Lens Cast Calibration. Each lighting scenario & shot?

Questions, comments and suggestions regarding Phase One P+ series and P series digital camera backs.

Lens Cast Calibration. Each lighting scenario & shot?

Postby Dillon » Sat Apr 23, 2005 2:33 pm

I have been shooting architecture for 20+ years, the last month with the P20 on a Rollei Xact. Every image I shoot requires some form of stitching, rise or fall of the rear standard, shift to the left and/ or right of the rear standard, often under mixed lighting scenarios. The angle of light hitting the chip at an angle creates a gradation of magenta to green. It is extreme when shooting with severe shifts and still noticeable with smaller shifts. Most of my shooting uses the entire image circle provided by the lens.

I promptly recorded a lens cast calibration for each of the 5 lenses that I use, with most possible shifts/ rises and falls for each. These total 63 combinations (about 6 hours of work). The lens cast calibration card worked well for my initial test and application to images. The calibration was done in open shade, outdoors. The image of the calibration white card, was recorded as 40 to 50% grey.

Questions: The calibrations that I have already recorded: Will these work for all lighting conditions, or do I need to record all 63 combinations for each possible lighting condition? Should I apply the LLC BEFORE, applying color temperature, white balance and exposure adjustments to the image? In the end, will I eventually shoot a white card and LLC after every shot on location?

I am hoping that someone has done this testing with the LLC. This could save me days (perhaps weeks) of calibration and testing.
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Postby Ashley11 » Sat Apr 23, 2005 9:58 pm

You shouldn't have to do it again.
Only other varible maybe if you tilt the lens.

I use the FlexBody quite at bit to do the same thing as you. Saved lens casts for the 40 & 50 at 5mm shifts.

Read Darren Bernaerdt's info on Perfectly Even Lighting for Copy Photos at:
http://www.dbphoto.net/techniques/

I find his technique to be first class when dealing with light fall off especially when using the 40mm lens and shifting.
I have saved a set of files at various shifts and apply these when needed.

Works great, a little slow but once you get the hang of it you'll love the results. No more centre filters required.

With the 40mm pushed in both directions one can get close to a 22mm lens, on a 35mm system, coverage.
Example - Pushed to the limit (+/- 14) in both directions – camera on it’s side:
Image
24”x 18” @ 300ppi = almost two (13.5 x 18 ) full frames.
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Postby Dillon » Sun Apr 24, 2005 2:03 pm

Thanks Ashley. Based on your experience, I am going to stop testing, and move on to having fun with assignments this week. I'm still suspicious about any changes to EV and color balance before applying the LCC. Stayed tuned...

By the way, nice work, nice web site. I like your use of people in the interior photos. Also, the wide angle \"look\" that you & I achieve with stitching, is superior to one-shot results from super wide lenses. I think the minimal distortion makes all the difference.

edit-O.K. I did more tests today. It makes NO difference what the light source is. The color value variance is uniform, regardless of the source.
Total LCC's in my collection are well over 100. These are for all the lenses in several positions, left, right, up and down. I'll put the LCC for swinging the rear standard on an as need basis.
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Postby Dillon » Sat Jun 04, 2005 7:51 pm

I am going to retract everything I said about the lens cast correction (calibration) working.

After shooting tests and calibrations for all lenses, over all possible shifts (180 total), I've determined the card must be shot under identical lighting. Even then, the results are questionable.

Interiors seem to be more forgiving. Exteriors have given me a very hard time. See results below. This happens with ALL lenses, particularly outdoors. The correction is good for the right portion of the photo, and not noticeable for the left portion. I've shot 5 or 10 entire projects like this, and need help. Desperately.

http://maylone.com/r6.htm[url][/url]
Last edited by Dillon on Sat Jun 04, 2005 8:31 pm, edited 1 time in total.
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Postby Ashley11 » Sat Jun 04, 2005 8:26 pm

Hi Dillon
Your LCC cards images are blue - they should be grey.
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Postby Dillon » Mon Jun 06, 2005 4:20 pm

Thanks Ashley, I am trying hard, but it is still not perfect. Skies are a key element for my shots and I would really like to perfect this. I am still getting a slight magenta to green shift.

Are you suggesting to \"make grey\" with C1?

