A Lens is a Lens is a Lens

[Mitch Gross]

I find myself explaining the concepts of lens coverage, focal lengths and formats and how they all relate again and again. I must have seen it explained in books and on the Web hundreds of times, often with metaphors and diagrams, but it can be very difficult to grasp if one's mind isn't quite wired that way.

So I figured that since these were all visual concepts I should try explaining it that way, and I used a couple of REDs and a few lenses to do it. I think the general concepts come through, and I hope that people find it useful. You can find this video on Abel Cine Tech's blog, Cine Technica:

http://blog.abelcine.com/2010/05/14/a-lens-is-a-lens-is-a-lens/

 

[Alan Zarnegar]

But is it a Lens?

Joke aside, thank you Mitch for your video. It is informative and educational as always.

P.S.
Any news on when the new Schneider Xenar Primes are coming out?

 

[Frank Weeks]

Nice presentation Mitch. Hopefully most of the folks here understand the basics but it's always a little easer when you can see it first hand.

 

[Scott Miller]

Well done. Thank you. Nice to get such a clear presentation of what is really a complex subject!

 

[Brian Ferguson]

That was a very good demo Mitch, as a director said once "talking about visuals, is like dancing about architecture."

I started my career in a still studio doing advertising illustration. We shot 35 mm, 2 1/4 sq. (6 cm x 6 cm) , 4x5" and 8x10" view cameras. You just learn to know the lenses that were wide, medium and telephoto for each format.

The need for people to equate every thing to 35 mm still cameras kind of drives me nuts. The thing to know is physics of lenses are the same for each - depth of field is based on lens length, and aperture and subject focus not the format. But those lenses on one format seem telephoto because the image area is smaller for example. So you pull back farther and the focus is different, and the DoF greater. People really underestimate the effect that lens to subject distance has on DoF.

The other thing you illustrated welll is that some lenses are say, 25mm- but designed to cover a different image area. But that does not change their DoF or any other parameters that you would enter in a DoF scale. The 16 mm and 35 mm are great examples because most of us have lived with them for so long. Also I equate 2/3" broadcast sensors as almost the same as 16 mm or on the RED One as 2K. On the 2/3" video cameras it does not matter if it is SD or HD for this discussion.

A very good illustration and I appreciate it, thanks.

 

[Frank Weeks]

"talking about visuals, is like dancing about architecture."

I love that quote

 

[Brian Ferguson]

my memory tells me that is what was said by Irwin Winkler in a ASC Magazine article in a story about "Life as a House" Starring kevin Kline, but memory is faulty. Anyway I have used that statement over the years and it gets clients out of the mode of verbalizing visuals, which is not very productive.

Well I thought that was the quote but a Google search shows up these also...
"Writing about Art is like Dancing about Architecture"
or attributed to Frank Zappa "Talking about Music is Like Dancing About Architecture"

Pretty much the same.

P.S. I just noticed that the only year with a gap for digital back issues on the ASC site is 2001

 

[Joseph Hutson]

Well done Mitch... :emote_popcorn:

 

[michael zaletel]

I have a question. If I have a 2x extender on a Canon 300mm lens and am shooting on a RED ONE in 2K, what's is my effective focal length. What is my effective cropping compared to shooting same lens without extender on a full-frame 35mm film camera?

Also, what happens to focal length and depth-of-field using same lens on a 5D2 vs. 7D?


-michael zaletel

 

[Brian Ferguson]

Do you equate full frame FILM (not still, one goes through the camera vertical the other horizontal) to super 35?

A 300 with a 2x on 2K would be 1200 mm compared to that setup RED at 4k at 600 mm. Or double it is easier to think in double or half instead of Millimeters I think.

The question sort of negates the meaning of this thread.
You need to work out DoF on a calculator based on your lens parameters and format. The point is, if you have the same millimeter lens and aperture and focus distance the DoF is the same. But your picture is completely different. This is where learning your craft kicks in. There are many things to be learned by experience. You need to see the video and try it for yourself, it will be a revelation.

