DoF comparison vs other formats

[marshallarts]

There is a project I've always wanted to do if I only I understood programing better to figure out to proper way of doing this.

We've all seen DoF charts, I own a few myself and understand the concepts. Between formats (sizes) DoF changes accordingly. If my charts are correct it would appear the correlation is easily computable and equivalent between formats. Example, my charts say:

35mm Film Standard 1.5, 50mm lens, subject @ 20ft = DoF of 5' 11" @ f/2
4x5" format, 176mm lens (Horizontal Equiv.) @ 20ft = DoF of 5' 9.3" @ f/8+1/2

My question is, is there anything else I'm not considering? To me it appears the DoF falloff is different in larger formats, and it is this characteristic that attracts me most. Is this just me or is there an explanation for this? The charts would leave me to believe DoF should be identical but to me it appears something is different.

Any helpful feedback would be greatly appreciated. Thank you very much for your help!

[John NYC]

There are already multiple iPhone apps an websites out there that do DOF calcs for all formats. DOF Master is one that I have.

I find them quite useless since DOF calculations are based on an idea of "acceptable sharpness" that is usually assumed to be a value much blurrier than I can tolerate.

[paulr]

Whatever standard for acceptable blur you choose, the DOF relationship between format sizes will stay the same. So you can use the same calculations.

[marshallarts]

Hey guys, thanks. Yes, I already have many different iPhone DoF calculators including the old charts before smart phones. My intention was not to recreate these charts, but to see if there are any anomalies not taken into account that I will notice when doing the calculations myself. I say this before despite all the evidence these charts suggest about DoF being the same between formats (when adjusting to the appropriate f/stop) I still can't help but feel the effects are different with larger formats.

My goal is to find what changes with larger formats that make the DoF characteristic more pleasing to me. The sense I have is MAYBE the falloff is different, at least that's the way it appears sometimes. Does anyone know?

[Drew Wiley]

As usual, I ignore the calculations are determine the effect I want simply with a loupe. After all, the final result is going to
be determined by looking at a neg or print, and not a calculator screen. The bigger the groundlgass the better. During initial
composition and focus I determine what the priorites of focus are going to be. I want something in the image to be dead on.
Then I will work with tilts & swings to bring the rest of the image into the degree of sharpness I hope to achieve at the final
working f/stop. Then I might view the effects of this halfway down. For instance, if my actual exposure is going to be at f45,
I check critical areas thru the loupe at f/22 first. After awhile this all gets very instinctive. In the field, one rarely has the
luxury of fully cooperative planes, like in tabletop photography. My own brother was all into hyperfocal forumlas etc, and then
at first I used the Sinar line system for yaw-free corrections etc. Eventually it all seemed like a waste of time. And the last thing I need is another damn battery-dependent gadget that can go wrong in bad weather, or take up unecessary space in the pack. What is technically correct during focus might be esthetically irrelevant or even distracting. I want control over that
variable too.

[Leigh]

There is absolutely no difference in DoF resulting from different formats.

Think about it. How does a lens know what film is behind it?

Take a photo on an 8x10 film. Note the DoF of various subject elements in a small area.

Then cut a 24mm x 36mm rectangle from the film containing those subject elements.
Look at them again. Has the DoF changed? Of course not.

The differences in the calculated results are the consequence of using different fudge factors
based on different "assumed" magnifications.

- Leigh

[paulr]

As usual, I ignore the calculations ...

As usual you ignore the question.

Marshallarts, there aren't any anomalies to consider. Defocus blur and diffraction both scale proportionally with magnification. If you have a sense of what apertures/focal lengths are working for you with one camera format, you can calculate the identical result with another format.

If you're looking for equivalence that considers more than depth of field, there are other factors. This (http://www.falklumo.com/lumolabs/articles/equivalence/CameraEquivalence.pdf) is somewhat confusing but thorough discussion on the topic.

[jp]

If you're using soft focus, DOF is different; usually greater, since it's a layering of sharp and out of focus images, then those out of focus layers are going to be in focus somewhere else.

As Leigh says, in practical terms a 300 symmar-s 5.6 at f8 is going to have the essentially the same DOF as a Nikon 300/2.8 at f8. It's just going to be a narrower field of view on a small sensor.

[Teodor Oprean]

The tables for depth of field are computed by plugging several values into a formula that has several input variables. You just need to look up the formula from reference books and do the substitution yourself with a scientific calculator. You will find that each published look-up table was obtained by making slightly different assumptions about the minimum acceptable circle of confusion. In addition to that inconsistency, the cut-off for the minimally acceptable circle of confusion is chosen differently between 35mm, medium format and large format. This is the reason why the equivalance you expected to find between 35mm and 4x5 is not confirmed exactly by the tables, but you have to agree that a DoF value of 5'9.3" is very close to 5'11" for a focus distance of 20 ft.

