Sony A900: A Closer Look at 24.6MP Resolution and Noise
by Wesley Fink on October 27, 2008 2:00 AM EST- Posted in
- Digital Camera
Full-Frame and 24.6MP Compared
The A900 is the first Sony to feature a full-frame DSLR. The sensor is approximately the size of a frame of 35mm film, which is 24x36mm, and 24.6MP is the highest resolution currently available in a full-frame camera.
Compared to most digital cameras today where the sensor size is closer to APS-C size, the lenses appearing to be 150% to 200% longer than the specified focal length. Other Sony DSLR cameras, like the A700, feature a 23.5x15.6mm sensor. 24.6MP is also double the sensor resolution of today's prosumer or advanced amateur models, which are generally around 12MP.
In the computer world smaller and smaller traces mean higher density, more transistors, and generally better and faster performance. However, the digital sensor is not a digital device. Instead, it is an analog device that gathers light and turns it into a digital signal. The reverse is true in sensors in that larger sensor size is almost always better, with everything else equal.
As you can see in the chart below, none of the APS-C sensors is even half of the area of a full-frame sensor. With a range of 28.1% to 42.4% of full-frame size, there is clearly a lot more information that can potentially be captured with a full-frame sensor.
| DSLR Sensor Comparison | ||||
| Camera | Effective Sensor Resolution | Sensor Dimensions and Area | % of Full-Frame | Sensor Density (MP/cm2) |
| Olympus E-520/E-3 | 10.0 | 13.5x18 2.43 cm2 |
28.10% | 4.0 |
| Canon XSi | 12.2 | 14.8x22.2 3.28 cm2 |
38.00% | 3.7 |
| Canon 50D | 15.1 | 14.9x22.3 3.32 cm2 |
38.40% | 4.5 |
| Pentax K20D | 14.6 | 15.6x23.4 3.65 cm2 |
42.20% | 4.0 |
| Sony A700, Nikon D300, Nikon D90 | 12.3 | 15x23,5 3.66 cm2 |
42.40% | 3.3 |
| Sony A350 | 14.2 | 15.8x23.6 3.72 cm2 |
42.90% | 3.8 |
| Nikon D700/Nikon D3 | 12.1 | 24x36 8.64 cm2 |
100% | 1.4 |
| Canon 5D | 12.7 | 24x36 8.64 cm2 |
100% | 1.5 |
| Canon 1Ds Mark III | 21.1 | 24x36 8.64 cm2 |
100% | 2.4 |
| Sony A900 | 24.6 | 24x35.9 8.61 cm2 |
100% | 2.9 |
The last column in the chart is the one that tells the story most accurately, however. Here the resolution of the sensor is divided by the sensor area to yield a sensor density. The lower the density, the larger the individual pixel size, and the more info that pixel can gather - all else being equal. There are a few surprises here, such as the Sony A350 being essentially the same density as the Canon XSi, and the new Canon 50D having the highest density of any current DSLR camera.
The last column does put into perspective the true potential of the full-frame sensor and sheds some light on the true meaning of Sony's 24.6MP A900 sensor. At 2.9MP per cm2 the A900 still exhibits a lower density and theoretically better high ISO performance than any current APS-C DSLR. This is very much at odds with the ridiculous claims many on the web are making about Sony going too high in resolution on the A900. In fact, sensor density on the A900 is lower than the 10MP Canon 40D, which is 3.1.
The point is that any issues the Sony may be found to have with noise are not the result of pixels being "too small". All else being equal the high ISO noise should be at least as good as an 8 to 10MP Canon sensor. Where the Sony does suffer is in comparison to sensor density of other full-frame sensors. In that metric the Sony has twice the pixels per cm2 of a Nikon D3/D700 and Canon 5D, and keeping up in high ISO performance with those cameras would be quite a feat.
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Heidfirst - Wednesday, October 29, 2008 - link
Wesley has already said that these are out of the camera jpegs at default settings & every manufacturer has different values for settings even if they may use the same names for them.If you want sharper out of the camera JPEGs then simply set more sharpness.
Or as a semi-pro you'll probably be using RAW & you'll be used to having to sharpen in pp anyway.
jamesbond007 - Thursday, October 30, 2008 - link
RAW images, which typically have zero sharpening (or any in-camera adjustments made) are sharper than the images provided.TariqGibran - Wednesday, October 29, 2008 - link
Your statement:"Sony sensor is double the resolution of competing full-frame cameras"
is technically incorrect. You also made this same statement in your initial review of the a900. In order to double the resolution, one must quadruple the megapixels, not just double them.
Wesley Fink - Wednesday, October 29, 2008 - link
You are talking about doubling the SIZE of the sensor, which is, as you point out, four times the resolution. Most readers would consider 12 MP double the resolution of a 6 MP sensor. I therefore stand by my description that the A900 at 24.6 megapixels doubles the resolution of the 12 megapixel full-frame competitors.sxr7171 - Monday, November 3, 2008 - link
I think that's a misconception that you should not perpetuate. It is the responsibility of the educated reviewer to inform people about linear resolution. That is the only number that truly speaks about how large you can print the image at a given DPI. Sure you could make the argument that twice the pixels is twice the detail and I wouldn't object but the ability to resolve two different points on either axis is a product of linear resolution. Please don't perpetuate the megapixel myth people are already clueless about the true meaning of the megapixel count and while the 24.8MP is nice for well lit, low action studio type shots most people really need something to shoot action with and talking about increased detail of 24MP vs 12 MP is something that is not important unless you want to shoot to make posters.TariqGibran - Wednesday, October 29, 2008 - link
You are incorrect. Look it up. You are only doubling the MP, not the linear resolution which is what we look at when we speak of resolution in regards to a digital sensor in this context. This is basic stuff any reviewer should be aware of and its sort of an embarrassment to the site and review that you do not know this. No knowledgeable reviewer would claim that 24MP gives double the resolution of 12MP. The fact that you stand by this inaccuracy is even more troublesome. Do some research.Wesley Fink - Wednesday, October 29, 2008 - link
I did try to look up your claim when you first made it in your comments to the A900 preview. I found nothing that corroborated your claim. We tried to look up your CLEAR definition of double resolution as 4 times the number of pixels again and found nothing to indicate that.It is not that we doubt you, but without any references to substantiate what you say it is not reasonable to make changes. If you have the "proof" you say is so easy to find please send us some links and we will take a look.
The 24.6 megapixel A900 does have twice as many pixels as the Nikon D3/D700 and Canon 5D.
melgross - Thursday, October 30, 2008 - link
Doubling the number of pixels increases the resolution by 39% (approx), but doubles the pixel DENSITY.This isn't in question.
lsman - Tuesday, October 28, 2008 - link
On first page,That resolution performance is amazing when you consider the 24.6MP Sony A700 is just $3000
You meant A900 is just $3000
Wesley Fink - Tuesday, October 28, 2008 - link
Thanks for bringing this to our attention. Now corrected.