 

[Nir Shelter]

I would say that the "magnification" is that of 1200mm but the DOF is of a 600mm as you lose F stops from the doubler and have to compensate. Then the 2k crop will further magnify the center of the image with no relation to optics.

 

[michael zaletel]

I would say that the "magnification" is that of 1200mm but the DOF is of a 600mm as you lose F stops from the doubler and have to compensate. Then the 2k crop will further magnify the center of the image with no relation to optics.

Wouldn't the effective cropping (not talking about pixel resolution here just the image in the viewfinder) with a 300mm and 2x extender at 2K on a RED ONE be similar to a 2400mm lens on a RED EPIC FF35 shooting 6K?

I'll do some testing. I have a 5D2 and a RED ONE w/Birger along with a 200mm Canon lens and a 2x extender. I'll shoot a still of something distant with the 5D2 and the 200mm lens, then another with the 2x extender. Then I'll put the 200mm lens on the Red with and without the 2x at 4K and then again at 2k. My initial guess is that to match the outside crop edges on the 2k-200mm-2x stuff, I would need a 1600mm lens on the 5D2 still shot. I understand from Mitch's video that the relative size of things at different distances would vary but I'm just talking about the frame cropping as if there was a 500 foot square green screen and only one object in front of it.

-michael

 

[Brian Ferguson]

I would say that the "magnification" is that of 1200mm but the DOF is of a 600mm as you lose F stops from the doubler and have to compensate. Then the 2k crop will further magnify the center of the image with no relation to optics.

Yes you are right and I forgot the obvious 2 stop loss from the doubler. It seems like hypothetical example from the start. Which further emphasizes my point is that there is a big difference between theory and practical application. Things on face value are different when you fire up the camera and look thru it.

The only opinion you should worry about is the one you formed from you own experience based on educated guesses. Any other statement is extremely subjective.

The question can not be answered until Full Frame is defined. Full frame still, or film? As asked originally. I still say if you want to shoot different formats you need to form your own opinions based on lens lengths that are appropriate for that format.

"I understand from Mitch's video that the relative size of things at different distances would vary but I'm just talking about the frame cropping as if there was a 500 foot square green screen and only one object in front of it."

That was exactly what he was talking about. You change any parameter and everything else changes.
It doesn't matter what you want it to be - you are bound by the laws of physics. It will be, what it will be. The point I have tried to make is - that with many variables the only correct answer is the one formula you make from your own observations. Maybe, just maybe, you can bend those physics laws to match what you hoped they would be.

 

[michael zaletel]

It seems like hypothetical example from the start.

Hi Brian:

I appreciate your feedback. My intention was not to pose a mere hypothetical. If I am say 200 feet away from a bird or other small animal and happen to be shooting on an EPIC FF at 6K, I am wondering whether I would need a 2400mm lens to get the bird to fill as much of the frame as it would fill if I were shooting with a 300mm lens and a 2x extender on a RED ONE at 2k.

The reason this is important is because I plan on owning an EPIC FF 6K camera some day and am wondering where in the hell I'm going to find a 2400mm lens. Sure I can use 1:1 to zoom in on the bird or animal for cropping but once I hit record, I zoom back out and can't follow the action or see what's going on on the monitor or Viewfinder. In cases like that I won't have time to lower the resolution on the EPIC FF and reformat my media.

The reason I shoot with a 300mm lens and 2x extender on a RED ONE at 2K in the first place is so I can get 120 frames per second. The EPIC FF 6K will eventually allow me to shoot 120 frames per second at 6K but I won't be able to see what's going on while the camera is recording if I'm using the same lens at the same distance... right?

-michael zaletel

 

[Stephen Williams]

Hi Mitch,

My only comment is you talk about 'apparent' DOF, DOF is about what's actually in focus as opposed to how soft the out of focus area is.

