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Author Topic: 24XL + Center Filter + Leaf back  (Read 9282 times)

rljones

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24XL + Center Filter + Leaf back
« on: October 02, 2006, 09:11:07 pm »

While running some tests on the Schneider Digitar 24XL lens with a Leaf 65 back (on an Alpa), I noticed something odd. I was trying to determine if I could simply use a home-brewed gradient to neutralize the fall off seen on the 24XL (as I've occasionally done with 4x5 scans), rather than use a center filter (CF). Since I have the proper CF for the 24XL, a comparison should be easy. I shot a neutral gray counter top, 8 to 12 inches away, keeping it out of focus with the lens set at infinity, at f5.6, f8 and f11 both with and without the CF. Exposure was adjusted appropriately (typically around 1/2 to 1/8 sec) with ISO at 200. All non-compressed Raw images were processed the same in Raw Developer.

Without the CF, odd colors were present in the center on the images at all f-stops. The very center has a faint cyan cast surrounded by an annulus of faint magenta which then radiated in all directions into a neutral-toned gray out to the periphery. These aberrations are subtle, and I did not notice them in any landscape shots I've made.

Meanwhile, the images made with the CF in place do not show these chromatic aberrations. I combined two of the images from the f8 exposure onto a single jpg and posted them at http://www.e-photoart.com/pics/24XL+-CF.jpg  (If this doesn't show up easily, I can try and accentuate the colors, but I was trying to manipulate the images as little as possible).

Misc. comments: I see no other problems with the Leaf 65 back (no centerfold, etc) and the 24XL appears to focus properly on the Alpa body. I also repeated this same test on a gray stucco wall outside and had the same findings (sky was overcast, so nice and even, indirect lighting), implying that the subject was probably not the source.

I realize the 24XL lens is pretty extreme for digital sensors, but I would expect any sort of fringing (not large bands of color) to be present in the periphery, not centrally. I am still waiting on a 47XL and have no other lens for the Alpa to test. I suppose I could repeat the test with some Mamiya lenses, but no retrofocus has as much vignetting as the 24XL (if this is the source of the problem) and I have no CF to fit those lenses.

If the problem were strictly due to the sensor, then the CF should make no difference. If there is an interaction of the 24XL and the sensor, why is the CF fixing the problem? Could a contributing factor be due to being too close to the subject (I could back off to 1m and leave the focus at infinity)?

I do not understand why am I seeing either the initial problem nor the fix from the CF. I was wondering if someone like Eric who shoots more walls than I do;) has noticed anything similar.

Thanks,

Robert
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mtomalty

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« Reply #1 on: October 02, 2006, 10:47:03 pm »

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« Last Edit: October 11, 2008, 12:56:16 am by mtomalty »
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peterhorsley

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« Reply #2 on: October 02, 2006, 10:54:30 pm »

Hi Robert,

Can I speculate there are two parts to the image defects?

Firstly I'm not sure the Centre Filter is getting rid of the colour mottling. In an effort to match the perceived brightness of both images and exaggerate the colours. I tweaked the brightness of the "With Centre Filter" image in Photoshop, then cranked up the saturation of the whole jpg.
There seemed to be a similar character in both images: a hot spot to the lower left of centre, and more 'colour' in the right of the image than the left.

I'd speculate that:
- the hot spot, in fact the general brightness-asymmetry is optical; the result of less-than-perfect colimination of the lens.
- the colour mottling looks like an interference effect to me.  I guess this could be from the sensor's surface structure, but I wonder if it is as simple as some residue from cleaning or manufacturing which has coated the sensor unevenly?

Have you tried shooting an image of a distant point light source without the lens, ie sensor only?  I'd imagine masking down a small flash unit would give exposure control without the shutter.

Have you tried cleaning the sensor?

Peter
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rljones

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« Reply #3 on: October 03, 2006, 12:16:29 am »

Mark,

I thought the monitor _might_ have an effect. My main monitor is Apple 30" and is calibrated. The problem is most noticeable here. I also see it on another iMac 17" and a MacBook 13". The least noticeable is on the iMac 17", which is not calibrated.

Peter,

Thanks for the suggestion about imaging a point source. I shall try this.

The sensor is spotless, and I've not cleaned it. The lens too appears spotless. When I look at the sensor (through the IR filter), it appears to be composed of 6 small panels. The color casts cover more than one panel, so I don't believe it is a sensor defect issue. As for lens collimation, this is perhaps possible. My optical knowledge starts to show its limits, but I thought the collimation was OK, based on seeing images that appear critically sharp over the entire field and point sources of reflection that have no comma or astigmatic distortions that I can see (although I've not examined it fully over every mm).

