Focal length, regardless of format

[rwreich]

Hello, all!

I'm very curious to know if there are any inherent properties of particular focal lengths that might contribute to the aesthetic qualities of an image regardless of film format. For example, on 35mm film, the 85mm focal length is considered a pleasing portrait focal length because the format and the focal length combine to create an angle of view that also contributes a reasonable amount of compression. On 120 film, say 6x7, with an 80mm lens (a very similar focal length), the field of view is wider because the film is larger, which changes the angle of view.

So, to restate the question using the example, are there properties of the 80mm / 85mm focal lengths that are applicable across formats?

I'm not an engineer, but I can read math. I'm not a physicist, but I love learning about and applying physical properties. Any helpful replies would be appreciated!

-Wayne

 

[ic-racer]

...35mm film, the 85mm focal length...create an angle of view that also contributes a reasonable amount of compression

You can achieve that same angle of view in any format.

 

[Ian Grant]

Only the focal length is common, after all a 90mm is a typical WA on a 5x4 camera, a standard for 6x7 and a moderate portrait lens on 35mm camera.

Ian

 

[Vaughn]

Perhaps consider it this way. If you are after a standard head-and-shoulder portrait, you got to stand far enough back so that the tip of their nose is not significantly closer to the lens than their ears. That keeps their nose the right size and not looking like some kind of Roman nose -- roamin' all over their face. To do that, one needs a focal length a bit little longer than the diagonal of the film...4x5 film has a diagonal of ~150mm, 5x7 is ~210mm, so portrait lenses for them are around 210mm and 300mm respectively. With lots of wiggle room.

 

[rwreich]

You can achieve that same angle of view in any format.

Only the focal length is common, after all a 90mm is a typical WA on a 5x4 camera, a standard for 6x7 and a moderate portrait lens on 35mm camera.

Ian

Right - so since only the focal length is common to all formats with different angles of view, is there a recognizable or distinguishing quality of a focal length that contributes to the rendering of the image regardless of format?

Maybe I'm not asking the right question, or I'm asking in the wrong way.

 

[Old-N-Feeble]

Forget focal length other than to do the math. Heck, even the math is relatively unimportant. What matters regarding depth compression is distance from lens to subject. Find the distance which provides the look you like then select a lens focal length that crops to the desired amount on film. The distance from lens to subject doesn't change regardless of film format. The only reason the lens focal length changes is due to the size of film. It's really that simple.

 

[rwreich]

Perhaps consider it this way. If you are after a standard head-and-shoulder portrait, you got to stand far enough back so that the tip of their nose is not significantly closer to the lens than their ears. That keeps their nose the right size and not looking like some kind of Roman nose -- roamin' all over their face. To do that, one needs a focal length a bit little longer than the diagonal of the film...4x5 film has a diagonal of ~150mm, 5x7 is ~210mm, so portrait lenses for them are around 210mm and 300mm respectively. With lots of wiggle room.

Yes - I completely understand this and agree when we're talking about presenting the same field of view on a given film size. Let me ask it this way:

I take three pictures of a model. Each with a different camera (35mm, 6x7, & 4x5). I use a 90mm lens on all three cameras and the distance to the model is the same: 4'.

I know the 35mm will be a tight headshot, the 67x will be more of a head & torso, and the 4x5 will be much wider. I also know that I can crop the larger formats to produce identical frames as the one given by the 35mm camera.

In real life, I know that 90 on 6x7, looks very different than 45mm on 35mm, and very different than 165mm on 4x5 even though they all share similar angles of view.

 

[Ian Grant]

Right - so since only the focal length is common to all formats with different angles of view, is there a recognizable or distinguishing quality of a focal length that contributes to the rendering of the image regardless of format?

Maybe I'm not asking the right question, or I'm asking in the wrong way.

If we used a 90mm f5.6 5x4 WA, a 90mm 6x7 standard lens and a 90mm 35mm camera lens at the same aperture (say f11 and shot from the same spot) and looked at the same area on each negative there won't be much difference, perhaps we'd expect the 90mm for a 35mm camera to have slightly better resoltion, followed by the 6x7 lens.

Rendering overall is down to lens design and here it's the differences between say a Tessar, a Plasmat, or a Triplet, then older lenses like Petzvals and Rapid Rectilinear. Also rendering can change significantly with aperture as well.

