parisian
Over the hill and far away...
Reged: 10/02/2002
Posts: 7721
Loc: Môn mam cymru
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I know that the area of apparent sharpness is less in front of the plane of focus than behind it.
Why does that happen?
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Hells pensioner - born to be mild
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daft_biker
Action Man!
Reged: 11/10/2006
Posts: 7540
Loc: Doon the glen
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It isn't. The point of focus should be in the middle of the area in focus.
It's half infront and half behind the focus point for DoF (for critical use at least!). Something to do with the laws of physics I believe....hopefully somebody more clued up than me can explain why!
In terms of apparent sharpness...if you use a small enough print size and a wide enough lens the effects of perspective may well make it look like more DoF extends behind ( guess!).
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Andrew (BSRIPN) ... Pics.
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parisian
Over the hill and far away...
Reged: 10/02/2002
Posts: 7721
Loc: Môn mam cymru
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Thank you Andrew, assuming you are right then I have been living under a misapprehension for years.
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Hells pensioner - born to be mild
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daft_biker
Action Man!
Reged: 11/10/2006
Posts: 7540
Loc: Doon the glen
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You wouldn't be the only one! It's an important factor in differential focus macro...on a good day I might have 2.2mm DoF to play with so it's important to try an make the most of it.
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Andrew (BSRIPN) ... Pics.
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Roger_Provins
Made-it Man
Reged: 22/10/2005
Posts: 3001
Loc: Gloucester, UK
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That seems to be so at macro/close-up distances but appears to change at longer distances. Try entering different parameters in this calculator. The results are quite enlightening 
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Rog
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Malcolm_Stewart
Carpal \'Tunnel
Reged: 11/07/2005
Posts: 2541
Loc: Milton Keynes, UK
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Here you are
If you inspect the results for subjects some distance away, 10m say, from the camera, you'll find the 1:2 rule applying. For the macro area, the results are as described by Andrew.
I'm too far away from O level maths to work out why it happens, but I'm sure I did years ago!
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Malcolm Stewart
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beejaybee
Marvin
Reged: 18/07/2007
Posts: 4851
Loc: Really Here In Name Only
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Quote:
It isn't. The point of focus should be in the middle of the area in focus.
It's half infront and half behind the focus point for DoF (for critical use at least!). Something to do with the laws of physics I believe....hopefully somebody more clued up than me can explain why!
Sorry but you're wrong. Though for the sort of work you do, the difference is infinitesimal.
The extreme example is hyperfocal distance focusing. The far point of the depth of field is infinity and the near point is half the hyperfocal distance. Clearly there is a great deal more distance in "sharp" focus behind the focus point than in front of it.
If anyone needs the mathematical explanation, I'd be happy to provide it.
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parisian
Over the hill and far away...
Reged: 10/02/2002
Posts: 7721
Loc: Môn mam cymru
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As Ridcully would say, "This is going to be about quantum isn't it?"
We seem to have mixed bag so far, any advance on yes AND no 
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Hells pensioner - born to be mild
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SqueamishOssifrage
veteran
Reged: 13/09/2006
Posts: 1458
Loc: Ayia Anna, Hub of the Universe
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Having programmed my HP 50g calculator with the various rigorous* formulae to calculate DoF (including a useful function to return maximum focal length for a required DoF assuming other factors are fixed) I can assure you that both you and beejaybee are correct. However, quite why this should be from an optical point of view, I am not sure - just that the maths says so!
No doubt Huw will be along shortly to explain it all to us folks of an inquiring mind. 
* I say rigorous as the formulae included in AP a few years ago were just useful approximations.
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'You people, you think I know duck nothing; I tell you: I know duck all.'
Credited to Michael Curtiz by David Niven
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Rhys
Sasquatch
Reged: 15/01/2004
Posts: 3099
Loc: York (home of the speedbump)
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Quote:
Quote:
It isn't. The point of focus should be in the middle of the area in focus.
It's half infront and half behind the focus point for DoF (for critical use at least!). Something to do with the laws of physics I believe....hopefully somebody more clued up than me can explain why!
Sorry but you're wrong. Though for the sort of work you do, the difference is infinitesimal.
The extreme example is hyperfocal distance focusing. The far point of the depth of field is infinity and the near point is half the hyperfocal distance. Clearly there is a great deal more distance in "sharp" focus behind the focus point than in front of it.
If anyone needs the mathematical explanation, I'd be happy to provide it.
I was always told the depth of field is two thirds behind and a third in front of the focal point.. Most reference book say the same. Never have I heard it is half and half (unless the area behind is the bigger half  )
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NRIPN (Officially Nuts..)
RGMP.co.uk (My Website.. well early stages anyway)
Benchinistas.org.uk The home of Benchism
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daft_biker
Action Man!
Reged: 11/10/2006
Posts: 7540
Loc: Doon the glen
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The smaller the circle of confusion you use the closer you get to half and half.
