Equivalence between Focal Lengths

D

David Evans

Well-known member
Hi,

I have a very simple question. For guys like me, that use 35mm adapters (SGblade), the focal lengths of the lenses I use is equivalent to the ones used in 35mm movies? I’m asking this because I always loved the widescope look of classical directors, and most of them are very fond of 28mm lens (Spielberg, for example). Now, I’m not trying to get picture like theirs (nor could I), but I do want that focal view. So, 28mm in my Sgblade would account for the same focal length as in movie cameras?


I have a 20mm Nikon and love it, but it still feels a little too wide…

Thanks

 
The SGblade uses a Full Frame 35mm rotating glass format ? ( I think it does, but I'm not 100 percent on this )

Most of the DOF adapters for 35mm still photo lenses use the still photo Full Frame 35mm imaging format, which is about twice the size of the motion picture Super35 format, so if Spielberg liked what a 28mm lens did for the Super35 format, then you would be looking for a 28mm x 1.4 = 39mm lens. ( when I used to shoot with a Full Frame 35mm Nikon F3 in the 1980's my "normal" lens was always a 35mm lens, which some people considered to be a "wide normal" lens )
 
Ok, is there a way to do the math? I use the usual up and down bars to create the scope like effect in post, but for that to work, it still has to be used in combination with the right focal length. Can someone help me with the math? :)
 
Liam Hall said:
Spielberg used anamorphic lenses on many of his movies, so might want to redo your math...
Click to expand...

OK, then if we factor in anamorphic lenses...

28mm x 1.4 x ( 1.78 / 2.35 ) = 29.69 mm lens

So roughly a 30mm lens on your SGblade, and you would crop your image to a 2:35:1 aspect ratio. ( assuming you are not shooting with anamorphic lenses with your SGblade adapter )
 
TheDingo said:
The SGblade uses a Full Frame 35mm rotating glass format ? ( I think it does, but I'm not 100 percent on this )

Most of the DOF adapters for 35mm still photo lenses use the still photo Full Frame 35mm imaging format, which is about twice the size of the motion picture Super35 format, so if Spielberg liked what a 28mm lens did for the Super35 format, then you would be looking for a 28mm x 1.4 = 39mm lens. ( when I used to shoot with a Full Frame 35mm Nikon F3 in the 1980's my "normal" lens was always a 35mm lens, which some people considered to be a "wide normal" lens )
Click to expand...

This is not as easy as you may think, haha. What camera are you using the adapter with?

Typically issues of formats are pretty easy, however, in your case, math will be nearly impossible to nail down.

Why? Lens adaptors, like SG Blade, Letus 35, RedRock Micro, etc, etc, are VERY difficult because they vary greatly. I used to have a RedRock unit way back in the day (2007, haha) and I was using it in conjunction with a taking zoom lens on my HVX200. I can tell you the math is almost impossible to calculate because the optical path is so complicated. Mainly because the taking fixed zoom lens on the camera is close focused to some sort of opaque screen (ground glass) upon which the front lens is projection onto. In my experience my HVX200 could zoom in and out into that image with some flexibility and thus greatly changing the FoV. So although we know these adapters give shallow DoF, depending on how much or little our taking lens is seeing of the adaptor screen, we could be wider, the same, or cropped into FF35mm format. Who knows!

I've never used the SGBlade, but I assume it works mostly like the others I mentioned above. I'll say there probably is a way to do the math by using some complicated formula using the sensor size of the camera, focal length of taking lens, and distance to the adapter screen (ground glass) and putting it in context... but I'm not sure.

If you were using an interchangeable lens camera system and wanted to compare with another, that is easy! The complication is that the ground glass the taking lens projects onto doesn't have a size. It's getting the whole lens image circle which can often be larger than FF35 and you're using a fixed zoom lens on the taking camera to crop into that unspecified image size to another unspecified format size. Where you land is greatly unknown to you. You can assume you're somewhere near FF35 but are you?








However, separate from your complicated lens adapter issue, I can say anamorphics are generally easy to diagnose. Anamorphic focal lengths are named the same as a spherical, however it's horizontal FoV is double. So a 50mm anamorphic lens and a 50mm spherical lens will look identical in framing from top to bottom. Meaning if you have a persons face filling the screen, top of the head to the chin, both carry the same composition. However, the anamorphic lens simply sees x2 wider.... the equivalent of a 25mm spherical. But we don't rename the anamorphic lens. A 50 is a 50. So depending on what you are shooting, it's not rather difficult to pick the spherical equivalent to any anamorphic lens, as they are named quite appropriately.
 