Could this be a sensor processing issue, with improper raw processing of this particular sensor?

I did play with the 'with CF' and did not notice as much of a cyan/magenta cast; again I was trying to keep image manipulation similar and do what I normally do for standard images. I'll re-check it. Prior to posting the images, I did adjust the lightness of each image using lightness values (Hue-Saturation-Lightness) in PS to equalize the upper left corner RBG to approximately the same values, letting the rest of the image fall where they may.

Maybe what we're seeing is the limitation of such a wide angle lens and today's sensor technology, despite the Leaf having one of the thinnest sensors around.

Thanks,

Robert
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rljones

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« Reply #4 on: October 03, 2006, 12:34:07 am »

Talk about fast and responsive dealers!

Steve Hendrix, from whom I bought the Leaf, emailed me just after my last post. He had a suggestion, feeling that the Custom Gain software may help. If so, then this is an individual sensor-software correction issue that would be applied for each lens, and would be slightly different for every sensor.

I'll re-shoot things tomorrow and re-post my findings as soon as possible.

No matter what, the detail and tonality from the 24XL on the Leaf is amazing to see. (Jeff at Badger Graphic has also been very helpful with the Alpa gear.)

Thanks,

Robert
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pixjohn

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« Reply #5 on: October 03, 2006, 03:46:55 am »

I can also tell you that the gain filter will help solve your problem. I sometimes see the same thing with my Aptus back. This is a leaf sensor problem! Leaf owners now know what its like to own a phaseone back.
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peterhorsley

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« Reply #6 on: October 03, 2006, 07:46:24 am »

Thanks for posting this Robert.  Looks Like I'm going to learn a lot about care and feeding of an MF sensor.

Peter
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rljones

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« Reply #7 on: October 03, 2006, 03:44:19 pm »

I made a calibration file at each f-stop (5.6, 8, 11). This is an image made with the a Leaf supplied diffusor that covers the lens being tested. The shutter on the 24XL was adjusted to maintain consistent exposure. These uncompressed, raw *.mos files are now the calibration files for the 24XL.

As a test, I ran Custom Gain on the calibration file against itself (that is, the calibration file was loaded both as the calibration and as the file to analyze with the fall-off correction set to 100% to remove vignette effects). The output of this was a wonderfully even toned gray image without visible vignetting. (I can post this file, but there's not much to see.) So far, so good.

Next, I used this same calibration file and loaded the problem f8 exposured file as described in the first post, above). The result is an image with no vignette problems, but the cyan-magenta cast is still present. Not so good.

I think this suggests some interaction of lens-subject. The only problem is I used two different subjects, under different lighting, and still had the color cast issue. However, the one common potential problem is that I put the camera/lens very close to the subject in an attempt to totally blur the image, and I'm wondering if this could be the source of the color disturbances.

If the sensor has its own distortions, the calibration file, which is essentially subtracted from an image, should remove these non-linearities. The proof is that the calibration file when run against itself does makes a neutral, flat image. This makes for a good control. The same should hold true for the lens, and it does: vignette effects are removed. However, when shooting a reflective subject at too close of a distance, perhaps the calibration file might not handle it the same as when it is imaging a diffused light (ie, the calibration process). This only thing I can think is to re-shoot at a greater distance, maybe 1m (I think the lens is focusable from 0.6m to infinity), leaving a sharp subject plane, use Custom Gain again and see if the matter is resolved. Other ideas?

Thanks,

Robert
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ericstaud

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« Reply #8 on: October 03, 2006, 05:51:58 pm »

Hi Robert,

In my experience I have only used the method of shooting a new calibration file after each shot.  I am having a new pocket sewed into all my shirts to hold the gain diffuser

Shooting a set of reference calibration files to be used later seems daunting.  Do you shoot 1 for each F-stop or for each third?  For shifts, do you shoot for every 1mm or every 5?  Do you shoot for different focus distances?

Be careful with the falloff control using your 24.  If your image circle goes dark in the corner the gain adjutment can add a lot of noise.  Don't throw away the originals before you know you are happy with the adjutment.  For many of my images I set the falloff correction to "0".

Also, if you make an 8 second exposure the add two stops to make the Gain File you are at 32 seconds.  The Gain adjustment process will add the noise from the 32 second exposure into your gain corrected 8 second exposure.

