I Need A Headache

Snapshot · May 4, 2014

Therefore, I would like need someone to explain to me why Depth of Field (DoF) is smaller in Medium Format given the same aperture ratio, focal length and subject distance than in 35mm format. Every calculator I've used shows that the DoF is greater (deeper) in 35mm. Yet, I keep hearing from many sources that DoF is smaller in MF than in 35mm when you consider the preceding parameters.

So, what am I missing? Are we also considering film area and/or referring to angle of view when comparing focal lengths when determing DoF?

:confused:

BrianShaw · May 4, 2014

I use a DOF dial calculator from a Kodak Master Photoguide (when not reading DOF off of the lens) and don't think about the whys and wherefores. No headache at all. But I really can't answer the question you ask either.

Peltigera · May 4, 2014

Depth of field is down to the lens only. It produces an image circle independently of the piece of film (or digital sensor) placed within that image circle.

If you use (say) a 50mm lens with 120 film, it will produce the same depth of field as that same lens used with 35mm film - but you will get a different angle of view.

If you keep the angle of view the same between different film sizes, then the focal length of the lens will also be different and so will the depth of field.

Snapshot · May 4, 2014

Peltigera said:
Depth of field is down to the lens only. It produces an image circle independently of the piece of film (or digital sensor) placed within that image circle.

If you use (say) a 50mm lens with 120 film, it will produce the same depth of field as that same lens used with 35mm film - but you will get a different angle of view.

If you keep the angle of view the same between different film sizes, then the focal length of the lens will also be different and so will the depth of field.

This is where the confusion for me begins. When the lens focal length is the same and the aperture is the same, all the calculators I have used produced DoF values that indicate the 35mm format produces a narrower DoF than a MF camera, given the same distance to the subject. So, I guess I'm wondering why the different values by these calculators. What is then they are factoring in?

markbarendt · May 4, 2014

The reason that DOF is in a practical sense shorter on MF and again shorter on large former cameras is that the angle of view is different and if we want a specific photo, like a head shot, we need to move the camera.

In order to match of the same composition the larger the format of the camera the closer to the subject we need to be. The closer we move the camera to the subject, with the same lens, the shorter the DOF.

cliveh · May 4, 2014

DOF is related to the circle of confusion and as the focal length increases, so does the circle of confusion. Does this help, or are you more confused?

Snapshot · May 4, 2014

markbarendt said:
The reason that DOF is in a practical sense shorter on MF and again shorter on large former cameras is that the angle of view is different and if we want a specific photo, like a head shot, we need to move the camera.

In order to match of the same composition the larger the format of the camera the closer to the subject we need to be. The closer we move the camera to the subject, with the same lens, the shorter the DOF.

Yes, that is my understanding as well. So, if we are suggesting that a MF camera has a narrower DoF over a 35mm camera, then we are comparing the same aperture, distance to subject and the lens angle-of-view. Lens focal length will not be the same for a particular angle-of-view for different formats.

In comparison, a 50mm lens for a 35mm format camera will have a similar angle-of-view as a 80mm lens for a MF camera. However, given the same aperture and distance to subject, the DoF value will be smaller for the MF camera.

Kawaiithulhu · May 4, 2014

Mark summed it up the reality of it quite nicely.

In the original question, however, where

given the same aperture ratio, focal length and subject distance

the DOF is exactly the same, but the picture between the two formats is vastly different because you see much more on 120mm than 35mm.

If you change the question to be "given the same subject coverage and aperture and subject distance" (swap focal length for distance) you end up with a shallower DOF on medium format because the lens itself has to be a longer focal length and that's what Peltigera is saying.

In summary: what confuses you is that most people imply but do not explicitly state that they are talking about a picture of the same coverage and not just whatever happens to be back there.

Snapshot · May 4, 2014

cliveh said:
DOF is related to the circle of confusion and as the focal length increases, so does the circle of confusion. Does this help, or are you more confused?

Are we speaking of the circle of confusion as it pertains to a lens or in my mind?

MattKing · May 4, 2014

Assuming same focal length and aperture in each system.

Depth of field depends on your choice of circle of confusion.

You choose a circle of confusion based on factors that are at least partially subjective - what will be "acceptably sharp" in the final output - the print or (in the case of slides or movies) the projection.

That which is "acceptably sharp" will depend at least in part in how much magnification is used between the negative or slide and the final output to achieve that final output. As medium format requires less magnification to get to the final output, you can start with a larger "acceptable" circle of confusion for medium format. If the "acceptable" circle of confusion is larger, then the apparent depth of field is larger.

Now take two aspirin, and call me in the morning.

greenbank · May 4, 2014

Snapshot said:
This is where the confusion for me begins. When the lens focal length is the same and the aperture is the same, all the calculators I have used produced DoF values that indicate the 35mm format produces a narrower DoF than a MF camera, given the same distance to the subject. So, I guess I'm wondering why the different values by these calculators. What is then they are factoring in?