Best,

Stephen

 

[michael zaletel]

Hi Mitch,

My only comment is you talk about 'apparent' DOF, DOF is about what's actually in focus as opposed to how soft the out of focus area is.

Best,

Stephen

Hi Stephen:

To make sure I have it right, DOF refers to the "depth of the field of focus" right? In other words, if I am shooting a bunch of objects spaced 1 foot apart between the camera and a backdrop 30 feet from the camera AND if I have a shallow or narrow depth of field and I focus at 10 feet, perhaps only the objects at 9 feet and 10 feet and 11 feet will be in focus and everything else will be out of focus.

If my depth of field is not shallow (is there a preferred term for that), then perhaps all the objects from 5 feet to 20 feet will be in focus.

An f/1.2 lens opened all the way up will have a much narrower depth of field of focus in front of and behind the distance mark at which the lens is currently focused than a f/4.0 lens opened all the way up. Right?

Thanks for helping me to understand the preferred terminology. Sometimes it's just semantics but I'd rather be on the right side of the semantics.

-michael zaletel

 

[Brian Ferguson]

Hi Brian:

I appreciate your feedback. My intention was not to pose a mere hypothetical. If I am say 200 feet away from a bird or other small animal and happen to be shooting on an EPIC FF at 6K, I am wondering whether I would need a 2400mm lens to get the bird to fill as much of the frame as it would fill if I were shooting with a 300mm lens and a 2x extender on a RED ONE at 2k.

The reason this is important is because I plan on owning an EPIC FF 6K camera some day and am wondering where in the hell I'm going to find a 2400mm lens. Sure I can use 1:1 to zoom in on the bird or animal for cropping but once I hit record, I zoom back out and can't follow the action or see what's going on on the monitor or Viewfinder. In cases like that I won't have time to lower the resolution on the EPIC FF and reformat my media.

The reason I shoot with a 300mm lens and 2x extender on a RED ONE at 2K in the first place is so I can get 120 frames per second. The EPIC FF 6K will eventually allow me to shoot 120 frames per second at 6K but I won't be able to see what's going on while the camera is recording if I'm using the same lens at the same distance... right?

-michael zaletel

My point is that it is a hypothetical question - because the Epic FF 6K is not in production. Until it is, it is a hypothetical, it is in development technically - no one can tell you yet for sure. Everything is subject to change count on it. It would be theory right now, and not practical experience, which is not something that has been tested.

I also asked twice, if you meant full frame still or motion, and you never answered the question.

If not getting closer than your lens allows, and shooting at a high image size, makes your lens too wide. Then you have to blow the image up in post until it suits your purpose. You won't be any worse off. Or get closer. There are not any outs here. You can't have closer images and bigger formats with the same size lens. A "normal" lens on a 4x5" view camera is 210 mm.

From a practical point of view there is only one viewpoint. If you increase your frame size 4 times you need to increase the focal length by the same.

This is the whole point of the thread, frame size and lens length relationship are exponential!
That is why Mitch was so frustrated and made a visual. It is not a description that is verbally simple.

If you see a difference between motion 35 digital and full frame still sensors you will see that same difference in sensors of the same relative size.

 

[Alexander Ibrahim]

An f/1.2 lens opened all the way up will have a much narrower depth of field of focus in front of and behind the distance mark at which the lens is currently focused than a f/4.0 lens opened all the way up. Right?

There is no such thing as depth of field of focus.

Depth of field and depth of focus are two separate things and should not be confused.

Depth of field is what you are asking about and happens out in front of the lens at the subject. It describes the distance ranges from the focal plane at which a subject is in acceptable focus.

Depth of focus happens behind the lens at the sensor. It is the range of distance at which a lens will project a suitably focused image onto a sensor (film or digital)

Your example of different lenses is confusing.

The only relevant factors when determining depth of field are:

the focal length of a lens
the current f-stop setting
the focal distance.

(Well that is over simplifying a bit... you have to choose a circle of confusion (CoC), the ASC manual assumes 0.0010 inches CoC, but you can safely ignore that for now.)