Here's my take on the topic: In real life, the depth of field formulas (whether accepted on faith in the form of pre-computed look-up tables or diligently recalculated from scratch using your own personal choice for circle of confusion) don't matter as much as you'd expect. What counts is that you make a series of test exposures to find out how the images look for the varius possible choices of subject distance, subject patterns, focal length, aperture and film format. That's what you need to become familiar with. I'm sure that cinematographers test their lenses with those permutations by exposing film and looking at the images. I don't think that they sit in a room crunching numbers from an approximate heuristic formula and plotting graphs for various cut-offs. This is the epiphany I had about depth of field when I was first trying to understand it. You don't need to stress about the math. You just need to accumulate a few rules of thumb about the approximate depth of field you can expect to get for a couple of typical values of focal length, aperture and focusing distance. Yes, you need to understand depth of field, but there's no need to stress about the math.

[paulr]

If you're using soft focus, DOF is different; usually greater, since it's a layering of sharp and out of focus images, then those out of focus layers are going to be in focus somewhere else.

As Leigh says, in practical terms a 300 symmar-s 5.6 at f8 is going to have the essentially the same DOF as a Nikon 300/2.8 at f8. It's just going to be a narrower field of view on a small sensor.

In practical use, you'd want to compare with a lens that gives the same field of view in different formats. A 90mm nikon lens at f1.7. would give the same depth of field at identical print sizes.

[Leigh]

Printing is a secondary operation that has nothing to do with the DoF of the image on film.

- Leigh

[paulr]

Printing is a secondary operation that has nothing to do with the DoF of the image on film.

- Leigh

Yes, you would come up with different answers if you were trying to figure out the DoF onthe film (for theoretical purposes) or in a print (for practical ones).

[marshallarts]

Thanks so much to everyone who has replied!
And thanks paulr for providing that link to that research paper-- I look forward to reading it because it looks exactly like the kind of study I wanted to do myself.

I believe we can agree the DoF calculations get us close to equivalency between formats and optically lenses would operate the same so there shouldn't be any anomalies causing different looks. It could very well be a perceived sharpness difference from the larger negative causing the large format to appear more startling compared to that of smaller formats.

[ramon]

Marshallarts, your numbers are wrong:



35mm Film Standard 1.5, 50mm lens,
4x5" format, 176mm lens (Horizontal Equiv.)



4x5" equivalent focal is 166.67mm (80x120mm) for 1.5x aspect ratio.



Page 19: "If the characteristic dimension is the short dimension of the format, the ratio of the 4×5 (96 mm × 120 mm) and 35 mm (24 mm × 36 mm) formats is 4."


Also take into account that ratio is 3.3333(period). Not number four as used in the paper.

80mm/24mm = 120mm/36mm = 3.33333....

---
(edited focal lenght was wrong)

[C. D. Keth]

Printing is a secondary operation that has nothing to do with the DoF of the image on film.

- Leigh

That's not entirely true. Printing size will affect one's perception of what is in focus and what is not. Printing 4x5 to be printed 4x5 feet will have much less room for error than printing 4x5 to be contact printed into little handheld gem photographs.

[Jac@stafford.net]

Acceptable Circle of Confusion (CoC) is one factor to consider. It is associated with degree of enlargement, which in turn is associated with anticipated viewing distance.

[ic-racer]

You can't make any DOF calculation without knowing the acceptable COC on the print.

[Mark Barendt]

I believe we can agree the DoF calculations get us close to equivalency between formats and optically lenses would operate the same so there shouldn't be any anomalies causing different looks. It could very well be a perceived sharpness difference from the larger negative causing the large format to appear more startling compared to that of smaller formats.

No, I'm not going to agree there.

Practical usage of our cameras throw more variables at us than you are allowing for here. Isolating just DOF is impractical.

If we assume for example normal angle of view and an essentially constant subject to camera distance then both focal length and format change, that means that both DOF and film characteristics will change.

You end up, for example, comparing "35mm film with a 50mm lens" to "4x5 film with a 150mm lens" or to "8x10 film with a 300mm lens".

Different lenses also have different characteristics at different apertures, have different coatings, have different corrections, had different quality standards and each of these effects the look.

All these "variables" are inextricably linked and each imparts its own signature on the final print.