Image magnification, I think. If the size of an object at the film plane is the same, no matter what the lens focal length, the depth of field should be the same. But vice versa: if the apparent angle of view changes (as it will between formats: different lens-to-film distance, so different image circle) the size of the image formed by the object will be different. Consequently the depth of field will also be different. See for example Simmons, Using the View Camera or Adams, The Camera; both have good technical discussions.

Snapshot · May 4, 2014

MattKing said:
Now take two aspirin, and call me in the morning.

Will do!

Roger Cole · May 4, 2014

Snapshot said:
Therefore, I would like need someone to explain to me why Depth of Field (DoF) is smaller in Medium Format given the same aperture ratio, focal length and subject distance than in 35mm format. Every calculator I've used shows that the DoF is greater (deeper) in 35mm. Yet, I keep hearing from many sources that DoF is smaller in MF than in 35mm when you consider the preceding parameters.

So, what am I missing? Are we also considering film area and/or referring to angle of view when comparing focal lengths when determing DoF?

:confused:

Well it actually isn't. Depth of field at a given f-stop depends entirely (yes, entirely) on image scale. And that's determined by focal length. An 80mm lens will have the same depth of field on 35mm as on 645, but it's slightly long on 35mm and normal on 645 thus "a normal lens" - that is, a lens producing an image scale that looks like a normal visual angle of view when seen on a piece of film of the size in question - will produce less depth of field on 645 than on 35mm, because it has to be a longer focal length. You can prove this easily if you have two cameras of different formats with the same focal length lenses. Take a shot on each from the exact same position then crop the one on the larger format (which will need to be cropped since it was shot with the same focal length onto a bigger piece of film, thus it will include more of the scene) so that you make identical sized and identically framed prints. At the same f-stop DOF will be identical.

Roger Cole · May 4, 2014

Peltigera said:
Depth of field is down to the lens only. It produces an image circle independently of the piece of film (or digital sensor) placed within that image circle.

If you use (say) a 50mm lens with 120 film, it will produce the same depth of field as that same lens used with 35mm film - but you will get a different angle of view.

If you keep the angle of view the same between different film sizes, then the focal length of the lens will also be different and so will the depth of field.

Close, but it's image scale, which of course is related to depth of field.

Take a shot with a 100mm lens on 35mm and another with a 50mm and then crop the print from the 50mm so that the image area and print size are the same as the one shot with the 100mm, and again DOF will be identical.

Roger Cole · May 4, 2014

greenbank said:
Image magnification, I think. If the size of an object at the film plane is the same, no matter what the lens focal length, the depth of field should be the same. But vice versa: if the apparent angle of view changes (as it will between formats: different lens-to-film distance, so different image circle) the size of the image formed by the object will be different. Consequently the depth of field will also be different. See for example Simmons, Using the View Camera or Adams, The Camera; both have good technical discussions.

Bingo.

And yes position - distance from the lens that its focused - affects DOF but it affects it identically given the same image scale (crop and see again.) The experiments I suggest must be shot from the same camera position with the same subject position to be valid.

Alan Gales · May 4, 2014

Snapshot said:
Therefore, I would like need someone to explain to me why Depth of Field (DoF) is smaller in Medium Format given the same aperture ratio, focal length and subject distance than in 35mm format. Every calculator I've used shows that the DoF is greater (deeper) in 35mm. Yet, I keep hearing from many sources that DoF is smaller in MF than in 35mm when you consider the preceding parameters.

So, what am I missing? Are we also considering film area and/or referring to angle of view when comparing focal lengths when determing DoF?

:confused:

A 100mm lens on a 35mm camera has the same depth of field as a 100mm lens on a 6x7 medium format camera at the same aperture.

The problem lies in that a 100mm lens on a 35mm camera makes a great head and shoulders portrait lens. To get the same equivalent focal length on a 6x7 camera you need a 200mm lens. Of course a 200mm lens has less depth of field than a 100mm lens at an equal aperture.

Clear as mud, right?

summicron1 · May 5, 2014

depth of field is independent of the f-stop and is wholly a function of how big the lens opening is as measured across the lens -- so while f8 on a 100 mm lens may transmit as much light as f8 on a 21mm lens, the hole through which the light passes in the lens is bigger.

That bigger hole means the circle of confusion at f 8 will be larger and more pronounced earlier, which means the depth of field is more narrow.

Alan Gales · May 5, 2014

summicron1 said:
depth of field is independent of the f-stop and is wholly a function of how big the lens opening is as measured across the lens -- so while f8 on a 100 mm lens may transmit as much light as f8 on a 21mm lens, the hole through which the light passes in the lens is bigger.

