Hi Bob,
interesting calculations, but I think you have one major flaw in your assumptions. You state: "Let’s say that the time to make all these exposures is 10 seconds."
With modern DSLRs this might be the case for long exposures and night shots but e.g. my EOS 40D is able to shoot at ~6 fps (the EOS 1D Mark IV even at ~12 fps) at full speed. So your 10 seconds change to .5 seconds for a series of -2EV,0EV,+2EV (0.25 or 1/4s respectively)!
In addition, I would assume that for most typical landscape shots clouds might be further away than a mile, so proper alignment should avoid stuttering.
If you want to push it to the limit, you could perhaps limit yourself to 2 exposures, which would push it to 1/8s which should even allow portrait shots (although perhaps with a higher reject rate - but shutter speeds of 1/8s and even 1/4s is not unusual in available light photography!)
Curious what you think about that.
Cheers,
Lars
My answer is as follows:
Hi Lars,
Whatever you're doing your gallery shows that it's successful. I did use a large number for the time it would take to make multiple exposures. Great cameras like yours allow for much less time to accomplish the same number of exposures so the problems I outlined would be minimized for you.
Two points:
1. I shouldn't allow myself to be drawn into the 'average cloud thing' but I can't resist. So, in the spirit of fun, let's say that the average cloud height is about 2000 feet. And let's say that the angle to the nearest cloud in the landscape photo that you're taking is 45 degrees. Simple trig tells us that the hypotenuse of the resulting triangle (the hypotenuse is the direct distance from the camera to the nearest cloud as defined above) is 2828.4 feet. If that cloud is moving faster than about 1.92 miles per hour (in parallel with the sensor plane) then that's a displacement of 1 pixel in half a second. (I'll leave the math to you, Lars). It's not unusual for a cloud to travel about 20 mph and those conditions would translate into a 10 pixel offset even in half a second. These numbers are full of assumptions and I'm not trying to prove that you will necessarily have a problem with your equipment. But the fact is that we see a lot of cloud displacement (and foliage displacement) in multiple image HDR. It must be caused by something (perhaps it's not coming from Canons?) and the math (even highly speculative math like this) suggests what it is. I'm just not personally optimistic about such a technique but that means nothing if you can make it work.
2. I want to approach this from a different angle. Let's say that you wrote me a letter and said "Bob, I invented this great new way to shoot photographs! Instead of using f8 at 250th of a second for normal lighting, I've pioneered what I call the 'slow shutter'(tm) technique. The benefits of a small f-stop are so obvious to me that I always shoot at f78 at one/half second even, I repeat, in normal lighting conditions." And then you would carry on, perhaps, about the incredible depth-of-field and that everyone should always shoot at speeds of 0.5 seconds or even longer. Well, if I received (or if you received) such a letter we would suspect that our correspondent was cracked. But that's what you seem to actually be advocating.
However, I know that the f64 school of American photographers (Weston and Adams and the like) actually did shoot this way so maybe I need to go back and review what they actually did and what problems they encountered. Perhaps you would be interested enough to look into it and write back about what you find? Thanks for your letter.
Best,
Bob
Hi Bob,
ReplyDeletelet me say two things before I come to your points.
First, I didn't want to put your calculations down, I am also interested why we see these artifacts in some (most?) HDR images and how to avoid them.
However, being a scientist I just was not completely convinced about the assumptions you make for the calculations. This does not necessarily mean that the conclusion is also wrong, but thats why we are discussing here.
Second, foliage displacement is a totally different thing since foliage is typically much closer than a cloud and there is a lot of random movement. My point was about the assumptions for cloud movement! ;-)
1.)
Again I am not convinced by the assumptions you make. Cloud heights can be from ~1000ft (anything not touching the ground -> fog) to 25000ft so an avarage of 2000ft is again (as you noticed allready) highly speculative. If the assumptions would be true you should frequently see cloud displacements of >100px for longer (5-10s) exposures. Did you encounter that?
Anyway, since I first read your post I've realized, that (cloud) displacement is a much larger issue than I thought it would be. So even if the assumptions are highly speculative or way off I have to admit that speed of movement is the most likely explanation. Sometimes you are easily fooled by even slow (looking) movements like e.g. in moon photography where you also roughly need a 1/focal length exposure time even on a stable tripod to avoid blurring due to moon-movement and earth rotation.
