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Author Topic: taking HDR exposures - when do you have enough  (Read 2821 times)

Marshallarts

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taking HDR exposures - when do you have enough
« on: August 20, 2009, 01:20:03 am »

This is a tough question to ask because I know I'm not going to get the response I'm looking for.  I've been focusing on HDR techniques simply to better understand certain principles, so please don't think too far into this.  I know many people may not like HDR but I don't want to discuss the aesthetic.  I also know most pro's would say "it all depends on what your subject is".  I'm sure those pro's can spot any beginner because the beginners probably haven't even thought about what the subject is, they just want HDR.  Personal opinions and techniques aside, if you want to capture an entire scene (like a landscape) and you know there is more DR than what your camera can capture.  When do you stop with the exposure bracketing?

Here's what I assumed:  If I have what I feel is an evenly exposed picture and I look at my histogram and instead of a slope going down at both ends (suggesting a good exposure) there is still a lot built up and cut off on both ends then I assumed that would indicate a candidate for HDR bracketing.  So I take an exposure -1EV and see if it slopes down in the shadows.  If not I go -2EV and so on until it is.  I do the same on the other end.

Is this correct?  Of course I'm assuming I want *everything* to be exposed properly (or at least not clipped) in this case.  If it were only a few specific subjects I could spot meter those alone and expose.  

I read a lot about people creating HDR with blanket generalizations like must be 3, 5, or 7 exposures, or -2Ev to +2Ev.  When using HDR programs like Photomatix is it anymore helpful to have more exposures within a certain EV range?  Are smaller increments really helpful?
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bill t.

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taking HDR exposures - when do you have enough
« Reply #1 on: August 20, 2009, 02:37:22 am »

It's pretty much as you suggest.  I get the best results when I have a bracket set symmetrical around the best possible single exposure, but a badly exposured bracket set will get me nowhere.  I like an underexposure that places the brightest objects about 4/5 of the way to the right of the histogram, and a corresponding overexposure that opens up the shadows.  In fact for most cases the results from processing 1 through 2 EV steps is pretty similar, but even when you process for a natural looking image 2EV scrunches look somewhat over-luminous to my eye.  Best to shoot the minimal bracket spacing to hold onto the bright areas.

In general the underexposed images are the most important to place properly on the histogram.  The eye accepts clipped shadows more readily than clipped highlights.  It is usually sufficient to simply apply a symmetrical overexposure for the shadows, but if the shadows are seriously clipped to the left either adjust the bracket step accordingly or rethink you choice of base exposure, or if you have "black hole" shadows like a deep cave entrance just accept it.

My best suggestion is to shoot 5 to 7 EV brackets with 1 stop steps until you get the hang of it.  With the Photomatix controls in roughly the default position experiment with selecting various subsets of those.  In theory when you are picking the "best" subsets you will have to make the least adjustments on the controls.  If the image is too dark, try a brighter overexposure element.  If the bright areas are clipped, try a darker underexposure element.  Try to stick to three or two input images, that's especially important if you are using blending rather than HDR.  You will soon develop a feel for what you need bracket-wise.
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MyEcholalia

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taking HDR exposures - when do you have enough
« Reply #2 on: August 20, 2009, 04:45:44 am »

In my experience, shooting a useful bracket set is actually very straight-forward if your camera has a feature to show blown-out highlights on the LCD screen after the shot. If you have this feature, you don't even need a histogram.

Here's how I do it:
- Set up the camera to automatically shoot a +/- 2 stop bracket set on exposure.
- Frame the shot and capture the 3 exposures.
- Check the "highlight warning blinkies" for the 3 shots on your LCD screen. You want no "blinkies" at all on the underexposed shot to make sure you have detail in the highlights, some blinkies on the middle shot, and a lot of blinkies on the overexposed shot, so that you get detail in the shadows.
- If you already have blinkies in the underexposed shot, trash this bracket set and use negative exposure compensation. Then shoot a new set.
- If you have no blinkies in the middle shot and only very few in the overexposed shot, trash this bracket set and use positive exposure compensation to brighten up the next set.
- Repeat until you have a bracket set that works

That's it. If you get some experience with this, it's really only a matter of seconds and usually doesn't need more than 2 attempts. Using this approach, a total of 3 shots +/- 2 stops apart is all I need, both for HDR or for blending work. Let me know if there are questions.

  Regards, Thorsten
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walter.sk

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taking HDR exposures - when do you have enough
« Reply #3 on: August 20, 2009, 09:48:56 am »

I am relatively new to HDR, and started out with the general guideline of starting with 3 images, 2 stops apart.  This worked well in some moderate and low contrast scenes, but not in scenes with sunny areas and deep shadows.  I found that 5 images so far have been sufficient for most high contrast scenes, and I vary the bracket from 1 2/3 stops per shot to 2 or 2 1/3 stops per shot and have got good results.  I start by shooting a test shot to determine whether I need exposure compensation.  If so, I dial it in and then arrange my bracket around that.  I have had no problem with noise in the shadows, as I make sure that the image with the highest exposure really brightens the shadow detail.

I don't always use the 5 pictures in the processing, and I am getting better at deciding which pictures can be left out.

A technique I have tried which seems to work for me is to spot-meter the brightest and darkest parts of the scene to determine the total contrast range.  I then assume a very conservative range for one image as covering 4-5 stops.  So, to lift the shadows off the left end of the histogram by a stop, and to keep the highlights just below the right end of the histogram, in a scene with maybe 9 stops of contrast, the bracket of 5 images at 2-stop differences gives me enough overlap between shots to make for smooth blending.  Again, this assumes that I have applied exposure compensation if necessary.

