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Solved: size of photo - pixel/inch?


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ms_khw's Avatar
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09-Feb-2007, 02:39 PM #1
Question Solved: size of photo - pixel/inch?
I have a photo that I'd like to size at 20x20". I've used 300 pixel/inch for all of my other photos, but to do that with this one would make the pixels # astronomical as well as the amount of ram I will need to work on it. What is the best way to go about this without losing any of the photos quality?
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09-Feb-2007, 03:36 PM #2
It is a help to put things in perspective. Read this article from the NY Times: http://www.nytimes.com/2007/02/08/te...ogue.html?8dpc If you do the math, his 5Mp print at 16 X 20 from a 3:2 format DSLR was from an image less than 120 PPI. I would hope I would have been one of the people who saw the difference between that and a higher image resolution, but it appears most people couldn’t. I can see the difference between my own prints at 120 PPI and a higher PPI.

Also keep in mind that the author had people inspecting the prints closely. Most large prints aren’t viewed from very close.

I have to use a loupe to see the difference over 180 PPI and can’t see any at all over about 240 PPI.

I don’t agree with doing an automatic resample to 300 PPI even when RAM and processing time isn’t a factor. Any resample degrades quality a little and it doesn’t improve the print that much, especially when you have plenty of pixels to start with. I don’t usually resample until I go well below 180 PPI. Then I resample to 300 PPI since I am doing a resample anyway.

For your very large print you can do most of the image editing before you resample. I like to do the sharpening after the resample, but everything else works fine on the original. I doubt you would see any difference over 180 PPI on a small print and you almost certainly wouldn’t on a large print that isn’t usually viewed from closer than a couple of feet. If you anticipate problems at a higher pixel count you might consider a resample up to 180 PPI for the print.

I think an exception to the generalization that people view larger prints from a greater distance is group photos like group or team photos. People tend to look closely at the individuals. But almost anything else doesn’t need high resolution. And a resample from a lower to higher resolution doesn’t help as much as some people seem to think.
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09-Feb-2007, 03:42 PM #3
20x20 inches @300 dpi isn't that big (6kx6k). I've worked with far greater sizes on machine with comparatively little RAM (1gb), so patience is key.

However, If the original image isn't that size (what size is it?), then resampling it will lose some quality for size, depending on the image, you'll be the judge of that.
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09-Feb-2007, 04:10 PM #4
Enjoyed the article. I've always been perplexed and wondered if there was a difference in camera mega pixel. I wondered if it did'nt really come down to the camera itself and the factors related to taking the photo. It appears as though this just may be so.

But now I'm even more confused. I took a photoshop class and was told that all photo resolutions should be set at the 300 pixel/in Unless of course you run into a situation size like the photo that I'm speaking of. I was left in the dark on that one. I understand that anything you do to the photo in PS would degrade it, even rotating it would do so. So in your opinion what would you set the resolution at? And you spoke of resample, would you select Resample when sizing it? Also would I be correct in setting it at Bicubic Sharper to keep its original sharpness when downsizing it?

Sorry for all of the questions. with thanks, msk
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10-Feb-2007, 11:19 AM #5
I seldom “set” the resolution. I set the print size and use the resulting resolution unless it is too low. Any advice to always resample to 300 PPI shows very poor understanding of graphics IMO. Perhaps you misunderstood the instructor.

Everything is most easily done with the crop tool. Select the crop tool and set the width and height of the print you want in the boxes. Put NOTHING in the resolution box. The crop tool will constrain the crop to the proper proportions and set the print size you have selected.

If you want to know what resolution you ended up with go Image > Image Size. The resolution listed reflects the resolution you end up with just using the camera pixels. If the resolution is too low you can then resample.

Bicubic is fine. Some people prefer Bicubic Smoother for an upsample but I don’t like it much. I use more sophisticated plug-ins for a resample, but bicubic works fine.

I would hope the instructor taught you to never overwrite the original image from the camera. That is your digital negative and shouldn’t be altered.
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10-Feb-2007, 11:40 AM #6
The image taken directly from the camera comes in at about 48x32" and I understand that downsizing it is fine, but never enlarge because of distortion as you say. Listner, I feel more confident in working on an image 20x20, the teacher thought I was nuts. And I never resave an image and work on it. My very first step is to place it on a disk - it becomes my negative as you say. Something that I was advised on here at the TSG. As for putting nothing in the resolution, I'm a little confused. It is set at 72 all of the time, regardless of what I open or crop? I had the understanding that 72 was fine for the web but not photos?? No, I didn't misunderstand the teacher as to 300 pix/in I went back through my notes. Our textbook or recommended reading was the Adobe Photoshop Bible, it says the same about 300 for images. So I am so confused.
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10-Feb-2007, 12:23 PM #7
Another point of confusion is that I don't know if the image size shown in PS is literal. I have difficulty believing that a photo could possibly be 48x32 when taken directly from the camera. Am I reading this correctly?

