Exposure Shift

[Tony Tremble]

Originally Posted by Alex Leith

Thanks for that explanation guys (and the clip Piotr). I had never really understood why softness from diffraction happened at different f-stops on different cameras. And now I do! Hooray!

And at the same time - woah! That's a fairly limited usable range we have!

Yeah, I think we could have done with a 3rd ND filter in the camera. Having said that I have noticed major softening was at F9.5. I shoot with the sharpness turned down to -2 anyway so that might have masked some softening at lower F stops. I usually aim to stay above f5.6 if at all possible but on a bright sunny day that can be a challenge without additional NDs.

Hopefully someone will develop a matte box that does not impinge on the IAF so adding additional NDs will be a cinch. I am not a fan of filter stacking screw-ons.

TT

[William Boehm]

i guess i need help understanding the diffraction issue. i am new at using an xh a1 and just returned from vancouver island shooting nature, eagles, seals etc along the coast...juan de fuca trail. having come from a well published still photo background...i understand pin hole diffraction at higher f stops of f16 or 22. but why is that point so low on f stops in the canon lens on the xh a1? does that really mean shoot at aperature priority 5.6 for all scenics and wildlife, and use a neutral density filter to decrease light on a bright day? i could not open up the example sent. so i need a simple explantation.

thanks ahead, this is a great forum. bill

[Richard Hunter]

Hi William. As you say, the same effect is seen on still cameras too. The reason it happens at much wider apertures on video cameras is that the sensor is much smaller than on any still camera. And because the A1 is fairly high definition, the individual sensor dots on the CCD are physically much closer together, therefore it does not take much bending of the light to cause a softer image.

Personally, I wouldn't shoot at aperture priority though. I tend to use Tv mode plus exposure lock. When you press exposure lock you get a readout of the aperture so will know when to add ND (or use faster shutter if absolutely necessary).

Richard

[Tom Roper]

Personally, I wouldn't shoot at aperture priority though. I tend to use Tv mode plus exposure lock. When you press exposure lock you get a readout of the aperture so will know when to add ND (or use faster shutter if absolutely necessary).

Exactly.

My MTF50 resolution measurements using the Imatest software support the finding of F4.0 being the boundary for diffraction. At the center, F3.7 is the measured peak, but corner sharpness improves at F4.0 and beyond. F4.8 is where the softening first becomes noticeable if only very slightly.

[Tom Roper]

The problem with aperture priority for video cams is not the effect upon the change of subject blur, but rather that it introduces flicker when the shutter speed changes. Better to put it in manual, or Tv mode. As the poster mentioned, if you press the exposure lock you can read out the aperture. Use the ND to get the exposure.

[William Boehm]

thanks for the replies. i comprehend the small sensor size, but then i would think aperature priority would be the way to go, unless you are panning or zooming and there is a light change and the shutter may flicker?

however i tried Tv just a few minutes ago, and panned around in the kitchen, and no matter where i landed, it showed with exposure lock f1.6...even in the darker areas at 1/60. the manual states exposure lock is in a plus or minus 2 stops range, which really seems to be a lot of variation. if f 4 seems to be the highest corner focus strength, and f3.7 the sharpest in the center...how do i keep the aperature in that range having to deal with the great plus or minus 2 in the aperature range? thanks again. bill

[Tom Roper]

Originally Posted by William Boehm

thanks for the replies. i comprehend the small sensor size, but then i would think aperature priority would be the way to go, unless you are panning or zooming and there is a light change and the shutter may flicker?

however i tried Tv just a few minutes ago, and panned around in the kitchen, and no matter where i landed, it showed with exposure lock f1.6...even in the darker areas at 1/60. the manual states exposure lock is in a plus or minus 2 stops range, which really seems to be a lot of variation. if f 4 seems to be the highest corner focus strength, and f3.7 the sharpest in the center...how do i keep the aperature in that range having to deal with the great plus or minus 2 in the aperature range? thanks again. bill

Forget about the kitchen.
Get some daylight. Set the camera to aperture priority f3.7, compose your scene, press the exposure lock and read what the shutter speed is. Unlock it and repeat after switching the ND. Use ND to lower the shutter speed where you want it to be, perhaps 1/125 or 1/60. Now switch modes to shutter priority and set it for 1/125 or 1/60. Although you can't lock the aperture or even know exactly where it is, you know it's going to be centered at f3.7, going up or down a little as you pan the scene.

So what you've done is use aperture priority and exposure lock to read out the shutter speed, that you're going to switch over to with the expectation that the iris will be centered at the point where you read the shutter speed.

The other way of doing it is to switch to manual. Set your aperture, then move the jog wheel to adjust the shutter speed until the arrow goes to the center of the scale on the light meter display. Once the arrow is at the center of the scale, read the shutter speed and switch to shutter priority mode and set it to the number you just read while in manual. As you pan the scene, the iris will open and close above and below but always close to your intended aperture center point. Although when shooting you won't know exactly where it is, the iris can open or close above or below the center point to maintain proper exposure without introducing the flicker. And it will be always close to where you would have locked it to in Aperture priority mode. If using the above methods the aperture deviates from f3.2 to f4.8, you will be within the sweet spot where the lens gives its best performance. It isn't necessary to lock the aperture rigidly to exactly f3.7.

[Tom Roper]

Re-reading your post, I understand what you mean about the 2-stop +/- range using exposure lock.

You get around that problem using the second method I outlined. Switch to manual. Set your aperture at f3.7. Then move the jog wheel for the shutter speed until the light meter goes to the center of the scale. You will note that the resolution of the shutter speed offerings in manual has more discrete values, 1/60, 1/75, 1/100, 1/125, 1/200, 1/250, 1/300 etc...

...whereas in exposure lock, they basically double, 1/60, 1/120, 1/250, 1/500 etc.

[William Boehm]

thanks tom for the information. that really helped. i was hoping i could read the f stop on the camera at all times...not quite sure that is an option on the display mode in either automatic of Tv. will research it. The clear footage this camera takes is amazing...bill

[Chris Hurd]

Originally Posted by William Boehm

i was hoping i could read the f stop on the camera at all times...not quite sure that is an option in either automatic of Tv.

Yes it is, sort of. When in A or Tv mode, press the Exp. Lock button to display the current f/stop setting. Press again to return to automatic exposure.