Actual Pixels and Cropping

[Dave Leroy]

I have a couple questions about definitions.
I have a 50D camera and shoot RAW. I convert into a 16bit TIFF file. When it opens in CS3, I see an image that the Navigator panel says is 12.5 %. If I hit the actual pixels button I get a 100% image.

The first question is about cropping terminology. If someone says they cropped 30% does this mean they cropped to 30% of actual? or they cropped 30% off one of the sides?

Second might be a bit more complicated. What does actual pixels mean? Is it monitor pixels, camera pixels? It seems to change with resizing.

I tried searching various spots on the web for answers with not much luck.

Thanks, Dave

[Stefan Minnig]

Dave,

A pixel is a basic unit of programmable color on a computer display. Thus, a unit of measure. A computer screen has the resolution of 72 pixels per inch (ppi). Of course, while it may look good on screen, printing out at 72 ppi is poor at best. The camera uses the same units of measurement in pixels that the computer uses to display an image in a programmable form. Obviously, the sensor will determine how many pixels it can capture. A 10 MP camera roughly outputs 10,000,000 pixels in a single image. Now when using a program like PS, and you resize smaller, PS will eliminate "unused pixels" to save file size. This is because you will not be able to see beyond 72 ppi on a monitor anyway. PS takes that into account. (Notice when you crop your original image in PS down to 1024 pixel by 800 pixels. After cropping, if you zoom back in, the file will be pixelated at a quicker rate than when zooming with the uncropped version. That is because Photoshop removed unneeded pixels.) If you resize bigger, PS will have to "add pixels" by using engineered formulas to "guess" what color pixels need to be added to keep the image looking the same. That increases the file size as well. So pixels are always the same size. Whether blown up in PS while zooming in (PS creates a blown up pixel by programming many pixels to display it like it is a single pixel for illustrative purposes), or on the camera (where the sensor determines how many pixels are in an image) a pixel will always remain constant as a unit of measure.

Confusing a little, but I hope this helps.

Stefan

[John Chardine]

Dave- the amount you crop is a difficult one to describe and you are right to be confused. One problem is that the area of a crop and the linear dimensions don't vary at the same rate. If you crop an 8"x10" image to 4"x5" you have halved the linear dimensions but you have reduced the original to only one quarter of its starting size (80 versus 20 sq. in.). When I say I've cropped by 50% I am intuitively referring to the linear dimensions rather than area. However, others may state this in a different way.

Only one thing to add to Stefan's response- modern monitors usually have better resolution than 72 ppi. One hears 96 ppi quite often. It is important though to set up your monitor to its native resolution so that the video card is not resampling the video output before you see it.

[Dave Leroy]

Stefan and John thanks for the replies.
John, it seems you have the same sort of query about cropping as I do. I had seen a thread a while ago on DPReview about pixel peeking. I had thought it meant looking at a picture at 600% or more. As it turns out, the more learned people said it meant looking at the photo at 100%. So I was wondering if there was a "definition" for saying how much a photo had been cropped. Is it linear, area, % available pixels removed or what...

Thanks to you both about comments about pixels. They were helpful. My question was poorly worded. In PS, on the tool bar, when you click on the Hand Tool three bars will apear above the image. One of the bars says "Actual Pixels". I was curious as to what Adobe meant by "Actual Pixels".
Dave

[Ed Cordes]

I could be wrong, but I always thought the "actual pixels" meant that each pixel of data from the sensor was represented by one pixel on your monitor. Therefore, you could examine any PS changes to your image on a pixel by pixel level. It is also why you get the square boxing effect when you go beyond 100% as the monitor pixels are then significantly smaller than one pixel of sensor data so the sensor data is kind of "spread out" around the point on the monitor.

[Dave Leroy]

Thanks Ed. I think that explains it to me and makes sense. So when we resize down and go for "Actual Pixels" we actually get a smaller image becasue there are less image pixels.

Do you have an opinion on the first question? When a person says they have cropped 30% what do they mean, is there a specific meaning or does it have various meanings?
Dave

[Ed Cordes]

Do you have an opinion on the first question? When a person says they have cropped 30% what do they mean, is there a specific meaning or does it have various meanings?
Dave

Dave, I always found this confusing. When I post Itry to estimate how much of the original image is left after the crop. So, for me a 70% crop means that the resulting image is about 70% of the original and I have eliminated 30% of it. Of course, this is an estimate on my part and others may look at it differently.

[David Thomasson]

I have a couple questions about definitions.
I have a 50D camera and shoot RAW. I convert into a 16bit TIFF file. When it opens in CS3, I see an image that the Navigator panel says is 12.5 %. If I hit the actual pixels button I get a 100% image.

The first question is about cropping terminology. If someone says they cropped 30% does this mean they cropped to 30% of actual? or they cropped 30% off one of the sides?


