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chachahavana
6-Jun-2008, 02:54
I currently use a 150mm f/5.6 on 4x5, usually shoot wide open.
Can any one suggest the dof outcome if I use a 300mm f/5.6 on 8x10, will I get a much shallow DOF than 150mm f/5.6 on 4x5 and is there a formula to calculate !?
Thanks for sharing !

chachahavana
6-Jun-2008, 03:55
I currently use a 150mm f/5.6 on 4x5, usually shoot wide open.
Can any one suggest the dof outcome if I use a 300mm f/5.6 on 8x10, will I get a much shallow DOF than 150mm f/5.6 on 4x5 and is there a formula to calculate !?
Thanks for sharing !

Robert A. Zeichner
6-Jun-2008, 04:31
How much depth of field you get at any given aperture will depend on magnification (a combination of focal length and subject to lens distance). Bokeh is a Japanese word used to characterize the appearance of the out of focus portions of photograph and while partially a result of the aperture used, really pertains more to the quality of those areas that are out of focus and not how little or how much depth of field you will get. The short answer to your question is yes, you will get shallower depth of field with the 300mm on 8x10, but only if you view both negatives of the identical composition from the same distance.

IanG
6-Jun-2008, 04:36
There are plenty of DOF tables on the internet and they'll save you the calculations. But yes you will have shallower DOF.

Think about what happens if you put a 300mm on your 5x4, or 100mm instead of a 50mm on a 35mm camera.

Ian

Ken Lee
6-Jun-2008, 04:38
All other factors being equal, depth of field is cut by 1/4, when you double the length of the lens.

To get the same depth of field with a 300mm lens, as you get with a 150mm lens, you have to stop down 2 stops more.

So if you shoot 150mm at f/5.6, then shoot 300mm at f/11 to get the same depth of field.

If the 300mm lens lets you open to f/5.6, then you will definitely get more bokeh or blur: 4 times the blur of a 150mm lens at f/5.6

Peter K
6-Jun-2008, 04:56
Why the same question in the camera- and in the lens-forum?

6-Jun-2008, 07:05
I suggest that you calculate the hyperfocal distances for the two set ups using the following equation taken from the "Introduction to Depth of Field" found at http://www.largeformatphotography.info/articles/IntroToDoF.pdf :

Hyperfocal distance = (Focal_length^2)/(F_number x Circle_of_Confusion)

But take into consideration; do you intend to make the same size print from both formats? and will you view the prints from the same distance? If so, then the acceptable circle of confusion in the 8x10 negative is twice as large as in the 4x5.

In example calculations with the acceptable circle of confusion in the print taken to be .25mm, I found the hyperfocal distance for the 300mm lens at f/5.6 on the 8x10 to be 211 feet, but for the 4x5 with a 150mm lens it was 105.5 feet. So yes, there is significantly less depth of field with the 8x10 when making the same size print.

Richard K.
6-Jun-2008, 07:18
I currently use a 150mm f/5.6 on 4x5, usually shoot wide open.
Can any one suggest the dof outcome if I use a 300mm f/5.6 on 8x10, will I get a much shallow DOF than 150mm f/5.6 on 4x5 and is there a formula to calculate !?
Thanks for sharing !

OK I may be wrong but I always thought that if you frame the same (i.e. same size image), the DOF is roughly (proportion in front to behind the exact focus in acceptable focus may change slightly) the same, independent of focal length of lens? This can be seen from DOF formulas, which have the f-stop in the formula, but not the focal length (or the product of aperture AND the focal length, which is the f-stop number). There is a common misconception that shorter focal length lenses have greater depth of field but if you even up the image size, that's just not the case. Not sure what this has to do with bokeh (signature rendition of the out of focus areas), though!
OK ket the arguments begin! :)

Richard K.
6-Jun-2008, 07:23
Er...product of f-stop and aperture is the focal length is what I meant! Unless of course that's wrong, then I meant the right thing....see how easy it is to get (circle of ) confusion! :o

6-Jun-2008, 08:56
...I always thought that if you frame the same {...} the DOF is roughly {...} the same, independent of focal length of lens? This can be seen from DOF formulas, which have the f-stop in the formula, but not the focal length...

