Exposure Histogram Okay, but Red Channel Highlights Blown
Sat, 12 June 2004 07:17
Barb Wayne was kind enough to provide me the link below. Basically, the
story is that it's possible to take a picture of a bright red object and
have apparent autofocus problems, especially with cameras using a CMOS
sensor. The cause has been traced to the red channel being oversaturated
(blown) in spite of the overall exposure as seen on the camera's histogram
(which is basically RGB luminosity) being right in the middle. There's a
DPReview reference and a picture that illustrates the phenomenon (with
accompanying channel-by-channel histograms) that are worth reading IMO.
Anyone have this problem with their flower shots? I'm pretty sure I have...
Sat, 12 June 2004 08:30
While the red channel very often is blown on my G3 images, even when the other channels aren't, I don't think that this necessarily has anything to do with the focus issue for the SLRS. (I believe on the compacts, such as the G3 the focus works on the contrast on the green sensors and looks for vertical contrast - as magenta type colours are as far away from green as you can get you would expect a contrast based focussing system to have more trouble with them (that's why the G series cameras can't utilise the AF light in the EX flash guns as they can't focus on the red light that it emits)). However I'm not sure that this is true for DSLR s which use a different, phase based, focus system.
If you don't want to have the reds blown out - it results in loss of detail in highly saturated areas- then shooting RAW is the way to go, as the clipping of one channel only, which isn't visible in the camera composite histogram, tends to be correctable with RAW.
Sat, 12 June 2004 08:50
Thanks for that post and the link. I wonder if that could be linked to
a problem that consistently bugs me in taking flower pictures. Reds are
particularly hard to get exposed correctly, so I think the red channel
issue is probably the reason. But, purple? I have tried, mostly in
vain, to capture good shots of my deep purple and deep almost midnight
blue iris and cannot get what I think is an accurate rendition. Tied to
the same problem, perhaps?
Now I have somthing to go on.
Sat, 12 June 2004 08:34
Susan, please say more about correcting a clipped red channel in RAW mode:
is that something you do during conversion with CS, or post-conversion? And
if it's really clipped, what can you really accomplish with it?
Sat, 12 June 2004 09:19
Chuck - Adobe's RAW converter in CS allows up to +/- 4 EV exposure adjustments during the RAW conversion process. This can give an amazing amount of data recovery in blown out areas. If just one channel is blown, usually the image won't be so overexposed that you can't recover some of the data by adjusting the exposure down a bit in the RAW conversion process. (I believe that for the DSLRs C1 can do the same thing - but it won't read the RAW from the G series cameras). I don't know how it works, but with RAW you can get highlight detail out that is totally lost in an out of the camera jpg with the same exposure - and of course having 16 bits of data to play with (and with CS the tools to allow you to play with them!) after the RAW conversion also allows more detail to be found without posterisation. If you are trying to shoot high contrast or highly saturated colour images, or in light where the white balance is uncertain, RAW is brilliant, IF you have the software to make the conversion painless. Canon's software does not qualify in this regard and CS doesn't seem to be a good solution for all cameras- and it is expensive. But it's working well for me.
Sat, 12 June 2004 09:24
Dick - dark purples are an issue with some camera sensors - both my G3 and my Kodak 2mpixel camera see certain dark purples as dark blue. It's very weird when you have jacaranda blossom lit by sunshine as the sunlit areas are correctly recorded, but the darker shadow areas show an abrupt shift to blue. My daughter has a witches costume with a dark purple cloak, which photographs as a deep blue, a hue shift of about 30 degrees. There's also a particular shade of forest green that the camera records as plain grey in certain lights.... The only solution I know of is a hue/saturation adjustment targeted to the right area!
Sat, 12 June 2004 09:53
Susan, eureka! Worked very well indeed. Now I'll have to go back and
correct all those images!
Thanks for the suggestion.
Sat, 12 June 2004 09:57
Dick - glad it helped.
Sat, 12 June 2004 09:45
thanks for the suggestion, I'm off to try it now!
Sat, 12 June 2004 11:06
Susan, that's what I needed to hear re CS; heading to the store to buy it!
Thanks for the explanation.
Sat, 12 June 2004 11:56
Thanks, Stu. I'm a registered owner of PS7, so the pain isn't
Sat, 12 June 2004 11:52
The manipulation possibilities with Camera RAW and the ease of use are simply incredible. I needed to upgrade from Elements to full PS for CMYK capability. I was going to save a few bucks (actually quite a few!) and but a copy of PS 6 on eBay; then I tried our Camera RAW, and CS became a must-have.
