Camera megapixel ratings - are we being fooled

[Law5Guy]

If you want more real world resolution, try a tripod before buying a new camera.

or... buy a camera with an optical image stabilization system.

Here is a review of the canon PowerShot S2 IS that shows the difference with osi and without... and a movie of the same as well. (about 14/ of the way down the page) Quite a difference:
http://www.dcresource.com/reviews/canon/powershot_s2-review/index.shtml

 

[Rinkopr]

I thank you for the additional info. I guess when I read about having to crop my photos that it was in-fact for the 4 x 6. I just know that it bugs the he&& out if me and makes me wish I hadn't switched to digital, but than again when I dusted off my old EOS 7E, I blew through 10 rolls in 2 hours......Digital is much cheaper in the long run...

 

[ScubaLuke]

There's a good graphic here (scroll down a bit) of film vs. digital resolution:
http://clarkvision.com/imagedetail/film.vs.digital.summary1.html

Luke

 

[Scuba_Jenny]

DPI doesn't make much sense when talking about cameras. If you have an image 720 pixels by 720 pixels and print it out or display it on a screen so that it is 10" square, than you have 720/10 = 72DPI. Take that same 720x720 pixel image and print it or display it as a 2.5" square, and you have 720/2.5"= 288 DPI.

In other words, the same data set or image can be 72 DPI low quality or 288 DPI higher quality, by just changing the output size.

For cameras it makes more sense to talk about total pixels in the image rather than DPI.

It has been my understanding that you cannot change the DPI simply by changing the size.
If I have a 2200x 1800 72DPI jpg and I scale it down to 600 x 400 it is still a 72DPI. Jpegs use lossy compression. From what I understand of this compression method is that it takes say 4x4 section of pixels, finds the "average" color and removes the remaining pixels. This is how jpgs are reduced. RAW and tiffs dont use lossy compressions.
This is true at least when using photoshop and other editing software, I would imagine it is the same if you are taking pictures and saving them as jpgs.

To prove this point. Take any large photo. Reduce the size to something small. Save it. Now open it and enlarge it. You will notice a lot of pixilation and loss of data.

 

[justleesa]

I am waiting for one of the companies to bring out a digital in a reasonable price range that maintains the proper aspect ratio. I don't remember the terms, as I only dabble in the camera arena, but when I bought my wife the 10D from Canon, the only ones that kept the correct size were Canon 1DNS, Kodak and Nikon (if i remember correctly). They had the CMOS chip the same size as a 35mm negative, but they cost almost $5k or more for the body. I just hate having to decide what part of the picture I am going to crop to get a true 8 x 10 or 11 x 17.
I guess I just frame the picture I am taking to tight to allow for adjustment....

The new Canon 5D also has that CMOS chip. It is so nice when I take a picture, get it printed and it's all there. Nothing gets lost by cropping

 

[markfm]

There's a good graphic here (scroll down a bit) of film vs. digital resolution:
http://clarkvision.com/imagedetail/film.vs.digital.summary1.html

Luke

Cool site, thanks.
I didn't notice whether the resolution of the objective (lens) was really addressed, though.
A common error is over-optimization, focusing on one particular parameter. You can start tossing more pixels into the CCD but it won't matter if you have reached the limits of the optics.

 

[Charlie99]

It has been my understanding that you cannot change the DPI simply by changing the size.
If I have a 2200x 1800 72DPI jpg and I scale it down to 600 x 400 it is still a 72DPI.

Yes, but if I have a 2400 x 1800 72DPI jpg (which means that it is 2400/72 = 33.3" wide) and resize it (WITHOUT RESAMPLING) to 8.3" width, then it will become 288DPI (2400/8.3" = 288DPI). OTOH, if I say that I want it to be 8.3" wide and 72DPI, then I'll not only have to resize it, but also resample or downsample it to 8.3" x 72 DPI = 600 pixels wide.

DPI is important only when you are talking about physical output devices, such as CRT screens and printers.

Pixel count, DPI, and physical size are tightly related --- DPI is nothing more than the pixel count divided by the size in inches.

Resampling and resizing are different, although related, actions. One can resize without resampling; just as one can resample without changing size. If you do NOT resample, then changing the "physical" size of an image will change the DPI in inverse proportion.

 

[ScubaLuke]

Cool site, thanks.
I didn't notice whether the resolution of the objective (lens) was really addressed, though.
A common error is over-optimization, focusing on one particular parameter. You can start tossing more pixels into the CCD but it won't matter if you have reached the limits of the optics.

I particularly like the way he has placed the different cameras on the chart. Thus justifying my lust for a D2X.

For SLRs and DSLRs the lens usually isn't the limiting factor. Good modern lenses will have a peak resolution of 60 to 90 lines per mm in the center of their field of view with some fall off towards the edges of image area. Usually the peak is between f5.6 and f11.

A very good film, like Velvia, will have a resolution of something like 40 to 50 lines per mm and that equates to about an 18MP sensor. Of course that's for an ISO 50 film. You start to see a pretty quick falloff in resolution as you move to higher ISO values.

Luke

 

[Scuba_Jenny]

Resampling and resizing are different, although related, actions. One can resize without resampling; just as one can resample without changing size. If you do NOT resample, then changing the "physical" size of an image will change the DPI in inverse proportion.

Ok, learn something new everyday. WOW. This little tidbit of information just turned my world upside down! For the better, mind you. Sometimes there are gems of info out there, and even though I was really skeptical of your post, I checked it out myself in photoshop and wa-la! incredible. You just made my job so much easier. Thank you.

 

[Puffer Fish]

I particularly like the way he has placed the different cameras on the chart. Thus justifying my lust for a D2X.

For SLRs and DSLRs the lens usually isn't the limiting factor. Good modern lenses will have a peak resolution of 60 to 90 lines per mm in the center of their field of view with some fall off towards the edges of image area. Usually the peak is between f5.6 and f11.

A very good film, like Velvia, will have a resolution of something like 40 to 50 lines per mm and that equates to about an 18MP sensor. Of course that's for an ISO 50 film. You start to see a pretty quick falloff in resolution as you move to higher ISO values.

Luke

If you check dpreview images for some of the camera's listed, it is rapidly becoming easy for digital sensors to exceed the 60 - 90 lpm of a very good lens. This, as you pointed out, was never an issue with film (Velvia is one of my favorites) Sony 828 is an excellent example of a lens limited camera.

Just take the size of the sensor compared to the resolution chart to see how this is becoming more and more of an issue.

It is interesting to note that a D70, for example, now easily being passed by fixed lens camera's (fuji S9000 or new Sony).

The Clarkvsion web site is very interesting and actually presents several ways that a smaller megpixel sensor could improve on it's performance, without just increasing the number of pixels. I think you will see this happening, at the same time that numbers are rising.

One of the things not mentioned in that web site is the ability to digitally work with images, in ways that could never have been done with film. Much as I love 4 x 5 images, the north rim of the grand canyon looks better with 10 (13 meg)digital images merged than it does with the best 4 x 5 image, and the the digital has better resolution. Same is true regarding a whole bunch of issues.

D2X is still one great camera - but a lot of others are closing in.