 - Super-Takumar 85mm f/1.9 - FA 77mm f/1.8 Limited - Cyclop 85/1.5 (Helios-40 innards) - Komura 80mm f/1.8 - Meyer Görlitz Primoplan 7,5cm 1:1.9 - Carl Zeiss Jena 80mm f/1.8 Pancolar - Canon 85mm f/1.8 S.S.C. - Canon 85mm f/1.2 S.S.C. Aspherical |
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LucisPictor
Joined: 26 Feb 2007 Posts: 17633 Location: Oberhessen, Germany / Maidstone ('95-'96)
Expire: 2013-12-03
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Posted: Wed Aug 26, 2009 2:22 pm Post subject: |
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LucisPictor wrote:
I don't think that bokeh highlighta appear rectangular because of the smaller mirror box. At least I can't confirm this experience.
I also have a lens with a huge rear lens, the Nikkor-S.C 1.2/55, and it does not cause rectangular highlights in my 40D. _________________ Personal forum activity on pause every now and again (due to job obligations)!
Carsten, former Moderator
Things ON SALE
Carsten = "KAPCTEH" = "Karusutenu" | T-shirt?.........................My photos from Emilia: http://www.schouler.net/emilia/emilia2011.html
My gear: http://retrocameracs.wordpress.com/ausrustung/
Old list: http://forum.mflenses.com/viewtopic.php?t=65 (Not up-to-date, sorry!) | http://www.lucispictor.de | http://www.alensaweek.wordpress.com |
http://www.retrocamera.de |
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Spotmatic
Joined: 18 Aug 2008 Posts: 4045 Location: Netherlands
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Posted: Wed Aug 26, 2009 2:28 pm Post subject: |
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Spotmatic wrote:
So this possible problem is purely academical? _________________ Peter - Moderator
Pentax K-5 + Pentax 645 + Canon 5D + Bessa RF 10,5cm Heliar, and a 'little' bag full of MF lenses. The lens list is * here *.
My fast 80s: Asahi-Kogaku Takumar 83mm f/1.9 - Super-Takumar 85mm f/1.9 - FA 77mm f/1.8 Limited - Cyclop 85/1.5 (Helios-40 innards) - Komura 80mm f/1.8 - Meyer Görlitz Primoplan 7,5cm 1:1.9 - Carl Zeiss Jena 80mm f/1.8 Pancolar - Canon 85mm f/1.8 S.S.C. - Canon 85mm f/1.2 S.S.C. Aspherical |
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poilu
Joined: 26 Aug 2007 Posts: 10472 Location: Greece
Expire: 2019-08-29
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Posted: Wed Aug 26, 2009 2:33 pm Post subject: |
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poilu wrote:
spot wrote: |
If the pixels cannot see the whole back element then it's vignetting |
digital is vignetting 40% more than film
it is not because of the back element but from pixels who cannot see light coming at angle
if you like nice borders, use film |
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Ballu
Joined: 28 Feb 2007 Posts: 912 Location: Columbus, OH. USofA
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Posted: Wed Aug 26, 2009 6:51 pm Post subject: |
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Ballu wrote:
Spotmatic wrote: |
It's also a common misconception that on a APS-C camera only the central part of a lens is used, this really isn't so. The light rays coming from a photographic subject never enter the front element of a given lens along the optical axis; instead light enters the front element from everywhere and at all angles. So the whole front element is always used... |
Its not a misconception, but a truth.. wrt sensor and final image.
Yes all light enters from the front element (from the corner too), but sensor captures the smaller part. Think of using hood on wide angle lens. FF camera will show the the hood related darkness.
Thats why, 18mm lens for FF is bigger in size (hence more distortion) but crop format has smaller size.
But big factor for size of lens is, "register distance" (retro focus design). _________________ -Ballu
http://balyanpage.blogspot.com/ |
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Minolfan
Joined: 30 Dec 2008 Posts: 3439 Location: Netherlands
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Posted: Wed Aug 26, 2009 9:52 pm Post subject: |
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Minolfan wrote:
If the rectangulair highlights only appear in this kind of pictures, why bother? |
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