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Author Topic: how to shoot HDR  (Read 18961 times)

RFPhotography

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Re: how to shoot HDR
« Reply #20 on: November 19, 2011, 04:26:50 pm »

Do a search on 'measurbator' and you will build in a second a list of people with a strong inferiority complex regarding the technical aspects of digital photography.

Sorry to disappoint you but I don't fit into that group.

As I said GL, I don't disagree with what you're saying.  Believe that or not, I don't care.  But, as difficult as it may be for you to accept, there are people out there who don't have the depth of knowledge you do.  As I also said before there are highly competent, accomplished photographers who don't have the depth of knowledge you do and who produce absolutely stunning photography.  Quite frankly, I'll take their work probably 99% of the time over someone who uses a strict 'photography by numbers' approach.  Website tutorials aren't intended to be doctoral theses on a subject.  At least I don't believe that's what they should be.  I believe they should be written for an audience with a wide range of knowlege and with a practical, real world approach.  If you don't agree, that's fine.  Again, I don't care.  If you read my tutorial, you'll note that I also said "just because you capture 9 or 11 shots in a bracket doesn't mean you have to use all of them in the HDR merge."  We're not saying a lot different, we're just saying it differently.  My tutorial also says "you don't need to capture every 1/3 or 1/2 stop."  Further down I said the following:  "As long as you’ve got the full brightness range covered with proper bracketing, 1 stop or even 2 stop separations between images give enough information for the HDR software to do its thing."  We're saying basically the same thing, again just saying it differently.  Some cameras also aren't capable of capturing at 2 stop increments in AEB mode.  So the limitations of what the camera can do also have to be taken into consideration.  What's faster and likely to cause less chance of ghosting; to shoot a 5 image bracet 1 stop apart with an AEB function and then use only the -2, 0 and +2 in the merge or to manually adjust exposure in between shots so that only 3 shots are made 2 stops apart?  What's going to be more likely to cause camera movement and thus ghosting in the image?  The AEB which is set once and can be fired with a cable release or having to manually adjust exposure by touching the camera in between shots? 
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Guillermo Luijk

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Re: how to shoot HDR
« Reply #21 on: November 19, 2011, 05:01:07 pm »

My comments about the number of shots and bracketing interval have 0 relation to your tutorial or to anything you could have said in this or in any other thread Bob. It's additional information that could be interesting to anyone learning about HDR and I pasted it from another forum (Eliminating noise in HDR tonemapped images), where a professional photographer claiming 100 times that he shoots 400 interiors a year, insisted on saying 3 shots are not enough for his scenes. Unfortunately he didn't provide any evidence of this nor wanted to share any of his RAW files to determine the real DR of his demanding scenes.

Regarding the discusion about 'measurbators', if you stop using that despective term you will probably not become a better photographer but you will certainly become a better person.

Regards

bjanes

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Re: how to shoot HDR
« Reply #22 on: November 19, 2011, 05:48:29 pm »

You know, we may actually agree on something GL.  Imagine that?   ;D

I agree that in an perfect world what you're advocating would be correct.  However what I've done is build in a bit of an 'error factor'.  Figuring out that precise ETTR point can be difficult for some who aren't as familiar with the concept or haven't done the necessary testing with their cameras to figure out where that point is.  So my goal was to give folks enough room for error that they could still end up with a good result (i.e., no blown highlights, no blocked shadows).  You can never shoot too wide a bracket set but you can shoot to narrow a bracket set.  And while I realise that LuLa is the internet home of the photographic measurebators, it's entirely likely that there are people who read this thread who may not fit into that group; hence the repeating of the same philosophy here.  It's the difference between writing for a more general audience and writing for the hardcore measurebators.

Bob,

Your tutorial gives some good advice, but you don't seem to have a good understanding of the technical aspects of photography and dismiss the advice of Bart and Guillermo (who are two of the most knowledgeable forum members in this area). Your lack of knowledge causes you to waste time taking an excessive number of shots, possibly allowing the elements in the subject to move or shooting conditions to change. I'm glad to see you do regard white balance as important. White balancing 9 or 10 shots according to the "season by taste" method could be a bit time consuming and inaccurate ;D.

