The simplest/shortest answer I can come up with is that the camera has to work a lot harder to process & store a higher ISO image than a low ISO image.
And the longer answer...
The full-speed burst-mode can only last as long as the in-camera buffer (high speed memory) has space for the next image. As soon as an image is processed, it will get transferred from the buffer onto the memory card. There's a few possible bottle-necks:
- Speed of generating new images (e.g. the 8-fps specification quoted for your camera)
- Speed of processing individual images (getting from the actual raw sensor data into an image file, be it RAW or JPEG, ready for writing to the memory card—even a RAW image needs to be compressed)
- Speed of writing from the high-speed buffer to a usually low-speed memory card (essentially the fps is the write speed of your memory card (MB/s) divided by the typical image size (MB))
Presumably you want all of these as high as possible (and you manually reduce the first one by lifting your finger off the shutter, when you don't need to be shooting, allowing the buffer to empty). It's worth noting that some cameras allow you to reduce the shooting FPS, letting you shoot more consistently (e.g. perhaps 8fps for ~2 seconds, 6fps for ~4 seconds, 4fps for ~8 seconds, 2fps indefinitely—just made up numbers of course), as that can be better than shooting 8fps for ~2 seconds followed by 2fps from that point onwards.
Some cameras (e.g. all the Canon DSLRs I'm familiar with) will tell you the number of images that will currently fit in their buffer in the viewfinder (based on the current camera settings). This number decreases as you shoot (at high-fps), and increases as the images are written to the card—so if it hits 0 the next shot won't be taken until it goes back up to 1 or greater. This makes it easy to estimate how many shots you can take at high-fps before it'll slow down.
The number is largely irrelevant if the 3 speeds mentioned above are all equal (you only really need space for ~2 images in the buffer if you can finish writing the 1st image before the 2nd is taken + processed). But that usually only happens at low resolutions, or low frame-rates (or maybe really expensive cameras). Presumably this number is effectively about 10-14 on your camera in the settings you described.
I have noticed that on several Canon DSLRs, the estimated size of images increases as the ISO increases. For example, on the 5D Mark III, increasing the ISO above 6400 not only reduces the buffer, but reduces the estimated number of images that will fit on the card. So this suggests that for one reason or another, higher ISO images take up more space, both in the buffer, and on the card.
Here's the buffer size as reported by the camera, and the number of remaining shots that will fit on the memory card (a blank 16GB CF), with settings kept constant other than ISO:
For highest resolution/quality JPEG:
- ISO 50–6400: buffer=37, remaining shots=1622
- ISO 12800: buffer=35, remaining shots=1616 (~0.4% larger)
- ISO 25600: buffer=25, remaining shots=1412 (~15% larger)
For full resolution RAW (interestingly lower resolution RAW actually reduces the buffer):
- ISO 50–6400: buffer=12, shots=442
- ISO 12800: buffer=10, shots=416 (~6% larger)
- ISO 25600: buffer=10, shots=386 (~15% larger)
A 15% increase in image size will mean the buffer takes 15% longer to write to the memory card, so this will certainly have some effect.
From memory, the effect on image size kicks in at much lower ISOs on other cameras I've used.
You might expect a direct correlation between the counts in buffer + memory card (since both counts are meant to represent how many images fit in a given amount of memory), but I suspect:
- its likely some of the compression/processing happens on the images in the buffer, so the image may start in the buffer as raw sensor data, and get turned into a RAW/JPEG file perhaps over a few steps, so it may not scale linearly
- they might have a different image-size estimate for the buffer vs on-the-card
- in both cases they will round down to the nearest integer, and this will be a more noticeable effect for smaller numbers (i.e. 10 vs 11 compared to 1000 vs 1001)
Buffer size is only part of the story, of course, and there are other factors that slow down the processing, like:
- noise reduction algorithms specifically designed for high ISO
- general image processing algorithms (e.g. peripheral or aberration corrections, even basic colour profiles could take longer with more noise/randomness in the image)
- compression of the RAW or JPEG data (more noise = more randomness, so will make for a larger image, and could equally mean longer to compress, depending on the algorithm)
Any of these which happen after the photo has been put into the buffer will potentially result in the buffer filling up sooner, if they mean the images take longer to become ready to write to the card.
Finally, some processing may be done in parallel (e.g. the Canon 7D has two DIGIC chips so most if not all of the processing can be run in parallel for 2 images at once), which can alleviate the processing bottle-neck to some extent. And likewise, a camera with two memory cards (like the Canon 5D Mark III) can write alternate images to separate memory cards, which alleviates the 3rd bottle-neck to some extent.
So its a bit complicated, but basically the camera is taking longer to process each image, and longer to write them from the buffer to the memory card, and eventually it runs out of buffer space to store new images.
