Focus Issue - red channel?

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James
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K guys, bit of an interesting one. I've been playing about with some close up shots of playing cards and I seem to have hit a problem. If what I think is happening is indeed happening I'll need some help. It seems that focus in the red channel is off. Let me present images to show what I mean.

cards_focus2.jpg


cards_focus1.jpg


When taking images of the red cards I hit focus on essentially every shot, hence my inference that the red channel is the problem. Intuitively this seems backwards since red is the obvious colour on show. The white (or nearly so) of the playing cards is up around 255 in the red channel, and the red in the heart shapes is up very close to that. The red channel shows pretty much all white when viewed on its own. You see the red colour primarily due to the absense of the blue and green.

In the black the problem is revealed, since all channels are now at very low values. To my eye I can't see any part of the black image that's in focus, and it isn't for want of trying, I've taken tens of images (compared to 3 our of 4 sharp for the red cards).

black and white from the individual channels from the black cards are shown below

Blue channel
cards_blue1.jpg


green channel
cards_green1.jpg


red channel
cards_red1.jpg


The blue channel looks pretty much sharp, the green channel slightly less so, the red miles out.

Is there anything I can do about this? Are my eyes deceiving me?

I don't know whats going on, this is just a bit of guesswork, so if I'm talking crap let me know or ask more questions if I'm not making sense. Thanks.


edit##

I should add, these are both 100% crops of much larger images.
 
Oh dear, it would seem my ability to describe the issue isn't great.

Anyway, after much thinking on the issue, the best I can come up with is simply that light of different colours refracts at slightly different angles and when you have all three sources of light from a bright source (ie the white), next to a dark source (the black), it will be emphasised in the image.
 
Alas it's 1.00am here but I'll get back to this in the morning.

Bob
 
The red channel doesn't look markedly softer than the other channels to me, but I do notice a lot less detail in the card face when compared to the other channels. Is the red channel definitely not overexposed?

We are looking at harsh transitions here with red poorly represented in Bayer interpolation. I suppose you could be right about the refraction of red light giving a different point spread function.
 
Mud said:
edit: reread what you said, err...effect of the Bayer interpolation? Red channel looks pretty overexposed too.
Click to expand...

No idea what bayer interpolation is so thats some reading for the morning. It's not over exposed though, max at 235 I think (or a snats over), can't remember right now. Certainly nowhere near getting clipped.
 
A_section.png


Well here's a section, not sure if it's just a trick of the mind...the human eye isn't RGB, it's more like green, an olivey green and violet I believe :cuckoo:
 
I think the colouring might be because the white balance is off, I shot in raw, auto white balance, and simply converted to jpeg as it was. I figured it wouldn't much matter for focus if the picture looked crap.



here is a link to a 400*400 tiff of the same image for anyone who wants it with the white balance set to tungstun which looks to be about right

It's about 400k so I didn't include it as an image
 
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