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Thread: Dof

  1. #1

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    Dof

    I have been shooting large format for a short while and reading everything I can about it when I am not taking pictures. There is a lot of talk about depth of field being shorter in 4x5 vs. smaller formats. Why is this? How does this effect everyone's large format compositions. I have spent most of my life shooting 35mm and I feel like I am still composing for large format as I would with 35mm.

  2. #2

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    Re: Dof

    There are three things that determine the dof of a particular format. Roughly here they are: 1) Distance from the lens to the subject (the closer, the less dof). 2) Focal length of the lens (the longer the lens the narrower the dof). 3) The f/stop (dof increases as one stops the lens down)

    As one moves up in format (35mm to 4x5 to 8x10) the "normal" lens for the format size increases -- thus giving people the idea that since a normal lens for 35mm is 50mm and normal for 8x10 is 300mm, then with a normal lens, 8x10 will have a shallower dof than a normal lens on 35mm.

    AA was asked for advise by Thomas Joshua Cooper -- should he stick with enlarging 5x7 up to 11x14, or go with a 11x14 camera. AA suggested staying with the 5x7 -- because the normal lens for the 11x14 would create too many dof problems.

    Comparing format sizes is also complicated by the degree of enlargement. Something about the circle of confusion and all that -- but I'll have to leave that to someone else -- I have not got that part into my head properly yet.

    Vaughn

  3. #3

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    Re: Dof

    The usual analysis goes something like this. Large format uses longer focal length lenses for the same angle of view, thus resulting in a smaller DOF. So, to get identical DOF in a larger format, you have to stop down more (for e.g., the DOF you get at f/8 on 35mm will require about f/22 on 4x5). Stopping down increases diffraction but since the larger format is being enlarged less (i.e., the circles of confusion are enlarged less), this becomes moot. So what you get with larger formats is not smaller DOF but a loss of speed (i.e., you will be using longer shutter speeds and may or may not be able to deal with movement etc ).

    Cheers, DJ

  4. #4

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    Re: Dof

    There is a lot of talk about depth of field being shorter in 4x5 vs. smaller formats. Why is this?

    Hello from France
    If "there is a lot of talk" it is probably because the problem is most of the times not well defined.

    It can be shown that if
    1/ the images are examinated on a print with the same final size
    2/ the images were recorded with top-class lenses stopped down to their best aperture, and not too much so that diffraction is not the limiting resolution factor,

    then, when both conditions are met, there is absolutely NO gain in depth of field for a given scene recorded from the same location in space with the same angle of view with different formats.

    This might be hard to accept, but it is a common situation that images recorded with small formats are printed to small size whereas images recorded with a large format are printed large size.
    It is unfair to compare DOF in those conditions !

    The proof for this is contained in 3 simple DOF formulae
    the hyperfocal distance is H=f^2/(N.c) where f is the focal length, N the f-number and "c" the acceptable circle of confusion. The limits of DOF p1 and p2 in object space for far distant objects depend on only on H and the nominal distance p, not the focal length.
    the formula is 1/p{1,2} = 1/p (+ or -) 1/H

    Condition 1/ and same angle of view imply that in the comparison the ratio f/c is constant between the different formats considerened in the test ;
    condition 2/ is more subtle ; a compilation of the best f-numbers for classical standard lenses from 35mm photography to the 8x10" format shows that the best f-number is roughly equel to f(in mm) divided by 8 mm for classical 6-element lenses, f(in mm) divides by 11 mm for top-nothec last generation lenses. Open the lens more than the best f-number, image quality is (slowly) degraded by residual aberrations, stop beyond the best f-stop, image quality is (slowly) degraded by diffraction.
    Hence the ratio f/N is constant at the best f-stop for different lenses of standard angular coverage (60-70░) all formats, from 35mm to 8x10" !

    Conditions 1/ + 2 together imply that the hyperfocal distance does not depend on the film format if the experiments are done according to those very strict comparison conditions.... not always fulfilled in real life, of course !

    And the origin of endless discussions on DOF come from the fact that different photographers consider different situations not as strict as stated above.

  5. #5

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    Re: Dof

    As pointed out, it's the focal length of lenses typically used in LF versus smaller formats that is the cause of shorter DoF; a 300mm lens has the same DoF in 35mm as it does in 8x10. You just have a wider field of view in LF.

