although using the aperture to create tha DOF i require is no problem for me, i still don't understand some things fully.

do all lenses with the same focal length at the same aperture have the same DOF?

why do larger formats shorten the DOF? or is it just the longer focal length required to get the same anlge of view?

is the DOF the same for a 50mm lens at 1.4 at 4m as with a 100mm lens at 2.8 at 2m?

i am confused...

-- stefan randlkofer (geesbert@yahoo.com), April 10, 2002

Answers

Hi,

Yes, all lenses with the same focal lenght have the same or very similar DOF at the same aperture. A 50mm lens is still 50mm lens wether you use it on a 35mm camera or on a medium format camera.

Frank

-- Frank (frank_bunnik@hotmail.com), April 10, 2002.

Try to read Erwin Puts website; he explains very clear how it works!

http://www.imx.nl/photosite/technical/DoF/DoF.html

Joop

-- Joop (mes@nat.vu.nl), April 10, 2002.

I am not sure that what Frank said is completely correct in practicality. DOF is calculated to give a certain circle of confusion depending on how many times you are going to enlarge a given sized film to achive certain sized print or projection and viewing distance. Technically speaking there is only one plane of focus that is sharp. Areas before or after it become progressively more unsharp. Depending on how many times you enlarge your film this area can be wider or narrower for a given focal lenght and aperture. I am sure somebody will give us the exact amount of circle of diffision allowed for different formats, I cannot find my own notes.

-- sait (akkirman@clear.net.nz), April 10, 2002.

As I understand it, depth of field, other things being equal, (eg the distance of the object being focused on) is a function of the ABSOLUTE size of the aperture (f stop is a relative, not an absolute measure, which is why say, f8 in a 28mm wide angle has much smaller diameter than f8 in a 135mm long focus lens.) Of course this is also why the same f stop in a long focus lens gives much less depth of field than in a wide angle - the "hole in the middle" is larger! Similarly f8 in a medium format camera is much larger than an equivalent lens in 35mm format - and has therefore less depth of field. As an aside, it also explains why with large format cameras, Ansell Adams could get away with using f32 or f64 (reduction in image quality caused by diffraction on stopping down is also determined by the absolute size of the aperture, not its relative size.)

-- Peter (peterm1@ozemail.com.au), April 10, 2002.

Harold Merklinger wrote some articles on depth of field for Shutterbug about 10 years ago. When they first came out I thought they were unnecessarily complex, but over the years I have come to realize that he has some very useful insights into depth of field and bokeh. They will repay careful study. Links to them are found at http://home.fox.nstn.ca/~hmmerk/HMbook14.html.

-- Ed Buffaloe (edb@unblinkingeye.com), April 10, 2002.

don't wish merklinger on anybody!! as for the original question, depth of field is dependent on two factors: actual iris size and magnification (either as a function of optical magnification or film to subject distance). however, the way these two factors affect dof varies. as noted already, there is only one point (or, more accurately, plane) of sharp focus. everything in front and behind that plane will be out of focus to some degree. the amount of defocus effect depends on actual iris size. with an infinitely small iris, you will have infinite depth of field (and this is why pinhole cameras seem to have infinite depth of field -- it's not actually infinite, but close to it). a very large iris -- as in a noctilux where the aperture is 50mm (remember f stop = focal length divided by aperture size) produces shallow depth of field. without going into any detail, this effect has to do with how lenses render off-axis versus on-axis light on the negative. the phenomenon is identical to the one that allows people with two eyes, spaced some distant apart to have depth perception. if our eyes were coincident (or if we just had one), we would not be able to record depth any better than a pinhole camera. another manifestation of the principle is the split ring rf in an slr lens that uses the aperture width as the base length for its rf.

however, how much we are able to see the defocus effect -- large or small -- depends on how much we magnify the images we are photographing -- either by getting closer or by using a higher mag (i.e. longer focal length lens). it may come as a surprise to some people, but at f8, all lenses will have approximately the same dof with respect to the same picture section. in other words if you take a photo of a group of people with a 50mm lens at f8 at distance X, such that the two people in the center of the group fill the whole frame, the amount of dof will be almost IDENTICAL as compared to a photo taken with a 20mm lens at a MUCH closer distance Y, such that the same two people fill the entire frame. the reason why the dof is not identical is that another property of wide angle lenses is that they distort perspective relationships, and so it will be impossible to get identical picture sections with identical magnifications of all the elements in those pic sections with the two lenses in the hypothetical i described.