Ian

 

[cliveh]

As Old-N-Feeble referred to, the camera to subject distance controls perspective, regardless of focal length.

 

[rwreich]

If we used a 90mm f5.6 5x4 WA, a 90mm 6x7 standard lens and a 90mm 35mm camera lens at the same aperture (say f11 and shot from the same spot) and looked at the same area on each negative there won't be much difference, perhaps we'd expect the 90mm for a 35mm camera to have slightly better resoltion, followed by the 6x7 lens.

Rendering overall is down to lens design and here it's the differences between say a Tessar, a Plasmat, or a Triplet, then older lenses like Petzvals and Rapid Rectilinear. Also rendering can change significantly with aperture as well.

Ian

Okay - Ian, I see that you're suggesting lens design has a much stronger impact than mere focal length. That makes sense to me and seems obvious now.

Shooting in different formats just has my mind working to reason why I prefer certain focal lengths. For example, I really like shooting 85mm and 135mm on 35mm. But I also really like shooting 80mm and 127mm on 6x7. I was trying to figure out if there was some reason why I was drawn to these focal lengths even though the angle of view was different for each format.

 

[rwreich]

Forget focal length other than to do the math. Heck, even the math is relatively unimportant. What matters regarding depth compression is distance from lens to subject. Find the distance which provides the look you like then select a lens focal length that crops to the desired amount on film. The distance from lens to subject doesn't change regardless of film format. The only reason the lens focal length changes is due to the size of film. It's really that simple.

As Old-N-Feeble referred to, the camera to subject distance controls perspective, regardless of focal length.

I do not disagree! But my thought process was less about perspective and more about focal lengths. My post one up from this might be more insightful as to why I was asking.

 

[Vaughn]

First example (model w/90mm lens); All shots will look the same (image size, perspective, etc) except for the the larger the negative, the more of the model that will be captured on the film. All you have done when you increased film size, is to sample (expose) a greater area on the same plane that is being hit with the same light.

"In real life, I know that 90 on 6x7, looks very different than 45mm on 35mm, and very different than 165mm on 4x5 even though they all share similar angles of view."

I will disagree with this -- I not found there to be a significant difference between those lens/film combos that can not be more accurately attributed to increased real estate (film size) and/or film proportions.

 

[cliveh]

I do not disagree! But my thought process was less about perspective and more about focal lengths. My post one up from this might be more insightful as to why I was asking.

As the focal length of the lens used is also related to print enlargement and viewing distance of said print, the subjects becomes more complex.

 

[markbarendt]

Right - so since only the focal length is common to all formats with different angles of view, is there a recognizable or distinguishing quality of a focal length that contributes to the rendering of the image regardless of format?

Maybe I'm not asking the right question, or I'm asking in the wrong way.

Ok, for one: in a general/theoretical sense; for a given lens, distance to subject, and aperture size you get a given depth of focus/depth of field regardless of format. Format only changes angle of view. Angle of view is also limited by lens design so typically a 300mm lens designed to "cover" 24x36mm film probably won't "cover" a larger piece of film, not even a 645 medium format but a 300mm lens from a large format camera system will cover any format up to 8x10" and very possibly a bit larger.

 

[markbarendt]

As the focal length of the lens used is also related to print enlargement and viewing distance of said print, the subjects becomes more complex.

Yes, and IMO viewing distance is the biggest factor.

 

[rwreich]

Really interesting thoughts. Thanks, everyone. I'm going to take a break and do some thinking...

 

[MattKing]

In real life, I know that 90 on 6x7, looks very different than 45mm on 35mm, and very different than 165mm on 4x5 even though they all share similar angles of view.

Not really - or at least not because of anything to do with focal length.

It is hard to construct a well controlled experiment, but if you were going to try, I expect a view camera with a variety of different lenses and a variety of different sized backs would be the closest.

You would still end up with slight differences between the films (35mm vs. 120 vs. sheet sizes) and differences with respect to grain size after enlargement, but if you were able to factor out those differences, only the qualities of the lenses themselves would cause a difference.

 

[Theo Sulphate]

When you think about it, format just specifies how you crop the image circle from a given focal length lens.

Take a lens, use it to project an image on a wall, then place film of different formats within that projected image.

Granted, a 150mm lens designed for 4x5 has a larger image circle than 35mm format. If I could mount a 150mm large format lens on a 35mm camera, it should give the same image as a similarly designed 150mm lens for 35mm format.