I don't know of any accepted meanings for talking this way about cropping. But I wonder why you raise this question in the context of viewing percentage. How much you crop off an image isn't determined by the viewing percentage.

Try this analogy: You use scissors to cut 30% off a sheet of typing paper. If you do the same thing while looking through a magnifying glass, you'll still cut 30% off the paper. Magnifying the view has no bearing on the result.

[Dave Leroy]

Hi David, The references to viewing %'s were more information for answering my second question, which I think Ed had answered quite well for me. My poor phrasing.

My question about cropping was to help me better understand what photographers were meaning when they said they were presenting a 30% cropped image for example.

It does not sound like it means anything other than 30% was cropped off a dimension which would then have produced a smaller print, all other things being equal.

Thanks for the reply, Dave

[Jerry van Dijk]

A computer screen has the resolution of 72 pixels per inch (ppi). Of course, while it may look good on screen, printing out at 72 ppi is poor at best.


Stefan, I use Corel photopaint for PP, which is by default set to 72 ppi. Does this mean that the images I have processed are not suitable for printing? What would your advice be to set the resolution to?

[Ed Cordes]

Stefan, I use Corel photopaint for PP, which is by default set to 72 ppi. Does this mean that the images I have processed are not suitable for printing? What would your advice be to set the resolution to?

The PPI for an image should be set at either the full or 1/2 the full native resolution of the printer. I think I am correct in saying that for Canon printers it is 600 ppi and for Epson it is 720 ppi. So, using Canon,I set my images to 300 ppi and let the printer interpolate to 600. I have tired to use 600 and found that the files are VERY big and take too long to load with no discernible difference in quality. An Epson user usually uses 360 PPI for image settings.

It is my understanding that if you use a PPI setting that is not an even interval of the native PPI of the printer the printer will interpolate to what it needs, but the interpolation will induce artifacts like extra stair stepping on curved lines.

Remember there is a difference between the PPI (pixels per inch) that you set in your post processing software and the DPI (dots per inch) that your printer sprays on the paper.

Hope this helps.

Ed

[Stefan Minnig]

Jerry,

Good points from Ed. Typically speaking, printing an image with a resolution of 72 ppi is going to give you a somewhat pixelated look. Deciding the right resolution varies with different kinds of printing methods, but 72 ppi will be poor for pretty much any print job. I usually try to aim in the 300 ppi range. When printing on newsprint, 200 ppi is sufficient. That is what is standard at our Newspaper company. I'm not familiarized with Corel photopaint, but if you have the option to set your resolution higher, I'd set it to around 300 ppi. Your quality won't be compromised and your file size will be reasonable. Hope this helps.

Stefan

[WIlliam Maroldo]

Note: I edited this post because Tony pointed out that I was confusing PPI and DPI. So i went ahead and deleted it. Also I failed to read earlier posts that quite adequately made the point I was trying to make anyway. However I did have something to add about crop percentages:
As far as determining a crop percentage. First determine the actual size of the image in megabytes. (bottom left in PS) Crop the image as desired. Determine ithe cropped image size in MB. Divide the cropped image size(Mb) by the un-cropped size(Mb) and multiply by 100 and you have percentage.
I am a long time both Photo-paint and Photoshop user, and talk about a default resolution has me confused as well. If you started an image from scratch, for example, the program doesn't assume 72ppi, you need to set it in the dialog box. If you import an image, from a digital camera for example, it will come in as a default ppi determined by the device, which could very well be 72ppi. At some point it is up to you to convert to a higher resolution, which of course reduces the image horizontal and vertical dimensions. Many programs have the ability(in preferences) to determine the import resolution. For example in Adobe Camera Raw clicking on the bottom center(where it lists Abode RGB:16bit: etc) you will be presented by Workflow Option Dialog where the resolution (pixels per inch) can be set. ~Bill

[Tony Whitehead]

Remember there is a difference between the PPI (pixels per inch) that you set in your post processing software and the DPI (dots per inch) that your printer sprays on the paper.
Ed
Reading this thread I think we need to remember what Ed has noted above. Pixels per inch(ppi) is very different to Dots per inch (dpi) and shouldn't be confused. In my understanding ppi relates to resolution of the image and is important to consider for various display purposes (lowish for web and highish for print) and dpi is a printer specification which is often quoted as a marketing tool by printer manufacturers.

[WIlliam Maroldo]

Tony. Duh! You are correct. I got them reversed. I meant to say pixels per inch instead of dots per inch, and it was dpi that should be disreguarded in printing. The funny thing is that when I adjust an image resolution I've been thinking (for a long time) "I need to adjust the dpi", and then adust the dialog that obviously says pixels/per inch! Oh well ~Bill