  6. #6
    Drew Saunders drew.saunders's Avatar
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    Re: Dof

    Here's a handy tool that takes into consideration circle of confusion (a term that causes confusion in many circles) as well as aperture diameter:

    http://www.dofmaster.com/dofjs.html

    A "portrait" lens for 35mm might be 90mm, while a reasonably similar angle of view for 4x5 would be from a 300mm (comparing 2:3 ratio film/digital with 4:5 makes the "equivalent focal length" math a little fuzzy). You can figure similar subject distance for those two lenses, at least similar enough to use that tool. For a head 'n' shoulders shot, you can figure about 5' or 1.5m subject distance. Plug in the numbers for a 90mm at f2, 2.8, 4 and 5.6 on a 35mm camera, then fiddle with the settings to see what f-stop you need to get a similar dof range for the 300mm on 4x5.

    Short answer for dof:
    A larger physical aperture gets you less dof. (That you get from the f-number and focal length of the lens).
    A greater degree of enlargement gets you less dof. (That's where you use the circle of confusion).
    Bigger cameras need longer lenses, with correspondingly larger apertures, but need less enlargement, so using a calculator that does both f-number and CoC is handy.
    Flickriver (to avoid Flickr's annoying new format): http://http://www.flickriver.com/photos/drew_saunders/

  7. #7
    hacker extraordinaire
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    Re: Dof

    Roughly here they are: 1) Distance from the lens to the subject (the closer, the less dof). 2) Focal length of the lens (the longer the lens the narrower the dof).
    In other words, the only thing that matters is magnification, since magnification is completely described by those two things.

    As pointed out, it's the focal length of lenses typically used in LF versus smaller formats that is the cause of shorter DoF; a 300mm lens has the same DoF in 35mm as it does in 8x10. You just have a wider field of view in LF.
    Convoluted explanations can be simplified by identifying the underlying factor...magnification. Photographers don't work with magnification too much, unlike microscopers. Fundamentally, the focal length of the lens and the distance from the subject affect dof, but only because those things change the magnification. It's simpler to realize that the real factor in play is magnification.

    Depth of field is a function of exactly three things--Magnification, Aperture, and circle of confusion. The size ratio between the size of the subject in real life and the size of the subject on the film is what matters. Blow it up larger, you will have less DOF. It doesn't matter if you blow it up by moving closer to the subject or by using a longer lens. Since we use bigger film in LF we typically end up magnifying the subject more. Thus we have less DOF at a given aperture.

    At the same level of magnification, all camera formats will have the same dof...the only difference is essentially a crop factor. If smaller formats seem to have more dof, it's because they tend to use short lenses that don't magnify the subject as much.

  8. #8

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    Re: Dof

    At this point of the discussion, I realize that we should always speak about APPARENT DOF.

    Depth of Field (DOF) is NOT the intrinsic property of a certain lens, focal length, or whatever combination of optical parameters if we do not start by putting clearly what will be the conditions of observation for the final image.
    Observed by a human with his naked eyes ? From 1 foot ? From 10 feet ?
    By a robot fitted with a top-notch diffraction-limited lens ?
    Observed through a scanning elecron microscope ?
    Seen from space by the Hubble space telescope ?

    Conventional values for the Cercle Of least Confusion "c" were defined in the good old days (where most cameras were fitted wth a non-interchangeable "normal" focal length equal to the negative diagonal) according to the followng viewing situation : you look at the print from a distance equal to the diagonal of the print and you define the value for "c" according to the miniumum visible feature size. I mean : visible by humans : one minute of arc for jet-fighter pilots, 2 minutes of arc for the rest of the world ;-)
    For standard photographic lenses (focal length = diagonal of the negative) this yields c= f/1720 for 2 minutes of arc; translate to: about 90 microns (3.5 mils) for a 150mm in 4x5", 180 microns for a 300mm in 8x10".
    For the jet-fighter pilot, you would take half of those values, 45 microns in 4x5" i.e. something considered as acceptable for old 6x6 cameras !!
    The only rock-solid figures are in fact those data for human vision, one or two minutes of arc, and even those numbers are fuzzy by a factor 2 !!

    If you want to print a large poster-size image, say 8 by 10 feet, for an exhibition, and if you insist that the public should experience the image from a distance of 1 foot, your depth of field calculations _have_ to be very different from what you would request for a letter-size image in a printed book seen from one foot of distance.
    Or for the advertising brochure of your local supermaket, with sales on corn-flakes at incredible prices...(yes, I know that in the good old days, corn flakes packages for serious sales brochures were always photographed by skilled professionals with a view camera in a professional studio ;-) )

    Hence in those discussions, the debate can be actually endless if the honourable assembly does not agree on the viewing conditions for the final images prior to start discussing !!