likewise, if you took both shots -- i.e. using the 20mm lens and the 50mm lens -- from the same distance, and merely enlarged the same pic section of the 20mm lens negative so that it matched the picture section of the negative taken withthe 50 mm lens, the dof would again be identical in the two negs.

remember, only things that actually lie on the plane of focus will be sharp. everything else will be unsharp to some degree. the dof tables used to mark modern lenses were compiled many years ago when resolution standards were much lower (film, paper, and lenses have gotten better over the years). do not make the mistake of thinking that because you are within marked dof limits you will get sharp pics. they should be acceptibly sharp according to some dead scientist's definition of acceptibly sharp. always focus as carefully as you can.

finally, as for the question about different formats, i hope the answer is obvious now. an 80mm lens at ten feet on 6x7 will produce roughly the same pic section as a 50mm on 35mm at ten feet. there will be less dof in the shot with the 80mm, but you don't have to enlarge the 6x7 neg as much to get to the same final enlargement size. thus 8x10 prints from the two negs should show comparable dof effects.

-- roger michel (michel@tcn.org), April 11, 2002.

"Do all lenses with the same focal length at the same aperture have the same DOF?"

Yes - shoot 35mm pictures with a Leica 75, a Rolleiflex 75, and a Schneider 75 Super-Angulon (for 4x5) - at the same aperture - and the framing and depth of field will be the same.

"Why do larger formats shorten the DOF? or is it just the longer focal length required to get the same anlge of view?"

Right - an 80mm 'normal' lens for 6x6 and a 150mm 'normal' lens for 4x5 will have less depth of field - at the same apertures - than a 50mm 'normal' lens for a 35mm. As mentioned - the final print enlargement will have some effect on PERCEIVED depth of field - so the larger formats have some advantage there, but less than they lose to the effect of the longer focal lengths, which is why you have to shoot view cameras at f/22-90 for most shots if you want everything sharp, even with a 90mm wide-angle - whereas you can usually get everything sharp at f/8 with a 28 on a 35mm.

"Is the DOF the same for a 50mm lens at 1.4 at 4m as with a 100mm lens at 2.8 at 2m?"

No - it's closer to the other way around - a 50 f/1.4 at TWO METERS will have the DOF (roughly) of a 100 f/2.8 at FOUR METERS - both wide open. But the image magnification will also play a part - the blurred circles in the background of the 100mm shot will be equally blurry, but also blown up twice as big by the telephoto framing - so they'll SEEM even more soft.

By the time you factor in focal length, aperture, format size, relative subject distance, and final print magnification - it becomes a really complex dance. =8^o

-- Andy Piper (apidens@denver.infi.net), April 11, 2002.

What Andy said. Plus: the viewing distance from the eye to the print (or the screen) is also part of the overall magnification, also affecting DOF.

-- Bob Fleischman (RFXMAIL@prodigy.net), April 11, 2002.

now i think i am closer to understand it. does that mean a 90mm summicron has (nearly) the same DOF as a 50mm noctilux, both wide open, with the distance adjusted?

-- stefan randlkofer (geesbert@yahoo.com), April 12, 2002.

Harold Merklinger wrote two books on depth of field

The INs and OUTs of Focus

Focusing the View Camera

Both has in depth discussion on Depth of field

-- martin tai (martin.tai@capcanada.com), April 12, 2002.

http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=004RVI

-- martin tai (martin.tai@capcanada.com), April 12, 2002.

if you read focusing the view camera, you would be well advised to seek alternate opinions on his suggestions for focusing algorhythms. there are a number of people who disagree with his results.

-- roger michel (michel@tcn.org), April 12, 2002.

I check out the result of Focusing the View Camera with analytical geometry and obtain basically the same result. Merklinger's method is correct.

-- martin tai (martin.tai@capcanada.com), April 13, 2002.