I also did some googleing about the 300 pixel/inch, which I shouldn't have because I think I'm more confused then ever!
This is one such site that I came across: http://www.design215.com/toolbox/megapixels.php Your thoughts on it?
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10-Feb-2007, 12:38 PM #8
The only thing that you need to consider is the Pixel Dimensions. You can set the resolution before you print or finalise your work.

As i said, 20"x20" at 300 ppi is a 6000X6000 (36 megapixel), so I doubt that's how big your images are.

However, 48x32" at 72 dpi is roughly equivalent to the output of an 8 megapixel camera. Changing the resolution to 300 DPI without resampling(image interpolation) will make the print size about 11.5 x 7.5".

Very generally, DPI has only relevance for printing or scanning. It does have relevance in Photoshop when working on images, especially Type size, so a working resolution of 300 is fine.

Last edited by lister; 10-Feb-2007 at 12:49 PM..
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10-Feb-2007, 12:50 PM #9
I will take into account the pixel dimensions as you suggested and then considered adjusting the resolution. I think For curiosity sake, I think try it both ways, one without touching the resolution as suggested by you Slipe and the other by changing it to a 300 resolution. As far as the size of my photo when I take it from the camera, this is what I get in photoshop. Is that for real?
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10-Feb-2007, 01:04 PM #10
Quote:
Originally Posted by ms_khw
As far as the size of my photo when I take it from the camera, this is what I get in photoshop. Is that for real?
Yes, the Pixel Dimensions indicate an 8 Megapixel camera - most images from digital cameras have a resolution of 72 dpi.

Bear in mind that if you just want to change the resolution without resampling, uncheck resampling.

Generally a higher resolution is used when the images are on a smaller scale and quality is important (Art prints etc). If you were working on a billboard or similar, then a lower resolution would be used (and probably some resampling).

Basically any resampling is undesirable as it uses an algorithm to add pixels to the image that didn't exist in the image beforehand, so detail will be lost. The 3rd party resampling software work better on large images, as they have more image data to begin with.
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10-Feb-2007, 01:31 PM #11
I think it is starting to make a little sense. If left at 72 resolution the image quality shouldn't be too bad when working with a print say 20x20 (example). When working on a smaller print, say 8 x 10 or 5x7 changing the resolution to 300 would be acceptable and possibly a plus in this case (remembering to uncheck resampling), am I correct?

Hopefully I haven't given you a headache. I really appreciate everyone's help on this. Oh, would I also uncheck resampling when working at a 72 resolution?
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10-Feb-2007, 02:57 PM #12
Something that might help you a little would be to go to the screen you posted and uncheck “Resample Image”. Change the width from 48 inches to 6 inches. The height should change itself to 4 inches.

Now look at the resolution and the pixel dimensions. Your resolution should be around 576 PPI and your pixel dimensions should be unchanged. You have not changed the image at all. It is still the exact same image you started with at 3456 X 2304. All you have changed is the size it is set to print, and the program is telling you that if you print that image at that size you will have 576 PPI.

If it would make you feel better about working on the image, make sure “Resample Image” is unchecked and just change the resolution to 300 PPI. You haven’t changed the image at all, but now you can say you are working with 300 PPI and not 72 PPI. But the PPI really has no meaning at all except when you apply it to a document size like a print.

Different cameras default to different PPI, but it makes no difference to the image. I think most Nikons default to 300, Olympus to 150 and most others to 72. If there is no resolution in the image file Photoshop will arbitrarily assign 72 PPI. What you have to understand is that the 72 PPI makes absolutely no difference unless you actually want to make a 48 X 32 inch print.

The reason I suggested using the crop tool to size your print is that 4 X 6 is the only standard print size your image can do without distortion. 3456 X 2304 is in a 3:2 width to height ratio. For any other standard print size you have to get the right width to height ratio. The easiest way to do that is with the crop tool.

If you crop a 20 X 20 from your image you will find you end up with 115 PPI – less if you crop any more from the short side than you have to. At 115 PPI I would resample, and I would go all the way to 300 PPI since you have to resample anyway. You would get the same resolution of 115 PPI if you printed a 20 X 30. You will understand why if you crop an image to 20 X 20 and then crop it to 20 X 30. Experiment – you can always go back a step in the history.

But say you cropped for an 8 X 10 print. You would find you ended up with 288 PPI after cropping without a resample. I wouldn’t resample 288 PPI. You don’t need more resolution for the print and you slightly degrade the image with the resample.
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10-Feb-2007, 05:30 PM #13
Slipe, Listner thank your for all of your help! Not quite as confused. I will do as you suggested slipe, play around with image-experiment. Maybe I'll get it right eventually.
with thanks, msk
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