depth of feild and focal length

[emz]

What is the links between depth of feild and focal length?

[Emmanuel BIGLER]

Ever heard about the hyperfocal distance H ?
Set the focusing distance to H and the depth of field will go from H/2 to infinity. At least in the classical geometrical model.

H = f*f /(N*c)
So the link between the hyperfocal distance and the focal lenght is : the f*f term in the formula.

Link to the depth of field limits : p_1 and p_2 being the limits of acceptable sharpness, p the distance setting, we get :
1/p_{1|2} = 1/p {+|-} (1/H)(1-f/p) where H = f*f / (N*c), f = focal length, N = f-number, c circle of confusion.
This summarizes everything you need.

[Paul Fitzgerald]

Hi there,

emz, longer lenses have a shallower depth of field for each focus distance than shorter lenses for general and scenic photos. For close range, tabletop and macro, the depth of field relates to the magnification ratio at the film plane. The extra bellows extension also changes the f/stop ratio and needs to be re-calculated.

At the same magification, longer lenses 'see' farther around the object than shorter lenses, so they give a different 'look'. Longer lenses also avoid the wide field distortion effect of shorter lenses and change the apparent difference between foreground, midfield and background. Just look at the groundglass to see the changes.

Hope it's a help.

[Neal Wydra]

Dear emz,

A good place to start with questions like these is the LF Photography home page.

Here's a link to an article worth reading. Slide by the math if you like, it's not necessary unless you are going to do the calculations yourself.

http://www.largeformatphotography.info/articles/IntroToDoF.pdf

[Leonard Evens]

The others have pretty much covered it. Let me just add that these considerations usually assume a fixed format size. If you change the format size, such as going from 35 mm to 4 x 5, and you keep the same angle of view, then the focal length necessarily changes. But so does the acceptable circle of confusion because you enlarge by a different factor for the same size final print. The net result in that case is that depth of field decreases, but linearly. Thus for going from 35 mm to 4 x 5, you might multiply the focal length by up to 4 to maintain the same angle of view, and you would have to multiply the f-number by 4 to maintain the same depth of field. That would be 4 stops difference. On the other hand, if you kept the same format, say 4 x 5, and multiplied the focal length by 4, going from wide angle to long, you would have to multiply the f-number by 16 (4 squared) for the same depth of field, a difference of 8 stops.

[Joseph O'Neil]

While there are many fine books and articles on the subject, theere is an easy and crude way to deal with depth of feild (DOF).

for 4x5, here's my own scale of things, with apologies ahead of time to those who have no sense of humour;

Lens Size --------------- DOF

75 to 90mm -------------- totally excellent dude

105 to 135mm ----------- not bad, but don't take things for granted

150 to 180mm ----------- not so great

210 to 300mm ----------- really sucks big time

over 300mm ------------- the only way you get any DOF at this range is to either measure it with a micrometer, or smoke some of the funky stuff and loose all touch with reality.

:)

joe

[James E]

"Ever heard about the hyperfocal distance H ?
Set the focusing distance to H and the depth of field will go from H/2 to infinity. At least in the classical geometrical model.
H = f*f /(N*c)
So the link between the hyperfocal distance and the focal lenght is : the f*f term in the formula.

Link to the depth of field limits : p_1 and p_2 being the limits of acceptable sharpness, p the distance setting, we get :
1/p_{1|2} = 1/p {+|-} (1/H)(1-f/p) where H = f*f / (N*c), f = focal length, N = f-number, c circle of confusion."

WHAT THE FUCK

[Ralph Barker]

James - language of that sort isn't appropriate here.

[Scott Fleming]

Ralph,

The quote? Or the WTF?

[Ralph Barker]

LOL, Scott. As that appears to be James' first post, and he uses a questionable e-mail address, we'll have to see if he's actually serious. If not, we'll clean up appropriately. I just hate it when tourists come through and spit on our floor.