Sat, 12 June 2004 12:34
I never had this problem, probably because I've shooting RAW from almost day
#1 with my 10D. Also, I've begun experimenting with Adobe RAW. Sure, it's
not C1, but it's not that bad. For the time being, it's saving me the
expense of having to buy another RAW converter.
So Chuck, go ahead, buy CS, you won't regret it ;-)
Sat, 12 June 2004 12:48
Thanks, Ray! Glad to see you're up and about!!
Sat, 12 June 2004 13:18
Everyone off and running to buy CS I see. Guess I'll be alone with my PE3 when the time comes....or are you all going to buy that too ?
Sat, 12 June 2004 13:20
I don't know, Jodi. I really like a lot of the doodads in CS, but I do feel more at home in my nice comfy PE.
Sat, 12 June 2004 14:01
Sat, 12 June 2004 14:04
Jodi, I don't know....you skipped PE2, so maybe I'll wait until PE4! (I
should live so long....)
Sat, 12 June 2004 14:07
Chuck, actually, my mil has PE2 and i used it enough to know I didn't need it. I always figured I'd wait for PE3 anyways. I do have full PS already as well. CS would be something I'd get if i could get it for a peanut and a smile :)
Sat, 12 June 2004 20:31
Susan, any recommendations on books for CS?
Sat, 12 June 2004 20:23
I played long and hard with the earlier demos of PS - but it was only CS that conviced me I *needed* photoshop rather than PS - mainly for the RAW converter. The rest of the things I Can do in Elements, although it may take a bit longer (I don't need CMYK). In fact I still haven't had time to sit down and learn any of the new bits of PS that I don't know (apart from the RAW converter), so I'm still using most of the same methods that I was in Elements.
Sat, 12 June 2004 20:49
Chuck, obviously I'm not Susan, but you might want to take a look at Deke's 'Photoshop CS One on One." There are things about it that are kind of irritating, but it focuses almost exclusively on areas/techniques that are different in CS than they are in PE.
It's a tutorial kind of thing, with a CD of all the exercises.
Sat, 12 June 2004 20:51
Oh, and a bunch of videos.
BTW, Chuck, when you get CS you get a total training video cd in the case with it, although it doesn't go into much depth.
Sat, 12 June 2004 20:51
I'm not Susan, but if I may...
The Photoshop CS Bible for Professionals
The Photoshop CS Book for Digital Photographers
Adobe Photoshop CS Creative Studio
Sat, 12 June 2004 20:54
> although it doesn't go into much depth.
It's kind of an understatement... ;-)
It's worthless, if you ask me.
Sat, 12 June 2004 20:59
Chuck - The best PS book for my purposes is Katrin Eisemann's book on retouching, but it's not CS specific - there's a good download chapter on her website that runs through the new CS features. I've got two doorstep books - neither of which I'm 100 per cent happy with - Bruce Fraser's Real World Photoshop book is very technical and aimed at graphic prepress pros, but it has some very interesting in depth material on things like sharpening and colour management and answers questions that I've not seen answered in any other books - it's very good on information that you need to set up a sensible workflow. But there's a lot of stuff on CMYK that I don't really need and it's not always terribly logically organised (although unlike many PS books it has a really good index so you can find stuff easily). I've also got Ben Willmore's Photoshop Studio techniques, which seems to cover all the tools and how to use them at a more basic/intermediate level with lots of examples. Neither book seems to cover the web based aspects of CS - in particular what image Ready can do -to any extent, and that's something I want to find in the fullness of time. For fun I've got How to Cheat in Photoshop which is all montage tricks and special effects.
Sat, 12 June 2004 21:18
Ray and Barbara, thanks. What I really need is a book that addresses only
the differences between PS7 and CS; I have a lot of books for 7. However,
you've mentioned at least a couple that I don't have, so...thanks again!
Sat, 12 June 2004 21:20
Susan, thanks - I have Eisemann's second edition and Willmore's PS7 version.
Will look closer at 'Cheat'; was thumbing through it at the local bookstore
this afternoon and it looked like fun.
Sun, 13 June 2004 03:59
It is fun ... take a look at the website <http://www.stevecaplin.com/> Wait until it fully loads before clicking on anything then run the mouse over all the bottles, books etc .... the graphics are amazing :) No click here buttons on this site.