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Multiple Exposures, HDR and the D90

Daniel Joder , Mar 24, 2010; 01:38 p.m.

Ok, I have just searched the threads and found some discussion, but nothing that completely satisified my curiosity. I have a D90 on the way from B&H (can't wait!) and one of the things I want to try is using the multiple exposure capability to create a sort of in-camera version of an HDR image. This would be with higher contrast landscapes and cityscapes mostly. Has anyone done this extensively with the D90 or any other DSLR? Is it worth it? Would you see a significantly better exposure with more tonal range as compared to simply taking one well-exposed image and adjusting curves, levels, etc. in Photoshop? I would think if it worked well I would see a lot more discussion about it as a landscape photography technique, no? Would such an image look more "natural" than a true HDR image? I am somewhat of a beginner with this digital stuff so maybe I am missing something here...???

Responses


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Dieter Schaefer , Mar 24, 2010; 01:52 p.m.

While you can take multiple exposures with your D90, there is no option to create an in-camera HDR with them. HDR involves creating a 32-bit HDR image followed by tone-mapping - no such capability in-camera with the D90.
Using multiple exposures too essentially lower overall contrast and dynamic range of a scene could be rather tricky in camera too - usually graduated ND filters are used for the purpose. You certainly have more flexibility by bracketing your shot and blending them later in post processing.

Daniel Joder , Mar 24, 2010; 02:17 p.m.

Yes, I knew that what you would get would not be an HDR image...I just wondered if it would be significantly better (a "poor man's HDR"?) than simply one well-exposed image on which you later worked curves/levels. From what you say, Dieter, the much better and easier answer, then, is to work the filters and/or blend several images later in Photoshop. So, multiple exposure technique not worth the effort? Maybe that's why I haven't seen much discussion of it... : )

Sjoerd Leeuwenberg , Mar 24, 2010; 03:10 p.m.

I think it is a much better idea to do the combination in photoshop or dedicated program.
But, I did have an idea I wanted to share. You could use multiple exposure (record menu) in combination with exposure bracketing, except that I just found out that nikon blocked that. No bracketing in combination with multiple exposures. Another way that might work (it works, but will the results be nice) is to use the retouch menu and there you combine two pictures into one (raw works best) and then use some exposure bracketing to take the pictures. i think you have to dial the exposure down a bit, because the exposures add up. Don't know if the result will actually contain more dynamic range. good luck.

Sjoerd

Dieter Schaefer , Mar 24, 2010; 03:12 p.m.

Daniel, I am not quite sure I understand what you are after. You can certainly use multiple exposures to build up one well-exposed image - but the sum of - let's say four exposures - will be identical to the one taken with a single exposure (motion effects excluded). With multiple exposures, you can't make selective changes to the overall images - or in other words, you can only add light but not subtract. If four exposures added together give a well-exposed image - but you'd like to get more light/detail in the shadows - then with adding a fifth exposure, you just waved your highlights good bye.
Lowering overall scene contrast while shooting requires graduated ND filters (or fill-flash). You can duplicate the ND filter effect by bracketing your shots and blending them later in post processing. Personally, I prefer the latter option as I don't like to handle filters in the field and feel I have more options and control doing it in post processing.

Daniel Joder , Mar 24, 2010; 03:35 p.m.

Ok, maybe I asked a question that was too advanced for my newby photo knowledge to allow me to understand! However, Dieter, your comment about "you can only add light" seems the key to the answer. So, I think I'll just keep working on getting the best possible exposures AND I'll especially keep working on my post-processing skills (a bit lacking at this point), as both you and Sjoerd recommend. Thanks for the help!

Rodeo Joe , Mar 24, 2010; 06:51 p.m.

Dieter, I'm puzzled by your reference to a "32 bit HDR file" above. Surely the only legitimate saveable file formats are 24bit or 48 bit? And all file formats are automatically converted to 24 bit when displayed on an RGB monitor anyway, so I don't see any reason why an HDR image can't be tone-mapped to 24 bit and saved as a Jpeg (Yukkk!).

I've created pseudo HDRs from 14 bit RAW files, which can easily cover a 12 stop SBR, and never had to work with 32 bit files. Simply create two "exposures" in 16 bit mode using ACR, open them in Photoshop, adjust the tone curves and then combine them using the layer value blend mode. The image mode can then be changed to 8 bit depth without a problem. Job done!

Rodeo Joe , Mar 24, 2010; 07:26 p.m.

Here's a quick demo of what can be done even starting from one jpeg exposure, using just an 8 bit application like the GIMP. The image was split into two superimposed layers, the top layer was "brightened" in the shadows using the curves tool and then the two layers were blended using "value" mode.

The two pictures side-by-side are the individual layers, and as you can see the highlights have been "blown out in the righthand layer, while the shadows are too dark in the left layer. The combination brings back the detail in the highlights as well as lifting the shadows. And all this was quickly done at 8 bit. Much better results and far more drastic shadow lifting can be done from a RAW file.


The individual layers

Rodeo Joe , Mar 24, 2010; 07:30 p.m.

And here's the combination.

Luke Kaven , Mar 24, 2010; 08:50 p.m.

Daniel, you've walked into some interesting possibilities.

1) Using multiple exposure in-camera -- multiple underexposed images: there is some slight noise reduction possible when stacking up multiple (underexposed) exposures, but they don't add up to much more than the results of one good exposure at a correspondingly lower ISO setting.

2) Using multiple exposures for averaging: This involves taking multiple exposures (all "properly exposed") and using an averaging function with them in post processing (eg., Photomatix). This will reduce noise somewhat even at lower ISOs, and it will in fact improve your shadow response quite a bit. The numbers will in fact be robust enough to begin working in HDR formats (32 bit floating point per pixel). From there you can do a lot with tonemapping.

3) Pseudo-HDR: Don't bother trying to fake out an HDR program with artificially bracketed exposures. It's barely worth the effort.

4) Real HDR: Bracketing even as few as 3 exposures (+-2EV recommended) will give you enough to create an HDR dataset that will allow good tonemapping.

It's a good time to learn some new things. For example, on your camera, ISO1600 will give you better shadows than ISO200. An image made from ISO200 (highlights) and ISO1600 (shadows) is much better than an ISO200 exposure, with the appropriate crossover function.

I'd strongly recommend picking up the demo version of Photomatix when you get your camera, and putting it through its paces. It will do averaging as well as HDR, and both have their uses. Welcome to the land of absolute light magnitudes, as opposed to the relative magnitudes of traditional photography which put black and white relative to a fixed exposure window.


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