Please, I am not trying to be argumentative but - What do you mean the focal length does not appear in the DOF formulas? The formula for Hyperfocal distance I quoted above has it in there - squared!

A possible source of this misconception is the formula for the F-number required to provide the necessary Depth of Field based on the depth of focus, or focus spread, given by Mr. Jeff Conrad in the source above as:

f-number = focus spread/(2 x CoC)

Focal length does not appear here explicitly but, if you consider how the "focus spread " is very much a function of the focal length of the lens then the dependency becomes clear.

It is not intuitive, but it is conventional wisdom (and correct) that the Depth of Field depends on the format of the camera. This is why point & shoot digital cameras with their tiny sensors and very short normal lenses are sometimes criticized for having no way to limit depth of field for portraits and other situations, and why ULF users have essentially none.

- Alan

Brian Ellis
6-Jun-2008, 09:40
OK I may be wrong but I always thought that if you frame the same (i.e. same size image), the DOF is roughly (proportion in front to behind the exact focus in acceptable focus may change slightly) the same, independent of focal length of lens? This can be seen from DOF formulas, which have the f-stop in the formula, but not the focal length (or the product of aperture AND the focal length, which is the f-stop number). There is a common misconception that shorter focal length lenses have greater depth of field but if you even up the image size, that's just not the case. Not sure what this has to do with bokeh (signature rendition of the out of focus areas), though!
OK ket the arguments begin! :)

If you change the focal length of the lens but frame the same then you necessarily have also changed your camera position, i.e. you've moved closer to or farther from the subject. Since distance from subject is one of the three factors that affect depth of field together with lens focal length and aperture, you've changed two of the three factors, not just one, and you've changed them in opposite ways. E.g. if you make a photograph with a 300mm lens, and then make the same photograph (as nearly as possible) with a 150mm lens, the shorter lens produces greater depth of field but you have to move closer to the subject to make the same photograph, which decreases depth of field. Ken Lee gave the relative proportions of each change in his message.

Shorter focal length lenses do produce greater depth of field than longer lenses if all you change is focal length, i.e. same aperture, same camera position, same enlargement factor, etc. etc. That isn't a misconception. The key to your misconception statement is that you said that in addition to using a shorter focal length lens you also "even up image size," which means you haven't left everything except focal length the same, i.e. to "even up the image size" you've necessarily either changed camera position when you made the photograph or changed the magnification factor when you made the print.

Richard K.
6-Jun-2008, 10:51
Shorter focal length lenses do produce greater depth of field than longer lenses if all you change is focal length, i.e. same aperture, same camera position, same enlargement factor, etc. etc. That isn't a misconception..

Well sure, but the image size is way smaller, objects in front and in back of the main focus are smaller and the COF is smaller, so yes there is more DOF BUT it's a different photo! I think that the misconception part is that for an identical composition (main subject image size the same) the shorter focal length lens will have greater DOF for a given f-stop. I'll concede that because of the size difference of objects behind and in front of the main subject, the proportion of the DOF in front and back changes but not the whole DOF - talking about regular (approximately infinity) photographs, not macros! I guess what I thought I was replying to was "if I take the same photo (same crop) in 4x5 and 8x10 at the same f-stop, with 150mm and 300 mm lenses (yes, I'm twice as far away now) respectively, will there be greater depth of field on the 4x5?" (If that wasn't the OP's question, then I'm not answering what I thought I was.)

I think it will be the same. Am I wrong? I guess what I think I'm saying is that DOF depends on f-number not focal length (directly) if the image size is the same, to a very good approximation. I'd be totally happy to be told I'm wrong by someone other than my wife and teenage sons! :)
Here is a simplified demo:

http://www.bluesky-web.com/dofmyth.htm

Richard K.
6-Jun-2008, 11:01
Please, I am not trying to be argumentative ...

Alan, me neither! We're just trying to learn, right? But there is a difference in hyperfocal distance formula (which clealy needs to factor in focal length) and DOF formula. DOF I think (see I'm not sure :) ) depends only on f-stop and image size for usual near infinity (which can be as close as 20 feet!) photography. Please see my reply to Brian Ellis ...

sparq
6-Jun-2008, 11:23
Richard, yes, you are wrong. Read Brian's post again, he gave you a good explanation why. You mix together DOF effects of varying FOV for a fixed format with DOF effects of varying format for fixed FOV - in both the focal length changes but the former one is irrelevant in the context of this discussion because, as you correctly said, you get a different picture.

