is there a real difference in speed of Sandisk CF III and iV, using
D3? the prices are different.
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The D3 is capable of utilizing the UDMA feature, which means it can write to the IV cards at about 40Mb/sec, versus about 20Mb/sec for the III cards.
Whether or not that speed is an advantage for you, will depend on what format you are recording to the card. At one extreme, we have a user recording large RAW files, either without JPEG, or with JPEG but with many of the image processing options turned off. In this case, the camera's processing time for each image is short, and the CF card write speed is the limiting factor; here, the IV cards will allow a sustained average frame rate twice as high as the III cards.
This was an important consideration for me last week, as I was shooting for Stars On Ice and wanted to record RAW files (normally I use JPEG), and I average anywhere from 50 to 120 frames per minute. With the smallest RAW files (12-bit compressed, about 10-11Mbytes each), that means a sustained write rate of at least 20Mbytes/sec was needed. Using Extreme III cards would have been very marginal, so I invested in new Extreme IV cards for the shoot.
However, if you are recording JPEG only, and have many processing options (e.g. high-ISO NR at high ISO, active D-lighting, image authentication, etc.) enabled, the camera's processing for each image will take much more time than the writing of the JPEG file to the CF card. In this case, you will not see much advantage to the Extreme IV cards vs. the III.