OR, shoot it as grey as possible? In either scenario, what is the optimal density value for the card? 125 to 200?

I have tried the following in different combinations;
• Reducing and increasing the saturation of the card image, in the exposure control.
• Clicking on the mid point, with the color control grey selector.
• Increasing and reducing contrast.
• Applying the same color balance to the card image, as the images that I am correcting.

Thank you in advance.
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lens cast correction

Postby Eve » Tue Jun 07, 2005 12:55 pm

I'm using a 35 mm lens on an H1: am I expected to do one lens cast calibration for the lens, or do I have to do various calibrations depending on the lighting situation?
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Postby Ashley11 » Tue Jun 07, 2005 1:40 pm

You may need to do a few at different f-stops but that should be it.
The two I use the most are f8 and f11.
Only really needed with the widest angle lenses, which in my case is the 40mm on the V system.
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Postby Dillon » Tue Jun 07, 2005 5:03 pm

And,
I am shooting F/16's and 22's with totally unacceptable results.

I'm beginning to wonder if 16's and 22's are uncorrectable.

Stayed tuned.
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Postby Ashley11 » Tue Jun 07, 2005 5:49 pm

Hi Dillon
Can't understand why your results are so off in the first place - most times I don't even bother with the LCC because the cast is so little - really nit picking stuff.
If I really pushing the lens via the FlexBody then it becomes more noticable but nothing like what I seeing in your results. It's more a light fall-off problem rather than a big colour shift thing. Slight colour shift but nothing major.

f22 / f16 - why stop down so much ?
Most lenses max at f11 and with a wide angle that's massive depth of field.

You can check max f stop quality very easily using you back. Before digital I was a f32 shooter.
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Postby Dillon » Tue Jun 07, 2005 7:57 pm

Ashley-

Ahhh, how easily I forget the days of F/32 in a dim auditorium. 2 minutes exposures, if lucky. Skip the Polaroids, I'd like to go home tonight please!

I'm sure you are aware, the optimal performance of the lenses is approximately the half way point, or so. With my architectural shots, I push my luck by shooting One and one-half stops under the minimum aperture. Not ideal, but required. Architects put depth of field somewhere on their check list of bad things.

i.e.
Schnieder Digitar 28mm
F/2.8 to F/22
I shoot it at 11 & 1/2

Rodenstock 45mm
F/4.5 to F/32
I shoot at 16 to 22

Nikkor-SW 65mm (formerly for film)
F/4 to F/45
I shoot at F/22 ish

Nikkor-SW 90mm (formerly film)
F/8 to F/64
I shoot F/22 to 32 ish

Nikkor-W 150mm (formerly film)
F/5.6 to F/64
I shoot F/22 ish

Rollie does not make an extension rail otherwise I would love to use my 300mm. I plan to buy the 35mm Rodenstock, as the 28mm has slight curvature on the edges. I have to cheat in photoshop to straighten the edges (just a little caution FYI). My main competitor in Detroit uses a very wide lens (47 on 5 by 4), hence I have occasionally resorted to using RealViz Stitcher for completing 300 to 360 degree panoramas rendered flat. I'll be glad to move to Colorado, where it looks like residential interiors might become my mainstay.

Oh yes, LLC. I have a million projects to complete in the next 45 days. My immediate tests will use a larger aperture for the LCC card shots. I'm going to remain optimistic. On a positive note; the P20 does an outstanding job on dusk shots.

How do you post example pics here?
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Postby Eve » Tue Jun 07, 2005 8:02 pm

Thanks for the clarification Ashley!
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Re:

Postby Ashley11 » Tue Jun 07, 2005 8:43 pm

Dillon wrote:How do you post example pics here?

When you post a reply you will see at the top of the box that you type into the following bottoms: B, i, U, Quote.... Img, URL.

B = Bold.
i = Italic.
Img = Image:
Image
etc, etc.

To find the image URL / http:
First, find the image on the web (you may need to upload it to your web site first), then open up a New Window / page and drag the image onto that window / page.
The new window /page should now just show that image.
In the Address bar, at the top, you now have the URL / http for that image.
Copy the URL and after clicking on the Img bottom, Paste it after the [img] letters appear.

Click Preview - to see if it worked.

Look forward to seeing images soon.

Cheers
Ashley
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