Auto-bracketing is convenient, but if you are shooting landscapes and using a tripod, you probably do want to use mirror lockup and this is not available on most cameras with bracketing. A sturdy tripod would prevent most camera movement, but auto-align present in most software packages can do a good job of removing it.

Regards,

Bill
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kencameron

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Re: how to shoot HDR
« Reply #23 on: November 19, 2011, 06:27:37 pm »

I don't think that anyone is saying that you have to have a deep mathematically based understanding of technicalities to take good photographs. If they are, I would respectfully disagree with them. It does seem clear that some helpful and friendly contributors to this and other forums (fora?) find a particular word offensive and distressing. I am respectfully wondering why that wouldn't be a good enough enough reason for you to stop using it. It is a pretty old joke, by now.
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Ken Cameron

RFPhotography

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Re: how to shoot HDR
« Reply #24 on: November 19, 2011, 07:03:42 pm »

Well then perhaps you need to phrase your comments more carefully GL so that there's less confusion.  But I'll absolutely agree that a 'hard and fast' 3 shots works in every situation mantra is flawed.  WRT the 'silly equation', it comes back to practical vs. technical.  The capabilities of the camera have to be taken into account in a practical example.  A 3 stop interval is something I'm not sure any camera can do.  Maybe there are 1 or 2.  I don't know the specs of every camera on the market.  But for the vast majority of cameras out there, it's not going to work.  So, in order to create that 3 shot, 3 stop interval, manual intervention is needed.  Touching the camera in creating a bracket set for HDR introduces all kinds of possibilities for camera movement between shots.  I'd much rather shoot the AEB at 1 or 2 stop increments without touching the camera than the 3 shot bracket having to fiddle with the exposure between shots.  Technically the 3 shots, 3 stops apart may be all that are needed.  Practically it presents problems.  But if you're working with a camera that has a max 2 stop AEB function and combined with the DR of the sensor that's not enough to cover the range of the scene then, practically, you have to make some compromises.  

Bill, I don't dismiss GL's or Bart's knowledge.  I'll grant that both have a fulsome knowledge of the behind the scenes math.  And that's terrific.  But largely irrelevant to, I'd venture, the majority of people trying to do HDR (or other techniques).  I have a more extensive knowledge than you seem to feel I do.  What I do is balance the technical with the practical.  What I write in my tutorials may not, actually, be what I do in practice.  Again, because I'm trying to write for a wider audience and don't want to weigh people down with very heavy, technical data.  But I don't need to know the math of, for example, how the Multiply layer blend mode works to know what it does and how to use it.  I've never, in any discussion, regarded WB as irrelevant or unimportant.  I don't accord it the same level of importance as you do.  Therein lies the difference.  Nice reference on the inaccurate.  ;)  

EDIT:  Ken, you posted while I was writing.  As far as old jokes go, some of Henny Youngman's stuff is still pretty funny and it's really old.   ;D  I still get a laugh out of Bill Cosby's 'face like a glazed donut' bit and it's pretty old.  That said, I will attempt to refrain from using that term. 
« Last Edit: November 19, 2011, 07:07:05 pm by BobFisher »
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bjanes

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Re: how to shoot HDR
« Reply #25 on: November 19, 2011, 10:25:11 pm »

So, in order to create that 3 shot, 3 stop interval, manual intervention is needed.  Touching the camera in creating a bracket set for HDR introduces all kinds of possibilities for camera movement between shots.  I'd much rather shoot the AEB at 1 or 2 stop increments without touching the camera than the 3 shot bracket having to fiddle with the exposure between shots.  Technically the 3 shots, 3 stops apart may be all that are needed.  Practically it presents problems.

I did learn something about my camera by participating in this thread, and began to think how I would use its features to implement HDR. My main camera (Nikon D3) allows AEB with up to 9 shots, but only at a maximum of 1 EV steps. If only 3 shots are needed one can set the camera to take shots at the nominal exposure, +1 EV, and +2EV. If 5 or more exposures are set, it brackets up and down around the nominal exposure, but one can use exposure compensation to counteract that so it effectively brackets only above the nominal exposure.

