[Dubesor]

Without getting into complicated optical discussions, what will the 'apparent' field of view for MF lenses be when used with APS-C? I've done lots of reading about this on the forum here, but it's still not crystal clear.

Am I correct in thinking it is about 3x for 645 lenses?

What I mean is [only as far as how 'zoomed in' it feels] - on my K20D, will a 25mm 645 lens look through the VF similar in zoom to a 50mm FF lens (such as my Takumar 50/1.4)?

I base this on:
25mm MF lens x 3.0 crop factor = 75mm 'perceived' focal length
50mm full-frame lens x 1.5 crop factor = 75mm 'perceived' focal length

Would love some explanation in simple numerical terms so I can figure out what MF focal lengths will translate to on my K20D. Am I getting it or way off mark?

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Last edited by Dubesor; 01-10-2010 at 05:08 AM.

[MattGunn]

Don't bother thinking about crop factors etc, just consider the focal length. Any 25mm focal length lens on APSC gives the same field of view as any other 25mm lens on APSC no matter what the lens was designed for (assuming the lens has the coverage for the larger formats). The field of view is only depent on the format (size) of the sensor you are using and the focal length of the lens you are using.
You only have to consider crop factors if you are trying to determine the field of view you would get with any given lens on different sensor formats. A 25mm lens on APSC will give a narrower field of view than the same lens on 35mm ff which will have a narrower field of view than the same lens on 645etc

[Dubesor]

Don't bother thinking about crop factors etc, just consider the focal length. Any 25mm focal length lens on APSC gives the same field of view as any other 25mm lens on APSC no matter what the lens was designed for (assuming the lens has the coverage for the larger formats). The field of view is only depent on the format (size) of the sensor you are using and the focal length of the lens you are using.
You only have to consider crop factors if you are trying to determine the field of view you would get with any given lens on different sensor formats. A 25mm lens on APSC will give a narrower field of view than the same lens on 35mm ff which will have a narrower field of view than the same lens on 645etc

I understand that, I think. But the question is how much narrower? Is there a way to quantify this? I'm looking for a "a 25mm lens on on APS-C feels as wide/tele as a ??mm lens on 645" kind of comparison ...

[pacerr]

Not sure exactgly what your question is but this may help sort it out. These are "close enough" approximations.

If you consider the FOV of a 50mm lens "normal" for 135 format, an 80mm would be about the same in 645 format and 35mm would come close for APS-C.

The Tamron web site has a useful, graphic lens FL/FOV comparison widget at:

Focal length comparison tool, Tamron USA

H2

[Wheatfield]

Focal length is focal length.
A 50mm lens for a 645 camera will have exactly the same field of view as a 50mm lens for a 35mm camera, will have exactly the same field of view as a 50mm lens for a 6x7 camera if they are all mounted on the same format camera.

This is why I rage at the numb nuts that invented the whole crop factor thing.

[wallyb]

Focal length is focal length.
A 50mm lens for a 645 camera will have exactly the same field of view as a 50mm lens for a 35mm camera, will have exactly the same field of view as a 50mm lens for a 6x7 camera if they are all mounted on the same format camera.

This is why I rage at the numb nuts that invented the whole crop factor thing.

Same field of view, absolutely, but whoo boy that vignetting will slow you down when using a 135 format lens on 6x7! Don't worry, I feel the same as you, I just don't want to see a surge of people saying "my smc takumar m42 vignettes and won't focus properly when i duct tape it to my 645 what do i dooooooo"

[rhodopsin]

we use "crop-factor" when comparing "APS-C" and "full-frame"; "crop-factor" can also be applied when comparing "full-frame" and "medium format"

crop factor of APS-C compared to full-frame is 1.6X.

crop factor of full-frame compared to medium format (645, 6x7) is close to 2X; evidence: 105mm medium format lens (edit) on medium format camera provides same angle of view as 50mm lens on full-frame camera.

to determine crop factor of APS-C compared to medium format, multiply the crop factors of APS-C/full-frame and full-frame/medium format: 1.6 * 2 = 3.2X

Example:

angle of view of 100mm medium format lens on APS-C camera is the same angle of view as 100mm * 3.2X = 320mm lens on medium format camera.