Ad your last post: How could DOF stay the same while hyperfocal distance increases? Think of it, that makes no sense. There's no magic there, just simple geometry.

Richard K.
6-Jun-2008, 11:27
Ooops, with 300mm lens on 8x10, I'm obviously the same distance away as 150mm on 4x5! However, I don't think there will be a difference in DOF but please explain if I'm wrong. But I'll stand by what I thought was being argued and just state that for a given format and a given subject framing and aperture, lenses of all focal lengths have exactly the same depth of field (yes the distance to the subject would have to change to keep the subject size the same). Wrong or right?

Richard K.
6-Jun-2008, 11:30
Richard, yes, you are wrong.

Yes, I misunderstood, I am wrong. I'll go back to things I know about...like spending money on more LF stuff!

6-Jun-2008, 20:51
Yes, I misunderstood, I am wrong. I'll go back to things I know about...like spending money on more LF stuff!

But don't be too hard on yourself. I just ran through the math and confirmed that your point is essentially true for the case where the f-stop, format, circle of confusion and image size (or subject distance to focal length ratio) all stay the same - as long as the subject distance is relatively short, say less than about 10 focal lengths. Changing to a shorter focal length lens gives very little increase in depth of field.

However, as the subject distance to focal length ratio becomes larger, the shorter focal length does increase depth of field significantly.

I did not know that.

Ken Lee
7-Jun-2008, 14:59
I should have mentioned the formula I suggested, I got on this forum, from Leonard Evens: one of our esteemed LF members, Professor Emeritus of Mathematics at Northwestern University, and all-around nice guy. It may be a rule of thumb, but it's simple enough that even I... can grasp it.

Here is an example that gives you a sense of the depth of field you get with a variety of focal lengths, each of which is a "normal" lens for 6x6, 4x5, and 8x10 respectively.

75mm lens at f/16 = 150mm lens at f/32 = 300mm lens at f/64

Frank Petronio
7-Jun-2008, 15:39
Well it's easy and cheap to find a 300/5.6 for an 8x10 but those 150/2.8 Xenotars for 4x5 are expensive... so the practical answer to his question is to just get an 8x10.

Alan Davenport
7-Jun-2008, 16:00
An assumption is made in depth of field equations, namely that we are making a "standard" 8x10 print, and that it will be viewed at a "normal" viewing distance. Since the OP is changing both focal length and film format, we also have to assume a different CoC.

A 150mm lens on 4x5 film will have the same angle of view as will a 300mm lens on 8x10 film. Since the 8x10 film will require only half as much enlargement to make any given print size, it can tolerate a CoC twice that of 4x5 film. The resulting depth of field in a print will be half as much with the 300/8x10 as with the 150/4x5.

Example: A subject 5 meters away, shot with a 150mm lens at f/5.6, on 4x5 film, and assuming a CoC of .1mm, will produce a depth of field of roughly 1.24 meters (in the subject space.) An 8x10 camera with a 300mm lens at f/5.6, because it will tolerate a .2mm CoC, will have about 0.59 meters DoF in the subject space. Thus the answer to the OP's question is one half as much depth of field with the 8x10.

Bernard Kaye
7-Jun-2008, 21:58
Ken, DOF is cut by 4, not 1/4, when you double focal length.
Bernie

Ken Lee
8-Jun-2008, 05:05
Ken, DOF is cut by 4, not 1/4, when you double focal length.

Right you are - Thanks Bernie !

chachahavana
8-Jun-2008, 06:56
Thank you guys for the very detail debate and discussion...actually I am not good at maths at all and it looks like I got the point but I am still a bit confused.

People always say a bigger film will get bigger bokeh… I don’t know if it is true. 150mm for 4x5 and 300mm for 8x10 is like standard lenses for both format, if my composition/framing of an object is identical on both format, according to the saying, the 8x10 should get a shallower dof. I am wondering if anyone has done some test otherwise I may find an 8x10 to try it out later.

Thanks again guys !!