So using Bart's and Guillermo's suggestion, I would set the nominal exposure as an ETTR exposure and then bracket for longer exposures until the DR of the scene is covered. Since real time histograms are not available, the easiest way to make an ETTR exposure without taking multiple shots is to take a spot meter reading from the highlight and add 2.5 to 3 EV. Most meters are calibrated for 12% average scene reflectance and would give 12% sensor saturation at the indicated reading. This allows 0.5 EV for highlight headroom. 100% saturation is 3 EV above 12% (log base2 1.0/0.12 = 3). One could use the built in camera spot meter or, even better, a 1 degree hand held unit to place the highlights. Allowing 0.5 EV of headroom to protect the highlights would probably be a good idea, and the camera histogram could be used for evaluation of the highlight placement (once one learns how the camera histogram relates to the raw file).  One could stop increasing the exposure when the shadows are well above clipping on the histogram, depending on the noise characteristics of the camera and the amount of shadow detail needed. This would entail making more shots than necessary but could be accomplished quickly using the electronic remote release. I did find that this works with mirror lockup.

Alternatively, one could make manual increments of 2 or 3 EV above the base exposure, but this would require touching the camera. One could use autoalign to take care of camera movement. In our host's Camera to Print and Screen tutorial, he states that he often shoots HDR hand held with good results using auto-align, so touching the camera on a stable tripod might not be all that bad.

How are others handling this in the field?

Regards,

Bill
« Last Edit: November 19, 2011, 10:28:03 pm by bjanes »
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Peter McLennan

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Re: how to shoot HDR
« Reply #26 on: November 19, 2011, 11:10:28 pm »

In our host's Camera to Print and Screen tutorial, he states that he often shoots HDR hand held with good results using auto-align

How are others handling this in the field?

Bill




Hand-held HDR panos are definitely doable.  This image from last month is three stitched horizontal frames, each of which is a three-image HDR.  It was shot hand-held.  No tripod was available in this instance so Photoshop and I did the best we could. 

D300 with a 16-85 @16mm.  Image is just under 8000X3000 pixels.
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RFPhotography

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Re: how to shoot HDR
« Reply #27 on: November 20, 2011, 07:34:53 am »

The issue, Bill, is that you're having to touch the camera between some of the shots.  That introduces the possibility of camera movement and a less precise merge.  Not to say it can't be done.  I used to do when I shot Canon because of the limitations on AEB with the sub-1 series bodies.  Peter's shot shows it can be done as well.  But, in general, touching the camera as little as possible is the better course.  You may also get a good alignment in the central part of the image but have less than perfect alignment toward the edges and some cropping may be required as a result. 

As far as the spot meter in camera, yes that's possible but in camera spot meters are % based rather than degrees.  So the amount of coverage is dependent on the angle of view of the lens.  Your spot with a wide angle lens will be larger than the spot with a tele lens.  Whether it makes a difference in practice will depend on the scene. 
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hjulenissen

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Re: how to shoot HDR
« Reply #28 on: November 20, 2011, 07:53:07 am »

Is the RAWs developed to TIF before HDR in photoshop? If so my guess is that the problem is that around such light source which is spread out due to clouds/haze there will be quite large areas where one or more but not all channels are clipped, which means that highlight reconstruction will take place in raw development. Highlight reconstruction aims to produce a good-looking result, but may not be that "true", for example you could get a darker highlight than it should be. When the HDR software then should combine these processed files it may not be able to match together the results. It is better to let the HDR software process the RAW file directly, then it can see exactly where clipping occurs and not take that into account when merging together the files.

If photoshop works directly on the raw files then I don't have a guess what the problem is though...
I am using the "export to HDR" option in Lightroom so that the images magically appear as a merged one in CS5. I would assume that given such control, Adobe would be able to take the technically best choices in the bacground.

-k
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Bart_van_der_Wolf

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Re: how to shoot HDR
« Reply #29 on: November 20, 2011, 08:21:35 am »

I did learn something about my camera by participating in this thread, and began to think how I would use its features to implement HDR. My main camera (Nikon D3) allows AEB with up to 9 shots, but only at a maximum of 1 EV steps. If only 3 shots are needed one can set the camera to take shots at the nominal exposure, +1 EV, and +2EV. If 5 or more exposures are set, it brackets up and down around the nominal exposure, but one can use exposure compensation to counteract that so it effectively brackets only above the nominal exposure.

