The eyepoint distance has also increased from 20mm to 22mm. This means you don't have to get your eye quite so close to see everything. This should make it easier to see the whole viewfinder at once and may make viewing easier for those who wear glasses.
If you click on the image above you should hear the sounds (it's an MP3 file). First is the EOS 20D, then comes the EOS 40D.
The 40D image is a 200% crop. The 20D image was rescaled slightly for direct comparison. As you can see the 40D does outresolve the 20D. The "2.8" pattern is cleaner and you can definitely see more detail in the "3.2" pattern.
Below you can see two shots of the same scene, one with highlight priority on and the other with it off.
The images in the upper row are crops from the JPEG generated by the camera with no adjustments made, while the images in the lower row were each adjusted by exactly the same amount to boost the contrast. As you can see, the highlight tone priority mode does bring out slightly more detail in the sky. Presumably this is done using a slightly different algorithm when doing the in-camera RAW conversion from the sensor data to a JPEG. I'd guess that it wouldn't affect the RAW image, though I haven't tested that yet. My conclusion is that the highlight tone preservation effect is slight, but real, and could help in some circumstances.
While Canon haven't yet provided any technical details of highlight tone priority for the 40D, the similar function in the EOS 1D MkIII is said to provide up to an extra stop of dynamic range in the highlight regions. The downside is that it may slightly increase shadow noise. Highlight tone priority is not available below ISO 200. When it is selected, ISO speed is displayed as, for example, "4oo" (small zeros) rather than the normal "400" so that you can tell at a glance if it has been enabled.
[• end of 09/09/07 update]
[• Update 09/28/07]
Well, I've now had a chance to look a little more closely at the Highlight Tone Priority mode, and my guess about RAW files (see above) turns out to be wrong. It appears that Highlight Tone Priority affects both the RAW file and the JPEG file. I'm not quite sure what Canon are doing, but it appears to be more than just different processing of the RAW file into a JPEG. It may be some hardware related feature but until Canon tell us, it's hard to say just what's going on. Looking closely at images shot with Highlight Tone Priority turned on and off, there is an increase in the shadow noise if you look closely enough. However in situations which need highlight tone preservation, the increased detail in the highlights far more than makes up for any slight noise increase in the shadows.
In the first set of images below there are a series of crops from the image of a step transmission target at the "highlight" end of the range. These were taken from RAW images shot at ISO 200, "developed" with a -2 exposure compensation in DPP 3.1 to better show the range of highlight detail. It's quite clear that with Highlight Priority on a greater range of densities is recorded (higher light levels still record as shades of gray rather than pure white), which translates to better highlight detail. The numbers in the top left corner of each patch are the average luminosity values.
In the second sets of images shown below we are looking at the other end of the target, the high density patches which correspond to deep shadow detail. The upper row shows these patches cropped from an ISO 200 shot RAW file "developed" with +2 exposure compensation to bring out shadow detail. As you can see the density range of both patches is similar which shows that using highlight priority hasn't lost and shadow detail. Note that you can't directly compare densities with the previous image as these were developed from the RAW files with a +4 stop relative exposure adjustment.
The lower 2 rows of patches in the figure above show the same images as the upper 2 rows, but with large changes made to the contrast and tonal range via histogram adjustments in order to show the noise. I think you can see that using highlight priority has boosted the noise slightly, though this noise probably wouldn't be seen in a normal print.
How much expansion of highlight range do you get? I'd say around 1 stop. My transmission targets are not precisely calibrated, but it looks like you can get away with about 1 stop more exposure without blowing out the highlights when Highlight Tone Preservation mode is switched on.
One way Canon might be doing HTP is to use some sort of amplifier with non-linear gain. Let's say you set the camera to ISO 200 with HTP on. You could then apply the gain of the ISO 100 setting (which may be a gain of 1) to the highlights (pixels with high values), but the gain of the ISO 200 setting to the shadows (pixels with low values). That would keep the highlights from blowing out while retaining shadow detail. You'd get the highlight detail of shooting with an effective -1 EC for the highlights, and the normal shadow detail for ISO 200 (no EC). It's possible you might then have to tweak the tone curve a bit for the best looking image.. This is just a guess of course, made on the basis of some forum and email discussions. It would explain why you can't use HTP at ISO 100 (if the "native ISO" of the sensor is 100, then there may be no gain applied at ISO 100, so you can't then apply less gain with HTP turned on!)