Use a big enough CoC, a wide enough lens and a small enough aperture and you can get 99% of the DoF behind 
Basically, the less critical you are (or need to be when using larger formats) of what is acceptably sharp the more DoF will be behind.
Personally I find half and half works better for small things but by the time I get to shooting something like a full length portrait it probably looks more like 60/40.
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Andrew (BSRIPN) ... Pics.
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Rhys
Sasquatch
Reged: 15/01/2004
Posts: 3099
Loc: York (home of the speedbump)
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The distance marks on older lenses were useful in this respect, you could focus then align the aperture marks with the distance marks to govern what was in focus. New lenses unfortunately don't have this option.
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NRIPN (Officially Nuts..)
RGMP.co.uk (My Website.. well early stages anyway)
Benchinistas.org.uk The home of Benchism
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daft_biker
Action Man!
Reged: 11/10/2006
Posts: 7540
Loc: Doon the glen
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Quote:
The distance marks on older lenses were useful in this respect, you could focus then align the aperture marks with the distance marks to govern what was in focus. New lenses unfortunately don't have this option.
Did I read on one of those links that those scales were often aimed at producing a 10x8 inch print? Probably just aswell they don't have them on lenses now many want to see their work at A3 (or go pixel peeping  ). The larger print sizes would need a smaller CoC for all of the "sharp" area to look sharp which would mean shooting at a smaller aperture than indicated on the lens scale. If you create the scales for those wanting A3 prints then it's overkill for those only wanting 6x4s.
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Andrew (BSRIPN) ... Pics.
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Rhys
Sasquatch
Reged: 15/01/2004
Posts: 3099
Loc: York (home of the speedbump)
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Quote:
Quote:
The distance marks on older lenses were useful in this respect, you could focus then align the aperture marks with the distance marks to govern what was in focus. New lenses unfortunately don't have this option.
Did I read on one of those links that those scales were often aimed at producing a 10x8 inch print? Probably just aswell they don't have them on lenses now many want to see their work at A3 (or go pixel peeping ). The larger print sizes would need a smaller CoC for all of the "sharp" area to look sharp which would mean shooting at a smaller aperture than indicated on the lens scale. If you create the scales for those wanting A3 prints then it's overkill for those only wanting 6x4s.
Wouldn't have thought it mattered what the print size was - on a neg whatever is sharp is sharp no matter how big you blow it up, would have thought that would be the case for digi as well?
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NRIPN (Officially Nuts..)
RGMP.co.uk (My Website.. well early stages anyway)
Benchinistas.org.uk The home of Benchism
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daft_biker
Action Man!
Reged: 11/10/2006
Posts: 7540
Loc: Doon the glen
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Quote:
Wouldn't have thought it mattered what the print size was - on a neg whatever is sharp is sharp no matter how big you blow it up, would have thought that would be the case for digi as well?
Traditionally (I believe!) different CoC values have been used to calculate DoF for different formats. The larger the format the less you have to magnify the negative to make a given size of print so you can use a larger CoC. The more you magnify the image to make a print the more you are likely to notice things that looked sharp at smaller print sizes so the requirements of what is "sharp" change.
For example I've got pics that would look sharp avatar sized but wouldn't look anything like sharp even at a modest 600 pixels for the forum. Am sure I also have pics that look sharp at 600 pixels that wouldn't look sharp printed at A3 Haven't we all?
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Andrew (BSRIPN) ... Pics.
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beejaybee
Marvin
Reged: 18/07/2007
Posts: 4851
Loc: Really Here In Name Only
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Quote:
Traditionally (I believe!) different CoC values have been used to calculate DoF for different formats. The larger the format the less you have to magnify the negative to make a given size of print so you can use a larger CoC. The more you magnify the image to make a print the more you are likely to notice things that looked sharp at smaller print sizes so the requirements of what is "sharp" change.
Yes, but with the distance/DoF scales engraved on the lens, if you want to be more precise just use the marks for the next wider aperture, if you intend to be sloppy use the marks for a smaller one - that's effectively the same as fiddling the CoC, by a factor of 1.4 per stop.
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zx9
old hand
Reged: 22/06/2007
Posts: 992
Loc: London
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Quote:
For example I've got pics that would look sharp avatar sized but wouldn't look anything like sharp even at a modest 600 pixels for the forum. Am sure I also have pics that look sharp at 600 pixels that wouldn't look sharp printed at A3 Haven't we all?
To an extent the size of enlargement is irrelavant for any given format, a 6x4 print is viewed at less than arm's length, an A3 print is often mounted and looks best / natural when viewed from say 1 to 2M away. Viewing a large print from further away reduces the impression of softness in the slightly out of focus areas. Though taking it to extremes, if you printed your little avatar file to A3 and viewed from a distance such that it appeared to be the same size as you see it on screen, say 4 to 5M away it would seem just as sharp.
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ZX9 (Keith)  
My Flickr Pictures
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