Hi Bill,

That's the usual way of doing it, bracketing with EV compensation to get a bias towards longer exposure times. Otherwise there will be too many exposures covering the highlights, where we actually need more photons to improve the shadows. The usual shortcomings of various cameras in facilitating an ETTR exposure determination based on Raw data keeps hunting us though.

Quote
[...]
Alternatively, one could make manual increments of 2 or 3 EV above the base exposure, but this would require touching the camera. One could use autoalign to take care of camera movement. In our host's Camera to Print and Screen tutorial, he states that he often shoots HDR hand held with good results using auto-align, so touching the camera on a stable tripod might not be all that bad.

There is something else that needs to be considered, and Guillermo and I probably have a different opinion about it (because we use different software), the large bracket steps. In my experience, using various HDR assembly applications (for building an HDRI from bracketed exposures), the larger exposure bracket steps can cause various quality issues. To name an obvious one, image alignment. When the individual exposures are several stops apart, the noise profile at the transition zones between the exposures may prohibit finding a good alignment automatically. Even shots taken from a tripod may benefit from a (sub-)pixel alignment. Another one would be the possibility to automatically address ghosting artifacts through pixel variance.

Recent tests I have done with SNS-HDR, which is basically an (unparalleled) exposure fusion application, show me that I can push the bracketing interval to 2 EV steps, but then beyond that inevitably I can see increases of noise occuring on smooth gradients (of course depending on the amount of postprocessing). The noise increase can probably be measured already at smaller intervals, but I focused on visual artifacts. The author of SNS-HDR, who knows what is going on under-the-hood, suggests to limit the intervals to 1 EV for the best results. I often use 1.33 EV intervals when I have to cover a large scene dynamic range in a somewhat limited number of exposures.

Other applications, notably the HDR assembly and camera response curve reconstruction types, can also suffer from large exposure increments to varying degrees. Gregory Ward Larson, one of the pioneers of HDR imaging also suggests to use 1 EV bracketing steps for the assembly of HDRI's. That doesn't mean that we shouldn't think about scenarios where larger steps are useful, but there are often compromises involved when we do.

Guillermo's ZeroNoise program is probably unique in the sense that it exploits the large EV step difference, but it also assumes minimal ghosting and perfect image alignment. I am slightly concerned about (shot) noise differences at the exposure transition zones. With e.g. 4 EV exposure differences, the noise varies by a factor of 2 (although it will be reduced some by gamma conversion), which could easily be picked up by the ensuing tonemapping operations, especially unwelcome on smooth gradients.

Some of the noise risk is minimized by shooting at low ISO's and by using the ETTR zones of the brackets), but that applies to all methods that will require significant post-processing.

Cheers,
Bart
« Last Edit: November 20, 2011, 01:22:06 pm by BartvanderWolf »
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hjulenissen

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Re: how to shoot HDR
« Reply #30 on: November 20, 2011, 02:04:54 pm »

The issue is, and it's particularly prevalent here on LuLa, that there is a not insignificant number of people who feel that in order to have any success as a photographer in the digital realm one has to have an absolute and complete knowledge of the science. 
Who are they, and can you point to any posts supporting your claim?

I have discussed such issues with you earlier, but I have been quite clear that I think that it possible to have great artistic and commercial success as a photographer without understanding all that much about what goes on inside the camera. For me personally, I have an interest in understanding stuff and not depending on any gurus unsupported claims about anything that I have an interest in. For those who are less interested in such things I can only advice to not read my posts or the kind of threads that I frequent.
Quote
...
Further I'd suggest that an over-reliance on those things and getting too caught up in the pursuit of 'accuracy' or 'perfection' can be detrimental to one's development as a photographer. 
I fully agree on this.

-h
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Guillermo Luijk

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Re: how to shoot HDR
« Reply #31 on: November 20, 2011, 08:42:44 pm »

I am slightly concerned about (shot) noise differences at the exposure transition zones. With e.g. 4 EV exposure differences, the noise varies by a factor of 2 (although it will be reduced some by gamma conversion), which could easily be picked up by the ensuing tonemapping operations, especially unwelcome on smooth gradients.

I think a 4EV interval can be considered too risky, but any modern camera provides good SNR in a good amount of stops below saturation, and this is the only thing needed to have invisible noise gaps at the transition areas.