Your question about the different lenses and their relevant DoF leaves out important information. I think you meant to keep all other factors the same, but you were not explicit.

So, if you were talking about two identical focal length lenses, say 50mm, the answer to your question is YES.

It also bears mentioning that it doesn't matter what the maximum aperture of the lenses are, only what the f-stop is actually set to. So, sticking with 50mm lenses you could have used two Zeiss 50mm primes, which can open up to f1.4, and just set one to f4, or you could use a zoom with a maximum aperture of f4, the results would be very much the same as far as DoF go.

Moving on...

A 50mm lens at f1.4 focused at 12 feet will have the exact same DoF as an 85mm lens at f4 focused at 12 feet.

The focal length of a lens is an important factor.

Other than that you appear to have the correct sense of the use of the phrase "shallow depth of field," i.e. when the eyes are in focus, but the nose and ears are soft.

When your depth of field is the opposite of shallow, its "deep." Just like your pool.

Now finally, different lenses can have different characteristics. When you look up a table of DoF you are getting an idealized version.

In real life a 50mm Zeiss ZE prime, a 24-70mm Canon zoom set to 50mm and 50mm Canon lens (and all set to the same f stop at the same focal distance) will exhibit different DoF, as well as other optical characteristics. In theory they are the same... but the realities of optical engineering mean that in practice they are in fact different.

Usually for most purposes these difference are small and can be ignored- but you should know they exist.

Hope that helps, but as has been noted elsewhere, writing about this can be like describing color via acupuncture.

 

[Stephen Williams]

Hi Stephen:

To make sure I have it right, DOF refers to the "depth of the field of focus" right? In other words, if I am shooting a bunch of objects spaced 1 foot apart between the camera and a backdrop 30 feet from the camera AND if I have a shallow or narrow depth of field and I focus at 10 feet, perhaps only the objects at 9 feet and 10 feet and 11 feet will be in focus and everything else will be out of focus.

If my depth of field is not shallow (is there a preferred term for that), then perhaps all the objects from 5 feet to 20 feet will be in focus.

An f/1.2 lens opened all the way up will have a much narrower depth of field of focus in front of and behind the distance mark at which the lens is currently focused than a f/4.0 lens opened all the way up. Right?

Thanks for helping me to understand the preferred terminology. Sometimes it's just semantics but I'd rather be on the right side of the semantics.

-michael zaletel

Hi Michael,

To simplify matters I will just talk about DOF which is about what is in 'Sharp' focus:-
Say I have a product shot, where the product is taking up half the frame. Shooting at F5.6 with a 50mm lens, unfortunately only 2/3 of the product name is in focus, the agency is not happy! If I change to a 25mm lens I have to move the camera closer (1/2 distance of the 50mm lens to the product) so the product is the same size in frame. Unfortunately the product name is still only 2/3 in focus at F5.6

DOF is related to F stop & Magnification only, lens focal length is irrelevant, the only way to increase DOF is to stop down.

Best
Stephen

 

[Stephen Williams]

Now finally, different lenses can have different characteristics. When you look up a table of DoF you are getting an idealized version.

In real life a 50mm Zeiss ZE prime, a 24-70mm Canon zoom set to 50mm and 50mm Canon lens (and all set to the same f stop at the same focal distance) will exhibit different DoF, as well as other optical characteristics. In theory they are the same... but the realities of optical engineering mean that in practice they are in fact different.
.

Hi Alexander,

Whilst what you say is true, if you look at the images produced by those different lenses the size on the sensor will be different. When you adjust the camera distance so that the size of the image matches at the same F stop the DOF will be identical.

The reason for this is lens thickness, DOF is calculated from the front nodal point of the lens not the film plane. Simple DOF tables assume a 'thin' lens. A Cooke 20-100 is clearly not thin at all!

As I said in the previous post F stop & Magnification are the only 2 components involved.

Best,

Stephen