[• end of 09/28/07 update]
"...Canon design engineers made the EOS 40D SLR's magnesium alloy exterior even more ruggedly dependable than its predecessors with upgraded dust- and weather-resistant construction, particularly around the camera's connection ports, battery compartment, and single-slot compact flash (CF) memory card door..."
The 40D does indeed have additional sealing around the CF card door and battery compartment door, but it's a thin foam type seal. It will no doubt keep out dust and some moisture, but it's not an "O" ring type waterproof seal. Even if it was, the EOS 40D, like all Canon consumer DLSRs lacks a waterproof seal on the lensmount and the popup flash isn't sealed, so water can get into the camera there. As far as I know the buttons and dials of the EOS 40D have the same seals as those on the 30D.
The 1D series bodies are weathersealed (though that doesn't mean 100% waterproof!). They have seals on the lensmount, no popup flash to leak, and additional gaskets on all the buttons and control dials to prevent water entering the camera body. The lack of such special seals on the 40D doesn't mean it will fail at the first drop of rain. Indeed the 20D and 30D (and Digital Rebels) have proven to be pretty good in light rain. However with any camera, especially ones not designed with full weathersealing, it makes sense to keep them as dry as possible. So the EOS 40D may be somewhat better sealed against dust and moisture than the 20D/30D in some areas, but it's not designed for extended use in extreme weather. For that you'll need an EOS-1D series body and one of Canon's weathersealed "L" series lenses.
[• end of 09/19/07 update]
The EOS 40D is much more than a minor upgrade of the EOS 30D. It's basically a new camera with a new CMOS sensor, a new 14-bit processing engine, a new AF system and new features such as Live View, Highlight Priority and High ISO noise reduction as well as improved ergonomics with the enhanced 3" LCD. Though it looks much like the 20D/30D it has a more "professional" feel - though I'm not sure why. Perhaps it's the quieter and faster shutter operation that simply makes it sound better!
In just about every respect the EOS 40D technically outperforms the EOS 30D and so I think it is a worthwhile upgrade for those who can afford to switch and who can use the new features. I didn't feel that way about the 30D, which was a good camera but for me didn't offer enough new features to persuade me to trade in my 20D and upgrade.
Based on using the camera for almost a month now, I'm very pleased with it. The larger LCD and revised menu structure makes the camera easier to use. The 3" LCD is very useful both for image review and data display (I can read it without putting my reading glasses on!). I really like the fact that the shutter is quiet in normal operation and even quieter still in Live View. The larger viewfinder and continuous ISO display again add to the ease of use of the camera.. Though I'd initially thought of Live View as a bit of a "gimmick", I'm actually thinking or more and more potential uses for it! AF is clearly better on the 40D than the 20D, achieving focus lock on subjects that the 20D simply refused to lock on to. The Highlight Tone Priority mode really does preserve highlight detail well with only a very slight increase in shadow noise. The ability to set 3 different custom shooting modes makes switching between commonly used settings very fast. Currently I have one set for high ISO work and one set for mirror lock up work, with the third one not yet customized. The anti-dust features of the sensor are so far working quite well too. After about 1 month of use, with fairly frequent lens changing, the sensor remains clean and essentially dust free.
Compared to other EOS cameras, the EOS 40D is much more like a "mini EOS 1D MkIII" than an "upgraded Rebel XTi".
The EOS 40D is the best yet of Canon's prosumer line of DSLRs (D30, D60, D10, D20 and D30). Each has offered more features than the last, and done it at a lower price. The EOS 40D ($1299) clearly follows in this tradition. It's a very good camera and I can recommend it. I didn't just review the 40D, I bought one myself, so that should tell you something!
I bought my EOS 40D from Amazon. They're reliable and have good customer service and a good return policy. AMAZON are showing the EOS 40D body is now shipping from stock (and they have a free shipping option), though the kit with the 28-135IS is still on backorder. ADORAMA are also shipping the 40D from stock, but again the kit with 28-135IS is backordered.
As of October 1st, if you click on the Amazon link for the Canon EOS 40D with EF 28-135IS you'll see that one of the Amazon marketplace vendors - Ritz camera - is advertising the kit as being in stock and you can order from them through the Amazon website.
Just don't get suckered by the scam artists advertising a 40D for $699, $999 or even $1199. Read this warning if you're even slightly tempted to believe an unbelievably low advertised price. If it sounds too good to be true, 99.999% of the time that's because it's not true!
Click on the links for current price and availability.