This is a genuine RAW histogram in EV divisions from my old Canon 350D (12-bit camera, 8 stops of effective DR, that is below any modern camera) from a 12 stops scene. Any optimum HDR software, given a 3 stop bracketing {0, +3, +6} (the histogram shows the 0 capture which corresponds to ETTR), should only use the 3-4 upper stops of the whole DR of the camera:



Any lower stops will be taken from the +3 and +6 shots, which will not be clipped. This means only information with a high SNR will enter the final composite and therefore noise gaps at the transition areas should be invisible even after strong post-processing (I did a test {0, +2, +4} vs {0, +4}, i.e. not using the central shot, and the noise differences were only visible in very small parts of the scene and after applying a radical contrast curve).

I agree with you this is the ideal situation, with optimum software. If SNS-HDR's author recommends 1EV intervals, it can mean that he prefers to be cautious and make sure you provide SNS-HDR with far enough information, or maybe the way he programmed it is not very optimun regarding building the composite. Have you done any test on SNS-HDR to find out how progressive it is? I mean, how wide the transition zones are. A good way to test this is to deliberately set ghosts in your scene (i.e. some moving object that appears at a different location on each shot), and see if they look semi-transparent (wide transition areas) or cut (zero progresiveness) in the output. The more progressiveness introduced by the software, the more added noise from undesired RAW files, and also the more loss of sharpness because of the blend.

Regards
« Last Edit: November 21, 2011, 04:36:03 am by Guillermo Luijk »
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hjulenissen

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Re: how to shoot HDR
« Reply #32 on: November 21, 2011, 03:57:27 am »

This is a genuine RAW histogram in EV divisions from my old Canon 350D (12-bit camera, 8 stops of effective DR, that is below any modern camera) from a 12 stops scene. Any optimum HDR software, given a 3 stop bracketing {0, +3, +6} (the histogram shows the 0 capture which corresponds to ETTR), should only use the 3-4 upper stops of the whole DR of the camera:Regards
I think what you are saying could be made clear with an analogy (if I may):
A circle may be described as a polygon in the limit N->inf. From your experience, using a finite number N (e.g. 3 stops) is sufficient so that no further visible benefits can be had by increasing it. The approximated circle looks round to the viewer no matter how she sees it :-)

By this clumsy analogy, I was hoping to make the point that using close brackets should provide a _theoretical_ advantage in improved SNR for parts of the tonal range ("making the circle smoother"), but as always, practical results trumps arm-chair philosophy.

-h
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Guillermo Luijk

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Re: how to shoot HDR
« Reply #33 on: November 21, 2011, 04:39:45 am »

A circle may be described as a polygon in the limit N->inf. From your experience, using a finite number N (e.g. 3 stops) is sufficient so that no further visible benefits can be had by increasing it.

Something like that. Less shots means a worse (lower) SNR threshold (i.e. there will be more visible noise in the noisiest parts of the composite). But if shooting at 3EV intervals that threshold is still invisible, no need to go to a higher SNR threshold through a narrower shooting interval.

In a modern camera (>9 stops of effective DR), using just the upper 3-4 stops means discarding as many as the lower 5-6 stops of captured information. That means we only make use of la crème de la crème of the captured information. Shooting at 1EV intervals we would only use the upper 1-2 stops and reject the lower 7-8 stops; doesn't it sound as a waste?.

Consider a regular scene of about 6 stops of DR we capture in a single shot (e.g. a portrait). There will be parts of the scene with a SNR corresponding to the 6th stop from saturation, and we find them OK. When bracketing for HDR at 3EV intervals, we are never going to use such a "bad" SNR. Why should we worry then?.

In addition to that, depending on how progressive is the fusion software, many shots can easily reduce overall sharpness if many shots are used and they are not milimetrically aligned, because big parts of the final image can end being a weighted average of more than one input RAW file.
« Last Edit: November 21, 2011, 08:18:08 am by Guillermo Luijk »
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Bart_van_der_Wolf

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Re: how to shoot HDR
« Reply #34 on: November 21, 2011, 08:11:40 pm »

Something like that. Less shots means a worse (lower) SNR threshold (i.e. there will be more visible noise in the noisiest parts of the composite). But if shooting at 3EV intervals that threshold is still invisible, no need to go to a higher SNR threshold through a narrower shooting interval.

