Why is shutter lag shorter with prefocus than with manual focus on Canon DSLRs?
Asked 6/19/2014
4 views
2 answers
0
I’m using Canon T2i and T3i bodies on a microscope, controlled from a computer, and I’m trying to automate captures at precise intervals. I found published shutter-lag figures that list different times for “prefocus” and “manual focus,” which seems confusing.
My understanding is that “prefocus” means the shutter button has already been half-pressed, so focus and other camera prep work are already done before the final trigger. In my setup the subject is already in focus, so I expected prefocus and manual-focus shutter lag to be the same.
Why are the published lag times different? Does half-pressing/pre-focus prepare the camera in ways that manual focus alone does not?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
1
The reason "prefocus" is faster even when it doesn't actually focus is that half-pressing the shutter does more than just focus, when you half-press the shutter the camera:
- Powers up any component that may have been turned off to save power.
- Focus (if in auto-focus mode)
- Meters the exposure
- Do all the preprocessing before taking the image, decide what the value of each automatically set value should be and set it
- Display selected values and focus confirmation.
Even if you are taking a sequence of images (so everything should be powered up), in manual exposure mode and manual focus mode you still have the metering and pre-processing (even if you don't use the camera's metering results the camera still shows you at least how far you are from the metered values).
For what you are doing all of this is probably meaningless, you can keep the shutter half pressed while you shoot you entire sequence but without testing this yourself you don't know how this effects shutter lag.
Originally by user2481. Source · Licensed CC BY-SA 4.0
user2481
11y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. “Prefocus” is usually faster because a half-press does more than just autofocus.
On a DSLR, half-pressing typically wakes the camera, starts/locks metering, prepares exposure settings, and readies other internal processing. If autofocus is enabled, it also focuses. When you then fully trigger the shutter, less work remains, so release lag is shorter.
Manual focus only means the lens focus is set by hand. It does not necessarily mean the camera has already done all the other half-press preparation. So a camera in manual focus can still show a longer shutter lag than a prefocused camera.
Also, published shutter-lag numbers are often of limited value for automation because what matters most is not the absolute lag, but how consistent that lag is from shot to shot. In your microscope setup, the camera/lens AF timing is largely irrelevant anyway; repeatability is the key specification to verify in your own setup.
Recommended products
UniqueBot
AI12y ago
Your Answer
Related Questions
What does AF-L do on a Nikon, and why use it instead of half-pressing the shutter?
Can a Nikon D3 refocus with AF-ON while the shutter is half-pressed for AE lock?
What’s the difference between Canon AI Focus and AI Servo autofocus modes?
How can I use CHDK for a timelapse with gradual exposure changes or overexposure protection?
On the Fujifilm X-Pro1, is full-pressing the shutter better than half-pressing to focus?