October 27, 2015

Yeah, 'Crop Factor is Math,' but This Video Will Help You Understand It

When fledgling filmmakers first start messing around with cameras and lenses, they run into a lot of confusing concepts, one of them being crop factor.

Michromatic's Mark Ryan Sallee helps simplify it all in this easy to follow video, in which he covers how sensor sizes crop images and change the focal length and angle of view of lenses, as well as, yes, a little bit of math that you might want to hang on to.

Crop factor is for understanding how the field of view of a given focal length is going to change as you move from camera system to camera system.

Understanding crop factor won't necessarily keep you from being able to make films like, say, not knowing how to hit the record button would, but it certainly will save you many headaches -- and money! It's a good idea to educate yourself on what the difference is between different sized image sensors, other than price of course, before you invest in lenses and a camera with a sensor that produces images that you weren't expecting.

And look! Now that you understand crop factor a whole lot better, you can participate in all of the arguments raging on the internet about full frame vs. Micro 4/3 vs. every other type of sensor that exists in the natural world. Delightful! Be safe and have fun.      

Your Comment

18 Comments

The 135 still frame has no significance for filmmakers. A 35mm movie frame isn't even close to a 35mm still frame. The film in a movie camera runs vertically, resulting in a frame that's roughly 1/2 the size of a 35mm still frame. Learn field of view. Learn what wide, normal, and telephoto focal lengths are on your camera. Crop factor is something somebody made up for amateur still photographers when newfangled digital SLRs had sensors smaller than the 135 film frame they were used to, and there's no need to understand it.

October 27, 2015 at 9:17PM, Edited October 27, 9:19PM

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It's useful for some (many). I got started with a full frame camera so it's what I'm used to, which makes crop factor handy for me. Plus lenses are standardized with their FF focal length so I can easily tell what it's going to do on different cameras. Don't act like you're better than "amateur" still photographers who use crop factor...

October 27, 2015 at 9:42PM

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Jeremiah Kuehne
Filmmaker
668

Standardized with their full frame focal length? What does that mean? A lens' focal length doesn't change depending on what camera is mounted to. This is exactly the kind of misinformation that confuses the issue.

October 28, 2015 at 7:44AM, Edited October 28, 7:44AM

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That's why it's called "crop factor" and not "focal length factor"...never said it changes the focal length just that you'll get a similar framing if you cropped into a FF focal length by the given amount of the crop factor. Sorry my response was a little vague. I can tell you what wide, normal, and telephoto are on any camera if you give me its crop factor, and to me, that's pretty handy. ;)

October 30, 2015 at 3:26AM

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Jeremiah Kuehne
Filmmaker
668

Absolutely agree.

Why even have numbers. After so long why do the conversions in my head. I'm sure I used to convert 24 to wide when thinking of which lens I want to use but now I think the brain skips the wide part and just thinks 24. But we don't need the numbers. We could label our lenses SUPER-WIDE, WIDE, MEDIUM, TIGHT, SUPER-TIGHT etc. We don't write on a script 24mm shot, we write wide shot.

But perhaps its not "cool" to write WIDE on a lens... Also next to the fashion industry the film industry is about the most pretentious in the world so actually writing WIDE on a lens might not cut it.

And I guess what happens when you put a full frame WIDE on a GH4.. Becomes a TIGHT, tight like a Tiger. I'm tired its late.

October 30, 2015 at 9:11AM

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Crop factor is relative to the image format you are familiar with. Lots of people have never shot with a 35mm Full Frame camera and may know nothing about how lens focal lengths relate to the FF image size.

For example if you've ONLY shot with cameras that have APS-C sized sensors, then you are going to have a completely different crop factor when determining how your APS-C lenses relate to Micro 4/3 lenses.

In todays film world the standard generally is the Super35 format which is slightly larger than the Canon or Nikon APS-C formats. ( if you want to complicate things even further Canon and Nikon APS-C cameras use different sized sensors, with the Nikon sensors being a little bit bigger than the Canon sensors )

And then there are SpeedBooster lens adapters that change the focal length and maximum aperture of a lens. So a Nikon 50mm f/1.4 lens becomes a 36mm f/1.0 lens when used with a typical SpeedBooster lens adapter.

The simplest way of getting to know how lens focal lengths relate to an imaging format is to just shoot with that format, and you will quickly figure out what each focal length does for the format.

If you're trying to match one format to another format, then the crop-factor for the two different formats can be useful.

October 27, 2015 at 9:35PM, Edited October 27, 10:27PM

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Guy McLoughlin
Video Producer
30569

I'm with you on the nonsense of full frame as standard, Guy.

But the optics of a 50mm/1.4 lens don't change, even with a SpeedBooster. The lens will have the same depth of field, with or without a SpeedBooster. The angle of view will change, in this case to match a 36mm lens, and the SpeedBooster will pass through an extra stop of light.

October 28, 2015 at 4:00AM

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Charlie K
1089

I generally think of the speed booster as magnifying the size and sensitivity of the sensor. I find that conceptually easier and more accurate (especially in regards to depth of field).

