Expodisk and red edges

[Schrödinger's cat]

Advertisement (gone after registration)

Cppied from the FAQ:

"Question:

How do I correct red edge in Capture 1 pro?
(Courtesy of Swamiji)

Answer:
First shoot an image with the lens in approximately the same circumstances as you wish to correct with something white opaque (I use a bit of white foam, but one can use white opaque glass, a white lid of a tin, even a sheet of white paper) but not an Expodisk."

 

What causes the Expodisk to be unsuitable?

[Advertisement]

The Leica Q3 is available for pre-order at B&H Photo Video and Adorama

[jaapv]

Possibly the prism structure? Just guessing. I never tried, as I find a folded piece of paper easier.

[pop]

The Expodisk appears to have a black rim which might make more difficult the detection of the red edges, I could imagine.

[Schrödinger's cat]

Possibly the prism structure? Just guessing. I never tried, as I find a folded piece of paper easier.

I thought that sounded like a good explanation, but when holding the Expodisk up to a bright light, the opaque white plastic prevents seeing the prisms from the lens side.

 

This is sort of an academic interest, since I generally use a coffee filter myself  (Granted, since it's a Leica, I use a Marks and Spencer coffee filter, but still..... )

[Schrödinger's cat]

The Expodisk appears to have a black rim which might make more difficult the detection of the red edges, I could imagine.

That sounded to me like a good theory, but with the lenses I tried the black rim cannot be seen by the lens.

[new2leica]

An Expodisc is not opaque. It is semi opaque allowing 18% light transmission.

[Schrödinger's cat]

Advertisement (gone after registration)

An Expodisc is not opaque. It is semi opaque allowing 18% light transmission.

It is.  However, how does that make it unsuitable for the process of correcting red edges?

[new2leica]

The FAQ's answer states to use something that is white opaque.  An Expodisk is not.  See http://blog.phaseone.com/get-perfect-snow-without-color-cast/

 

Scroll down to where a reference file is created for color correction.

 

If an Expodisk is used the image won't have the necessary 'whiteness' instead it will be 18% grey.  So I would infer that is the reason.

[Schrödinger's cat]

 

 

If an Expodisk is used the image won't have the necessary 'whiteness' instead it will be 18% grey. 

Ah.  That simply never occurred to me, but it sounds like the perfectly obvious explanation.  However, in reading the article at the link you posted, the author states :

 

"Hi Robert, 

Yes, an ExpoDisc should work.

 

All the best,

Niels"

 

I gotta admit though, I find your line of reasoning is compelling.

[jaapv]

What is the difference between white and grey? Only the exposure, the colour channels are balanced in both cases. The camera will expose the both differently to render the same density.

Shooting through white paper and an expodisk should produce exactly the same file if the camera is on "A".

So that cannot be a reason not to use it.

[luigi bertolotti]

Ah.  That simply never occurred to me, but it sounds like the perfectly obvious explanation.  However, in reading the article at the link you posted, the author states :

 

"Hi Robert, 

Yes, an ExpoDisc should work.

 

All the best,

Niels"

 

I gotta admit though, I find your line of reasoning is compelling.

I'd say that in practice the expodisc, though not "white", works rather fine, considering that its "greyness" is uniform and not biased to some color , which would alter the real red shift at the edges.  Anyway, a normal A3 white matte sheet is imho the simplest tool .. and the lightining of it is more important than the "purity" of its white.

[mjh]

What is the difference between white and grey? Only the exposure, the colour channels are balanced in both cases. The camera will expose the both differently to render the same density.

Shooting through white paper and an expodisk should produce exactly the same file if the camera is on "A".

So that cannot be a reason not to use it.

Indeed an 18% grey is perfectly suited for white balance, better than white in fact. If the image appears white rather grey, some RGB channel may be clipped, skewing the WB result.