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Peter Märtin of Vantage Film, makers of Hawk Anamorphic lenses, explains. “Anamorphic lenses use cylindrical elements to squeeze the image in one axis only—the width, not the height. What means an anamorphic lens has different focal lengths: the horizontal part of the image is the wider focal length and the vertical is the longer focal length. Also, the lens has two nodal points. (The nodal point is where all light beams converge when going through the lens.) One nodal point is for the horizontal part of the light rays, and the other one is for the vertical. Essentially, the lens records the image in a sort of three-dimensional way.

“It’s similar to looking at a landscape with one eye closed. If you hold up your hand and move it closer to you, your hand will covering more of the background. Move side to side, and you reveal different perspectives behind your hand. You get information about the three-dimensionality of the room. Anamorphic lenses do something similar: providing the two dimensional sensor a part of the three-dimensional information. It’s almost 3-D, perhaps 2.5D.”

There's something inexplicably appealing about anamorphic lenses, and it’s not inextricably tied to blue line streaks or oval bokehs.

Peter continues, “The anamorphic look is very elegant. The lens is not a neutral, technical, observer. Instead, it is subjective. It changes the scene slightly, adding out of focus areas, providing depth to a sequence. It’s very appealing for faces, good for beauty. It gives actors a beautifully cosmetic, elegant, interesting, different look. With a long spherical lens, the face might look flattened, which is not always flattering. The anamorphic lens gives you depth and is pleasing. A lot of cinematographers are using anamorphic lenses mainly because they look so beautiful for faces.”

A good way to select the appropriate anamorphic lens for a specific scene is to think in terms of the vertical focal length. Use the same numbers as you would for spherical. A 100 mm anamorphic lens gives you the same headroom as a 100 mm spherical lens. Of course, the 100 mm anamorphic will be twice as wide as the spherical 100 mm — equivalent to a 50 mm spherical in its horizontal eld of view. If you were thinking in terms of a 100 mm spherical lens and wanted the same horizontal field of view in anamorphic, you’d choose a 200 mm anamorphic lens. Of course, the vertical axis would be “tighter” because the vertical angle of the anamorphic is the same as the spherical.

http://www.filmanddigitaltimes.com/wp-content/uploads/2013/08/Anamorphic-Math-FDTimes.pdf
 
To the original poster:

If I were you, I would say the simplest solution to your problem would be the following:

I would do the following test:

1) Acquire a Canon 5Dm3 camera and put it into video mode for 16:9 ratio
2) Find a large white wall and measure (with precision) 10' away from the wall.
3) Put the 5Dm3 sensor at that exact point, 10' from wall.
4) Put a 50mm lens on the camera (or anything really,, but let's pick a common one)
5) Using painter's tape or paper tape, with precision, tape off the corners of the frame on the wall. (You might want to connect to a TV or monitor, or conscript another person to make this easier)
6) Remove the 5Dm3 from the tripod.

7) Put your camera with SG BLade adapter on the tripod.
8) Put the SG Blade Ground Glass (in the adapter) exactly 10' from the wall (NOT the camera sensor)
9) Put a 50mm lens on the SG Blade (or whatever you chose for step 4)
10) Using the taking zoom lens on your camera, zoom slightly in and out of your ground glass to align with and match the 5Dm3 framing chart on the wall.
11) Take note of the camera/adapter setup and write down the zoom value of the taking lens.

You should now know your camera is at FF35mm FoV (at least to Canon specifications).

NOW, you may proceed forward and do the more simple math of format to format FoV calculations. Abel Cine's calculator will be of great use. If your camera has been calibrated as I have instructed above, simply use the 'Canon 5D' option on the site for your camera (http://www.abelcine.com/fov/)
 
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Ryan Patrick O'Hara said:
Mainly because the taking fixed zoom lens on the camera is close focused to some sort of opaque screen (ground glass) upon which the front lens is projection onto. In my experience my HVX200 could zoom in and out into that image with some flexibility and thus greatly changing the FoV.
Click to expand...

So people didn't standardize on the 35mm Full Frame image area ? ( at least in terms of image width )

Otherwise it's like cropping into your image in post, which will obviously change the FOV for the finished image.
 