I have not had the need for a bracket of 7 shots yet.
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Guillermo Luijk

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taking HDR exposures - when do you have enough
« Reply #4 on: August 20, 2009, 09:55:59 am »

I would make it simple: the easier end of the light range to be identified as correctly captured by a digital camera thanks to the clipping information provided are the highlights, correct? so start by the highlights.

1. Expose as much as you can right before starting to clip the highlights of interest and shoot.
2. Now look at the camera display: are there important areas of the scene that you wish to lift in postprocessing to show their textures, that still display underexposed?
    YES?: then you need more shots. Increase shutter by +2EV (or even +3EV), less is a waste, and shoot again. Repeat 2
3. Mix the information from the shots in an optimum way

That's the method. Since it's very convenient not to touch the camera between the different shots, the best thing is use AEB, for example the {-2,0,+2} scheme, and adjust it so that the least exposed shot, i.e. the one named '-2', corresponds to the shot calculated in 1.
To do that you just need to increase shutter by +2EV right after calculating the exposure in 1, and then activate the AEB. Always M mode of course.

If 3 shots were not enough (which is very rare in real life situations if the first shot was a proper ETTR), go on shooting with increasing shutter exposure.

Regards.
« Last Edit: August 20, 2009, 02:52:00 pm by GLuijk »
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Luis Argerich

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taking HDR exposures - when do you have enough
« Reply #5 on: August 20, 2009, 11:05:27 am »

The -2, 0, +2 recipe works sometimes but is a nonsense as a catch-all. The problem is not the intervals but the center, centering at 0EV is right only in some scenes. Guillermo's method is much better with the only exception that sometimes you can deal with some burnt highlights, the sun (if included in the scene) or some street lights can be burnt in some images.
IMO the key is to make sure the shadows are well lifted in the more overexposed shot, I see a lot of failed HDRs due to underexposure, specially at night.

Marshallarts

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taking HDR exposures - when do you have enough
« Reply #6 on: August 25, 2009, 01:25:54 pm »

Thanks for all the responses and I'm so sorry I haven't replied back yet, I've been very busy and haven't had time.  

Guillermo actually changed my opinion about HDR all together in responses he had to an earlier thread I started.  His technique loosely described here and elsewhere using "Zero Noise" appear right on in terms of a noise free image.  But when I came to learn HDR I believe I was originally after the HDR aesthetic of high-dynamic range (though I don't like the phony look achieved using certain methods).  I get confused with terminology here.  Would this HDR be "globally larger DR" or is it technically reduced DR? it seems with more DR in an image that is still 8-bit you in fact reduce contrast globally even though it appears the DR is increased... My apologies, I find it hard to convey this and terminology trips me up.

It seems Guillermo's method really addresses the issue of noise, being to properly expose highlights then shooting to gain detail in the shadows.  But from what I've seen it doesn't produce the HDR style images I've seen (and am trying to explore), it just looks like an perfectly exposed normal image .  I realize as I learn more experimenting with HDR I will probably begin to realize I don't like it.  I don't feel I'm on the level of those who prefer exposing for noise free versus HDR like Guillermo.  I would love to be enlightened if you have feedback on this philosophy!  

I wish I could list all the ways I've found, the past few weeks through research, to create HDR-like images (even the term HDR has been subject to debate).  I've seen the tone mapping methods with HDR.  Exposure blending (I feel more recently they call this exposure fusion--what's the difference?).  WLD or WDR?  Pseudo-HDR from one exposure (in my testing it seems very obvious processing multiple exposures from one photo does a better job at pseudo-hdr than relying on programs like Photomatix to do it automatically from one photo----but I'm not sure why it can't do it by itself on it's own.)  Each method appears to have it's own look which is overwhelming.  Others, like yourself, seem to not address the HDR I'm speaking of and refer to the process of correcting exposure--through layers or other techniques that create awesome photos but not that expanded DR look I've seen in HDR (people have said this expanded DR look actuality flattens the tone curve.  As you can see I'm having a disconnect here!)

But specifically my situation now, I am making time lapses and want expanded DR. I find the workflow of HDR easier since I need to do this as a batch of 500 images at a time.  What would you do in this example?  The first is exposed average for the whole scene--the sky was blown out but without spending too much time correcting and for the sake of this example I just used Auto exposure in ACR.  The second is a (-2, 0, +2) bracketed photos I tone mapped with defaults in Photomatix (Its default, I didn't bother to work with it to make it more realistic but it'll give the expanded DR that I'm talking about)

I hope this makes sense or at least you can see my many questions and confusing here.  I would greatly appreciate some guidance not only with your process but addressing my confusion.  Thank you!
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Luis Argerich

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taking HDR exposures - when do you have enough
« Reply #7 on: August 25, 2009, 02:03:30 pm »

What is your question then?

Marshallarts

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taking HDR exposures - when do you have enough
« Reply #8 on: August 28, 2009, 01:14:14 pm »

Quote from: luigis
What is your question then?
Well, after rereading all this feedback it sounds like my question has been well answered.  Instead of asking more questions I should be trying out my techniques.  Because it takes a long time for me to do this I get frustrated after I finish when I learn a better way I should have done it in the beginning.  So I sort of obsess about knowing it all 100% before I start.... something I need to get over.

thanks everyone for all your feedback!
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