Memory card size and class for video

Basically the speed it can read and write data to the card. Class 10 is faster card and less chance of dropping frames or data.

Class basically almost means nothing anymore - pretty much always get a class 10, but also check the speeds for both read and write. You need to get SDXC UHS-1 Class 10 Memory Cards for 4K. The SDXC is what to look for as well as 90mbs or greater. Unfortunately the industry will advertise in some cases as follows:
Sony 16GB SDHC/SDXC Class 10 UHS-1 R40 Memory Card

Notice the SDHC/SDXC - it is not even close to SDXC yet they advertise this. The R40 is the speed (40mbs) - crap. :)

I have had good luck with my GH4 and the Kingston 32GB UHS-1 SDHC Memory Card (Class-10). You can see it here http://www.bhphotovideo.com/c/product/1038708-REG/kingston_sda3_32gb_32gb_uhs_1_sdhc_memory.html

It does depend on the camera. The Sony AX-100 when shooting 4K does not need 90mbs. In fact the Sony recommends Class 10 SDXC memory. I bought a couple SDHC 90+mbs and the camera rejects them for 4K unless your reformat them in your computer first using exFat format. It appears with the AX-100 much cheaper (slower class 10) cards are all that is needed. Different than most or all the other 4K cameras.

I use a hacked Panasonic GH2 up to 176mb and have never used anything other my Panasonic 16 GB SDHC-UHS-1 Class 10, Read 90, write 25 cards. On a typical video shoot involving a few hours, I'll take along 2 cards and very rarely need to use the second. They are pretty inexpensive at B&H Video.

Don, Sony recommends SDXC not SDHC. Everywhere I have looked and researched this a 40mbs card won't work will the full codec of 4K on the AX-100. If you are writing mp4, as it is soooo compressed, it may be okay, but using the XAVC S codec they only recommend SDXC class 10 - it's on page 19 of your manual.

That's what I said, Sony recommends SDXC. Unless I'm reading something wrong you agreed with me completely. The 90mbs is not needed with 4K UHD on the AX-100. If a person chooses to use the SDHC they need to computer reformat using ExFat. The XACV S codex is very compressed. A camera saved 4K file of about 200Mb will be about 2Gb Photo JPEG after processing. Great to agree ;)

Don, Read the bold text above each box....
Recording images in the XAVC S format requires SDXC

Recording images in a format other than XAVC S can use SDHC or SDXC (of course) etc,,,

From http://store.sony.com/4k-camcorder-with-1-sensor-zid27-FDRAX100/B/cat-27-catid-All-Advanced-Camcorders

Media Type : SDXC Memory Card(Class 10) XAVC S; *Class 10+ SDXC memory card is required for recording in XAVC S Memory Stick PRO Duo (Mark 2);Memory Stick XC-HG Duo; Memory Stick PRO Duo (Mark 2); SD/SDHC/SDXC Memory Card(Class 4 or Higher)

Andy, stop the fighting for no reason. Read my lips. We agree completely on SDXC. It is possible and works just fine to use SDHC as long as you reformat it in exFat. Do you own one and tried it? I have taken several hundred 4K with the AX-100 with the higher speed SDHC and they work perfect, no problems at all after re-formating in exFat. Go read the forums about AX-100 and you will see either card works just fine "when re-formated to exFat". I have no argument with the owners manual, in fact I own the owners manual that came with my AX-100 that I shot 150 4K UHD clips with today using a reformatted SDHC 90 card.

For those looking to buy the AX-100, both Andy and I have said the same, no need to buy the SDHC for 4K, the normally cheaper SDXC card is all that is needed in the AX-100. If you have or want to buy the faster SDHC 90 cards just remember to reformat it first ;)

"You need to get SDXC UHS-1 Class 10 Memory Cards for 4K." Agree Andy, that is all that is needed but not the only one that effectively works. ;)

Why do you consider this fighting for no reason? I am merely stating the specifications as per Sony. But it seems if anyone points out facts you consider it a fight. Sony has a reason for their specifications. It has merely to do with reliability as per their labs testing. I would guess that they only state SDXC because they understand not everyone has the capability of reformatting to exFat. I am merely clarifying it for those that do not have all those capabilities. Price per GB is about the same between the two so that is not the issue. This information is merely explaining the correct info for those with less understanding and knowledge. You may have a more advanced understanding, but that does not mean everyone else does.