-Eric
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peterhorsley

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« Reply #9 on: October 08, 2006, 08:04:34 am »

Quote
... I used this same calibration file and loaded the problem f8 exposured file as described in the first post, above). The result is an image with no vignette problems, but the cyan-magenta cast is still present. Not so good....
[a href=\"index.php?act=findpost&pid=78961\"][{POST_SNAPBACK}][/a]

Hi Robert,

I'm curious about the problem but don't have any knowledge or experience to offer.

It does sound like you've taken the lens-sensor interaction out of the 'picture' but it sounds weird that the subject could be the problem in two cases.

It could be one of things where you now know that if you ever have to shoot an out-of-focus-subject-very-close-to-the-lens it could be trouble.

Charge extra.

Peter
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rainer_v

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« Reply #10 on: October 09, 2006, 02:46:52 pm »

here the rodenstock 28HR without centerfilter at f8, shot with the Sinar e75, konverted with a white referenz file in Brumbärs eMotion Konverter.
the image was even shifted up 8mm.

http://www.tangential.de/sinar/greyshot28mm-hr.jpg
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ericstaud

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« Reply #11 on: October 09, 2006, 03:15:32 pm »

Hello Rainer,

Can you describe your workflow?  Do you shoot your exposures for each subject and then shoot the reference file?  Have you attempted to create a set of reference files for each lens which you keep on your computer?  And lastly, how do you make the reference exposure if you are shooting at dusk where your original image is captured in the 10 to 30 second range?

Thanks,

Eric
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rainer_v

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« Reply #12 on: October 09, 2006, 03:30:21 pm »

Quote
Hello Rainer,

Can you describe your workflow?  Do you shoot your exposures for each subject and then shoot the reference file?  Have you attempted to create a set of reference files for each lens which you keep on your computer?  And lastly, how do you make the reference exposure if you are shooting at dusk where your original image is captured in the 10 to 30 second range?

Thanks,

Eric
[a href=\"index.php?act=findpost&pid=79702\"][{POST_SNAPBACK}][/a]
i shoot for every motif a reference file before the photo. this ref. files are konverted than in a batch to the white ref. the programm uses to crrect the images. as next step i convert all the photo raws outs o f the e75 to dngs,- the ref. files are taken at the right position in the batch automatically, the dng files are already corrected.

i further have a set of ref. files for all my lenses which i can use if no ref. shots can be taken.
usually i am able to create also at dusk a ref file. its not nessesary that the ref. file is taken at the same exp. time, but should be taken at the same shift setting. if the focus is set different the vignetting will not be corrected so 100% as you can see in my shot. its no püroblem if the ref. file is underexposed, if not too much.

at all the workflow is fast and uncomplicate, much more than my description. the possibility to make batches and to fly in here the references automatically in the right position makes the converting easy and fast.
the results are perfect i.m.o.
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evonzz

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« Reply #13 on: October 11, 2008, 12:32:44 am »

picking up an old thread here, i have recently used the 24XL (without Center Filter) w Cambo wide DS and Leaf 75.

My results with the 24 were not as great as other lenses.  Unfortunately i had no center filter, but did make a white reference file and used the CGA in the leaf software to get a more or less even xposure.  In the corners of the image i noticed some heavy duty coarse and colourful noise.  The images were of interiors with mixed lighting.  Thankfully the noise was on fairly even tone areas and i could smooth it out.  Some chromatic aberation was present, but mostly corrected with the CGA file.

However for future shoots, i'd like to gain a better understanding of dealing with this problem.  Reading through this thread, it appears that some of this noise may have come from a reference exposure. I was ISO 50, and from memory reference exposure at 8 secs / F8.  If I increased ISO to, say, 200, in order to gain a faster exposure, how different is the noise created?  [this is a rental kit i used so a bit limited to do to too many tests]

Next time i hope to have the center filter for the 24XL, which i have told rental company to order.  Would the center filter help to eliminate the noise from the corners of the image or s this a problem with this lens/back combination?

Appreciate any comments and sorry for hashing out an old thread.

Cheers


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yaya

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« Reply #14 on: October 11, 2008, 01:03:21 am »

Quote from: evonzz
picking up an old thread here, i have recently used the 24XL (without Center Filter) w Cambo wide DS and Leaf 75.

My results with the 24 were not as great as other lenses.  Unfortunately i had no center filter, but did make a white reference file and used the CGA in the leaf software to get a more or less even xposure.  In the corners of the image i noticed some heavy duty coarse and colourful noise.  The images were of interiors with mixed lighting.  Thankfully the noise was on fairly even tone areas and i could smooth it out.  Some chromatic aberation was present, but mostly corrected with the CGA file.

