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Calculating Depth of Field
#1
Taking some portaits of kids last weekend I found it quite difficult to get tack-sharp focus on their eyes... that's partly due to the autofocus (back-focus) problems I've been having with my D7000, which is now with Nikon for calibration.



That got me wondering about calculating depth of field. I know there's a depth of field scale on my 85 f/1.8D but it's the only one of my lenses that does have, and I don't know if it's accurate when using with DX sensor... is the depth of field the same?



When it's stated that the depth of field of this lens wide open (f/1.8) is equivalent to f/2.8, is it because a 130mm focal length lens at f/2.8 has the same depth of field as an 85mm lens at f/1.8? How is the aperture equivalence calculated?



Back to the portraits: how do I calculate what aperture will give me a 30cm / 1 ft depth of field for different lenses (50mm, 85mm) at different distances, used on a DX sensor?
#2
Try these:



[url="http://dofmaster.com/dofjs.html"]http://dofmaster.com/dofjs.html[/url]



[url="http://eosdoc.com/jlcalc/"]http://eosdoc.com/jlcalc/[/url]



You will have to take into account that due to the way sensors work, pereceived DoF is probably a little less than calculated DoF. At least, that is my opinion. YMMV.



Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
#3
[quote name='wim' timestamp='1321716271' post='13036']

Try these:



[url="http://dofmaster.com/dofjs.html"]http://dofmaster.com/dofjs.html[/url]



[url="http://eosdoc.com/jlcalc/"]http://eosdoc.com/jlcalc/[/url]



You will have to take into account that due to the way sensors work, perceived DoF is probably a little less than calculated DoF. At least, that is my opinion. YMMV.



Kind regards, Wim

[/quote]



Brilliant, thanks Wim

85mm, f/1.8 on DX at 6 feet: 5.95 to 6.05 feet. No wonder that was difficult...! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/tongue.gif' class='bbc_emoticon' alt='Tongue' />

And even at f/4, it's only 5.89 to 6.12 feet... so I hope the camera comes back with the autofocus spot-on...



85mm at f/1.8 gives same DoF as 127mm at f/2.8 (near as dammit): 5.96 to 6.04ft at 6 feet



What's the maths for calculating this? I bet it would make my head hurt..! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='Wink' />



I also found this, for lens / sensor equivalents: [url="http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm"]http://www.cambridge...sensor-size.htm[/url]







Ian
#4
No one carries around a tape measure when photographing.



You are kinda working backwards. You need to work out which lens do what, rather than worry about dof scales.



For example, if someome has a largish face and you want to flatten it somewhat, you can shoot them with say a 200mm lens. Whether the left eye is slightly softer than the right is not really a big deal as long as the closest eye is in focus.



How the face looks is more important!
#5
[quote name='Studor13' timestamp='1321725352' post='13038']

No one carries around a tape measure when photographing.



You are kinda working backwards. You need to work out which lens do what, rather than worry about dof scales.



For example, if someome has a largish face and you want to flatten it somewhat, you can shoot them with say a 200mm lens. Whether the left eye is slightly softer than the right is not really a big deal as long as the closest eye is in focus.



How the face looks is more important!

[/quote]



Hmmm...

I know which lenses I want to use... (50mm and 85mm on DX sensor)

What I wanted to know was how shallow the depth of field is with those lenses at a typical shooting distance with particular apertures

...and no, I wasn't going to use a tape measure!! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />
#6
Just calculate the DOF for your lenses at the typical distances for portraits and see which aperture you can reasonably use to get the face sharp if your focus is on the near eye. Use spot measuring for your AF. Try a lot and soon you'll have it by instinct.



#7
[quote name='Studor13' timestamp='1321725352' post='13038']

No one carries around a tape measure when photographing.....How the face looks is more important!

[/quote]



pardon me being a little light headed, but actually in the world of cinematography the focus puller does (did?) carry around a tape measure for focus :-)
#8
I agree with Studor13, DOF "scales" and calculation tools are a bit nonsensical.



You use a certain aperture size for either the look of the photo, or the light conditions. Not to get a "measured" depth in focus (which is nonsense anyway).



If you do not want a blurry background, no need for a 85mm f1.8 lens. If you need a whole person somewhat in focus, no need to fiddle with DOF blah either, just set the lens to f5.6 and you will be fine.



85mm on your Nikon gives approximately the field of view of a 85 x 1.5 = 127mm lens.



The aperture of the lens is 85mm / 1.8 = 47.2mm. For that 127mm lens that means: 127mm focal length, 47.2mm aperture: 127 / 47.2 = f2.69



Now that did not make your head hurt, I am sure.. Very simple really <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />
#9
[quote name='Brightcolours' timestamp='1321732917' post='13043']

I agree with Studor13, DOF "scales" and calculation tools are a bit nonsensical.



You use a certain aperture size for either the look of the photo, or the light conditions. Not to get a "measured" depth in focus (which is nonsense anyway).



If you do not want a blurry background, no need for a 85mm f1.8 lens. If you need a whole person somewhat in focus, no need to fiddle with DOF blah either, just set the lens to f5.6 and you will be fine.



85mm on your Nikon gives approximately the field of view of a 85 x 1.5 = 127mm lens.



The aperture of the lens is 85mm / 1.8 = 47.2mm. For that 127mm lens that means: 127mm focal length, 47.2mm aperture: 127 / 47.2 = f2.69



Now that did not make your head hurt, I am sure.. Very simple really <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />

[/quote]

Sorry, but your last statement is wrong. On crop the FL doesn't change, neither does the actual aperture.



However, what changes, due to the smaller format, is the CoC, and that is what causes DoF to change. This because DoF and sharpness is all about definitions, involving sharpness at a certain image size and viewing distance, from such a format.



Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
#10
[quote name='Brightcolours' timestamp='1321732917' post='13043']

85mm on your Nikon gives approximately the field of view of a 85 x 1.5 = 127mm lens.



The aperture of the lens is 85mm / 1.8 = 47.2mm. For that 127mm lens that means: 127mm focal length, 47.2mm aperture: 127 / 47.2 = f2.69



Now that did not make your head hurt, I am sure.. Very simple really <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />

[/quote]



Yep, that is simple... thanks BC. I didn't know it worked like that.



[quote name='Brightcolours' timestamp='1321732917' post='13043']

I agree with Studor13, DOF "scales" and calculation tools are a bit nonsensical.



You use a certain aperture size for either the look of the photo, or the light conditions. Not to get a "measured" depth in focus (which is nonsense anyway).



If you do not want a blurry background, no need for a 85mm f1.8 lens. If you need a whole person somewhat in focus, no need to fiddle with DOF blah either, just set the lens to f5.6 and you will be fine.

[/quote]



Yes, but it's useful (to me) to know how shallow the depth of field is for a given focal length at typical distances, at a given aperture. Yes, I want to blur the background, but how much? And is there a balance between how much to blur the background and leeway in the depth of field...



If I knew I could always get the intended point of focus spot-on and had 3cm in front and behind the intended focus plane which were going to be sharp, then I'd trust sticking to f/1.8... but until I'm sure of the equipment, maybe I'll stop down a bit to get a bit more leeway, but still blur the background (though obviously not as much as wide open).
  


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