Frame size and how it relates to lens focal length

[George Odell]

Here is something I'm trying to wrap my head around

Check out these images. There are many similar charts like this but these two are easiest to follow.

http://eyepatchfilms.com/wp-content/...comparison.jpg

http://cvp.com/images/uploaded/sensor_table.gif

So, in the top image, if the cyan colored Full Frame box on the left if what the 5D covers it would seem that all other cameras out there... including the very pricey Red Epic... cover way less of a given focal length 35mm format lens.

That being the case, the focal length marked on a given 35mm format lens is true only for a full frame 5D camera. All others will need to multiply the focal length by some factor to get the actual FL based on image sensor size.

The full frame sensor dimension (the far left box in the second image) is 36 x 24mm. The 35mm motion picture film image area is also 36 x 24mm. So any lens... PL mount, Canon, Red etc. that covers this industry standard "full frame" format is only correct in focal length on a camera like the 5D. On all others the coverage on the sensor is less so the focal length is actually longer.

For instance, with my Canon 60D I need to multiply a Canon EOS lens by 1.6 to get the FL for my cropped frame APS-C camera sensor. The $60K Arri Alexa* has an aperture, when recording, of only around 25 x 13.5mm. That is almost identical to what my Canon 60D is. So the lens correction is also quite large for this high-end camera.

This is really amazing when you think about it.

*http://www.arrimedia.com/alexa/technical-data ... for 16 x 9 shooting

[Dave Baker]

George, the actual focal length stays the same no matter what the sensor size, so a 50mm on a full frame will still be a 50mm on a crop sensor.

The confusing bit is that you have to apply the crop factor to the 35mm focal length, by that I mean 50mm in the example, to get the field of view of the lens on the crop sensor. So 50mm x 1.6 for our 60Ds comes out at 80mm. So the field of view is the same as an 80mm lens, but the focal length is still 50mm.

As far as using it is concerned, it's easier to think of it of an 80mm lens since that's the effect.

Dave

[Seth Bloombaum]

Originally Posted by George Odell

...The full frame sensor dimension (the far left box in the second image) is 36 x 24mm. The 35mm motion picture film image area is also 36 x 24mm...

I believe that's incorrect. A 35mm still camera has an image about that size, but there the film's width is the image's height ("landscape").

But in a motion picture camera, the film's width is the image's width as well. That is, the film runs vertically in motion pic cameras, and horizontally in still cameras.

Therefore, a 35mm motion picture image is apx. 24-26mm wide, depending on the camera.

Video camcorders that approximate this spec are "Super 35" sensors, very close to your Canon APS-C sensor size.

In the quest for "filmic" shallow depth of focus, full-frame shooters have gone beyond what 35mm motion picture cameras were capable of. Filmic... what?

[George Odell]

But would not the focal length or what I should have called the crop factor (thanks Dave) remain the same either way... film to video.

The diagonal would still be the same for either film or full frame video and that is what affects coverage does it not?

[Seth Bloombaum]

Focal length x crop factor = equivalent focal length (when I put this 50mm lens on my APS-C cam it looks like an 80mm (EFL) angle of view).

Here's a motion picture chart:
35 mm film - Wikipedia, the free encyclopedia

You'll find a nice graphic on the right-hand side of the article, that indicates the Super-35 image area as 24x18.6mm, which would give a 30mm diagonal. (IIRC, the "super" refers to no mag or optical sound stripe, giving the image area the nearly full width between sprocket holes.)

Canon 60D APS-C 22.3×14.9mm = 26.8mm diagonal.
Super35 24x18.6mm = 30mm diagonal.
Full frame 36x24mm = 43mm diagonal.

[George Odell]

Originally Posted by Seth Bloombaum

Canon 60D APS-C 22.3×14.9mm = 26.8mm diagonal.
Super35 24x18.6mm = 30mm diagonal.
Full frame 36x24mm = 43mm diagonal.

According to your chart, a full frame sensor like that found in the Canon 5D has a diagonal of 43mm. If you have a 50mm lens made for a full frame camera to cover the 43mm diagonal in full... no corner cropping... it can only be a 50mm lens for full frame (43mm diagonal) sensors.

Any other camera that does not use a 43mm diagonal (like the Canon C300, the Red or the Arri Alexa or the "cropped frame" DSLR cameras) would need to have a crop factor applied.

Correct?

[Dave Baker]

Correct except for one small point. Sorry to be pedantic, but I quote "it can only be a 50mm lens for full frame (43mm diagonal) sensors." It should be "it can only act like a 50mm lens for full frame (43mm diagonal) sensors."

You've got it now!

Dave

[Seth Bloombaum]

Yes, those are the concepts, exactly right.

Originally Posted by George Odell

...it can only be a 50mm lens for full frame (43mm diagonal) sensors.

This terminology isn't correct. A 50mm lens is always 50mm, which is a property of its optical design that's independent of the camera its mounted on. So, we use this term Equivalent Focal Length, EFL, which is, um, indexed to what portion of the projected image a 43mm diagonal sensor would see.

A 50mm "normal" lens on my 60D has an EFL of 80mm. If I wanted a matching shot on a 5D, I'd need to put an 80mm lens on it (or an available 85mm and move that camera back from the subject). (This EFL is a classic portrait angle of view, good for a head & shoulders shot at a practical working distance without much telephoto artifacting)

Related, a "normal" focal length for a 6x7 film camera, a stills cam that produces an image apx. 6cm by 7cm is 105mm, or a *crop* factor of .48.

The use of these figures is mostly important to those jumping around between formats. Was talking with a shooter who always works in Super35, he's got a firm grasp of wide-angle, normal, and telephoto on that format and never does crop-factor math.

For me, these looks are engraved on my brain from years of 35mm still photography, and I'm frequently working in different formats, so I'm often thinking about crop-factor and EFL.

The world of lens choices according to me may be found in post #7 in this thread:
http://www.dvinfo.net/forum/canon-eo...ml#post1753396

It's EFL that enables the discussion of "how wide is wide", etc.