However for future shoots, i'd like to gain a better understanding of dealing with this problem.  Reading through this thread, it appears that some of this noise may have come from a reference exposure. I was ISO 50, and from memory reference exposure at 8 secs / F8.  If I increased ISO to, say, 200, in order to gain a faster exposure, how different is the noise created?  [this is a rental kit i used so a bit limited to do to too many tests]

Next time i hope to have the center filter for the 24XL, which i have told rental company to order.  Would the center filter help to eliminate the noise from the corners of the image or s this a problem with this lens/back combination?

Appreciate any comments and sorry for hashing out an old thread.

Cheers

The diffuser filter provided makes you loose about 2 stops, so for the gain file, when possible you need to increase your exposure time by 2 stops. You can also use a flash gun or similar if longer exposures are not possible.

Hope this helps

Yair

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evonzz

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« Reply #15 on: October 11, 2008, 04:07:35 am »

Quote from: yaya
The diffuser filter provided makes you loose about 2 stops, so for the gain file, when possible you need to increase your exposure time by 2 stops. You can also use a flash gun or similar if longer exposures are not possible.


Yair


Yes, i did increase exposure in time by 2 stops, hence i my question about whether a 30 second Diffuser reference shot would add noise when using the CGA tool.
As for using a flash gun, the flash would render my diffuser shot useless as i am measuring ambient colour.
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rainer_v

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« Reply #16 on: October 11, 2008, 07:00:11 am »

Quote from: evonzz
Yes, i did increase exposure in time by 2 stops, hence i my question about whether a 30 second Diffuser reference shot would add noise when using the CGA tool.
As for using a flash gun, the flash would render my diffuser shot useless as i am measuring ambient colour.
different color temperature does not change the result of diffusor shots. it works with inverting the colors relative to a grey point, so the relative shift e.g. to magenta or to green is recorded and than inverted and subtracted from the image. this shift is allways the same, it does not depend in the motiv nor in the color temperature the shot is taken.
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evonzz

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« Reply #17 on: October 11, 2008, 11:35:59 pm »

Quote from: rainer_v
different color temperature does not change the result of diffusor shots. it works with inverting the colors relative to a grey point, so the relative shift e.g. to magenta or to green is recorded and than inverted and subtracted from the image. this shift is allways the same, it does not depend in the motiv nor in the color temperature the shot is taken.

i thought that when using the diffuser, a file is created that not only measures the colour shift, but also measures the differences in colour temperature as a result of the nature of the subject. So for example if i was shooting an interior, all the various temperatures emitted from different light source is then mapped to the actual photo, thus neutralising the various colour temperatures.  On this assumption, if a flash was used, i'd imagine that perhaps the ambient lights may not be corrected.  

So if using a flash, is the flash bounced out from camera to the subject?
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yaya

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« Reply #18 on: October 11, 2008, 11:56:18 pm »

Quote from: evonzz
i thought that when using the diffuser, a file is created that not only measures the colour shift, but also measures the differences in colour temperature as a result of the nature of the subject. So for example if i was shooting an interior, all the various temperatures emitted from different light source is then mapped to the actual photo, thus neutralising the various colour temperatures.  On this assumption, if a flash was used, i'd imagine that perhaps the ambient lights may not be corrected.  

So if using a flash, is the flash bounced out from camera to the subject?

Rainer is right, the diffused image is diffused so that the relative shift (Magenta/ Green) can be corrected. We also use it to correct luminance falloff (vignetting) but the colour temperature doesn't take part in the gain process.

To neutralise the image you can use a Grey card and balance the raw file after you've corrected the shifts.

Yair
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Murray Fredericks

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« Reply #19 on: October 12, 2008, 10:52:31 pm »

Quote from: evonzz
Yes, i did increase exposure in time by 2 stops, hence i my question about whether a 30 second Diffuser reference shot would add noise when using the CGA tool.
As for using a flash gun, the flash would render my diffuser shot useless as i am measuring ambient colour.


Rod,

that noise in the corners is due to the correction required (2 stops for the 24mm) being so extremem - it has literally ripped the file apart in those areas. It is worse in dark interiors as shadow noise is worse at the extremes...

I shot the 24mm without the cf for a while and had the exact same problems. Now I use the cf always.

I would not advise changing the 'kind' of light you use (eg making the gain file with flash when you are shooting ambient) as the shifts seem to be affected by the quality of the light being shot. This is why Rainer makes a gain file for every image - as I do. This is very different from colour temp - which has nothing to do with the gain file.

If it's too dark for the gain file, you can push up the iso for the gain file and drop it again for the shot proper.

Cheers

Murray
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