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Macro DOF


KatieR 17 6.2k 6
16 Feb 2008 5:29PM
I read in a magazine today (yes, it was one on photography Tongue), that while Depth of Field normally extends twice as far behind the point of focus than it does in front, this "rule" doesn't apply to macro situations.

I don't think I've ever heard that before. Is it true or is it more that the depth of field can be so incredibly tiny when shooting close-up that any difference is imperceptible?

Sorry to be so picky, but, hey, there's nowt on telly Smile
JohnHorne 16 1.0k
16 Feb 2008 5:39PM
It's true. For close-up work depth of field extends approximately equal distances in front of and behind the plane of focus.
KatieR 17 6.2k 6
16 Feb 2008 5:44PM
oooooh! how exciting!

So when does the rule change?
AnthonyM 16 428 2 United States
16 Feb 2008 5:44PM
Can't imagine how. Physics doesn't give up its laws for certain people. There is only a thin pane that is "in focus". The rest "appears acceptably sharp" based on distance from that plane.
As things come closer we get more detail and are more critical of "acceptably sharp".
As things get further away, there is less detail and "acceptably sharp" can cover a greater range.

The area of focus may be so thin that 1/3 v.s. 2/3 may be fairly imperceptible.

Sorry, cannot recall all the technical jargon, but physics don't care about "jargon" either.

Of course, I'd be interested to see the physics that prove otherwise.
KatieR 17 6.2k 6
16 Feb 2008 5:45PM
Oh, I take that back then...

would you like to discuss amongst yourselves?
AnthonyM 16 428 2 United States
16 Feb 2008 5:48PM
Actually thinking about it and with what I have seen myself, 1/3 v.s. 2/3 is just an estimate, and I believe it changes to some extent with focal length and focus point.

I am sure there is greater DOF behind the subject but the difference probably becomes further decreasing as you get closer with longer focal lengths to the extremes.

Does that sound right?
There was a great page on the details of how that works, but I cannot seem to find it.
KatieR 17 6.2k 6
16 Feb 2008 5:56PM
Yes, the 2/3 - 1/3 thing is only a rough estimate, so perhaps the reality should not be surprising.

I hate it when "rules" are whipped away from right in front of me!
chrissd 15 304 United Kingdom
16 Feb 2008 6:08PM
when you are in super macro..as with flowers...to create an effect...say at f2.8, your depth of field is measured in mm, for me what is in focus thru the lens is on all else is out, which helps create some stunning effects 2/3....1/3 its all the same, what you see is what you get, i dont allow rules to run the course, all rules are meant to be broken
JohnHorne 16 1.0k
16 Feb 2008 6:30PM
If you are interested you can look up formulae for calculating nearest point in focus and furthest point in focus based on focus distance, focal length , aperture etc.

Put your chosen values into the formulae and you will find that the closer the focus distance the more equally the depth of field is distributed.

For a particular focal length, aperture etc, you might then calculate the following values:

Focus at 50mm, nearest point in acceptably sharp focus is 49.95mm, furthest point 50.05mm (depth of field is approximately 50% in front, 50% behind)

Focus 100mm, nearest 99.8mm, furthest 100.2mm (dof approx 50 / 50)

Focus 1,000mm, nearest 980.4mm furthest 1,020.4mm (dof approx 49 / 51)

Focus 5,000mm, nearest 4,545mm, furthest 5,556mm (dof approx 45 / 55)

Focus 10,000mm, nearest 8,333mm, furthest 12,500mm (dof approx 40 / 60)

Focus 20,000mm, nearest 14,285mm, furthest 33,333mm (dof approx 30 / 70)

Focus 30,000mm, nearest 18,750mm, furthest 75,000mm (dof approx 20 / 80).

So, stating that the ratio is 50 / 50 for close-ups is a lot closer to the truth than saying that it is 1/3 / 2/3 for longer distances. (For most practical purposes focusing on any more than 20m to 30m is to focus on infinity.)

The ratios calculated would be different for a different focal length and aperture, but the conclusion would be the same.

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