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04-04-2009, 11:04 PM   #1
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Boosting High ISO Performance

While playing around with the K20d's "Multiple Exposure" feature (p. 103 manual) I accidentally discovered something. There is more to learn I am sure, but it is immediately clear that shooting multiple exposures greatly enhances image quality, even @ high ISO.

To demonstrate this effect, I have below 2 large crops, from 2 separate images, both of which were shot @ ISO 6400 on the K20, with identical exif i.e., same aperture, same shutter speed, %100 controlled lighting. The only difference is one of the images is the result of using the Multiple Exposure feature of the K20. With this feature, you can use up to 9 shots, piggy backed to create one image. But, to demonstrate here, I only took 4 shots for the ME image below. Having experimented, I am sure more shots bring even better results. Just look @ the crops (close to %100) @ 6400. I think this is interesting and it implies lots of potential. I am not sure how much it improves high ISO shooting, but it is by a whole lot. Also, even at 100 ISO, dynamic range and noise seems improved.

I would appreciate feedback from anyone who understands the ME tool, or from anyone who just wants to comment. I next want to see if this tool will allow me to ME @ multiple apertures. There is no post processing here.


Last edited by Jewelltrail; 09-25-2009 at 09:06 AM.
04-05-2009, 12:49 AM   #2
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Multiple exposure averages several images so it improves the noise/signal ratio.
Noise is random so the more shots you average the lower the average noise. (for example the same pixel might have as a result of noise lower green value in one image and higher green value in the other, so it averages out to approximately zero). Ideally if you took a very large amount of pictures, the random noise would be 0 (but you would still have some noise that is not random, but caused by sensor and electronics defects etc.).
Signal on the other hand (the actual image of the scene) is always the same (unless camera moves etc.).
04-05-2009, 12:54 AM   #3
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You can also use image stacking software, or, just good ol' photoshop to achieve the same effect.

However––and I remember posting this same thing somewhere... maybe here, maybe on Flickr, I forget––but anyway, the thing is I see very limited use to this, because in order for this to work, you would have to be shooting at still life; nothing in your frame can move.

But, if that's the case, why not use a tripod and shoot @ ISO100 and long exposure?


The only use I can see for this technique is if you're shooting something (still life) when you don't happen to have a tripod (or are not allowed to use one) in a dark place. But then, you would have to use something (like Photoshop) to align your photos, as you would be shooting handheld.


So perhaps if you're in a museum with dark lights, where they allow photography, but no tripods––then, this technique (not multiple exposures for 1 frame, but taking multiple photos and later aligning and stacking them) will become useful.

With no disrespect––I think it's definitely a cool concept (a very similar concept of which is used a lot these days in astrophotography), just of limited utility in practice.
04-05-2009, 01:21 AM   #4
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Umm...

In principle, your proposed technique is somewhat similar to the multiple pass mode of the film scanner, which averages out noise for more samples.

However, as for Multiple Exposure to be used, you have to turn down the EV for underexposing each shot. The more number of shots for the ME, the more underexposure will need.

With more underexposure, each shot with very high ISO can be even more noisy, as they are exposed to the left. So, adding noisy shots would not really give you a less noisy combined picture, but the reverse is probably true.


QuoteOriginally posted by Jewelltrail Quote
While playing around with the K20d's "Multiple Exposure" feature (p. 103 manual) I accidentally discovered something. There is more to learn I am sure, but it is immediately clear that shooting multiple exposures greatly enhances image quality, even @ high ISO.

To demonstrate this effect, I have below 2 large crops, from 2 separate images, both of which were shot @ ISO 6400 on the K20, with identical exif i.e., same aperture, same shutter speed, %100 controlled lighting. The only difference is one of the images is the result of using the Multiple Exposure feature of the K20. With this feature, you can use up to 9 shots, piggy backed to create one image. But, to demonstrate here, I only took 4 shots for the ME image below. Having experimented, I am sure more shots bring even better results. Just look @ the crops (close to %100) @ 6400. I think this is interesting and it implies lots of potential. I am not sure how much it improves high ISO shooting, but it is by a whole lot. Also, even at 100 ISO, dynamic range and noise seems improved.

I would appreciate feedback from anyone who understands the ME tool, or from anyone who just wants to comment. I next want to see if this tool will allow me to ME @ multiple apertures. There is no post processing here.


04-05-2009, 02:42 AM   #5
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If you compare a stack of 4 ISO 6400 exposures, you should compare it to one ISO 1600 exposure.

Already mentioned is that this technique is very common in astrophotography, also for reducing bad seeing and tracking errors.
Free software for stacking is registax.

George
04-05-2009, 04:36 AM   #6
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Time to go back to school and learn more about the Pentax K10D/K20D, Rice High. It does compensate EV automatically, and your reasoning about more noise is flawed.
04-05-2009, 05:01 AM   #7
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@RiceHigh:
Two easy steps in correct order:
1) Try
2) Talk
04-05-2009, 06:35 AM   #8
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Thank you, everyone, for your comments. I agree there is limited utility in this feature, and that is one reason I posted: I am hoping to get ideas for more utility. I can see using the feature in Night shooting, perhaps of cityscapes. Or, when trying to capture lightning strikes; 9 successive shots should increase odds of a good capture. What about the increased Dynamic Range--is it my imagination, or does this process improve that? If it does, woudn't that alone be cause to shoot with this feature in certain situations? Landscapes come to mind.

