Setting fixed aperture with auto iris lens

[emag]

I use Av mode 99.99% of the time and have since my first 'decent' camera, a Nikon F2 in 1974 and still do today with a 60D and 40D.  When I couple a camera to my telescopes I shoot manual for astro and Av for daylight work.  Recently I've encountered a situation that has me a trifle stumped - given time I might find a solution but I suspect there is a simple solution and I needn't reinvent the wheel.  With an auto iris lens I want to set the opening fixed for timelapse.  Pressing the stopdown preview, then the lens release and turning the lens ever so slightly, I can keep the iris fixed and take a shot.  BUT, I get an ERR01 (lens communication) message after the shot.  A half shutter press clears the error message and I can then take another shot (after releasing the half shutter press - i.e., it take two shutter presses).  This won't work for timelapse.  Anyone know a workaround?  I have ML but have not fully explored its capabilities, perhaps the solution is there.  Maybe it's in the manual, in which case I deserve a slap on the back of the head and a 'RTFM'.

[lol]

Am I missing something here, why can't you leave the lens connected normally and use Av mode?

If for some reason you can't do that, you were half way there will the partial twist method. Set desired aperture, press and hold DoF preview, and twist off the lens which will remain at the set aperture. Leave the partially twisted lens in place. The camera wont know its there and can't change the aperture then.

[emag]

Yes, I could use Av mode, but I am trying to have the lens remain at the desired aperture setting rather than stopping down for each shot, it reduces the possibility of flickering due to the iris not closing to -exactly- the same position each time. You've given me a thought - I may need to twist the lens a bit more or remove and then partially remount it.  Thanks.

[paul13walnut5]

Apart from anything else, even if the aperture did stop down exactly the same each time, it's a hell of a lot wear.
300 stop downs for 10s of timelapse?  Not good.

It does sound like you haven't twisted the lens enough, about a third should be enough to totally disengage the comms pins but keep the lens secure.

I also would recommend looking into ND faders.  I'm not a fan generally, but this is one situation where they can be good, shoot with the lens wide open, and control the exposure with the ND fader.

As your timelapse will probably be resized to 1920x1080 at best the loss of sharpness shooting wide open isn't that much of an issue.  And theres no stop down!

ND faders dont work well on UWAs or teles above say 100mm equiv, but if you are using a typical walkaround kit zoom then they'll be fine.  Just be sure to check images on playback before you set off your timer.. live view sometimes doesn't entirely reflect whats on the card with uncoupled / manual lenses.

[mws]

What about a older manual lens with an adapter? There are lots of great older lenses on ebay for not allot of money.

[Mt Spokane Photography]

It should not flicker due to the aperture in manual or AV.  If the lighting changes, and you have a fixed aperture, you are forcing it to flicker.
 
The aperture does stop down in live view, but it will vary with available light.

[paul13walnut5]

@Mt Spokane

It should not flicker due to the aperture in manual or AV.  If the lighting changes, and you have a fixed aperture, you are forcing it to flicker.
 
The aperture does stop down in live view, but it will vary with available light.

Hi, for standard video this is almost correct (although even 'constant' aperture lenses twitch a little when zoomed, at max aperture or stopped down) for timelapse it is false in practise, no matter how right it sounds is theory.

The issue is that -like most SLRs with non-stop down metering- the EOS cameras leave the aperture diaphragm (or to use the video term, iris) wide open until the point of exposure.   This is to assist composition and focusing.

When the image is taken the iris momentarily stops down then releases.  Great for stills.  For timelapse this can be a pain because:

-the iris has to stop down exactly the same each time or you end up with flicker.

and

- if you shoot say 10s of timelapse, thats 250 or 300 stop downs, a lot of wear on the iris diaphragm blades, that can easily be avoided

It should not flicker due to the aperture in manual or AV.  If the lighting changes, and you have a fixed aperture, you are forcing it to flicker.

Timelapses are most effective at dawn or dusk, when the light changes dramatically over a very short period of time.  If you set your camera up properly with enough exposures (I find 0.8s @ 2s intervals works best -avoids the NR and stops the sensor getting too hot) you get nice even exposure ramping.    Full manual is the only satisfactory way to control the camera for this.

The aperture does stop down in live view, but it will vary with available light.

Live view also has exposure compensation, which cannot be disabled on some cameras, so what you see on the back on the camera isn't always what will be on the card, particularly with uncoupled lenses, manual lenses and where exposure times are longer.

Here's an example of the flicker. I'd done everything right except uncouple the lens.  This was consistent manual expsoure settings, and although you'll see the desired exposure ramping, you will also observe the slight aperture flicker as the lens doesn't stop down exactly the same each time.

http://www.flickr.com/photos/scott1shpau1/5658874949/
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