More Pixels; Good or What ?

[andy70437]

I`m very confused by the pixel count and what it actually means with a cropped and enlarged photo. If I have a 10Mp photo and I crop it by 50% and then enlarge the cropped photo by 100%, will having the more pixels to begin with give a better enlarged cropped photo ? ie sharper ?
Sorry if this appears very obvious to some out there but I coud do with the info to settle some arguements at work.
Thanks,
andy

[RAH]

The hole in your idea is the enlarging part. You lose quality in an image when you enlarge it. Just thinking about it will tell you why. What the image manipulation program has to do is insert pixels throughout the image. It uses a certain amount of logic in doing so, but even so, it is manufacturing pixels out of thin air. The result is a degraded image.

Decreasing the size of an image (I don't mean cropping, I mean the opposite of enlarging - a resize/resample to an image of smaller dimensions) throws pixels away and also degrades an image, but perhaps not as much.

[Recurvirostra]

Depends what you mean by "enlarge it". If you mean that you printed it out at the same size as you would have printed the original photo, then (all else being equal) it will be of the same quality as an uncropped photo taken with a 5Mp camera.

[gordon g]

Lets say you take an image which is 3000 by 2000 pixels in dimension ie 6Mp.
Crop it to 50% size - ie 1500 by 1000 pixels = 1.5Mp
Already you have lost 75% of the original information (which of course might be useless background if you are cropping to highlight a particular part of the image).
Then extrapolate this 1500 by 1000 image in photoshop or whatever to double its linear dimensions back to 3000 by 2000 pixels. Yes - you have got back to 6Mp, but 75% of them have been invented by the extrapolation algorhythm. This will have consequences on image quality, especially on sharpness, detail and colour gradation.
The best solution is to lay the desired image across as much of the sensor as possible in the first place - in other words get closer or get a longer lens. This will give you more pixels to play with initially, and hence better image quality.

[BelgianPhil]

Lets say you take an image which is 3000 by 2000 pixels in dimension ie 6Mp.
Crop it to 50% size - ie 1500 by 1000 pixels = 1.5Mp
Already you have lost 75% of the original information (which of course might be useless background if you are cropping to highlight a particular part of the image).


Small typo: you don't crop the image to 50% of its size, you halve the length and width, ie cropping to 25% size.
All the rest is spot-on.

[Leicaman]

Small typo: you don't crop the image to 50% of its size, you halve the length and width, ie cropping to 25% size.
All the rest is spot-on.

Mathematically quite correct. Still its 50% I have to type in my resize box.

The two attached pictures show the effect of reducing to 50 % then enlarging the result to 200% to bring the size (but not the quality) back to the original.

[gordon g]

Small typo: you don't crop the image to 50% of its size, you halve the length and width, ie cropping to 25% size.
All the rest is spot-on.

Yes - sorry. 'Size' is a bit impresice a term. I should have said crop to 50% pixel dimensions to make it clearer.

[andy70437]

Thanks Gents for all the info.
I follow your logic and maths !
So, if I want to have a half decent slide show on my computer is the expense of a 10Mp camera worth it ?
Andy

[gordon g]

Thanks Gents for all the info.
I follow your logic and maths !
So, if I want to have a half decent slide show on my computer is the expense of a 10Mp camera worth it ?
Andy

Not necessarily. It also depends on quality of pixels on the sensor. There is an argument that states small tightly packed pixels - such as 10Mp on a 1.6 crop sensor - can cause problems with image quality as well. 8Mp on a 1.6 crop sensor would for most purposes be virtually indestinguishable from 10Mp. (Various threads have debated this - most recently 1Ds image quality and 350D vs 400D - sorry, dont know how to put links in here)
However a higher pixel count on a full frame sensor eg EOS 5D or similar, undoubtedly gives excellent image quality, but it does come at a financial cost.

Ultimately it depends on the application the images will be put to and financial constraints as to whether full frame is worth it for you.

[Recurvirostra]

So, if I want to have a half decent slide show on my computer is the expense of a 10Mp camera worth it ?

For a slide-show on a computer, you could crop your 10Mp photo linearly by 50% and its resolution would still be better than that of most computer monitors - without doing any resizing/resampling at all. So therefore no quality would be lost due to the cropping.

If you're going to print that cropped image at a large size then, yes, there may be a difference in quality, but the higher megapixel count image (assuming equal pixel quality) will always give at least as good, and usually a better, result than the lower megapixel count camera.

[RAH]

This is really just a matter of math and the device you want to use your image on.

Most monitors have resolutions of 800x600, 1024x768, or 1280x960. So, you know how large an image is required to fill the screen - i.e. an image 800x600 will fill a monitor screen that has a resolution of 800x600. This is a very small image. Even a 1280x960 image is a pretty small image.

So, as has been pointed out, to do a "slide show" on your monitor, you do not need large images, since all you are trying to do is fill the monitor screen. The same is true with web images, which are also displaying on a screen.

However, if you want to PRINT an image, this is where you need more pixels. The general rule of thumb for inkjet printers is that you should print at least 200 pixels per inch (sometimes called dots-per-inch - DPI). Soooo, if you want to print an image at 4 x 6 inches, you need an image at least 800 pixels (4x200) x 1200 pixels (6x200). If you want to print an image at 8 x 10, you need an image at least 1600 pixels (8x200) x 2000 pixels (10 x 200).

That's all you need to know, really, except that when you are using an image manipulation program to resize an image for PRINTING, you should NOT use what they call "resample". Your goal is to change the dpi that the image will print at, NOT the number of pixels in the image.

[gordon g]

So, as has been pointed out, to do a "slide show" on your monitor, you do not need large images, since all you are trying to do is fill the monitor screen. The same is true with web images, which are also displaying on a screen.

Yes - if that's all you want to do. I had in mind a slide show via projector, in which case higher pixel dimensions would, I think, make a difference in avoiding posterisation - with fewer pixels available, each would be projected correspondingly larger.

However, if you want to PRINT an image, this is where you need more pixels. The general rule of thumb for inkjet printers is that you should print at least 200 pixels per inch (sometimes called dots-per-inch - DPI). Soooo, if you want to print an image at 4 x 6 inches, you need an image at least 800 pixels (4x200) x 1200 pixels (6x200). If you want to print an image at 8 x 10, you need an image at least 1600 pixels (8x200) x 2000 pixels (10 x 200).

For printing I would aim for 300 pixels per inch for smaller prints, although you might get away with 250ppi at A4 and above. The reason for this is that the eye can resolve aprrox 300ppi at close viewing distances, but larger prints are generally viewed from further off, and so the perceivable resolution drops.

[andy70437]

Many thanks again for all your information and advice. You have certainly opened my eyes and I can see now what I need to do to maximise the quality of my photos.
andy