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DOF question
Old 07-18-2006, 09:01 PM   #1 (permalink)
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OK , by now most of you know I am questioning my camera , skills , luck etc. about how blurry I personally think my own images are .
So I was looking over old posts and found a link to a DOF calculator

I typed in Fuji S3 + 127mm( 85+1.5x) + f4.0 + 8ft. = dof of 7.91 to 8.09 ft.

IS THIS RIGHT ???
the area of 'sharpness' is about 2-3 inches ??
Heck , this would explain a lot !

if so even at 72mm(48+1.5x) DOF is only 18 inches ???

Am I just to dense for this to sink in ,or is the calculations off ??
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Re: DOF question
Old 07-18-2006, 10:26 PM   #2 (permalink)
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Actually, I think it's about 5 inches.
(assuming .2 CoC, 85mm focal length, F4.0, and 8 feet focus distance)
when you calc DoF, don't apply the multiplier for angle of view (magnification)
check this out:
http://www.photozone.de/3Technology/depth2.htm
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Re: DOF question
Old 07-18-2006, 10:42 PM   #3 (permalink)
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DOF is ~5 inches. Here's a link to an online calculator.

http://www.dofmaster.com/dofjs.html

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Re: DOF question
Old 07-18-2006, 11:04 PM   #4 (permalink)
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[Thanks for replying , I guess I don't need to keep blaming the camera for going bad.
All this time it was just user error

5 inches isn't very good for keeping a person sharp !

Stan
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Re: DOF question
Old 07-18-2006, 11:11 PM   #5 (permalink)
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That was the one I found , but I used the 1.5x
still.... 3 or 5 inches is not good for any models ( I don't know any THAT thin )

I guess I would be better off shooting wider and getting MORE DOF and then crop the image closer
rather than zoom in and lose the DOF trying to get a close-up

Here I am blaming the equipment and all along it was the button pusher
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Re: DOF question
Old 07-18-2006, 11:24 PM   #6 (permalink)
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5 inches of DOF will get the whole face in focus from the nose to just in front of the ears, so it should be plenty. Here's ~3 inches, earlobe to eyelashes.



In most cases, if the eyes are sharp the viewer is less likely to notice softness in the other facial features.

If you're shooting with strobes, then you may want to make sure your camera is focused (or focusing) on what you think it is. Perhaps either the camera or lens has a focus calibration problem? If you're using available light then watch for camera shake, mirror slap, or the like.

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Re: DOF question
Old 07-18-2006, 11:50 PM   #7 (permalink)
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I assume that your image is probably near full size ( meaning you got in close at a medium Zoom ? )

where as I was using a 28-85 zoom from about 8ft away - neither full body or close-up

I have been blaming the equipment and such but I also took the uv/haze filter off for half the pics of the last shoot I did ( my newest theory )

I think I got good focus on most of those INCLUDING this one I took with natural light ( I am guessing at 1/20 and f 5.6 )
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Re: DOF question
Old 07-19-2006, 12:46 AM   #8 (permalink)
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If your shutter speed is down around 1/20th. (and you regularly find yourself shooting with a shutter speed slower than 1/90th.) then I'd be looking at camera shake or mirror slap as the culprit. Are you aware of the hand-held shutter speed guidelines for lens focal lengths? Basically, you should keep your shutter speed around 1/focal length. 1/30th at 28mm, 1/90th. at 85mm, etc. when shooting from your hand. Image stabilization lenses can help here, but there's also mirror slap - the vibrations initiated by the mirror when it hits the top of the mirror box. Some cameras are more susceptible to it than others. It manifests itself at slower shutter speeds. You might want to try setting your ISO such that you keep your shutter speeds higher and see what that does for your image sharpness.

Another thing you can try is to measure off an exact distance, set the lens to that distance, shoot, and see what comes out. That is, use a tape measure to measure exactly 8 feet (and place your subject on that spot), set the focus mark on the lens to 8 feet, put the camera on a tripod, set the shutter to 1/60th. or faster, shoot, and see what you get. If that's not sharp then I'd have my lens looked at. Next, leave the camera on the tripod, set the camera to autofocus, and take another shot. If the image is blurry at this point then the autofocus in the camera probably needs adjustment. If, on the other hand, the image is still sharp then I'd begin to consider operator error.

My image was taken from ~3' away with a 120mm macro lens on a medium format body, equivalent to a 75mm lens on 35mm. Magnification was around 1:5 life size, so yeah - close in with a telephoto.
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Re: DOF question
Old 07-19-2006, 07:11 AM   #9 (permalink)
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Hey, Chip -

I know the 1/focal length guide from the old days (I also remember f/16 at 1/ASA for bright sunny days - those old days). I know it's only a guide. What approach do you recommend with digitals that aren't full-frame sensors? Do you think that the minimum shutter speed estimate should be determined by the true focal length or by the 35 mm equivalent?

I know guides like these aren't carved in stone. Thanks.
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Re: DOF question
Old 07-19-2006, 05:32 PM   #10 (permalink)
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Quote:
Originally Posted by StanTom
OK , by now most of you know I am questioning my camera , skills , luck etc. about how blurry I personally think my own images are .
So I was looking over old posts and found a link to a DOF calculator
I typed in Fuji S3 + 127mm( 85+1.5x) + f4.0 + 8ft. = dof of 7.91 to 8.09 ft.
IS THIS RIGHT ???
the area of 'sharpness' is about 2-3 inches ??
Heck , this would explain a lot !
if so even at 72mm(48+1.5x) DOF is only 18 inches ???
Am I just to dense for this to sink in ,or is the calculations off ??
I often shoot portrait type shots with my Sigma 70-200 2.8 lens and at 2.8 so that I can blur the background and foreground. It works like a champ. But yes, you only have a few inches of DOF at most portrait distances. But with a longer lens you can get back further. I might be at 20 feet away at 200mm when shooting that portrait at 2.8. But even at 20 feet, I'll only have about 4 inches of DOF. And by the way, it does make a difference if you are using a camera with a crop factor. That needs to be dialed into the equation. If you check any of the DOF calculators and you dial in a full frame 35mm vs a 1.5 or 1.6x crop factor, you'll get different results. You have more DOF at the same distance with full frame than with a smaller sensor size.

The other factor that often causes loss of sharpness is camera movement. If you are shooting handheld at lower shutter speeds, then you introduce another level of unsharpness. So when I shoot that shot at 200mm at 2.8 I try for a shutter speed of 1/300th at the very minimum, or I get out the tripod.

But even a few inches is adequate for portraits. You want your eyes to be very sharp, so focus on the eyes. Even if you get slight softness in other parts of the head, it will still work great for portraits.

Cheers,
rfs
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