October 28, 2015 at 7:51AM

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A SpeedBooster is also known as a Telecompressor or Focal Reducer, and WILL reduce the effective focal length and F-stop of any lens they are attached to. So adding a SpeedBooster to a f/1.4 lens with a 50mm focal length will convert the lens to a 36mm focal length with an f/1.0 aperture.

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

October 28, 2015 at 10:26AM, Edited October 28, 10:28AM

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Guy McLoughlin
Video Producer
30569

That Wiki page does not contain any math to prove or endorse your comment.

A lens can never get wider then it is. Period.
Trying to shoot wider then a lens is capable of is one of the causes of Vignetting.

What a Focal Reducer does, is to reduce the Crop Factor.

Cropped MM = Real MM divided by ( crop factor multiplied by focal reducer )

Say I took the Nifty Fifty from an EF camera, put in front of a Metabone EF-to-M4/3s Speed Booster, and put that in front of a GH4... ( http://www.metabones.com/products/details/MB_SPEF-m43-BM2 ) ...., a .071 ratio focal reducer

MM = 50mm / ( 2 x .71 )
or,.
MM = 50 / 1.42
= 35.21mm

There is no sum where any combination of focal reducer and cropped sensor can get a wider image then the lens is capable of, because you Divide the lens by the Crop Factor.

November 2, 2015 at 12:31PM

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Stewart Fairweather
Cinematographer
359

Nice cat

October 27, 2015 at 11:37PM

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Danny T
Photographer
490

Oh yes! Crop factor. I manage a rental house and I always ask my client what format are they used to work with (before looking at different lens options.) A lot of client (dp, 1st ac) don't know anything else than S35 or S16. Then, if their director wants a Red Epic Dragon shooting at 6K, I'll be telling them that a 35mm lens will be quite wide at this format and that if they go in lower resolution for higher fps, it will also change the field of view. (then fun explanations starts!)

October 27, 2015 at 11:49PM, Edited October 27, 11:50PM

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Danny T
Photographer
490

I love when some of the writers here do their own articles. Speak about their experiences, share first hand knowledge or share an interesting video that they shot.

But a lot of NFS posts recently have been these copy paste youtube videos.

October 28, 2015 at 2:01PM, Edited October 28, 2:01PM

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This brings up a good question I haven't quite figured out yet and maybe somebody here can chime in. I use a Sony A7s and a Tamron 24-70mm for most of my work. On shoots where I have to move quickly, I will shoot at say 28mm and then switch to the crop mode via the menu to "turn that 28mm into a 45mm" as far as field of view goes. What I would like to know are what are the differences between cropping to get the 45mm vs zooming in on the lens.

My assumptions are:
1. Cropping: the DOF stays the same as it was on 28mm | Zooming: the DOF gets shallower and true to a 45mm
2. The compression of space that you get from going to longer focal length is the same for both options (this is the one I am not 100% sure about). In other words, aside from less shallow DOF, the picture should look just like a real 45mm would look like on that same camera.

Is this correct or am I missing something? The reason I'm asking is because I'm considering getting the Raven when it comes out, which will require me to get a wider lens like a 15-30 to get the same approximate range of fields of views. Aside from a deeper depth of field, will anything else change about my image from using 15mm to achieve my usual 28mm, and using 24mm to achieve my usual 45mm?

October 28, 2015 at 8:52PM

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Tim Milgram
Director / Cinematographer
296

A 100mm lens is telephoto on 35mm, normal on medium format (or IMAX) and wide on large format. At no point is it anything other than a 100mm lens.

October 29, 2015 at 7:36AM

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The compression (or perspective and proportion) that you get from a 28mm lens will not look the same than the one you get from a 50mm lens. If you use a smaller sensor size that changes your field of view with the 28mm, you'll still get the same compression, regardless of the sensor size. In the end, what you really do is simply using the center part of your image 'cause that's what your sensor sees.

Red Raven is just shy of S35. 35mm focal is the standard 39 ish degree field of view on S35 sensor. (A 50mm focal is the standard 39 ish degree field of view on a fullframe sensor.) So maybe looks into lens from the starting point that a 30mm will be your standard 39 degree on the Red Raven.

October 29, 2015 at 8:50AM

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Danny T
Photographer
490

Yes, in principle only depth of field changes. But lenses aren't perfect, and any image quality issues will either be magnified or cropped out.

- A vignette that's present at full frame 45mm might get cropped out at 28mm, giving an even exposure.
- A lens that resolves 1080p at full frame might look soft when cropped in.
-A lens with soft corners at full frame might have good edge-to-edge sharpness cropped in.
- Chromatic aberration might be negligable at full frame but apparent when cropped in.

November 5, 2015 at 12:24AM, Edited November 5, 12:31AM

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Anton Shavlik
Cinematographer
74

Great summary of the issues around this. It'll be nice when we just all have full frame sensors, but until then, it's good to keep this stuff in mind. Also, as a visual effects artist whose, if your 3D the work ends up being integrated into footage shot on a camera that the crop factors are relevant, it's vital for matching elements. Most 3D programs assume traditional film framing and lens lengths. Odd really, but that's the way it is.

October 30, 2015 at 8:50AM, Edited October 30, 8:50AM

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Douglas Bowker
Animation, Video, Motion-Graphics
215