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Old Jun 20, 2004, 10:25 PM   #1
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How is crop related to zoom ?

If i crop a 4" x 5" portion of a 8 MP raw image taken at 1X optical zoom and increase the size 200% is that like taking a 4 MP shot of the same image taken at 2X optical zoom (since 2X it wont need to be cropped and increased 200% since i had 200% more zoom).

Another question

Is the raw image of a 2048 x 1536 camera always 8" x 6".

2048/256=8", 1536/8"=6"
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Old Jun 20, 2004, 10:45 PM   #2
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Digital zoom anyways...that's what the camera does when it does a digital zoom...it crops and then enlarges the picture.

Many advanced/serious photographers disable the digital zoom on their cameras because using it destroys the original optical image.
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Old Jun 20, 2004, 11:35 PM   #3
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Using zoom or wide angle lenses distort a scene. Sometimes the distortion is pleasing and sometimes not so pleasing. Cropping a photo taken with less zoom will look different than a photo taken using the full zoom.

When comparing quality of photos, the size of the image in pixels is important. I think using inches to compare will just confuse things. If you start with more pixels in the image you can do more things with the image. Adding more pixels after the image is out of the camera is possible, but it's not as good as starting with more.
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Old Jun 20, 2004, 11:38 PM   #4
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There are several errors/flawsin your statement.

1) MP does not scale the way you specified it. If you cut out the middle portion of the image in the 1x version you wouldn't get 4MP. Sony 8MP 828 has this resolution:

3360 x 2460 = 8265600 pixels / (1024*1000) = 8MP (if you do fuzzy marketing math)

I completely don't understand where you're getting the 4"x5" from. So I'm going to ignore that. Is What you really mean is that you'll take the middle 1/2 of the X and Y sides of the sensor and enlarge is? Yes? 'Cause if you do, you don't have 4MP.

(3360 / 2) * (2460 /2) = 1680 * 1230 = 2066400 / (1024*1000) = 2MB

Sensors don't scale linearly. The "2x" in the zoom actually increases the image in two dimentions, not one.

Therefor it is not the same to use the middle 1/2 of a 8MP sensor at 1x optical zoom enlarged then it is to use a 4MP camera (because the middle portion of the 8MP sensor is only 2MP.)

2) But lets just pretend that you actually took a centered 4MP crop out of the 8MP image (whatever that dimention would be) and you enlarged it to match the same picture as the 2x optical zoom. The answer is that the 2x optical zoom image would look better. To enlarge the 4MP crop you'd have to have software create data that wasn't there. They do a decent job but its still fake data. Real data is always better.

3) Pixels (2048 x 1536 in your example) have absolutely nothing to do with a physical measurement. You can tell the computer to print that many pixels to any dimentions you want. You could make a 20" by 15" picture if you wanted (at 100 "dots per inch", which would be better named "pixels per inch") or you can make it 10" by 7.5" at 200 dots per inch. That is completely up to you and not the camera.

So to answer your second question "Is the raw image of a 2048 x 1536 camera always 8" x 6"." The answer is no, its whatever size you want to print it. Most people thing that somewhere around 150-250 dots per inch produces good results. But it's picture dependent.

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Old Jun 27, 2004, 9:38 PM   #5
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Thanks Eric

I noticed today that my raw image has a size of 11.38 x 8.53. These were pics i took on 2048 x 1536 resulation at Superfine compression.

So is this correct

raw image length is 2048/11.38=180 dpi = 180 pixel per inch*

raw image width is 1536/8.53=180 dpi = 180 pixel per inch*

I conclude my camera records at 180 pixels per inch by length and width.

*Are you saying a DPI= pixel per inch (interchangable terms) ?

So if I crop a 2 1/2" x 3 1/2" box, I am cropping a 450 pixel (2.5 x 180) x 630 pixel (3.5 x 180) image ? If i blew this up to 5 x 7 would it just be bigger dots (i.e. 90 dpi resolution).

I also notice the ratio of 11.38: 8.53 is 4:3. Most prints are something other that 4:3 ratio. So i take i guess every print requires some type of cropping and resizing.

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Old Jun 28, 2004, 9:41 PM   #6
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I think I understand what you are asking. Your asking if the image quality is the same between a cropped 8mp wide angle picture of a scene or a zoomed 4mp picture of EXACTLY the same scene.

Well, I'm not sure. But what I did, is I took two pictures of my bookcase, one is a wide angle full resolution shot (2048x1536) The other is a zoomed shot taken at one of the cameras lower resolution settings (1200x900) I then cropped out a 1200x900 section of the wide angle photo.

Judge for yourself.


I'm not sure how valid this test is. Because I have a feeling the low resolution setting on a high resolution camera is going to look better then a low resolution camera. but it's still a interesting idea.
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Old Jun 29, 2004, 10:43 AM   #7
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Hi, obrien. On your last post, you stated that your RAW image had a size of 11.38" x 8.53". I am wondering what is telling you this. Are you looking at them in Photoshop, and getting the data from there? The reason I ask is because you can set the PPI in Photoshop (or other software) to whatever you want. According to your calcs, it is currently set at 180 DPI or PPI. Your statement that your camera records at 180 pixels per inch is essentially meaningless. The PPI only matters when you make prints. Your camera could care less - it just records an image with a certain number of pixels in it. If you're getting this from the EXIF data, I find it useful to generally ignore it.

I always work in pixels when I touch up or crop pics. It is likely that I will make prints at various PPI, depending on what size the prints are and how much cropping I've done. I just try to remember that 150 PPI is about as low as I want to go for a good quality print. Sometimes, if I am creatinga background image for my computer, I will resize it to my screen resolution, or Windows will crop it. But this is always a resize to a smaller number of pixels, and I am still working in pixels, not inches.

As for enlarging anything to 200%, I'm not clear on how you are doing this. As someone stated, if you use software to actually enlarge the image by adding pixels, it is basically guessing what color those pixels should be and results may vary. I never do this.

Your original question was about crop and zoom. By using the OPTICAL zoom of your camera, you can frame a shot with your full camera resolution (all the MPs). If you don't zoom, but instead crop out what you want in the frame, then you lose the pixels that you crop out, and this might affect the quality of larger prints. But if you are only printing 4x6's and you have a 3MP or higher cam, you can do fairly significant cropping and get away with it.


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