[ozlizard]

I read on here somewhere that by using multi exposure on the K 20 you can acheive the same effect as using an ND filter with long exposure times. An example of this is to get that silky water look. I have tried this technique out myself and was very impressed with the results using 9 exposures. Anything moving (water, clouds) had that dreamy look and anything static (rocks etc) were as sharp as a tack. Great effect! Now there is a question at the end of all this....how does this feature differ from the 3 shot HDR on the K7?
(BTW I explained this technique to a local Canon dealer and he couldn't believe it...never heard of a camera that could do that he said...chalk another one up for Penatax and the fab K20!)

[bymy141]

I read on here somewhere that by using multi exposure on the K 20 you can acheive the same effect as using an ND filter with long exposure times. An example of this is to get that silky water look. I have tried this technique out myself and was very impressed with the results using 9 exposures. Anything moving (water, clouds) had that dreamy look and anything static (rocks etc) were as sharp as a tack. Great effect! Now there is a question at the end of all this....how does this feature differ from the 3 shot HDR on the K7?
(BTW I explained this technique to a local Canon dealer and he couldn't believe it...never heard of a camera that could do that he said...chalk another one up for Penatax and the fab K20!)

The silky look with moving water etc is done with either multple exposures (with the same exposure levels) or with a long shutter time on a tripod.
HDR is something different. It has been discussed several times before on this forum.
HDR stands for high dynamic range. Dynamic range is measured in "stops".
In the old days a lens would have aperture stops, while on the camera there would be a dial that had shutter speed stops. Changing 1 stop creates less or more what we now call Exposure Value (Ev). A typical APS-C sensor has approximatly 10 to 12 stops dynamic range.

A camera sensor can only "see" a limited range of light and darkness at any one time. Your eye can see a much wider dynamic range. And LCD screens and printers are even much worse in their dynamic range.

The idea of HDR is that by under-, right and over exposing an image you can combine these images into one image. The underexposed one will add extra high light DR, and the overexposed extra dark/shadow area details.
In order to enable more DR in an image also a special file format was introduced. Your Pentax camera is not using that.

After the HDR image is created, you have all this extra high light and shadow detail. Now you want to see it in a JPG image on your screen. Since both JPG and much more your screen / printer cannot cope with that DR, you will need to do dynamic range compression (tone mapping). Most HDR software packages have plenty bells and whistles to this, creating this special HDR look.
My K-7 has no parameters to manipulates its hdr algorithm. Unfortunalty.

If you'd use your K20D and an HDR software package, you will see that the results can be much better than what comes straight out of a K-7.

I hope this helps you understand HDR somewhat better.
Here is a K10D HDR example:



- Bert

[ozlizard]

Thanks for that explanation but that was not my question. I understand how HDR works and have tried a few "experiments" but what I really wanted to know was how does the multi exposure function on the K20 differ from the HRD funtion on the K7?

[Lowell Goudge]

multi exposure is deliberate under exposure by 1/n where n is the number of shots you take, so that the total accumulated light is correct for the entire scene. Exposure over the entire scene must be within the 4-6 stops of good dynamic range of the sensor / film

HDR is multiple exposures where each physical section of the shot is exposed correctly, with the other areas being either over or under exposed, then the shots are overlaid, using the detail where it exists in each of the shots.

as a result, you would only get the silky parts you see with long or multiple exposures where there was detail present in all 3 layers of the HDR shot

[ozlizard]

Thanks Lowell, I find that using this technique produces shots with perfect exposure, probably similar to the results you may get from the extended dynamic range function on the K20 only I think much better. Very impressive results. Example below taken with 9 exposures.

[insulinguy]

i have tried this a couple of times with the K20 - with little success. does the camera merge them together or do you do it PP and if so - what are you using? i know stacking layers in CS3/4 is one way to do it - but havent tried. when you take the 9 shots does the k20 then make 1 file??