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Last edited by rhodopsin; 01-10-2010 at 11:21 AM. Reason: clarification 105mm on MF body

[Wheatfield]

we use "crop-factor" when comparing "APS-C" and "full-frame"; "crop-factor" can also be applied when comparing "full-frame" and "medium format"

crop factor of APS-C compared to full-frame is 1.6X.

crop factor of full-frame compared to medium format (645, 6x7) is close to 2X; evidence: 105mm medium format lens provides same angle of view as 50mm lens on full-frame.

to determine crop factor of APS-C compared to medium format, multiply the crop factors of APS-C/full-frame and full-frame/medium format: 1.6 * 2 = 3.2X

Example:

angle of view of 100mm medium format lens on APS-C camera is the same angle of view as 100mm * 3.2X = 320mm lens on medium format camera.

The problem with the term is, it is being used by people who have never picked up a film camera in their lives, they get confused because they have no basis for reference, and then the same confused posts crop up time and time again.

[Wheatfield]

Same field of view, absolutely, but whoo boy that vignetting will slow you down when using a 135 format lens on 6x7! Don't worry, I feel the same as you, I just don't want to see a surge of people saying "my smc takumar m42 vignettes and won't focus properly when i duct tape it to my 645 what do i dooooooo"

At least then we can start explaining image circle rather than the very tired and ready to be put to bed crop factor.
It amazes me that the term has stuck around for as long as it has considering how very few people now shoot film at all.
As much as I dislike the term, it had it's place 10 - 5 years ago when DSLR cameras were the new kids, but now, with so very few people shooting film, and even less shooting multiple formats, it's really time to just shoot it and bury it.

[Marc Sabatella]

What I mean is [only as far as how 'zoomed in' it feels] - on my K20D, will a 25mm 645 lens look through the VF similar in zoom to a 50mm FF lens (such as my Takumar 50/1.4)?

No. A 25mm 645 lens will have *exactly* the same FOV of view on your K20D as a 25mm FF lens or a 25mm APS-C lens. The so-called "crop factor" is not something one uses in comparing different lenses on the same camera; it's something you use in comparing different cameras with the same lens. If you don't shoot a 645 camera, then there is no crop factor - all 25mm lenses have exactly the same FOV on your camera.

[rhodopsin]

The problem with the term is, it is being used by people who have never picked up a film camera in their lives, they get confused because they have no basis for reference, and then the same confused posts crop up time and time again.

I also bemoan use of the term "crop factor"; "crop factor" is meaningless without knowing "compared to what." I imagine some techie drew a diagram showing the size of APS-C sensor on a full-frame sensor and concluded "looks like a crop; lets call it the crop factor."

[Ole]

The APS-C field of view can be found in the data table in lens review database, for example:

Pentax Lens Review and Specification Database - 45mm F2.8

Not all entries have been augmented with this information yet, but we're getting there.

[Venturi]

This is why I rage at the numb nuts that invented the whole crop factor thing.

I propose a ban on any "technical" term quipped by marketing teams.

[rhodopsin]

Another factor when using medium format lenses on APS-C camera is lens formula. For example a medium format wide angle lens, such as the 45/4 lens for Pentax 67, when used with APS-C sensor will give much more barrel distortion than a 45mm lens designed for APS-C sensor due to difference in wide angle and normal lens optical formulae.

[Wheatfield]

Another factor when using medium format lenses on APS-C camera is lens formula. For example a medium format wide angle lens, such as the 45/4 lens for Pentax 67, when used with APS-C sensor will give much more barrel distortion than a 45mm lens designed for APS-C sensor due to difference in wide angle and normal lens optical formulae.

Do you know this for a fact or are you making this up?
I'd like to see some citations please.
With the 6x7 lens, you will be using something like the central 1/4 of the image circle, and are well away from the edges where barrel distortion is likely to rear it's ugly head.