ND - how to adjust DOF?

M

mikecazzx

Well-known member
Ok, I built a DIY ND filter. It seems to work in movie mode, but in photo mode it seems to cause discoloration spotting.

I am a bit confused as a newbie.

So how does the ND filter allow me to get a shallower DOF?
Will the camera always produce a deeper DOF in bright sunlight?
 
The size of the iris (the f-stop) determines the DOF. The more open the iris is, the shallower the DOF will be.

Bright sunlight shots are usually deep DOF because the iris has to be closed down to deal with the amount of light. A neutral density filter absorbs some of that light, allowing you to open the iris up more, thus getting shallower DOF.
 
Barry_Green said:
The size of the iris (the f-stop) determines the DOF. The more open the iris is, the shallower the DOF will be.

Bright sunlight shots are usually deep DOF because the iris has to be closed down to deal with the amount of light. A neutral density filter absorbs some of that light, allowing you to open the iris up more, thus getting shallower DOF.
Click to expand...

Ok, I seem to need some clarification when working with adapted lenses.

I understand the brighter the light, the smaller the hole, the deeper the DOF.

On the GH1 with an adapted Canon FD 50MM 1.4...

I turn the silver ring to attach it to the adapter. This ring dial controls blades within the lens. Whether the blades are open or nearly closed seems to make no difference in the light I see on the screen. So do I have something set wrong on the camera? Is an automatic gain taking control?

Also, I do not see any difference in DOF whether set to 1.4 or even 16 on the lens.
Again, have I set something wrong when using an adapted lens?
 
Am I not getting something here? DIY ND filter? We have alway used glass in ND 3, 6, 9, 12 for light reduction. What is this. I understand aperture of course.
 
If you first mount a circular polarizer onto the lens and the mount a linear polarizer on the circular, it can create a DIY "variable" nd filter. Although, you may see issues with color, especially right before the darkest setting. Google on the net and read until your completely confused, then try it. Some have reversed the glass on one of the filters when they had issues. Mine seems to work ok in movie mode, but generates color blotches like purple in still mode.
 
mikecazzx said:
Ok, I seem to need some clarification when working with adapted lenses.

I understand the brighter the light, the smaller the hole, the deeper the DOF.

On the GH1 with an adapted Canon FD 50MM 1.4...

I turn the silver ring to attach it to the adapter. This ring dial controls blades within the lens. Whether the blades are open or nearly closed seems to make no difference in the light I see on the screen. So do I have something set wrong on the camera? Is an automatic gain taking control?

Also, I do not see any difference in DOF whether set to 1.4 or even 16 on the lens.
Again, have I set something wrong when using an adapted lens?
Click to expand...

What you see on screen is LIVE data off the sensor. During the time you are not taking pictures, the shutter is OPEN CONTINUOUSLY and light passes through freely. Even when changing aperture, a lot of light will be hitting the sensor, and the GH1 will be compensating by giving you a view that is gained appropriately for composition.

If you want to see what the exposure will look like taking into account the shutter speed, you need to turn the camera's shutter speed effect preview on. In order to do this, you'll press the aperture/trash button, then you'll see a message something like "Shutter speed effect OFF, press [display]", if you press and hold display at this point, you'll see OFF change to on and you'll get a preview of what the actual exposure will look like. Note if you want to the shutter speed effect to stay on even when you release the button, there is an option the menu for that.

As for your question about DOF, you also won't get a true DOF preview if the camera is not in shutter speed effect mode, even if you have a manual iris on your lens. Keep in mind that the DOF has to do with the aperture, but also the distance to the subject and subsequent distance to the background (e.g. your focal point versus the stuff you want to blur into a nice bokeh). If you focus on things far in the distance, or infinity, the depth of field will be sufficiently wide such that the background you are trying to blur will remain in focus. A good way to test if everything is working, is to go to an open area with a nice background, set up a subject, and move so that the subject is as close as you can focus. Take a snapshot or video at that point and I'm sure you'll see the DOF effect.

-kap
 
Hi Barry in another thread I started someone wrote me on a question about iris and HD performance that at higher values of F/4 the HPX400 does not perform HD quality due to diffraction. This means that HPX300 only delivers HD in a very smaal iris range (between 4 and 1.6). Do you have the same experience ?
 
Diffraction isn't something that's related to camcorders, it's related to lens optics and physics. The exact same diffraction issues will affect every 1/3" camcorder ever made, and every 1/4" camcorder, etc. The smaller the chip and the higher the pixel density, the more likely you are to experience diffraction. You can also see the effects of diffraction on large-chip DSLRs, when they're crammed full of dozens of megapixels.

The standing rule in HD is - avoid tiny apertures. On a lower-res camera you can get away with deeper apertures; on an HPX170 I can push it to about f/9.6 before diffraction starts to noticeably strip away resolution. But on a higher-res camera, you'll notice the effects at more-open apertures.

On an HPX300/HPX370, you'll probably start seeing diffraction at about f/5.6 or so. Red will be affected a little earlier, maybe as early as f/4, but red only makes up a small portion of the ooverall video signal and it's sampled at half-res anyway due to the 4:2:2 color sampling, so I'd say you're safe to f/5.6.

If you want the very sharpest images possible, stay above f/5.6 on any 1/3" HD camcorder.
 
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