[ozlizard]

Yes it takes 2 to 9 shots (whatever you choose), I just leave it on auto ev and you then press the shutter however many times you selected then it merges all the images into one. This image was enhanced using Topaz Labs clarity setting which is pretty mild. What impresses me is the good exposure. I only use 9 shots to get the water effect but 3 seems to really work well for improved dynamic range.

[pentaxian_tmb]

Sorry for beating a dead horse, but now I am confused. I thought the multi-exposure function allowed you to exposure bracket a shot -1, 0, +1 and then overlaid all three (or however many you took) images on top of one another. --Essentially doing in camera what you can do in photoshop.

Assuming each of those three shots correctly exposes the various parts of a scene--how is that different than HDR?

[mithrandir]

Another bonus of using the multiple exposure mode is that it lowers the noise considerably. With the camera setting the exposure for each shot to cumulatively add up to the correct exposure for the final combined shot. Example 9 shot multiple exposure at 100 iso means the camera is combining (9) 11 iso (or thereabout) shots to get a final product with 100 iso.

[mithrandir]

Exposure bracketing produces individual frames (it doesn't combine them in camera). HDR mode on K-7 does combine three shots to produce a wider dynamic range in the resultant picture. Multiple exposure mode does not combine different ev levels, it just divides the ev (1/n) by the number of shots so that the final (combined picture) equals the total of all the 1/n's.

[pentaxian_tmb]

Ahhh. I understand that if you set to auto EV and take three shots, each shot will be at 1/3 exposure of the final image. ...But, isn't it possible to uncheck the auto ev feature and then change the exposure so that you shoot 0, -1, +1 ? In the latter case you would wind up with 3 seperate images of the same scene at three different exposure levels merged into one image in camera.--right?

[StigVidar]

Ahhh. I understand that if you set to auto EV and take three shots, each shot will be at 1/3 exposure of the final image. ...But, isn't it possible to uncheck the auto ev feature and then change the exposure so that you shoot 0, -1, +1 ? In the latter case you would wind up with 3 seperate images of the same scene at three different exposure levels merged into one image in camera.--right?

That's correct, but you will end up with a sum of all images and in that case it will be even more overexposed than the single +1 image. HDR isn't like this at all. HDR is using dark parts of one image and ligth parts of another. This isn't a sum of all the light levels, but more like the mean. The image is flattened and appear to have less contrast.

[pentaxian_tmb]

I underdstand now, thanks.

[insulinguy]

ok - i didnt have the auto ev checked - therefore it was making 9 different files...thanks for the explanation - now it makes perfect sense.

[falconeye]

Thanks for that explanation but that was not my question. I understand how HDR works and have tried a few "experiments" but what I really wanted to know was how does the multi exposure function on the K20 differ from the HDR funtion on the K7?

There is a unified view on this.

Both, multi-exposure and bracketing, gather more light than a single exposure can do.

In the case of N exposures it is clear: you gather N x as much light.

In the case of 5x2EV bracketing, it is like 1 + 4 + 16 + 64 + 256 = 341 x as much light. For N<341, the bracketing method should win.

(The K-7 HDR does 3x3EV like 1 + 8 + 64 = 73 x.)

However, bracketing can lead to artifacts at contrast boundaries and the longer exposures may need a tripod. Therefore, multi-exposure can yield great HDR results (I use N=16 in such cases (just burst them away )).

So, in the first step, multi-exposure and HDR both yield high dynamic range pixel data. In the case of multi-exposure, by adding all pixels and dividing by N. In the case of bracketing, by cleverly choosing and normalizing the "best" pixels.

Now, the K20D/K-7 multi-exposure stops here and writes a final JPG by linearly(*) cutting the high dynamic range pixel data into 8 Bits.
(*) Well, JPG uses sRGB which compresses 3000 linear levels into 8 Bit...

The K-7 offers the second step in HDR processing as well: tone mapping.

The high dynamic range pixel data are non-lineearly mapped onto the 8Bit output space, by both compressing global contrast and expanding local contrast. The K-7 offers 2 different "strengths" to chose from.