 

[480sparky]

Once you start figuring out how to 'convert' a given FL when shooting a given format to another FL/format, then you run into another problem..... DOF.

A 50mm on a 35mm film camera may have the same DOF as a 100mm on a MF, and the same for a 150mm on a 4x5..... they will each have a different FOV when all three formats are shot at the same aperture.

 

[Ian Grant]

A 50mm on a 35mm film camera may have the same DOF as a 100mm on a MF, and the same for a 150mm on a 4x5..... they will each have a different FOV when all three formats are shot at the same aperture.

There's no logic in that statement At the same aperture the 150mm lens (on a 5x4) will have considerably less DOF compared to the 50mm (on 35mm), you've also chosen FL's for each format that have a relatively close FOV.

If you were to state: "A 50mm on a 35mm film camera may have the same (approx) FOV as a 100mm on a MF, and the same for a 150mm on a 4x5..... they will each have a differentDOF when all three formats are shot at the same aperture." Then that makes sense.

Ian

 

[Ian Grant]

Granted, a 150mm lens designed for 4x5 has a larger image circle than 35mm format. If I could mount a 150mm large format lens on a 35mm camera, it should give the same image as a similarly designed 150mm lens for 35mm format.

There are LF lens sold with mounts for 120 & 35mm cameras, the 360mm f5.5 Tele Arton for instance, I've seen them with a focus helical to fot M$2 and Exacta cameras and also for Pentacon 6. There's one for sale at the moment on this forum, they actually cover 7x5

One of the Leitz lenses, I think the 135mm Hektor, can be used as an LF lens, it's not a telephoto design and the head unscrews from the barrel. I've tested vintage LF lenses by mounting on a set of M42 bellows and a DSLR, I was looking primarily at contrast & flare between different design types rather than resolution.

Ian

 

[rwreich]

After thinking about the issue, I'm imagining that there may be other factors involved. I completely agree with the concepts and thinking that Theo presented. Matt, when you said:

Not really - or at least not because of anything to do with focal length.

It is hard to construct a well controlled experiment, but if you were going to try, I expect a view camera with a variety of different lenses and a variety of different sized backs would be the closest.

You would still end up with slight differences between the films (35mm vs. 120 vs. sheet sizes) and differences with respect to grain size after enlargement, but if you were able to factor out those differences, only the qualities of the lenses themselves would cause a difference.

I don't know if you were considering that the apertures would have to change to aproximate DOF, or if you were sayibg that apertures would remain locked at f/5.6 or whatever. Shorter focal lengths on smaller formats only appear to have the same DOF as longer focal lengths on larger formats if the aperture is changed.

Ian - absolutely. DOF is mathematically constant given a focal length, aperture, and distance to subject. It is not dependant on the size of the film, though it may appear that way.

I think that I prefer a certain level of compression that is derived from the focal length and the working distance between camera and subgect, regardless of field of view.

And it took reading other people's ideas to clarify that.

The sweet spot, for me, begins around 65mm and continues upward from there. At a given aperture, working distance, and focal length, the level of compression and DOF is the same regardless of format. So, if 80mm is telephoto on one format, normal on another, and wide-angle on another, that's fine. Perhaps I'd rather use three different formats instead of changing lenses.

 

[Ian Grant]

Ian - absolutely. DOF is mathematically constant given a focal length, aperture, and distance to subject. It is not dependant on the size of the film, though it may appear that way.

That's what I've said all along However in practice DOF tables change for the same FL at the same aperures beteen 35mm, MF and LF because it's assummed that larger format negaives need less enlargement and so the COC used in calculations is increased in the calculations.

I think that I prefer a certain level of compression that is derived from the focal length and the working distance between camera and subgect, regardless of field of view.

The compression is purely down to the working distance.

Ian

 

[markbarendt]

think that I prefer a certain level of compression that is derived from the focal length and the working distance between camera and subgect, regardless of field of view.

Visual compression (making the background look closer to the foreground) is enhanced by narrow angle of view (takes away the viewers peripheral vision clues) and large depth of field (so no focus clue of how far away things are).

 

[markbarendt]

The compression is purely down to the working distance.

Ian

I'm not tracking your thought here, compression (mountains behind look close to person in foreground) or flattening (smaller nose v bigger nose in portrait)?