Help me understand the correlation of image circle and film format.

rhyno0688 · Jan 17, 2016

So most of us know that the wider open the lens the more out of focus or thinner depth of field. As you go up in medium the glass gets bigger allowing more light through regardless of the lens saying f/5.6 in LF, 2.8 on a Mf lens and 1.4 on a 35 mm camera. The aperture number is dependent on the length of the lens and the size of the opening in said lens not the amount of photons passing through.

So my question is, when I use my gx680 with lenses made for movements and the lens has a larger image circle, is the Dof going to be more similar to a larger camera/lens like my Crown graphic or is it going to be closer to my 67 cameras at a similar length lens and aperture?

I have seen people mount Mf lenses to a smaller format camera but haven't seen a comparison.

I'd try it to find out but I'm currently sitting in a coffee shop waiting for a friend. And I'm bored so my mind goes into over drive.

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Ian Grant · Jan 17, 2016

The DOF is related to the FL not the format so a 90mm lens for 35mm has the same DOF as a 90mm on a 120 camera or a 90mm on 5x4 or 7x5.

However in practice smaller formats get enlarged more so the out of focus areas become more noticeable, when DOF tables are compiled they assume a smaller COC (Circle of Confusion) for smaller formats to take this into account. You can look DOF and COC up on Wikipedia.

Ian

markbarendt · Jan 17, 2016

Ian gave you a great practical description.

Here's a description of image circle.

https://en.wikipedia.org/wiki/Image_circle

A "normal" 150mm lens on a large format camera might have an image circle of 210mm/8.26inches. https://www.schneideroptics.com/inf...arge_format_lenses/symmar/data/5,6-150mm.html (Different designs have different circles.)

The sensor/film size you put behind that lens doesn't matter to DOF. Instead, switching just sensor/film size, defines the angle of view for the photo. If you use 5x4 film the film behind that 150mm lens you will see a normal angle of view, with 35mm film you will see a very narrow angle of view (IMO this is mistakenly called telephoto), with your 6x8 medium format film you get something a bit narrower than normal, if you use 7x5 film the 150mm lens would create a slightly wide angle photo.

A "normal" 150mm lens will focus at infinity when the center of the lens is 150mm from the film.

A 150mm "telephoto" lens might focus properly at 100mm from the film.
Tele photo designs allow the lens to stay closer to the camera. For example on a 35 mm camera, a 150mm lens might only stick out 100mm.

A "Retrofocus" 150mm lens might focus when 200mm from the film. This type of lens is normally found on 35mmSLRs with short lenses, the center of a normal 28mm lens would sit about 1-1/8" from the film, that can't happen because the mirror has to have room to flip up so those lenses are designed to sit farther from the film.

rhyno0688 · Jan 17, 2016

Thanks for the recommendations on where to go and the explanations. I have a little reading to do now.

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ic-racer · Jan 17, 2016

The determinants of depth of field are magnification, aperture and acceptable circle of confusion size. Not focal length or film format size.

MattKing · Jan 17, 2016

ic-racer said:
The determinants of depth of field are magnification, aperture and acceptable circle of confusion size. Not focal length or film format size.

This is correct, but .......

We tend to achieve the magnification we seek, for the film format chozen, by varying the camera to subject distance and focal length.

Dan Fromm · Jan 17, 2016

MattKing said:
This is correct, but .......

We tend to achieve the magnification we seek, for the film format chozen, by varying the camera to subject distance and focal length.

Interesting. MF zoom lenses, anyone?

I set magnification by varying extension given the lens' focal length, focus by moving the camera/lens assembly.

MattKing · Jan 17, 2016

Dan Fromm said:
Interesting. MF zoom lenses, anyone?

I set magnification by varying extension given the lens' focal length, focus by moving the camera/lens assembly.

Most likely though you also make an initial decision about which lens and focal length to use.

I don't use large format. But when I am most involved with issues of magnification (generally with close focus work) I usually am also choosing between lenses of different focal lengths, based on considerations of factors like flat field performance and comfortable working distance.

More generally though, it seems clear that all the variables are interconnected.

