When I think of a wide angle lense ie: 21mm 24mm I usually think of "wide angle" or the horizontal plain left to right as seen through the viewfinder. But doesn't it stand to reason these wide angle lenses also offer more clarity of focus (depth of focus) than a normal or tele lense? If both a wide and normal/tele are stopped down completely, a wide angle lense creates a smaller aperture than a longer lense, further CONDENSING LIGHT thus making objects sharper at staggered distances. Am I right or just thinking too hard?

-- Jim Ford (fordhayes01@aol.com), March 29, 2001

Answers

But it really isn't a smaller hole, because it's closer to the film, which makes it appear, from the film's position, to be the same size as a large hole farther away.

Now I hope we don't get into that whole depth of field discussion. Your assignment for tonight is to shoot the exact same picture from the same spot, with your 50mm and your 21, both at the widest opening that they both have in common--probably f/4 or f/2.8--and then blow the 50 shot up to 8x10, and crop that area from the 21 negative, and blow that area up to 8x10, and tell me what you see.

And yes, you undoubtedly are thinking too hard, but that's just a general comment, based on the idea that if you have to ask. . . . :-) But that doesn't mean you should stop. :-)

-- Michael Darnton (mdarnton@hotmail.com), March 29, 2001.

If I understand you right, I think you're right. While the exposure would be the same for a given f/stop (ref. your example of the lens being stopped down completely), depth of field would increase the wider the lens, and decrease the longer the lens because depth of field is independent of focal length. It is dependent upon the actual size of the aperture. This is why, for example, the 15mm lens on the 8x11 Minox, which produces about the same angle of view as a 50mm lens on a 35mm camera, has a staggering depth of field that extends from about 6 feet to infinity when the focus is set to about 11 or 12 feet. This is also why the small format camera (the 35mm) is such a good choice for quick/candid/general photography over, say, a medium format or a large format camera: For a given angle of view, the depth of field is less, therefore less versatile in terms of focus.

OK, did I understand your question properly, or did I just go 180 degrees off in the other direction? And for what I _did_ just explain, am I right about that?

-- Tony Rowlett (rowlett@mail.com), March 29, 2001.

I knew I had to mess SOMETHING up in my answer above. What I meant to say about medium or large format cameras is that THEIR depth of field is less, for a given angle of view, than the 35mm camera, and that is why the 35mm camera is more versatile in terms of depth of field, for the candid/casual/general photography category.

-- Tony Rowlett (rowlett@mail.com), March 29, 2001.

Is it not correct that the depth of field depends only on the focal length and the aperture? In other words, if you could put a 50mm lens on a 5 x 4 camera, then the depth of field would be identical to that on a 35mm camera at the same aperture. The field of view would be much greater on the 5 x 4 but the range of sharpness would be the same.

There would be no increase in image sharpness in absolute terms on the film.

-- Wayne Murphy (wayne.murphy@publicworks.qld.gov.au), March 29, 2001.

Obviously, no one is bothering to actually DO the experiment. You're all thinking too much, acting too little. :-)

-- Michael Darnton (mdarnton@hotmail.com), March 29, 2001.

Obviously if you do the experiment, you will find that the small section from the 21mm image when enlarged to 8x10 is much less sharp than the identical area from the full image of the 50mm frame. This of course assumes that both lenses produce roughly the same resolution in line pairs/mm on the film. Theoretically, the 50mm lens is capable of much higher resolution at a given aperture than the 21mm lens at the same aperture (f-number) but the realities of lens design tend to reduce this maximum possible resolution to something much more modest.

-- mark (mramra@qwest.net), March 29, 2001.

Jim, when thinking hard we could conclude :) that an F-number is a relation of an aperture diameter to a focal length. The shorter focal length, the smaller aperture we see, and it doesn’t depend on lens design: tele-, retro-focus, or obvious.

A DOF, which is expressed as an acceptable value of a COC, depends directly on a distance to a subject, and depends inversely on an F- number (1/1, 1/2,....1/64..., and so on) and a focal length.

