Crop Factor VS Focal Length

alexdhanneman · Feb 25, 2015

Hey everyone. This is going to be my first post on this site, but expect more to come.

First I would like to introduce myself so that this post will make a little more sense. I am a digital compositor, but I am trying to learn the film side / photography side as well. Sometimes we have to undistort film plates, where it is necessary to know what the focal length of the shot is.

My question is when dealing with focal length, if I shoot on a camera that is not full frame, what do I say the focal length actually is?

If I am telling someone in post production what the focal length is, do I tell them the true focal length of the lens that was on the camera or do I include the crop factor? For example, maybe I have a basic zoom lens 18-55mm, and it's zoomed to about 21 mm. Lets say it's on a Canon EOS camera, so it is not full frame.

Do I tell the person 21 mm, or do I tell them 21 mm * (crop factor, say 1.6 for APS-C from Canon) which would bring the equivalent focal length to around 33 mm.

Just trying to wrap my mind around focal length and crop factor. Thanks.

Derrel · Feb 25, 2015

I would use the "effective focal length" as stated with reference to 35mm film or 24x36 digitial, aka FX in Nikon-speak, or FF in Canon-speak. So, if a 50mm lens is used on a small-sensor Canon camera, call that 50mm x 1.6x as the focal length used. If the shot comes from a camera with an even smaller sensor, call it 50mm x 2.0 effective focal length.

This would be pretty important, since on full frame, a 24mm lens is a fairly wide wide-angle lens, and the background would tend to render "small", and "far away looking"...on a micro 4/3 sensor a 24mm lens would render things much more as a normal field of view lens, with an almost neutral degree of foreground/background magnification. If a 24mm lens were somehow fitted to an iPhone, it would act as a super-telephoto lens.

photoguy99 · Feb 25, 2015

I can't answer the question of what various professionals expect to be told, but most accurate would be the focal length of the lens used to take the shot (which does not change with format, it is a property of the lens by itself) and the physical dimensions of the sensor or film.

The lens projects a little picture on the back of the camera. That picture has nothing to do with the sensor, it's all about the lens.

The film or sensor sits in the back of the camera and records a chunk of that projected little picture.

Ray Hines · Feb 25, 2015

Thank you photoguy99, I couldn't have put it better myself. Now I'm going to run away and hide before all those togs that like to convert to FF come along.

Derrel · Feb 25, 2015

photoguy99 said:
The lens projects a little picture on the back of the camera. That picture has nothing to do with the sensor, it's all about the lens.

Uh, sorry, but that's not accurate at all. Not. At. All. What the picture looks like is determined to a HUGE degree by the capture format's size. There is an actual relationship between the type of image that is made, based on the size of the capture medium and lens focal length!

On 4x5 sheet film, a 90mm lens is a true wide-angle lens, with things that are far away being made to look verrrrrrrry small, and foreground objects look large. On an iPhone, a 90mm lens would be a little under a 30x uber-telephoto, and the background objects would be rendered as BIG on-film, with extreeeeeeme "telephoto compression" look to the images.

You're spreading utter misinformation by saying the picture has nothing to do with the sensor....the fact is that the sensor's size plays a HUGE role in what each lens creates! You ought to know this. Have you ever shot 4x5 sheet film on a view camera?

alexdhanneman · Feb 25, 2015

Thanks for the quick responses guys.

Yeah I suppose it is good just to know what camera it was so that I can look up the film back size, so either way I can have the true focal length as well as the cropped equivalent.

Confusing stuff at first. Still trying to wrap my head around all the terms and understand what is actually happening within the camera. Have to know all of the technical side because a lot of the work I do is trying to replicate things happening inside the camera on computer generated objects placed into a live action scene.

curtyoungblood · Feb 25, 2015

If you're specifically looking to correct distortion, I would think that the actual focal length would be more important than the effective focal length. I think distortion is much more related to the optics of the lens than to the sensor that is capturing it.

photoguy99 · Feb 25, 2015

The picture projected onto the back of the camera, Derrel, NOT the picture captured by the sensor.

I admit that I am gliding casually past image circle size issues, but I don't think they matter here.