[rdenney]

DOFMaster is based on an assumed 8x10 print. But even the iPhone version allows you to select whatever circle of confusion you prefer. Generally, I select on about a half to a quarter of what they say they use for a given format in their help screen. They use a 0.1mm as the maximum acceptable circle of confusion for 4x5, which only gets a 2x magnification to reach 8x10. I use 0.025, for reasons discussed below.

Leigh is right that the circle of confusion created by a lens of a given focal length at a given aperture is the same no matter what format film is placed behind it.

But he is wrong that the depth of field is the same. Depth of field is defined as the defocus blur being slight enough so that it appears to be acceptably focused on the print. That requires defining what appears sharp on the print, and then working back through the enlargement ratio. If I don't want the fuzzy spot rendered by an out-of-focus point to be bigger than, say, 0.2mm on the print, then using a circle of confusion standard of 0.1mm works fine for a 2x enlargement--as suggested by DOFMaster.

The problem in comparing formats is that we have to make assumptions that may or may not be realistic. For example, we may assume the same print size, which means the circle of confusion we use will need to be 0.2mm divided by the enlargement factor (if 0.2mm on the print is what we want). With that assumption, and using an 8x10 print as the anticipated size, 4x5 gets a circle of confusion standard of 0.1, because .2/2x = .1. But 35mm would get 0.025, because .2/8x = 0.025.

If we assume that the enlargement ratio is the same, then we can use the same circle of confusion for both formats. But that circle of confusion will only look the same on the print if they are the same degree of enlargement. So, with that assumption, we'd be comparing an 8x10 print from 4x5 to a 2x3 print from 35mm.

For smaller prints, we assume the print size will be the same. But as print size increases, we get to the point where other factors prevent further enlargement. I would not generally want to make 16x20 prints from 35mm negatives, for example. 17x is just too much enlargement to maintain my standards. And the 0.012mm circle of confusion runs into the effects of diffraction, so that diffraction will control what looks to be in focus, not depth of field. So, while 16x20 is my standard-size print from 4x5, I just won't go that big with small format, so they can't really be compared with either of these assumptions.

For me, though, the 0.2mm fuzzy spot on the print is too big. I'd rather limit it to 0.1mm. And I standardize on a 16x20 print for 4x5--the biggest print I can make at home. So, 0.1mm / 4x enlargement = 0.025mm. I end up stopping down a lot. And at 4x, diffraction just never becomes a practical limitation, so I'll stop down as necessary. With 4x5, being out of focus is likely to be a much bigger effect than diffraction in any case.

We should also remember that it's not a hard line. It's a standard we set, but the sizes of those out-of-focus details become smoothly larger as their lack of focus increases. It may be that 0.1 is what I want for the important subject material, but not quite as important for other parts of the print, even if I still want the print to look sharp overall.

Generally, my strategy is to stop down as far as I need to for depth of field, and then print as big as the negative will go. If I needed so much depth of field that I had to stop down too far, then I just won't print as large. That happens with small format, but not so much with large format, until the prints get much bigger than I can make.

Rick "lack of depth of field and defocus blur are not the same dimension" Denney

[jb7]

Rick, if you were to post the first reply to every thread, this forum would be a lot more concise.

There are some calculators on this page that might be helpful to the op, particularly the first and last. Perhaps it's similar to the converter you were hoping to build?

http://www.tawbaware.com/maxlyons/calc.htm

However, that last converter seems to deal with converting infinity values, not often the case when you add in extension with larger formats for closer subjects, so there might be some additional scribbling to be done. I'm impressed by it, though I don't have the inclination to test the math. Still, it's nice to plug in some slow old large format lenses, and find that the 35mm equivalent would require a lens of f/0.6 to achieve the same result- in a kindof a peculiarly perverse sort of way...

[paulr]

Also take into account that ratio is 3.3333(period). Not number four as used in the paper.

Well, that depends on what dimension is most important to you. There's no absolute answer, since the aspect ratios are different. I find it odd that the paper in question uses height, but it's imaginable that to some photographers that would be the defining dimension. Others use width, and others split the difference and use the diagonal.

I use width. But I find in practice 3.333 is not quite accurate, because film holders don't let you use the entire negative. My holders give me 3.75x4.75" image area. I find after cropping in to avoid edges of the film guides, my useable area is more like 3.625 x 4.625. This makes the ratio closer to 3:1.

[ramon]

Also take into account that ratio is 3.3333(period). Not number four as used in the paper.



Well, that depends on what dimension is most important to you. There's no absolute answer, since the aspect ratios are different. I find it odd that the paper in question uses height, but it's imaginable that to some photographers that would be the defining dimension. Others use width, and others split the difference and use the diagonal.