2.)
I actually didn't want to advocate to shoot slower than possible, but question your statement, that multi-exposure HDR is impossible for people and landscape.
Maybe we can agree on the following: multi-exposure HDR is
-impossible in sports photography
-almost impossible if fast and close movement is involved (foliage)
-difficult (producing a lot of potential rejects) if clouds are involved
For shooting people it highly depends on the scene. If you can arrange a still scene like Ryan Brenizer is using for his amazing low DOF shots[1] where people mustn't move for about 20-30s it can easily be done. For normal portraits in really difficult lighting situations I'd still advocate for the option of shooting multi-exposures perhaps with -1,0,+1. This not only increases the chance of having a correct exposed image but also leaves the option for HDR if no movement is present or pseudo-HDR from the most suitable exposure.
So, whats the verdict? We know why there are a lot of HDR pictures of the typical non-moving subjects and scenes and why many serious photographers stay away from HDR except from pseudo-HDR from a single image which can still be beneficial for some scenes. As a possible solution on the software side I could think of a correction brush, with which you can mask regions of the image that should only be taken from a given exposure to correct for movement. Would involve some additional work, but you could even deal with foliage. That's basically the technique Trey Ratcliff aks stuckincustoms used for his popular Times Square HDR[2] but since there is no correction brush (yet) he had to do it offline with Photoshop and layer masking.
For now my conclusion is that I'll stick to pseudo-HDR for most of the cases now (the pseudo-HDRs at the end of the HDR-tutorial[3] pretty much show what's possible allready) and only shoot bracketed exposures when I am pretty sure about little movement in the scene, or know that it is really worth spending time masking moving objects in the post-processing. Not a too big problem since I don't like the overprocessed, artifical look of many HDR images anyway. Once there is a software available with "movement correction brush" I'll read it on your blog give it a try then! ;-)
Cheers,
Lars
[1]
http://bit.ly/soGXa
http://bit.ly/ghsDU
[2]
http://bit.ly/9OImBP
[3]
http://bit.ly/4EVyWZ
Hello Lars,
ReplyDeleteI know you weren't dissing my calculations. The use of math in these circumstances is just to allow us to get an idea of the size of a problem and it does not allow real precision. I used the word 'speculative' about my math but it isn't. It just has to make many assumptions. Each person will want to perform these calculations for his own equipment and situation. When I look at multiple image HDR I usually see an irritating fuzziness and this results from movement between exposures and movement in the subject itself. My calculations will, as I say, let us know what the result will be for errors in n pixels whether the camera moves or whether the subject moves.
Remember that these statements of mine are just my opinions and, as I said before, if you can make the technique work in a way you like for either landscapes or portraiture or anything else then that's great.
You know, I started this blog because I saw a lot of immature and defective HDR work (blurry and oversaturated) and yet people were raving about it. Right away it struck me that MI HDR would never catch on because it's like making a movie in which you bet that nothing moves. Some HDR practitioners seem to me to be forming a clique in which shoddy work is praised to the skies by the in crowd just because it's got some HDR look. I want this blog to critique pictures accurately and be honest enough to say what the problems are and I hope to encourage my readers to do the same. But I'm not a dictator either. It's not my intention to tell people what they must do but I do want to make suggestions about what can be done.
I want to thank you for introducing me to Ryan Brenizer whom I didn't know about. He has a wonderful look. This is just my ignorance but why do his shots take 20-30 seconds to produce? Probably there's something I should have read which I missed.
I really don't believe that post-camera HDR (either in single or multiple image forms) has much of a future. The higher end cameras are already improving in the DR they can capture and some also feature a way of mapping DR extremes into a more moderate range. The future of enhanced DR capture is in the camera. At a guess, at the end of five years (to 2015, say) we'll find extreme DR handled entirely in the camera and the silly mating of multiple images will (once again) be a thing of the past.
Trey Ratcliff is a fabulous photographer but remember that there's a lot more going on in his photos than HDR. He's a master craftsman and a fine artist. Half of his magic consists of infallible scene selection and – I suspect – ruthless editing. I read the tutorial you pointed me to and I couldn't suppress a smile. This complex process which is really just a form of anti-ghosting (i.e., another way of saying 'single image HDR') makes my point about the futility of MI HDR crystal clear.
Thank you for your very interesting and informative letter.
Bob