A DOF doesn’t depend on a film/sheet format and on an angle of coverage of a lens.

-- Victor Randin (www.ved@enran.com.ua), March 30, 2001.

Reduce focal length by half, the depth of field increases FOUR folds. For example, A 24mm lens has about FOUR times the depth of field of a 50mm lens, assuming both lens are set the same fstop.

Alternatively, you can use FOUR time as wide aperture in a short focal lens to get the same depth of field as a lens which has twice the focal length For example, a 24mm lens at about f2.8 can achieve the same depth of field as a 50mm standard lens at f11 ( 11 /4 =2.8)

-- martin tai (martin.tai@capcanada.com), March 31, 2001.

If we take a box camera with a hole as a lens, the distance of this hole to the film plane, and the size of the aperture hole would make the F/number, (150mm focal distance and a 1mm aperture hole makes a F/150 stenopeic camera) the actual size of the aperture hole will draw the points of ligth, the bigger the difuser, the smaller the sharper, the size of format doesn´t change this rule.Shorter focal length smaller aperture; 24mm F/4= 6mm, 200mm F/4= 50mm, camera makers neather change this rule.

-- R Watson (al1231234@hotmail.com), April 04, 2001.

Martin, what you wrote holds true assuming the same camera to subject distance for both lenses. But if you move in correspondingly closer with the shorter focal length, until the same subject area is being photographed (the subject is rendered at the same image size on the film) then the depth of field advantage fades to almost nothing. For example, the Leica M compendium gives the DOF of a 35mm lens at f/1.4 and one meter, as 0.97 to 1.04 meters. The 75mm at the same distance must be stopped down to f/8 to give the same DOF. But now back away with the 75 to 2 meters, so as to cover the same image area in the plane of focus. At f/1.4, the 75mm gives 1.97 to 2.03, almost as much depth as the 35 at one meter.

So we may conclude that the DOF advantage of the wider lens accrues from rendering the image smaller on the film at the same subject distance. If the image is smaller (less magnified), unsharpness is magnified less as well, making it more acceptable. No?

-- Bob Fleischman (RFXMAIL@prodigy.net), April 05, 2001.

Exactly.

-- Mani Sitaraman (bindumani@pacific.net.sg), April 05, 2001.

Rigth; phisics are some times hard to explain but very easy to understand.

-- R Watson (al1231234@hotmail.com), April 05, 2001.

DOF in terms of Image ratio

Bob raised an interesting question, what is thd DOF of lens with regards to the proportion of object vs image ? As a matter of fact, at the same Object-size/image size, the DOF is ALMOST independent of focal length. In other word, what Bob illustrated is correct, a wide angle lens and a long lens, if the image covers the same frame, and using the same aperture, the DOF of both lenses are nearly equal.

Examples:

A 60mm amd a 200 mm at fstop = 8 and circle of confusion of 0.03mm and an object/image ratio of 20 has a DOF of 203.03mm and 201.73mm respectively (the object distances are 1.26m and 4.2m respectively.

We can see that the difference in DOF of a 60mm lens and a 200mm lens at Object/image ratio of 50 is quite small.

However, when the object/image ratio is large, for example when object/image =50, the DOF become 1277mm vs 1228mm

DOF for MACRO

At close up or macro photography cases, the DEPTH OF FIELD DEPENDS ONLY ON OBJECT/IMAGE ratio, fstop and circle of confusion, and has nothing to do with focal length.

Example a 60mm Macro-Elmarit and a 200mm lens, at f8 and coc=0.03mm, if the Object/image ratio =5 (at distance of 36cm and 120 cm respectively ) has a depth of field in both cases= 14.4 mm

For close up and macro situation, the DOF can be represented by the following equation:

DOF = 2R*(R+1)*FSTOP*COC Where R=ratio of object size vs image size

In which there is no focal length.

substitute the numbers we have

DOF= 2*5*6*8*0.03 = 14.4mm

-- martin tai (martin.tai@capcanada.com), April 07, 2001.