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Old Dec 29, 2005, 5:22 AM   #1
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In order to be able to shoot more pics on my next long trip (one month) I am considering (besides purchasing addition SD cards) to shoot in 5 MP at compression FINE.

I have read an article which mention actually that the getting down from 7MP to 5MP (called downsampling) actually has a very negative impact on the end result, and much worse than using a higher compression of the JPEG. Explanation for this is that the compression algorithm remove 'unrelevant' pixels while downsampling applies an uniform pixel removal. (http://www.neocamera.com/feature_size_vs_quality.html).

My question is the following: am I better to take 7MP at NORMAL or 5MP at FINE ? According to the reviews, average size of 7MP at NORMAL is approx 950 KB and average size of 5MP at FINE is 1.4 MB.

Which of these settings will provide me with the best quality ? I expect to print at 13x18 cm, but I may want to enlarge the good shots.
Thanks a lot.
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Old Dec 29, 2005, 8:19 AM   #2
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I am surprised to here that the A620 compresses 7 MP down to 1 MB. For my taste that's a bit too much compression. As a consequence you may get jpeg artifacts. Thus I would recommend this compression only with ISO 50, when noise is minimal. Otherwise 5 MP isn't bad at all. It will still be better than the A610, because demosaicing artifacts are lower. By downsizing you will reduce noise, demosaicing artifacts, and sharpen at the same time.
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Old Dec 29, 2005, 1:02 PM   #3
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We have a SD550, 7.1MPand I bought two 512 MB memory cards for it. Shooting at full 7.1 MP resolution in the lowest compression (super fine) will give 156 photos per card or 312 photos total. Plus the nine full res photos the included 32MB card can hold totals 321 photos. I believe the A620 will yield about the same number of photos at full resolution. I will delete the bad shots every night before sleeping.

If we take more than 321 "keeper" photos, I plan to switch to our Olympus Stylus Epic 35mm P&S. This film camera takes super sharp photos due to its fixed, non-zoom lens and is considered by many to be a classic 35mm camera. It's easy to buy 35 mm film anywhere and I believe that one should always have a backup camera on trips anyway.

The way I look at it is I bought the 7.1 MP camera for its resolution, plus it allows cropping down to 5MP which increases the effective optical zoom from 3x to maybe 4x. (wish I knew how to figure the exact amount of effective zoom by cropping). So why compromise our purchase by shooting at a lower resolution or compression?

Though I don't plan to shoot at lower quality, if I had to, I would shoot at 7.1 MP fine instead of at 5MP super fine. I figure a camera is designed to shoot as fast as possible so it does not have a lot of time to do excellent downsizing like a computer can but can do a better job at jpg compression using hardware. (unless of course the camera only uses a 5 MP area of the sensor with no downsizing. I have no clue how the SD550 or A620 handles that.)

$.02, Skylark
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Old Dec 30, 2005, 4:41 AM   #4
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@skylark:
You seem to believe that the image processor first generates a full 7 MP picture and then downsizes it to 5 MP. This is not the case. By far the most difficult task of the image processor is demosaicing. The pixels of a CCD are only sensitve for one color. 50% are sensitve for green and 25% each are sensitive for blue and red. The image processor has to interpolate the remaining two colors for each pixels. This process is called demosaicing. Because there are less pixels for blue and red there is more noise is in these colors. Demosaicing at full resolution is most difficult and generates a lot of softness, which is compensated by sharpening, which then amplifies noise. Using raw mode the PC can use much more sophisticated demosaicing algorithms and you get significantly better results at maximum resolution. Unfortunately the A620 doesn't have a raw mode. Demosaicing is simpler, when you combine it with downsizing and it generates considerably less softness. This procedure is not only much faster, but the information loss is also smaller than with the lenghty 7 MP + downsize procedure. Thus a 5 MP picture shot with a 7 MP CCD is usually better than a 5 MP picture shot with a 5 MP CCD.
Otherwise I agree with your assessment. If 7 MP with fine mode is small enough, I would use it. However, I am somewhat reluctant to use the high compression 7 MP normal mode, which generates pictures with a size < 1 MB. That is simply to much compression and I would prefer 5 MP fine instead.
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Old Dec 30, 2005, 8:40 AM   #5
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Kassandra,

Thanks for the info. I'm still a bit confused. Using round numbers and referring only to size not compression:

When a7 MP camera like the A620 records 5 MP images on the memory card, does it capture the image on the 7 MP surface of the CCD and then down size it to 5 MP or does it capture the image on 5 MP worth of the CCD and use that to record to the memory card?

Thanks, Skylark
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Old Dec 30, 2005, 10:21 AM   #6
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The camera always uses the entire CCD to capture a picture in raw format, but it doesn't generate a full 7 MP pixel picture from the raw picture, if you have chosen, say, a 5 MP resolution. Rather it directly generates a full 5 MP pixel picture from the 7 MP raw picture, i.e interpolation and downsizing is done in a combined processing step. I am not sure what happens in movie mode. Because time is very tight, it may only use a smaller part of the CCD for movies. Also my Powershot S1 movies are rather noisy while an ordinary 640x480 picture is absolutely noise free. The noise indicates that indeed only a small part of the CCD is used.
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Old Jan 17, 2006, 5:30 PM   #7
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I have done the tests, made some havy cropping and sent the shots for print at the mini-lab.
I found out actually that there is no real difference between 7MP at normal and 5MP at fine. I would say even that the result is a bit better on 7MP at normal (highest compression).

So as I don't really see difference, I will choose 7MP at normal as it save almost 40% of the picture file size.
Anybody has some other experience about this issue ?
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