Ryan Patrick O'Hara said:
To the original poster:

If I were you, I would say the simplest solution to your problem would be the following:

I would do the following test:

1) Acquire a Canon 5Dm3 camera and put it into video mode for 16:9 ratio
2) Find a large white wall and measure (with precision) 10' away from the wall.
3) Put the 5Dm3 sensor at that exact point, 10' from wall.
4) Put a 50mm lens on the camera (or anything really,, but let's pick a common one)
5) Using painter's tape or paper tape, with precision, tape off the corners of the frame on the wall. (You might want to connect to a TV or monitor, or conscript another person to make this easier)
6) Remove the 5Dm3 from the tripod.

7) Put your camera with SG BLade adapter on the tripod.
8) Put the SG Blade Ground Glass (in the adapter) exactly 10' from the wall (NOT the camera sensor)
9) Put a 50mm lens on the SG Blade (or whatever you chose for step 4)
10) Using the taking zoom lens on your camera, zoom slightly in and out of your ground glass to align with and match the 5Dm3 framing chart on the wall.
11) Take note of the camera/adapter setup and write down the zoom value of the taking lens.

You should now know your camera is at FF35mm FoV (at least to Canon specifications).

NOW, you may proceed forward and do the more simple math of format to format FoV calculations. Abel Cine's calculator will be of great use. If your camera has been calibrated as I have instructed above, simply use the 'Canon 5D' option on the site for your camera (http://www.abelcine.com/fov/)
Click to expand...

Or simplify it even further: choose your lens not based on mathematics or focal length or what your adapter is doing; choose your lens (and your f-stop) based on whether the composition, DOF, and perspective makes the visual statement you want it to make.

I did like the comments on choosing an anamorphic - interesting about horizontal vs. vertical FOV - but I imagine someone who has shot with anamorphics knows which lens to grab for a given shot.

I can't really wrap my head around "what lens will look like Speilberg on my sensor/adapter" - I'd prefer "what lens is closest to my vision for the shot".
 
Michael Carter said:
I can't really wrap my head around "what lens will look like Speilberg on my sensor/adapter" - I'd prefer "what lens is closest to my vision for the shot".
Click to expand...

Except, you get different 'vision' with an anamorphic; depth of field, bokeh, sharpness etc, all very different. And that's the point...
 
TheDingo said:
So people didn't standardize on the 35mm Full Frame image area ? ( at least in terms of image width )

Otherwise it's like cropping into your image in post, which will obviously change the FOV for the finished image.
Click to expand...

While it is true that one can change the FOV in post... one can not really change certain characteristics about a 'shorter' vs 'longer' lens focal length relative to the original FOV.

For example, at 10' one may have a face filling the frame with a lens of length X. To be sure one can use a lens of 2x, 3x, Nx, adjust the camera subject distance, and then crop down to the face, but the angle subtended by the 'object ' (the face) will be different and different portions of the background will be covered... not to mention the resulting loss of 'sharpness' for the amount of 'enlargement' required to fill the display frame.

Also, what type of 35mm Full Frame are we speaking... Motion Picture Full 35mm frame (and perhaps attendant different formats, Academy, Super35, a variety of other croppings? ), or 35mm Still Frame which there was limited Motion Picture usage?
 
Thank you all for your help! I must say I never tried an anamorphic lens, but I've been reading about people who use them and say they give an unreal look that makes them feel very cinematic. I think it's probably something to do with that widescope feeling that I'm also looking for. Excuse me for asking, but can I use anamorphic lenses on my SGBlade? I'm completely new to anamorphic, never used one...
 
Michael Carter said:
Or simplify it even further: choose your lens not based on mathematics or focal length or what your adapter is doing; choose your lens (and your f-stop) based on whether the composition, DOF, and perspective makes the visual statement you want it to make.

I did like the comments on choosing an anamorphic - interesting about horizontal vs. vertical FOV - but I imagine someone who has shot with anamorphics knows which lens to grab for a given shot.

I can't really wrap my head around "what lens will look like Speilberg on my sensor/adapter" - I'd prefer "what lens is closest to my vision for the shot".
Click to expand...

I understand what you are saying but let's not get aggressive at OP. I understand the importance of not making your films by simply emulating how other's work, but for those learning, (which is all of us) we can all appreciate reading American Cinematographer and being able to put their work into context and maybe incorporate some of that theory into our work without having to take trial and error guesses over and over. With experience comes greater grasping of imagery, but even in my case, it's nice to have standards and format consistencies upon which to make my decisions with greater ease and accuracy.

I say these questions are perfectly valid.



TheDingo said:
So people didn't standardize on the 35mm Full Frame image area ? ( at least in terms of image width )

Otherwise it's like cropping into your image in post, which will obviously change the FOV for the finished image.
Click to expand...

