John,
I was not looking for a progression of lenses. And yes I do follow you.
All I was trying to do was understand if I could expect a lens mathematically dead center between 90 and 150 (which is 120) would provide a view that was exactly dead center of the look that a 90 and 150 would provide. If so, the "progression" would be linear. If it was non-linear, 120 would not be the answer.
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It’s certenly not linear.Originally Posted by AdamD
A 90mm on 4x5” will give a horizontal angle of view of 66 degrees. A 150mm on 4x5” will give a horizontal angle of view of 43 degrees. The "middle" lens should give 54,5 degrees. The lens that will give you that angle is a 115mm.
Question goes back to perspective as rendered-recorded on film or image recording device. IMO, focal length steps with 4x5 film format of 90mm, 115mm/125mm, 150mm is small. It might be more effective to move the camera position as needed to achieve the image composition in mind instead of trying to apply lens focal length to achieve image composition in mind. This could be a zoom lens image making habit that has followed into LF sheet film image making from using zoom lenses common in modern digital and 35mm cameras, where this habit might not, should not apply.
As for zoom lenses, they are used for a different effect in cinema and video.
https://www.youtube.com/watch?v=maz9KP76VvI
Angenieux 12- 240mm f/3.5 with side prism finder, circa 1972.
Note the front to back object size differences and perspective differences as the zoom demo scenes in this video. These are a effective demo of different focal lengths in a given scene with a fixed camera position. It is common to dolly-trolley-rail the camera rig to alter camera position while zoom is applied to aid in the sense of motion in cinema or video.
Visual differences between dolly-trolley-rail (moving the camera position) -vs- zoom (often fixed camera position) images are discussed in this video.
Still images have a different set of requirements than "moving images".
Bernice
Adam...thank you for clarifying. As thus, your interpretation of "linearity" would give you a multiple (from 90mm) of about 1.29X (not perfect, but close) - which would give you an FL of 116.1, and multiply this by 1.29 to get 149.77 - with the closest available FL which would ascribe to such linearity therefore being 115mm. Does this make sense?
Another way to look at this multiple would be to first originate from 90 and try to land as close to 115 as possible...then use the same multiple for this result and see where it lands you. Thus...90 x 1.278 = 115.02, then 115.02 x 1.278 = 146.995. Does a 147mm lens exist which covers 4x5? I cannot think of one offhand...which leaves you, again, at 150mm.
Which leaves us to split the difference...and use a multiple of 1.284 to realize (going from 90mm) an FL of 115.56, then applying this multiple again which gives us 148.37. Of course, you can continue to split hairs here - I guess my point being that there is no single multiple which would comply with your idea of linearity...but in terms of progression - that which goes from 90 to 115 to 150 - its pretty darn close to linear!
am I missing the posts that speak of angle of view differences from various lens design with same focal lengths
and what is minimum focus, vs 'normal' which is...
then we have various format rectangles which change
Tin Can
When asking whether something is "linear" you always have to qualify, linear in what variable?
In this case, for example, focal length, image size on the film, field of view in degrees, or perceptual feeling of wideness or depth in the image? The latter is presumably the most important thing to think about, but it's not easily quantified and depends on what kind of photography one is doing.
Just to give an example, many photographs of architectural interiors use a very wide angle lens because they need the field of view and have a basically rectangular subject. But if you use a very wide angle to photograph a group of people, the people at the edge of the field will have elongated faces/heads due to rectilinear perspective. You don't notice converging verticals or elongated heads when looking with your eyes because your brain adjusts, but once forced onto the flat paper or screen, they become obvious and sometimes jarring. There may be a lens that is just wide enough to capture the group while allowing you to stand far back enough to minimize the egg-head effect, but it depends on your photographic aims.
A 120mm lens is somewhere in the middle between a 90 and 150mm, but trying to define the "middle" precisely is not really useful without context.
Holy smokes!! Everytime I think I ask a "simple" question, you guys manage to blow it up into all kinds of detail I could never imagine existed!!
So, the question was, "Is it a linear progression between 90mm and 150mm?"
And the answer is........wait for it.......No.
If it were a YES, the answer would be 120mm but John Layton clearly shows us that it's not exactly linear, but, it's really close.
Not to make this thread totally blow up, but based on the math John shows, the divergence from a linear line would become more and more noticable as you get longer in the FL. So I think the 50mm to 90mm is probably pretty flat on the curve and between 90 and 150 is when the progression starts to be noticable, but somewhat negligible and I'm imagining that between 150 and 300 it's real, and certainly by 450 is probably substantial.
Anyone want to plot a graph?
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Anything in life worth having is worth sharing.
How about a screenshot from a viewfinder app?
I have some LF lenses with ACTUAL FL hand written on barrel which varies by sample
Obviously done on optical bench as somebody thought it important
Tin Can
Not with a 90mm example. But it gives a impression from different focal lengths on 4x5"
https://garymulder.nl/fotografie/focal_length_examples/
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