    I've seen large colour prints from a 8x10" slide, poster size about 3x4 feet, where each postcard-size portion of 4x6 inches is as sharp as a good postcard.
    This is what a good view camera lens is capable of delivering, this is so amazing that it defies the imagination. If you only allow the public to look at the image from a distance equal to the diagonal of the poster, then, if the image was taken with a 300 or 360mm in 8x10, stopped down to the best f-stop of 22 or 32, there is no gain or loss in DOF with respect to an image taken on 35 mm film with a focal length of 45-50mm used at its best f/stop, about 5.6
    However the image from the 35mm film will (hopefully for LF geeks) appear terrible compared to the LF image, and if somebody whispers : eh, Emmanuel, wait a minute, do not forget about digital ? then enters another player in the game : noise, plus top-secret built-in image sharpening tools hidden inside digital camera proprietary sofwtare !
    But this is another story for another endless debate ! ;-);-)

  9. #9

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    Re: Dof

    I think Emanuel's response must be correct, but as written, it may be subject to misinterpretation.

    The issue is not just DOF; it is exposure time. With a 4 x 5 camera, you typically end up using much longer exposure times than you would with a 35 mm camera, assuming, that is, that you want to produce basically the same final print, with the same things in focus in the print. I think this is perfectly obvious in practice to anyone who does both types of photography, but the explanation requires some theory. If subject motion is an issue, this can put constraints on how much DOF you can obtain.

    Let me explain.

    The basic fact is that the hyperfocal distance is directly proportional to the SQUARE of the focal length while being inversely proportional to the diameter of the acceptable circle of confusion (coc) and also inversely proportional to the f-number.

    Say you go from 35 mm to 4 x 5 format. To produce a final print the same size, you need to enlarge one fourth as much. That means you should be willing to accept a coc four times larger, which reduces the hyperfocal distance by a factor of four. If you want the same angle of view in each case, you need to multiply the focal length by a factor of four, so, for example, if you used a 75 mm lens in 35 mm, you would have to use a 300 mm lens in 4 x 5. The result is that the hyperfocal distance gets multiplied by a factor of 4 SQUARED or 16. Combining the effects, the hyperfocal distance gets multiplied by a factor of 16 divided by 4 or by a factor of 4. Since the hyperfocal distance is now greater, you have LESS DOF, if you use the same f-stop.

    But therein lays a subtle point. You are not likely to be using the same f-number for 4 x 5 as you would be using for 35 mm. Indeed, you wil likely end up using an f-stop with f-number multiplied by 4 compared to what you would use for 35 mm, nd that is fine as long as the image is static.

    Suppose as is typically the case, you want to include both a near point and a far point in focus, but you don't care if you include even more. DOF theory tells you the smallest f-number which will work according to the above assumptions. On the other hand, as you stop down further, i.e., increase the f-number, you increase the likelihood that diffraction effects will be visible and degrade the image. This also is affected by the degree of enlargement. The result is that for any format, there is a range of acceptable f-numbers for which every thing you want in focus will remain in focus and diffraction won't limit you in the final print. Assuming as above, that you keep the same size final print and the same angle of view, that range shifts upward as you increase the format size. For going from 35 mm to 4 x 5, you need to use a factor of four at both ends which means a shift of eight f-stops in where the range starts and stops, but the total number of acceptable stops remains the same.

    As noted before, this is fine as long as the image is static. But, there is a real difference, which I alluded to at the beginning. If you have to stop down further by some number of stops, then you must also increase your exposure time to compensate. That means that for exactly the same picture, with the same size final print and same angle of view, you will be using significantly slower times with 4 x 5 than you will with 35 mm. If subject movement is a consideration, then pictures you might easily take with 35 mm may be very difficult with 4 x 5 to capture. This can be a factor even for a static landscape if wind rustles the foliage.

  10. #10
    the Docter is in Arne Croell's Avatar
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    Re: Dof

    Quote Originally Posted by Leonard Evens View Post
    I think Emanuel's response must be correct, but as written, it may be subject to misinterpretation.

    The issue is not just DOF; it is exposure time. ....

    ..... If you have to stop down further by some number of stops, then you must also increase your exposure time to compensate. That means that for exactly the same picture, with the same size final print and same angle of view, you will be using significantly slower times with 4 x 5 than you will with 35 mm. If subject movement is a consideration, then pictures you might easily take with 35 mm may be very difficult with 4 x 5 to capture. This can be a factor even for a static landscape if wind rustles the foliage.
    But that assumes you use the same film speed. Following the same argument, you can allow a grain that is roughly 4x as large linearly in 4x5 vs. 35mm for the same size print with the same graininess (that this is usually not desired for LF is a different matter). So by going from ISO 25 in 35mm to ISO 400 in LF you have the same shutter speed...

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