That bigger hole means the circle of confusion at f 8 will be larger and more pronounced earlier, which means the depth of field is more narrow.

I read what you wrote and didn't understand it so I did some research. Apertures are not the same size at same F stops for different focal lengths. The F stop measures light transference and not actual aperture diameter. Interesting!

dorff · May 5, 2014

Snapshot said:
Yet, I keep hearing from many sources that DoF is smaller in MF than in 35mm when you consider the preceding parameters.

If you are aiming to achieve the same output from two different format sizes, you have to:

use lenses that have equivalent focal lengths (i.e. they produce the same angle of view,
you have to have the same perspective (the cameras have to be in the same place) and
the lenses' effective diameters have to be the same.

The easiest way I can explain this is by way of an example. Let's say one format is 4 times the size of the other. You use 50 mm f/2 for the smaller of the two. The effective diameter is f/d, 50/2 = 25 mm. The lens that will give you the same output on the larger format is 50 x 4= 200 mm. The f-stop is 200 / 25 = 8. Lens symmetry (or P-number) can affect the amount of field depth in front of and behind the focus plane, i.e. it can distribute the available focus depth forward or backward. The above applies in the pure sense therefore only to perfect lenses (P = 1), but in a practical sense may be used for most lenses.

I suggest you read Paul van Walree's website, where this is explained and illustrated nicely and where you'll find a DOF calculator as well.

dorff · May 5, 2014

dorff said:
If you are aiming to achieve the same output from two different format sizes, ...

By output, I mean you scan to a file of the same dimensions, or you print both to the same paper size.

In an absolute sense, 50 mm f/4 lenses produce the same DOF on the film, but on medium format it is much less enlarged onto the final print than 35 mm would have been, so that in the output the DOF is no longer the same. What happens is that the difference in DOF between recording and enlarging exactly cancel one another when the recording lens's effective diameters are the same, as explained in my first post.

TimFox · May 5, 2014

I had not paid attention to this fact until I bought a used 8x10 inch camera and duplicated some shots I had previously done with a 4x5.
The overall fields were roughly the same, so the 8x10 used a lens with twice the focal length as used with the 4x5.
Specifically, I was shooting from the Adler planetarium peninsula (in Chicago), focusing on the huge sundial with the skyline in the background.
To get proper exposure (daylight), I used the same shutter time and aperture (f-number), but the depth of field was noticeably shorter on the larger image:
the sundial was sharp in both images, but the skyline was much softer in the 8x10.

John Koehrer · May 6, 2014

summicron1 said:
depth of field is independent of the f-stop and is wholly a function of how big the lens opening is as measured across the lens -- so while f8 on a 100 mm lens may transmit as much light as f8 on a 21mm lens, the hole through which the light passes in the lens is bigger.

That bigger hole means the circle of confusion at f 8 will be larger and more pronounced earlier, which means the depth of field is more narrow.

This is the only correct post so far.
The f stop is a simple ratio of aperture diameter to focal length.
I has no direct indication of physical lens opening,just a RATIO.
Graphic Graflex Photography from the 40's 50's summed it up nicely............One sentence, NO math!
Paraphrased 'cuz I have CRS disease.

A 15mm opening will give the same DOF regardless of negative size or focal length.

Sirius Glass · May 6, 2014

MattKing said:
Assuming same focal length and aperture in each system.

Depth of field depends on your choice of circle of confusion.

You choose a circle of confusion based on factors that are at least partially subjective - what will be "acceptably sharp" in the final output - the print or (in the case of slides or movies) the projection.

That which is "acceptably sharp" will depend at least in part in how much magnification is used between the negative or slide and the final output to achieve that final output. As medium format requires less magnification to get to the final output, you can start with a larger "acceptable" circle of confusion for medium format. If the "acceptable" circle of confusion is larger, then the apparent depth of field is larger.

Now take two aspirin, and call me in the morning.

Yep, That is the correct answer. So now you know and you can get over it.

Roger Cole · May 6, 2014

John Koehrer said:
This is the only correct post so far.
The f stop is a simple ratio of aperture diameter to focal length.
I has no direct indication of physical lens opening,just a RATIO.
Graphic Graflex Photography from the 40's 50's summed it up nicely............One sentence, NO math!
Paraphrased 'cuz I have CRS disease.

A 15mm opening will give the same DOF regardless of negative size or focal length.

Same in terms of measures of circle of confusion yes. But if one has to be enlarged ten times and the other only twice, the first will enlarge that COC five times more at the actual viewing scale, reducing the effective visual DOF.

Alan Gales · May 6, 2014

John Koehrer said:
The f stop is a simple ratio of aperture diameter to focal length.
I has no direct indication of physical lens opening,just a RATIO.

Thanks John. You said it clearly.

I was wron. I mean wronnn. No, I mean wronnnnnn. How did Fonzie ever say that word?

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