Basic Question: Calculate DOF for HMC40

D

Denniscasey

Well-known member
I am thinking depth of field for camcorders could involve sensor size and other factors beyond just iris opening like in my old 35 mm still camera days. In this sense DOF could almost be camera or sensor size specific--HMC40 different than HMC150.
If this is case, is there a way to calculate DOF for HMC40 assuming you know the iris opening and are not into gain and so forth--i.e. shooting in bright daylight. When I shoot fast moving objects (back to race cars again) I leave HMC40 in auto focus because I can't manually focus fast enough. Auto focus on HMC40 is generally very good, but there are still several instances of going in and out of focus--I don't know what causes that to happen in some cases.

But, if I know, for instance, that at f 4.0 HMC40 will have objects from 20 to 60 feet in focus I could lock focus and avoid camera hunting for focus (when left in auto-focus) in whatever situation causes it to go in and out of focus.
 
Dennis,

I don't know that trying to calculate the DOF is going to work, but in an hour or two, you could test and make a chart for yourself. Note that the DOF in foreground and the background from the point of focus will not necessarily be equal. Another thought on how to handle your issue is to use auto-focus to preset on a spot, then switch over to focus lock to hold the setting. As bad as the evf and flip out are, I do this fairly often when in a hurry, or out in the field. I don't much care for AF while shooting for the reason you have already mentioned; AF will at times jump around, and always when you don't want it to.

Grant
 
You could use this basic DOF calculator
http://www.dofmaster.com/dofjs.html

Use a sensor size of 1/4".

The problem with the iris is that it doesn't do what you think it does. It only stops down to f/2.8, any further stopping down won't actually change the iris, it'll instead involve the ND filter, bringing in more and more ND to simulate the exposure that those smaller iris readings would give. But in reality the iris is stopped at f/2.8. Once you get past about f/6.4, the iris does start stopping down again, but -- that's bad! Unlike everything you've been taught in stills photography, deeper f-stops do not (after a certain point) result in more being brought into focus! Instead, the smaller the f-stop, the more everything goes out of focus, because of diffraction. When dealing with a tiny-sensor HD camera, you want to keep the iris as open as you possibly can, and really, f/2.8 is about the optimal iris for maximum DOF and maximum overall image sharpness.

So, with that said, feel free to stop down to about f/6.4 in the display's readout, but be aware that stopping down any further will result in losing sharpness. And if you're staying above f/6.4, use f/2.8 as your iris for purposes of calculating DOF.

With that said, at f/2.8, at 100% telephoto, and focusing at 23 feet, you'd have a DOF of about 3 feet total, so in-focus would be from about 22' to about 25'. If you want to get 20 to 60 feet in focus, you can get there, but you'll have to zoom out. At a zoom position of 21mm, and focused at 31 feet, everything from 21 feet to 60 feet would be in sharp focus. So you could zoom anywhere from full wide angle (4mm) up to about halfway through the telephoto range (21mm) and if you had manual focus set at 31 feet, everything from 20' to 60' would be in sharp focus when zoomed in, and of course when zoomed out, everything from 2' to infinity would be in focus.
 
Barry, this link to DOF calculator is a great learning tool--just what I wanted!! Lets me "see" impact of various settings before trying in field. Your comments showing DOF impact of 100% zoom (48mm focal length) vs no zoom (4mm) as I also experimented with in the calculator was quite a graphic reminder of zoom impact--sometimes helpful to see quantitative data.

The reminder of way HMC40 stops down below 2.8 by using ND filter and impact that has on DOF very helpful. From your book I remember now what you said about reasons for staying in f2.8 to f6.4 range--need to re-read several parts of the book (admit to skimming just parts I need to learn about at certain times). Your book has said all this, but sometimes playing around with a tool like this DOF calculator helps bring it all into focus as one sees how the settings numerically change DOF. Thanks again for your contributions and help to HMC community (and me). Dennis
 
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