I had a conversation about this a couple of years ago with someone who shot exclusively in a single format - so-called full frame digital.

When you work with a fixed format, the relevant equations simplify out, and focal length can be used instead of magnification. The person I was talking to based what he was saying on the assumption that people wouldn't have a choice of "sensor size". I don't think he was happy with me when I indicated the contrary.

ic-racer · Jan 17, 2016

MattKing said:
This is correct, but .......

We tend to achieve the magnification we seek, for the film format chozen, by varying the camera to subject distance and focal length.

One can change both and have the same magnification, which would have no effect on DoF.

barzune · Jan 17, 2016

"...I have seen people mount Mf lenses to a smaller format camera but haven't seen a comparison. ..."

"...A 150mm "normal" lens on a large format camera might have an image circle of 210mm/8.26inches. https://www.schneideroptics.com/info...5,6-150mm.html (Different designs have different circles.)..."

Very clear and good information but,( and I apologize for what may be a dumb question), when I use a lens on a larger format camera, the image circle does not cover the "sensor" (ie, a 50mm AIS lens on a 6X6 camera),
so I am assuming that this is not a "normal" design?

Dan

Dan Fromm · Jan 17, 2016

barzune;1953836980Very clear and good information but said:
on a larger format camera[/B], the image circle does not cover the "sensor" (ie, a 50mm AIS lens on a 6X6 camera),
so I am assuming that this is not a "normal" design?

Dan

Terminology time. "Normal lens" has several meanings. One commonly used convention is "lens whose focal length is the format's diagonal." Under it the normal lens for 35 mm still is 43 mm, normal for 6x6 is 80 mm, and so on. Rarely used with 35 mm still and smaller formats, usually used with formats larger than 35 mm still.

Another commonly used convention is "focal length usually delivered with the camera." Under it the normal lens for 35mm still's focal length is somewhere between 50 and 60 mm, normal for 6x6 is 80 mm, and so on. Most commonly used with formats no larger than 6x9.

A "lens of normal construction" is neither telephoto nor retrofocus. In other words, its rear nodal point is near its diaphragm. Telephoto lenses have their rear nodal points a useful distance in front of the diaphragm; this reduces the minimum extension at infinity -- for cameras with the focusing mount integral with the lens, the flange-to-film distance at infinity. Retrofocus lenses have their rear nodal points a useful distance behind the diagram; this increases the minimum extension at infinity, makes it possible to use short focal lengths on SLRs.

None of this has anything to do with coverage. Coverage is determined by design.

The typical normal lens for 35 mm still -- 50 - 60 mm focal length -- is a 6/4 double Gauss (or derivative) lens no slower than f/3.5. There are exceptions. Anyway, these are relatively narrow angle lenses, typically cover no more than 50 degrees. By an odd coincidence that won't bear close examination, this is a little more than needed to cover 43 mm. And by another odd coincidence these lenses are of normal construction, i.e., neither telephoto nor retrofocus.

In general, lenses for 35 mm and MF cameras are made to cover the cameras' formats and little more. The exceptions are for cameras that have movements.

Dan Fromm · Jan 17, 2016

MattKing said:
Most likely though you also make an initial decision about which lens and focal length to use.

Matt, I have three Micro-Nikkors, all AIS; 55/2.8, 105/2.8, 200/4. The 55 shoots a little better than the 105, the 105 shoots much better than the 200. I use the shortest one possible given the situation, dial in the magnification desired and then move the camera/lens assembly to focus. "The situation" comes down to working distance possible given the subject. For a variety of reasons -- skittish subject, physical possibility of access -- short working distances are often impossible.

I also shoot closeup with medium format cameras, usually a 2x3 Graphic. With them I don't have the luxury of using any of several focal lengths in the field, typically shoot at 1:1 or less. In fact,with them 1:1 is the highest magnification that's practical in the field. Again, I choose the magnification I want, set extension to get it and move the camera/lens assembly to focus.

