HDR Exposure

Skiddley

Skiddley

Skiddley

Soldato
Joined
1 Aug 2003
Posts
3,797
Location
Cheshire
Hi - not posted here before and only recently started playing with photography on my Lumix LX2 so forgive my nooby questions :D

So HDR rendering, I take bracketed shots of -1,0,+1 EV and photomatrix does its thing. Lovely. But why can't you just programatically over and under expose a single, normally exposed image to create the final HDR render? Or better still, why can't HDR algorithms simply work out the final image from a single frame.

I take it there is 'more information' in a set of three distinct images than a single frame can provide if you were to under and over expose it in CS2 say, but that sugests to me that the exposure adjustment in CS2 is a hack and doesn't really do what it says on the tin. Am I missing something?

Thanks :D
 
Last edited:
No offence but you seem to have a bit of lack of understanding of digital imaging, otherwise you'd understand why your question is a bit wrong.

If you could just adjust the image in photshop then there would be no point in HDR or any of that sort of thing. Look up histograms, tonal range, curves in various places and it may be a bit clearer.
 
whitecrook said:
No offence but you seem to have a bit of lack of understanding of digital imaging, otherwise you'd understand why your question is a bit wrong.

If you could just adjust the image in photshop then there would be no point in HDR or any of that sort of thing. Look up histograms, tonal range, curves in various places and it may be a bit clearer.
Click to expand...

What a thoroughly informative post!

It is perfectly possible to create a HDR image from just one RAW shot, the problem is that you do lose detail at the extremes of the exposure range or end up with lots of unpleasant noise.

CS2 does have a "Merge to HDR" feature but this only works with the three-RAW solution (without a lot of jiggery-pokery). The photomatix software doesnt care what you use so is much better in that respect.

I am sure cykey will be along shortly with a heap more info for you :)

EDIT: well, he beat me to it but his guide is rather good :)
 
DRZ said:
It is perfectly possible to create a HDR image from just one RAW shot, the problem is that you do lose detail at the extremes of the exposure range or end up with lots of unpleasant noise.
Click to expand...

Umm that's the point - if you are losing details in the highs and lows, it's not HDR.
 
Someone correct me on this if I've got it very wrong:

Here's a pointless powerpoint (hah!) guide to why hdr works.

hdr.gif


A standard RGB jpeg image has 8 bits of information per chanel. As a result, it can only display so many colours.

Fig 1 shows a brightness scale of a 'perspective' - what you look at down the viewfinder of the camera. Everything you can see in the viewfinder has a brightness somewhere on the scale. There's really dark stuff, and really light stuff. There aren't enough colours (dynamic range) in an 8bit jpeg to capture the complete range of colours in real life. Therefore, the jpeg can only show a part of this range. How you do the exposeure determines which part of the brightness range you capture. Now figure two shows what we capture if we do a long exposure - Everything darker than the darkest recorded just comes out black, and everything brighter than the brightest it can capture comes out white. Therefore we lose detail (or "information") for all the points that are brighter than our range that we're capturing. Alternatively, like Fig 3, we can do a short exposure, so we're capturing the brighter end of the range. Now anything that's darker than what we can capture comes out black.

To get a bigger range, we need to sample more of the available dynamic range of reality. So we take lots of pictures, at different exposures (Fig 4). Once that's done, we run it through a nifty algorithm that basically takes the complete range captured and squeezes it down into a standard 8 bit image.

And that's why you need different exposures.

I should point out that cameras are actually capable of capturing a much larger range than can be contained in an 8bit image - hence the advantage of using a camera that can capture in RAW. Using a RAW editor, the exposure can be adjusted and the results saved out as JPEGs to give the same effect without taking separate images.

Go read cyKey's guide :)
 
whitecrook said:
Umm that's the point - if you are losing details in the highs and lows, it's not HDR.
Click to expand...

Erm, detail, not Dynamic Range. You lose detail in blown highlights and clipped shadows but if you escape either of those perils you still have the problem of noise. You can still produce an image with a higher dynamic range than just normally processing the one frame.

Perhaps you ought to calm your posting style down and look at whats going on before berating others? :)
 
Thanks guys,

Like I said, never claimed to be an expert (white ;)) I'll go read cy's guide. Nice to be encouraged and not just shot down :p. Growse, the pic was good, thanx.

Should probbably find some technical references to digital photography too (appature, shutter priority modes etc..) to bootstrap my learning not to mention composition guides. The only reference I have is the camera manual :rolleyes:. I'll do a search - sure there has been discussion on decent books\websites already.

So I'm putting myself through an intensive few weeks of learning the technology, getting familiar with my camera, photoediting processes and the like before I go travelling (Canada) in a few weeks. What would be really handy is a syllabus, a step-by-step guide to photography. I really want to gain a fundamental understanding of the concepts - it's all too tempting to go shooting without knowing what you are doing....

Skidd.
 
The book that keeps getting recommended here for learning the concepts is Bryan Peterson's "Understanding Exposure". I'm after a copy of this, but have been ordered not to buy it by my better half. Think she's stocking up Xmas ideas!

For getting used to the camera, I stumbled across this and found it rather a fun way to learn what a camera can and can't do. (Written for a 350D and kit lens, but adaptable to most cameras with some manual control)
 
Thanks dude, I'll check it out. From the reviews on <BOOK SHOP NAMED AFTER A RIVER> it seems well respected, only 15 notes too :D
 
growse said:
Someone correct me on this if I've got it very wrong:

Here's a pointless powerpoint (hah!) guide to why hdr works.

hdr.gif


If one is taking 3 photos of the same subject it must be quite difficult to keep the camera steady enough? Even minor movements should affect the outcome?

I believe there are some cameras which will take 3 (bracketed?) exposures, ie the one set by the user, and then + and - one stop. Is that what you guys do?

Would it not be possible to load the original JPEG into PhotoShop, duplicate the layer a further twice and then using levels adjust two of the layers to simulate the same effect? (that way you wouldn't have problems with camera shake).

I wonder if the follow-on to Canon's 3S IS will have RAW because I haven't seen any x (times) 12 zoom, non DSLR IS cameras with the RAW feature (unless you guys know different?).
Click to expand...
 
Belly said:
If one is taking 3 photos of the same subject it must be quite difficult to keep the camera steady enough? Even minor movements should affect the outcome?
Click to expand...
Called a tripod :)
I believe there are some cameras which will take 3 (bracketed?) exposures, ie the one set by the user, and then + and - one stop. Is that what you guys do?
Click to expand...
That, or use RAW.
Would it not be possible to load the original JPEG into PhotoShop, duplicate the layer a further twice and then using levels adjust two of the layers to simulate the same effect? (that way you wouldn't have problems with camera shake).
Click to expand...

Fundamentally not. Levels adjustments just moves the colours around in the image. Lightens and darkens. You can't lighten white and you can't darken black. You need information about what was lighter than whatever's white in the image and about whatever's darker than the black parts of the image.
 
growse said:
Called a tripod :)

That, or use RAW.

.........................................

Thanx :)

Yes, I just bought a cheap 2nd hand tripod on fleabay so I can "have-a-go" at panorama shots (prior to that the only connection I ever had with something with 3 legs was a bar stool and the next door neighbour's cat)! :D

Just being naturally clumsy and pessimistic (pessimists are rarely dissapointed you know) I would have thought that even with a tripod one would have slight movements, so obviously RAW would be the better choice.
Click to expand...
 
You must log in or register to reply here.
Back
Top Bottom