1D Mark III Aperture/FPS Issue

[AirborneAngel]

Hey everyone,

I'm having a sort of strange problem with my Canon 300mm F4L IS and 1D Mark III, I'll try to articulate it as best I can.

Earlier today I was shooting NCAA Tennis Finals and everything was going swimmingly (the lens was sharp, autofocus was fast, etc.) until I tried to stop down from f4 to anything smaller (f4.5, f5, f5.6, etc.).

With the lens at f4 in AI Servo mode (continuous AF) I was able to make use of my body's 10fps, but when I stopped down the lens, without changing any other settings, my framerate dropped noticeably from 10fps to probably around 6-7fps. I do the vast majority of my shooting in manual mode and am confused by what caused this - I tried increasing my shutter speed and ISO, but nothing seemed to get my FPS back up to 10 apart from opening the lens back up to f4.

I don't have this problem with any other of my lenses on this body, and tested to make sure by immediately swapping the 300 f4 for my 70-200 f2.8, which achieved 10fps at f5.6 (as it should). And no - I did not have the IS on the 300 f4 turned on! (Lens was set to AF, IS off, 1.5mm minimum focusing distance while 1D was set to 1/1600, f4.5, ISO 640, continuous high speed motor drive).

Has anyone ever seen anything like this before? Or am I just being a complete idiot on a public message board by overlooking something obvious? I'm hoping this is a "doh" moment.

 

[rebby]

I can't find the documentation on this for the 1D III but I do remember reading about it at one point or another.

What you are experiencing is a lag created by opening and closing the aperture during the focus process when in AI Servo. Here is what is going on when you shoot at f4 w/your 300f4;

- Camera obtains initial focus at f4
- Camera fires shot
- Camera refocuses at f4
- Camera fires shot
- Camera refocuses at f4
- repeat...

Now, let's shoot at f8. Here is what happens;

- Camera obtains initial focus at f4
- Camera adjusts aperture to f8
- Camera fires shot
- Camera adjusts aperture to f4
- Camera refocuses at f4
- Camera adjusts aperture to f8
- Camera fires shot
- Camera adjusts aperture to f4
- Camera refocuses at f4
- repeat...

Because the refocus always occurs at a wide open aperture, you're really complicating things by stepping down and, as a result, this slows things down (this is one reason why you always want your "sports" lenses to be so good wide open). You can minimize the lag if your lens is capable of adjusting aperture very very fast and this is exactly what is happening with your 70-200f2.8. At the end of the day though, your 300f4 isn't fast enough at adjusting (and readjusting) aperture so you notice the lag.

It's also worth noting that shutter speed, ISO, and focus mode (AI Servo is slower for obvious reasons) can also affect the maximum number of frames per second.

This exact topic is briefly discussed on page 111 of the 1D X manual. The 300f4 is even specifically mentioned.

 

[MCAsan]

I think you might have more feedback on this question if you go to a site focused on Canon photography. I would suggest Canon Digital Photography Forums....photography on the net.

http://photography-on-the.net/forum/