HMC-40 F-Stops

BobDiaz

BobDiaz

The Professor
Just for the fun of it, I decided to try and calculate what the F-Stops would be on the HMC-40. (For now, money is tight, so I have to wait before I can afford one.)

Anyway, my math comes up with odd numbers, so I'm not sure what it would be.

From OPEN --> F2.0; I calculated F1.782, F1.888, and F2.0
In this range I'm guessing that it appears as: OPEN, F1.9, and F2.0 Is that correct?

From F5.6 --> F8; I calculated F5.656, F5.992, F6.349, F6.726, F7.126, F7.549, and F8.
My guesses are: F5.6, F6.0, F6.3, F6.7, F7.1, F7.5, and F8.

If anyone has the time and interest, I'd love to see how well I did on guessing what the F-Stops are. I regret that when I had my hands on a unit at DV Expo, I didn't think to use the video end of my still camera to record the different F-Stop settings.


Bob Diaz
 
Open
1.9
2.0
2.1
2.3
2.4
2.5
2.7
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.5
4.8
5.0
5.3
5.6
6.0
6.4
6.8
7.2
7.6
8.0
8.5
9.0
9.6
10.0
10.5
11.0
close
 
Hello Bob,

This is approximately what is displayed on the LCD:

Open, F2.0 then .1 increments to 3.0 - .2 inc to 4.0 - .2, 5.0 - .3, 6.0 - .4, 7.0 - .2, 8.0 - .5, 9.0 - .6, 10.0 - .5, 11.0, Close.

I hope this helps your research.
 
Barry_Green said:
Diffraction -- what is it, and why do you care? Diffraction is something that's caused when the aperture is too small for the light to effectively squeeze through cleanly, so it gets scattered. The result is that the smaller your camera's aperture is, and the more finely focused the light has to be to render a good image, the more likely you'll run into diffraction.

On a big-chip camera with great big pixels, it's not a problem.

On a tiny-chip camera, especially a tiny-chip camera with super-tiny super-dense high-definition chips with miniscule tiny pixels, diffraction can have a noticeable impact on your image's sharpness.

We've already discussed how the HMC40's neutral density filter works, automatically, to keep the camera's iris open as wide as possible for as long as possible, to prevent diffraction from happening. And as such, my recommendation is to keep the camera at f/6.4 or more open (i.e., use the range from OPEN on down to f/6.4, and I think it looks just fine even down to f/8, but try not to go past it into the range of f/10 or f/11, etc).

So what happens if you do stop down further? You lose some sharpness. How much? Well... not as much as I was thinking would happen, actually. There's a drop, especially at f/10.5 or f/11, but it's milder than I was expecting. (obviously it's nowhere near as bad as one would think, because of the nifty ND filter trick; it means that at a stated f/11 the camera's iris is really, truly, at around f/5.6, which minimizes the potential for diffraction to happen).

Diffraction-Combined.jpg
Click to expand...
From: http://www.dvxuser.com/V6/showthread.php?t=182865
Message #4.

With the iris, up to F2.8 is a real F-Stop, but from F3.0 --> F6.4, the F-Stop doesn't change. The ND slides into place. (At maximum the ND is 2 and 1/3 stops.)

Going back to Barry's research on diffraction, I wondered what the real F-Stops were above F6.4...

6.4 = F2.8
6.8 = F3.0
7.2 = F3.2
7.6 = F3.4
8.0 = F3.6
8.5 = F3.8
9.0 = F4.0
9.6 = F4.2
10.0 = F4.5
10.5 = F4.8
11.0 = F5.0

When the camera is at F8.0, it really is at ND + F3.6; not surprising that this shows just a tiny loss of sharpness, considering that this is a 1/4" image sensor. By F9.0, we really are at ND + F4.0; this would have been OK on a 1/3" image sensor, but is a bit too much for a 1/4" sensor.

If I remember correctly, the HMC-150 should have an upper limit of F5.6 before diffraction starts to appear, but because of the lower pixel count in the sensors, one could go to about F8 before the diffraction starts to appear.


I know this doesn't change things, but it's just nice to understand what exactly is going on.


Bob Diaz
 
Diffraction can start affecting the red wavelength as soon as about f/5.6 on a 1/3" camera, yes... maybe even f/4. But overall it's not that big of a deal because a 4:2:0 color sampling system is going to throw out 75% of the red anyway, and of what remains, remember that only about 29% of the HD full-color signal comes from red. Green is a tighter wavelength and doesn't get affected quite so early as red does.

But yeah, an actual f-stop of f/5.0, that does all add up and make sense...
 
Hi Dan,

Last night I downloaded your Grand Canyon footage and played it on my 37" 720p HDTV, great stuff you did.

Now if I try to estimate what happens with diffraction in 720p mode, I would guess that because of the lower resolution, one could stop down the lens even more before it starts to really show up on 720p footage.

My guess would have been 1/2 a stop from F8; F9.6, but in looking at Barry's charts, even F10.5 (really ND + F4.5) appears to be 800+ lines. A 720p video does not have 800v pixels to resolve all those lines. So, it appears that even F11 would not have a major impact on sharpness in the 720p mode. (Anyone want to test this?)


Even then, the loss in sharpness in 1080p mode may be hard to spot with 900+ lines at F2.8 ---> F6.4 compared to 800+ lines at F10.5 setting.


As a side note, if Barry had run the diffraction tests using only red light, then using only blue light, he would have gotten different results. Red is a longer wavelength and diffraction occurs sooner with red light than with blue light, which has a shorter wavelength.


Bob Diaz
 
Anybody know what the outer edge size of the panasonic wide angle is, so I can add a nd filter to it? I've got the 43mm nd filters, but I noticed I get vigneting with two filters. Anyone think it would work if I got an outer ringless filter to the end of the wide angle?

Chris
 
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