Iris closes even when aperture is set fully open, why?

I love my 50mm 1.4, with that large glass opening you can see clearly all the way to the sensor as you take a photo (of your face ).

Generally, the iris is fully open to allow for metering and focus. When the picture is being taken the iris closes. Of course at f/1.4 the iris really doesn't move that much...

One thing I noticed is that when the image is just about to be exposed the iris closes down all the way then back open, even when I am using an aperture of f/1.4.

I'm sure there is some technical reason, Any ideas why this happens?
Perhaps to help eliminate light coming in?? Or is this just a type of system reset/calibration between photos?


I was just thinking recently, there is a lot happening between each image being taken. If all these hardware movements were eliminated I bet the camera could take some wickedly fast sequences of photos. Assuming the sensor, processor and memory could keep up, of course.

interesting observation and curious behaviour. I just tried the same with my EOS 300D and it seems this 'recalibration' routine doesn't happen with the Canon

I'll check it out... but how on earth can you tell?
You'll see the mirror going up, but I've not seen anything on any of my lenses that does that
I'll look more closely later...
Matt

he, he, I only notice it on my 50mm f1.4. My other lenses are too deep and I cannot see if that is happening.

Use the 50mm f/1.4. Set shutter speed to half a second. Set Aperture to 1.4.
Take a picture. You'll see the mirror flip up then down but then just afterwards, the iris flashes closed and open again.

FYI, The behavior is the same on my istD.

I can see this with the 50mm A 1.7 too.

My old Canon AE-1 film slr required you to cycle through the full f-stop range manually after previewing depth of field. It sound as if the Pentax is doing something similar. Maybe this is to minimise the effect of backlash (slop) in the lens diaphragm?