The first crop posted is the ME shot, the second one is, of course, the single shot.


I have a question. It looks like the features also allows me to take multiple shots, at multiple aperture settings. The trouble I have here is adjusting the aperture value forces me to touch camera (wheels) between shots, which creates slight movement--hence, blurry final image. Any ideas here?

I am aware of software which allows us to do a similar thing, without the ME feature. Also, it seems clear this feature is not completely unlike the concept in HD processing. Thanks for the comments about astrophotography--I wasn't aware of this.

QuoteQuote:
Geporge: Already mentioned is that this technique is very common in astrophotography, also for reducing bad seeing and tracking errors.
Free software for stacking is registax.
Thanks George--I do not have any software for this.

QuoteQuote:
procyon Multiple exposure averages several images so it improves the noise/signal ratio.
Noise is random so the more shots you average the lower the average noise. (for example the same pixel might have as a result of noise lower green value in one image and higher green value in the other, so it averages out to approximately zero). Ideally if you took a very large amount of pictures, the random noise would be 0 (but you would still have some noise that is not random, but caused by sensor and electronics defects etc.).
Signal on the other hand (the actual image of the scene) is always the same (unless camera moves etc.).
Thank you, very much, for all this information!


Last edited by Jewelltrail; 04-05-2009 at 06:41 AM.
04-05-2009, 06:39 AM   #9
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QuoteQuote:
Rice High: In principle, your proposed technique is somewhat similar to the multiple pass mode of the film scanner, which averages out noise for more samples.

However, as for Multiple Exposure to be used, you have to turn down the EV for underexposing each shot. The more number of shots for the ME, the more underexposure will need.

With more underexposure, each shot with very high ISO can be even more noisy, as they are exposed to the left. So, adding noisy shots would not really give you a less noisy combined picture, but the reverse is probably true
.

Rice High, did you take a second to look at the 2 crops I posted? Please do this, and you will see the first one has much less noise than the second one does.

Also, it is not my "proposed technique." It is a feature of the camera---see p 103 of the K20d manual. Clearly, you are %100 wrong about the final image--it does produce shots with lower noise. I think you would be the only person to argue this. Have you ever used the K20?
04-05-2009, 06:48 AM   #10
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QuoteOriginally posted by flyer Quote
Time to go back to school and learn more about the Pentax K10D/K20D, Rice High. It does compensate EV automatically, and your reasoning about more noise is flawed.
How come the school has the K10/20 to learn?

Whilst I do appreciate your information and correction, I don't appreciate the leading words you use, which are unfriendly and redundant.
04-07-2009, 12:07 PM   #11
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I just tried this out and 9 shots with ev compensation at iso 6400 looks almost exactly like a iso 1600 image to me noise wise... which is pretty good really.

so 1600 x 9 should come down to 400 iso

and 800 iso should come down to 200 level.

However cool...outside of star photos with some kind of tracking device I dont really see the point of it. (and even then with a tracking tripod you could just use long shutters again?)

We need to have a brainstorm for potential uses of this!
04-07-2009, 01:35 PM   #12
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QuoteOriginally posted by RiceHigh Quote
How come the school has the K10/20 to learn?

Whilst I do appreciate your information and correction, I don't appreciate the leading words you use, which are unfriendly and redundant.
Strange how quick you can be at criticizing, but how you hate to be criticized. When you keep sending crap right and left, you shouldn't be surprised when the wind turns around and brings it back.
04-07-2009, 10:58 PM   #13
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QuoteQuote:
Werticus: I just tried this out and 9 shots with ev compensation at iso 6400 looks almost exactly like a iso 1600 image to me noise wise... which is pretty good really.

so 1600 x 9 should come down to 400 iso

and 800 iso should come down to 200 level.

However cool...outside of star photos with some kind of tracking device I don't really see the point of it. (and even then with a tracking tripod you could just use long shutters again?)

We need to have a brainstorm for potential uses of this!
I did many shots at ISO 100 & the final pic was so clean of noise that I was amazed--it looked almost not real, and seemed to lack sharpness. Thanks Werticus, I too am searching for a practical use for this--I too thought brainstorming here might get us one.

I believe Night scenes, since dark shadows really highlight noise, would be a boon for this kind of shooting. Sure, you could take longer exposures, but there would still be heavy noise in the shadows.

I also think there would be an advantage in Dynamic Range in ME shooting--can anyone with technical expertise explain id this is so or not? Thanks.
04-07-2009, 11:07 PM   #14
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I think D range would benefit as well. since that seems to add a bit of noise imo

also you could take multiple low iso images with d range, and long exposure and high f stop for crazy clean images.

I might take one now Just to see how clean it can be.
04-07-2009, 11:11 PM   #15
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QuoteQuote:
Rice High:

How come the school has the K10/20 to learn?

Whilst I do appreciate your information and correction, I don't appreciate the leading words you use, which are unfriendly and redundant

高的米: 您有许多好想法,但是我建议您是有點關於Pentax的更多positve。 因為您是很快投入Pentax名字下來,許多論壇成員不严肃对待您。 嘗試更加支援的方法,并且您將使更多人民听您,不用他們变得很惱怒。 歐內斯特

" 当水制服fire.",人类制服无情; Mencius 6A :18.
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