Northwoods Bill wrote in post #17341458
Miy new 100-400 just arrived, and I will get a chance to play with it tonight. Can't wait!!
I was reading through the manual on the IS settings and a question came to mind. The directions are to press the shutter button half way to engage the computation for the IS and then all the way to actually engage the IS as it is I am sure on all other IS lenses. What it got me to wondering about was back button focus. I use back button focus almost exclusively as many do. When using BBF I tend to depress the shutter from nothing to fully depressed in one move, does that allow the stabilization time to compute and work? Sorry for what is probably a dumb question.
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https://photography-on-the.net …showthread.php?p=17341451 First of all, I'm going to assume it's a 100-400mm IS Mark II you are asking about... in which case, I haven't used it.
However, I have used a dozen or more other IS lenses on my Canon for the past 13-14 years and almost exclusively use BBF. IS lenses I have now include two 70-200 (f2.8 and f4), 300/4, 300/2.8, 500/4 and a couple 28-135. I don't have, but have used the 100-400 first version, among other IS lenses.
With all those, IS starts and runs as soon as the BB is pressed. It also starts and runs with a half press of the shutter release, with all the lenses on all the cameras I've used over the years. It's not delayed until the shutter release is fully pressed, the way you describe.
However, the 100-400 Mark II might be different. Let me explain. Some of the other, newer IS lenses now have IS Mode 3 (in addition to Mode 1 "Normal" two-axis correction and Mode 2 "Panning" one-axis correction). This only activates IS the instant the photo is taken. I'm not entirely certain the purpose of this, because I like having IS stabilizing the image in the viewfinder before the shot is taken. The only thing I can think of is that sometimes IS causes some slight "drifting" of the image, which might be a small issue if carefully framed on a distant object. It also might save a little battery power, not using IS until the instant it's needed (however I often shoot sessions where I get upwards of 1200-1500 images per battery with a fresh charge, using IS nearly continuously throughout the day... so it's not like it's a power hog or anything).
Now, I know the 100-400 Mark II only has Mode 1 and 2... It doesn't have this new Mode 3 that's found on 300/2.8 Mark II and a few others. However, if IS is fast enough operating on those lenses to stabilize the instant a photo is taken, then it's likely similarly quick on the 100-400 II. Maybe the new 100-400L simply uses the same "instant IS" all the time, instead of "spinning it up" in advance of the shot.
I set BBF up on all my cameras... have done so for years. I certainly haven't used all models, but maybe a dozen or more different ones over the years and have never seen a Canon model where you could set the BB to only activate AF and not metering and IS too. On all of them, especially the current ones with more user-assignable control functions where you unassign AF from the shutter release half press to enable BBF, by choosing something like "start metering" in the control assignment grid... Even though that only refers to metering, the BB (and shutter half press) definitely also start IS working. When there is no background noise covering it I can hear it operating on my lenses. Once BBF is set up, pressing the BB starts all three: AF, metering and IS... While shutter half-press only starts metering and IS. This was how it has worked on EOS-3, 1V, Elan 7, EOS 620 film cameras, 10D, 30D, 1D MkIIn, 1DIII, 5D, 50D, 60D, 5DII, 7D DSLRs... that I can recall at the moment. (Note: On the older models before Canon implemented the AF-On button on both camera bodies and vertical grips, we reassigned the * button to perform BBF... This meant either setting it up without AE Lock or with AE Lock on full time... But otherwise worked the same as with current cameras/grips with the AF-On button.)
The only alternative way of using the AF-On button that I have seen is that you can set it up to stop AF, rather than to start and run it. Someone doing a pre-focus technique might want to use this.
I have found the Canon manuals to be misleading at times... downright wrong in a few rare instances. For example, in nearly all the manuals for IS lenses they suggest turning IS off when using the lens on a tripod, but without any real information about why. With a few lenses this is necessary because more primitive forms of IS don't "self-detect" and turn it off themselves when there is no movement. And, because of this, their IS can go into a feedback loop of sorts where it is actually causing movement (and image blur) rather than correcting it. This really only occurs with a few lenses and only happens when they are solidly locked down on a tripod where there is absolutely no movement (doesn't occur on a monopod or when using the lens on a tripod with a "loose" gimbal mount, for example). And, even then, IS can be effective against internal camera vibrations (such as shutter "slap"). So the only real reason for nearly all the Canon manuals for IS lenses to include a suggestion to turn IS off on a tripod is to conserve a small amount of battery power.