In a modern camera (>9 stops of effective DR), using just the upper 3-4 stops means discarding as many as the lower 5-6 stops of captured information. That means we only make use of la crème de la crème of the captured information. Shooting at 1EV intervals we would only use the upper 1-2 stops and reject the lower 7-8 stops; doesn't it sound as a waste?.

Hi Guillermo,

From a signal processing point of view, I agree. By all means let's exploit the best Signal to Noise Ratio (SNR) parts of our captures, while at the same time looking for efficiency (reduce the number of exposures (= shooting time and storage space) needed).

However, and that's also something to consider, from a practical point of view the overlapping transition/boundary regions are not all bad. In fact they can be put to good use (automatic ghost removal, or noise reduction by averaging).

Quote
In addition to that, depending on how progressive is the fusion software, many shots can easily reduce overall sharpness if many shots are used and they are not milimetrically aligned, because big parts of the final image can end being a weighted average of more than one input RAW file.

Well, assuming stationary subjects (e.g. architecture, especially interiors), alignment should not be that much of an issue. For other scenarios, subject or camera motion can be either a blessing or a curse. In most exposure fusion scenarios it allows for a natural blending by assigning a weighting to the individual exposure brackets, either based on luminosity or contrast. This allows for smooth blends even for handheld images, where the weighted averaging can reduce noise in the transition zone, or increase sharpness in a focus stacking scenario.

Partially variant image content can help with automatic ghost and/or focus detection (or even facilitate super-resolution), or local noise reduction, so it's not only bad news.

So what I'm basically saying is that it depends on the software that one uses for the exposure/focus fusion how well the overlaps are exploited.

Cheers,
Bart
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Guillermo Luijk

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Re: how to shoot HDR
« Reply #35 on: November 22, 2011, 04:07:55 am »

Hi Bart,

In theory averaging can reduce noise. I.e. 2 images, even if one has much greater SNR than the other (because it had more exposure in capture), provide improved SNR if they are properly weighted. In practice I did the calculations of real SNR improvement, and once the exposure offset is 2 stops or more the contribution of the least exposed shot to this improvement becomes negligible compared to the possible drawbacks in sharpness because of the progressive blend. Moreover, to get that improvement the two shots have to be optimally weighted (there is a precise pair of weights that statistically maximises SNR improvement), so arbitrary weighting could even make the result worse than the most exposed image considered alone (e.g. a 50% blend of two shots 3EV apart, has worse SNR than the most exposed shot). Tonight I'll show you the numbers I worked out.

Regarding the loss of sharpness, even shooting on a tripod with mirrorlockup and remote shutter there can be micro-misalignment between the shots, and this doesn't need to be greater than one pixel to reduce output sharpness. Imagine you spend 1.600 EUR on a Canon 24mm TS-II because it can really take advantage of your 2.000 EUR 5D2's 21Mpx sensor, and you loose that peak of sharpness and investment just beacuse your software or workflow is based on a progressive blending.

Regarding the weighted averaging according to luminosity you refer to, I have used TuFuse (Enfuse) which uses this approach. This is no problem at all as long as you provide TuFuse (Enfuse) with images where the sharpness problem has already been solved. For example I blend my RAW files with nearly zero transition areas (>90-95% of the total pixels of the image come from a single RAW file), and make copies of the output at different exposure values letting TuFuse do the difficult fusion task. We have the best of both worlds: max sharpness and control in the fusion process, and automated tonemapping.

I don't see how progressiveness can help reduce ghosting. IMO it's just the opposite, since progressiveness spreads ghosts over wide areas without control over them. I prefer to have cut ghosts and completely eliminate them by acting on the fusion map. This is as simple as forcing all pixels in the area come from a single RAW file, ideally the most exposed non-clipped RAW file in that zone.

Regards!
« Last Edit: November 22, 2011, 04:24:35 am by Guillermo Luijk »
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Guillermo Luijk

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Re: how to shoot HDR
« Reply #36 on: November 22, 2011, 09:21:50 am »

This scene was shot {0, +2, +4}:


100% crops:

.


In the initial fusion, the cars in the road produced ghosting. I only needed to paint in plain gray colour (which corresponds to the middle shot) the street in front of the building in the fusion map (the trace of my rough painting is clearly visible):

« Last Edit: November 22, 2011, 05:16:19 pm by Guillermo Luijk »
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