No. See the first post. He wants the exact equivalent. So we need to crop 4x5 negative to have the same aspect ratio as 35mm. 35mm has a ratio of 1.5 (36mm/24mm).

So It's a mathematical answer. And there is an absolute answer: to use width. If you use height your picture is 6x9 no 4x5. Do the maths:

If you use width: 120mm / 1.5 = 80mm so image is 80x120mm and ratio is 3.33333x as I said before. So lens equivalent is 167.67mm (50mm x 3.3333)

If you use height: 96mm /1.5 = 64mm so image is 64mm x 96mm and ratio is 2.6666x lens equivalent would be 133.33mm but then we are not talking about 4x5 anymore (It's like a slightly bigger 6x9)




I use width. But I find in practice 3.333 is not quite accurate, because film holders don't let you use the entire negative. My holders give me 3.75x4.75" image area. I find after cropping in to avoid edges of the film guides, my useable area is more like 3.625 x 4.625. This makes the ratio closer to 3:1.

Then with your film holders: if your width is 4.625, your ratio is 3.26319 (Not 3:1).

[paulr]

So It's a mathematical answer. And there is an absolute answer:

I don't agree. The math needs to be based on the measurements actually used. The width of your crop is what precisely determines your angle of view. The size of the film or sensor outside the crop is irrelevant.


Then with your film holders: if your width is 4.625, your ratio is 3.26319 (Not 3:1).

Not sure where you get this. I'm using an uncropped measurement for 35mm, because I use the full frame (as do most people using a dslr). This is exactly 1.5 inches.
4.625 divided by 1.5 is 3.0833.

The real point here isn't the numbers, it's that you have to find your own numbers based on how you work. Is width most important to how you see space? Height? Some of each? And you have to figure out your actual format size based on the crop you use. Numbers derrived by other means won't apply precisely to your work.

[ramon]

35mm film is 24x36mm (or 0,9449" x 1,4173")

Your film is 3.0833" x 4.625"

You have a multiplier ratio (different from xy ratio) of 3,263.

If you want the same picture from a 50mm focal lenght into a 3.0833" x 4.625" film you need a focal lenght of 163.15mm (50mm x 3.263)

That number comes from the diagonal ratio, when we use the same X-Y ratio.

Diagonal of 35mm film format with X-Y ratio of 1.5 (36/24) is 43.27mm. (square root of 24x24 plus 36x36).

Your film has a usable width of 4.625" (that's 117.475mm)

So if you want the same xy ratio of 1.5 your height should be 4.625" / 1.5 = 3.0833" (that's 78.3167)

Diagonal of film with size 78.3167mm x 117.475mm (xy ratio = 117.475/78.3167 = 1.5) is square root of ( 78.3167x78.3167 + 117.475x117.475) is 141.187.

So 141.187 / 43.27 = 3,2629304367922348047145828518604

117.475 / 36 = 3,2631944444444444444444444444444
78.3167 / 24 = 3,2631958333333333333333333333333

So your ratio is 3.263

I uploaded a Excel file (compressed because the forum does not allow .xls). You can change the X and Y dimensions (size) for some predefined focal lenghts.

[paulr]

Ramon, you didn't read my posts. Yes, your math works if we're going by film diagonals. Going by diagonal may be appropriate for some people. For me and for a lot of others, going by width makes more sense. This really has to do with how a person sees / uses space in their work ... it's not an absolute. I stipulated this when explaining my reasoning.

[C. D. Keth]

To expand on Paul's answer, Ramon, your method would be quite appropriate for architecture indoors or landscapes deep in a canyon where both width and height play heavily into one's perception of a lens' "wideness." Move outdoors into the open, however, and you might find a landscape photographer might only care about the wide dimension because shooting a little more or less sky doesn't change his perception of wideness much if at all.

[rdenney]

Ramon, you didn't read my posts. Yes, your math works if we're going by film diagonals. Going by diagonal may be appropriate for some people. For me and for a lot of others, going by width makes more sense. This really has to do with how a person sees / uses space in their work ... it's not an absolute. I stipulated this when explaining my reasoning.

Furthermore, the aspect ratio that is used for the comparison should be dictated by that of the intended print, which should be dictated by the interpretation of the subject material. If I'm making a 12x18" print, then the comparison uses the width. If I'm making an 8x10" print, then the comparison must use the height. The only time I can use the diagonal is when both formats are the same aspect ratio, but then I can use any linear dimension.

Rick "there is no one right answer, but there are plenty of wrong ones" Denney

[C. D. Keth]

"there is no one right answer, but there are plenty of wrong ones"

...and enough gray areas in between right and wrong to really drive you nuts sometimes.