Green Magenta shift using Hasselblad HTS

[jonathan.lipkin]

Hey everyone. You may be familiar with my ongoing search for a digital camera system with some movements. I have it narrowed down to a Rollei XAct with a Phase One back and a Hasselblad hd3 with an hts adapter. Took the Hassy out for some testing this weekend. I haven't processed all the images yet, but on some, but not all, of the images with extreme shift, there is a noticeable color cast. Starts as green at the top, the middle is normal, then the bottom is magenta.

OK, I know what you're saying - don't use the shift. But, the manual specifically describes using shift of up to 18mm in each direction. The Hasselblad tech I spoke to thought this would be corrected in Phocus, but it doesn't seem to be doing so. I have an email in and will let you know what they say, but would be interested in hearing if anyone else has the same experience. I suppose I could shoot a white diffuser over the lens to create a correction map, but seems a pain to do.

[Bart_van_der_Wolf]

... I suppose I could shoot a white diffuser over the lens to create a correction map, but seems a pain to do.

Hi Jonathan,

You should indeed use a color cast correction. That's what it's for, to compensate for color cast (and vignetting and light fall-off).

Cheers,
Bart

[ejmartin]

Hi Jonathan,

You should indeed use a color cast correction. That's what it's for, to compensate for color cast (and vignetting and light fall-off).

Cheers,
Bart

+1

Capture One and RawTherapee (the current dev builds) can both perform this type of flat field correction.

[David Grover / Capture One]

Hi Jonathon,

It is pretty easy in Phocus to use Scene Calibration to correct for this.

Phocus will automatically correct for Vignetting, CA and Distortion, but there are too many variables to accurately remove casts without using the Scene Calibration tool.

Hope that helps and that you enjoyed using the HTS.

Also you can easily place your H3D or H4D on the Rollei xact, fully compatible with manual, or electronic shutters from Schneider or Rollei.

David

[jonathan.lipkin]

Thanks, everyone. Scene correction tells me that the image is too complicated.

[Barkeeper]

and a Hasselblad hd3

Which one did you use? 31? 39? 50?

Thank you.
Barkeeper

[jonathan.lipkin]

Metadata tells me h3dII-39MS. Don't think I enabled any of the MS capabilities, but couldn't find a manual for it.

[David Grover / Capture One]

Thanks, everyone. Scene correction tells me that the image is too complicated.

Did you capture the scene through an Opaque filter as per any other MF system?

MS capabilities are enabled when shooting tethered to a computer.  Look in the Camera Control tool to switch between Single and Multi.

I guess you are buying second hand?

David

[JerryReed]

"Opaque" filter?  Meaning no light passed through the filter?

Jerry

[jonathan.lipkin]

"Opaque" filter?  Meaning no light passed through the filter?

Jerry

LOL - you mean a lens cap?

David - No, I did not use a filter. Had heard about them, but didn't realize it would be an issue. The manual and brochure had not mentioned anything about the tint.

[Nick-T]

You need to capture a reference by taking a shot through a translucent filter (i.e. a bit of plexiglass), Phocus (and Capture one/Leaf capture do the same thing) than uses that reference shot to remove the colour casts. You can't just use the image as the reference as Phocus has no way of knowing what is cast and what is part of the scene. Hope that makes sense. As an aside I once did a copy shot for an acquaintance (i.e. for free) and didn't feel like creating a proper copy set-up so I just side lit it and used scene calibration to even up the light fall off. Worked perfectly and made me feel marginally better about shooting for free...

[jonathan.lipkin]

You need to capture a reference by taking a shot through a translucent filter (i.e. a bit of plexiglass),

Yup - just got off the phone with Hasseblbad tech support and that's what they suggest.

[David Grover / Capture One]

LOL - you mean a lens cap?

David - No, I did not use a filter. Had heard about them, but didn't realize it would be an issue. The manual and brochure had not mentioned anything about the tint.

Whoops.  Opaque really not the right word.  Excuse me. 

In Phocus, if you goto Help > Show User Manual then you can read more about Scene Calibration there.  Otherwise drop me a message and Ill send you a tutorial.

David

[jonathan.lipkin]

In Phocus, if you goto Help > Show User Manual then you can read more about Scene Calibration there.

Interesting. David, were I to roughly duplicate the lighting conditions (outdoor, sunny) and lens conditions (35mm lens, HTS adapter set to x tilt and x shift), do you think I could duplicate the tint and retroactively create a calibration image?

[David Grover / Capture One]

Interesting. David, were I to roughly duplicate the lighting conditions (outdoor, sunny) and lens conditions (35mm lens, HTS adapter set to x tilt and x shift), do you think I could duplicate the tint and retroactively create a calibration image?

Well, it has been spoken of at times. 

I am not 100% convinced it would be foolproof but worth a try.

[Bart_van_der_Wolf]

Interesting. David, were I to roughly duplicate the lighting conditions (outdoor, sunny) and lens conditions (35mm lens, HTS adapter set to x tilt and x shift), do you think I could duplicate the tint and retroactively create a calibration image?

Unless the software does something strange behind the scenes, there should be no problem when you try to get the lens/focus/aperture and X/Y shift the same as when the actual shot took place. I don't think the lighting conditions matter, unless there is something uncommon(*) going on in the software, or your lighting conditions were exceptional (e.g. Sodium lighting).

(*) Common procedures in Astrophotography use a normalized colorbalance for the Flat files.

Cheers,
Bart

[ejmartin]

Bart, the purpose here is to even out a response imbalance across the frame (similar to vignetting, but rather a linear gradient rather than radial).  This can be done per channel using the calibration image (this is how Rawtherapee does it), so is independent of white balance which is a global equilibration between the channels.

[Bart_van_der_Wolf]

Bart, the purpose here is to even out a response imbalance across the frame (similar to vignetting, but rather a linear gradient rather than radial).  This can be done per channel using the calibration image (this is how Rawtherapee does it), so is independent of white balance which is a global equilibration between the channels.

Hi Emil,

Yes, there are 2 possibilities. The channels of the cast correction image can be individually normalized to 1.0 from the maximum value of each channel itself, or from the maximum of all channels. The latter could introduce a White balance shift when dividing, so I assume it is not implemented to do it that way.

Cheers,
Bart

[jonathan.lipkin]

I just got myself a piece of white, semi-opaque plexi and have done some preliminary testing with Scene Calibration. So far, so good. It removes the green/magenta cast well. My print viewing board (which I'm shooting to test) isn't evenly illuminated enough to do super-precise testing, but the cast is very apparent in non-corrected images, and nearly completely removed in corrected images.

The 'Equalize intensity' function works less well on the more extreme shifts (+15) but there does not seem to be much intensity unevenness.

Further, I tried retroactively creating a scene calibration image and it worked pretty well. I shot my print viewing station which is lit by daylight flourescent bulbs at the HTS settings of the images I had shot a couple of weeks ago on the beach. I then created Scene calibrations from them and applied to the beach images. Not perfect, but a huge improvement.

[Dinarius]

=> David Grover

Hi David,

Rather than start a new topic, I thought I might simply ask the question here.

I'm Windows 10/Phocus 2.9.1

When attempting to create a scene calibration, I am sometimes getting the error "Failed to create correction. Correction error." Repeatedly clicking Create makes no difference. Some files (with the same exposure) work and some don't.

This error, as far as I can see, is not mentioned or dealt with in the Phocus Help pdf.'s.

Any idea what causes this and how to avoid it?

Many thanks.

D.