I'm not sure I'm understanding the question. Lens adapters vary greatly. They offer large sensor DoF to smaller sensor cameras but do so in a way that does not have any standards. Like at all. The restrictions and variations are also the result of how the taking camera interacts with the adapter.

David Evans said:
Thank you all for your help! I must say I never tried an anamorphic lens, but I've been reading about people who use them and say they give an unreal look that makes them feel very cinematic. I think it's probably something to do with that widescope feeling that I'm also looking for. Excuse me for asking, but can I use anamorphic lenses on my SGBlade? I'm completely new to anamorphic, never used one...
Click to expand...

Yes, anamorphic lenses are MORE than just aspect ratio. Bern posted earlier in this post with a quote that explains some of the magic outside the obvious oval bokeh, tendency to horizontally flare, and etc.
 
28mm anamorphic35 has the horizontal field of view of a 14mm spherical and fullframe35 (36mm) format is 1.63x wider than anamorphic35 (22mm) so you'd use a 23mm lens and letterbox to 2.35 in post. Not aware of any 23mm FF35 lenses but I'm sure a good 24mm, 21mm or 20mm would be within the margin of error. Now the real issue here is why would anyone intentionally use a ground glass lens adapter after 2009? (not counting frazier/periscope/snorkel lenses)

Keep in mind that Spielberg and others love those wide lenses because they have huge budgets and awesome production values in their top-class production design, lighting, casting, and scheduling, and a nice immersive wide lens really shows those things off... You won't be able to hide or cheat anything on a 23mm lens.
 
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Short answer: Imitate Spielberg on your 35mm adapter by shooting between 35mm and 50mm, preferably 40mm if you can find one (a pancake lens?).

Long answer: Spielberg has shot in three different formats: Anamorphic, Flat 35, and Super 35.

Anamorphic. Anamorphic after being desqueezed has what I call an optical frame size of 42 x 17.5 mm, which makes a diagonal of 45.5 mm, which is a tad bigger than the diagonal of full-frame 35mm stills film (43.27 mm). It is the diagonal dimension that I think contributes most to the overall feel of a frame, and what you should use when comparing frames of different aspect ratios. 28mm on anamorphic would be very wide. I don't have an example of 28mm in anamorphic, but Chinatown was shot almost completely on an anamorphic 40mm, and as you can see it looks pretty wide angle:

file_563974_chinatown-polanski_03262012_150014.jpg

An example of a movie that Spielberg shot in anamorphic is Raiders of the Lost Ark. I haven't found a record of exactly what focal lengths he used, but I don't think he went as wide as 28mm. Most of the shots look no wider than the 40mm above. He maybe used 50mm a lot.

Flat 35. From the mid 1950s until the end of the 20th century, if you weren't shooting anamorphic, you were shooting Flat 35. Flat 35 is just a crop of the movie film to make it widescreen, 1.85:1. In fact the cameras simply shot the whole Academy 1.375:1 frame. The crop was made by inserting a metal plate in the projector at showtime. This way you could have extra vertical margins while shooting, to take extra care that the dang boom mic doesn't come into shot. Nice.

Flat 35 is about 22 x 12 mm, which makes a diagonal of about 25mm, which is not much bigger than the 4/3 format. A normal, medium lens on Flat 35 is 30mm. They don't make 30mm cinema lenses. They also don't make 28mm cinema lenses. So whoever said Spielberg's favorite lens was 28mm has misinformation. David Mullen, a member of the ASC, says that Spielberg seems to like 27mm and 29mm (which average to 28mm, yes) and lines up with the look of Spielberg's movies, which is to say mildly wide.

An example of a movie that Spielberg shot in this format is Jurassic Park.

Super 35. It wasn't until the beginning of this century that many people started shooting Super 35. But it's not the Super 35 like on the Alexa, FS7, etc. (24.89 x 14 mm) with a 16:9 aspect ratio. It's a little smaller, 24.89 by 10.41 mm, to yield a 2.39:1 aspect ratio. So it's the same-shaped rectangle as anamorphic but with spherical lenses --- just cropping the frame (like Flat 35 did, but using more of the original negative). A normal lens in 2.39 Super 35 is about 33mm.

An example of a movie that Spielberg shot in this format is Minority Report, and for this film we have hard numbers from Spielberg's cinematographer that they used only a 17mm, 21mm, and 27mm lens. On your 35mm adapter this would be roughly like using a 28mm, a 35mm, and a 40mm lens.
 
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