John Koehrer · Jan 17, 2016

The Graphic Manual says. Paraphrasing here: Regardless of focal length the DOF at a given DIAMETER(not f stop) of aperture is the same"

As an example, a 20mm diameter aperture gives the same DOF with any focal length.

markbarendt · Jan 17, 2016

barzune said:
"...I have seen people mount Mf lenses to a smaller format camera but haven't seen a comparison. ..."

"...A 150mm "normal" lens on a large format camera might have an image circle of 210mm/8.26inches. https://www.schneideroptics.com/info...5,6-150mm.html (Different designs have different circles.)..."

Very clear and good information but,( and I apologize for what may be a dumb question), when I use a lens on a larger format camera, the image circle does not cover the "sensor" (ie, a 50mm AIS lens on a 6X6 camera),
so I am assuming that this is not a "normal" design?

Dan

The angle of view that 50mm lens might be 60 degrees.

Focus a normal 50mm lens, 50mm from center of lens to the film, and the 60 degree spread makes a small image, you are right, just enough to cover say a 35mm frame.

Focus a 90mm lens 90mm from the film and the image circle covers 6x8 nicely.

Focus a 150mm lens at 150mm and it covers 4x5 nicely.

300mm 8x10

600mm 16x20

Dan Fromm · Jan 17, 2016

Mark, it depends on the lens' design. A few examples:

6/4 double Gauss types and derivatives, all lenses of normal construction, typically cover ~ 50 degrees, not 60.

f/14 Perigraphes, dagor derivatives and lenses of normal construction, cover as much as 115 degrees at infinity (claims in the catalog, I can't test). My little 90/14 Perigraphe's catalog coverage is 5x7 at infinity. That's 100 degrees and the lens allows movements.

If the VM is to be believed, my beloved 12"/4 TTH telephoto, not a lens of normal construction, just covers 4x5, not 8x10 as you say a 300 mm lens should.

That a lens' focal length is normal for a format has no implications for its coverage.

markbarendt · Jan 17, 2016

I Agree Dan, was just trying to get the idea across, not the specifics.

lxdude · Jan 18, 2016

Pentax makes adapters for its 6x7 lenses to go onto its 645 and 35mm cameras, and for 645 lenses to go onto its 35mm cameras.
A 6x7-mount 200/4, a 645 mount 200/4, and a K-mount 200/4 will all have the same DoF on a 35mm Pentax at the same aperture.

Ian Grant · Jan 18, 2016

lxdude said:
Pentax makes adapters for its 6x7 lenses to go onto its 645 and 35mm cameras, and for 645 lenses to go onto its 35mm cameras.
A 6x7-mount 200/4, a 645 mount 200/4, and a K-mount 200/4 will all have the same DoF on a 35mm Pentax at the same aperture.

Not strictly true they will only have the same apparent DOF if enlarged by the same amount, because DOF depends on the COC, enlarged to give the same sized full frame prints the smaller negatives will have less apparent DOF. So the same DOF on the negatives but we need to take into account the degree of enlargement as well.

It's not uncommon for some lenses to be sold for MF and 35mm, after all the fast f2.8 180mm CZJ "Olympic" Sonnar was sold for 35mm and later the Pentacon 6 as was the longer version.

The Schneider LF 360mm Tele-Xenar sold to cover up to 7x5 can also be found in an M42 focusing barrel. One of the Leitz longer lenses (from memory a Hektor) also covered 5x4 when the cell is removed from the focus barrel.

Ian

lxdude · Jan 18, 2016

Ian Grant said:
Not strictly true they will only have the same apparent DOF if enlarged by the same amount, because DOF depends on the COC, enlarged to give the same sized full frame prints the smaller negatives will have less apparent DOF. So the same DOF on the negatives but we need to take into account the degree of enlargement as well.

What I said was, they will "all have the same DoF on a 35mm Pentax at the same aperture", to illustrate that the basic optical properties are the same regardless of image circle.

The existence of the actual lenses and adapters makes possible a simple demonstration of this, and the fact that they are all telephoto designs of the same focal length and maximum aperture, with the main optical difference being image circle, makes it easier to visualize it.

Help me understand the correlation of image circle and film format.

rhyno0688

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Help me understand the correlation of image circle and film format.

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