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medium format focal lengths
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PostPosted: Thu Jan 24, 2008 7:20 pm    Post subject: medium format focal lengths Reply with quote
for a 75mm lens mounted on a crop camera (at correct registration distance) what will be the focal length?

see here if you are wondering why I ask.
http://forum.mflenses.com/diy-bronica-etr-to-eos-mount-t4943.html


PostPosted: Thu Jan 24, 2008 7:21 pm    Post subject: Reply with quote
same than 35mm lens. 75x1.6 if you have Canon.


PostPosted: Thu Jan 24, 2008 7:23 pm    Post subject: Reply with quote
thanks I have a new 120mm f2.8 Very Happy


PostPosted: Thu Jan 24, 2008 8:32 pm    Post subject: Re: medium format focal lengths Reply with quote
hacksawbob wrote:
... what will be the focal length?

see here if you are wondering why I ask.
http://forum.mflenses.com/diy-bronica-etr-to-eos-mount-t4943.html


The focal length will always be the same. (OK, that's perhaps kind of hairsplitting, but this "mistake" is made so often that we should be really sensitive about it.)

What changes is the angle, the "Field-of-View" since the rectangle that records the image uses a smaller part of the projected image.

So Attila is right (when he says like a lens for 35mm format, I guess that's what he meant). A 75mm lens (not matter for which format is was built) will be a medium tele photo lens at a crop DSLR, approx. like a 120mm lens for a 35mm format cam, and thus you're right as well. Very Happy


PostPosted: Thu Jan 24, 2008 8:40 pm    Post subject: Re: medium format focal lengths Reply with quote
hacksawbob wrote:
for a 75mm lens mounted on a crop camera (at correct registration distance) what will be the focal length?

see here if you are wondering why I ask.
http://forum.mflenses.com/diy-bronica-etr-to-eos-mount-t4943.html


As everyone took joy in reminding me, the focal length is unaffected by the camera its mounted on.

If you are asking for the 35mm equivalent focal length (ie the focal length which would give the same field of view) then the crop factor is around 1.6 from 645 MF to 35mm. If you are asking for the crop factor from 645 to DX crop, its 2.45. I don't have the sensor size for the Canon handy, but its a little smaller than Nikon so the crop factor will be a bit larger.


PostPosted: Thu Jan 24, 2008 9:59 pm    Post subject: Reply with quote
right maybe "the effective focal length" not feild of view as that would be measued in degrees wouldn't it? like the tamrons say on their lenses.


PostPosted: Fri Jan 25, 2008 2:15 am    Post subject: Reply with quote
As Gertrude Stein would have put it:
"A 75mm lens is a 75mm lens is a 75mm lens". Laughing

It's interesting to notice how in most human minds, the logic of different film/sensor surfaces is reversed into a supposed issue with metamorphic lenses. The lenses never change. A 75mm lens is a 75mm lens is a 75mm lens. What change are the recording surfaces, or, to use a synecdoche, the cameras.

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PostPosted: Fri Jan 25, 2008 6:24 am    Post subject: Reply with quote
Orio wrote:
What change are the recording surfaces, or, to use a synecdoche, the cameras.


Well put, Orio. I love synecdoches. Cool

Cheers!

Abbazz


PostPosted: Fri Jan 25, 2008 6:43 am    Post subject: Reply with quote
It's all about the circle, imagenthat is. Wink Wink


PostPosted: Fri Jan 25, 2008 5:18 pm    Post subject: Reply with quote
Quote:
What change are the recording surfaces, or, to use a synecdoche, the cameras.
per Orio

Interchangeable cameras?
Very Happy Very Happy


patrickh


PostPosted: Fri Jan 25, 2008 8:10 pm    Post subject: Reply with quote
patrickh wrote:
Quote:
What change are the recording surfaces, or, to use a synecdoche, the cameras.
per Orio

Interchangeable cameras?
Very Happy Very Happy


patrickh


That's absolutely the right way to think about it Patrick!

Partly cos I'm a nerd and partly because I want to put this eternal question to rest, these are the diameters of the image circles for different image formats (same as the diagonal of the image):

6x9 dia = 10.82cm
6x6 dia = 8.49cm
6x4.5 dia = 7.5cm
35mm dia = 4.33cm
Canon 400D sensor dia = 2.67cm

In theory if you try using a lens for a small format on a bigger format camera you'll end up with massive vignetting. If you use a lens for a large format on a smaller format camera, you'll only see a part of the image circle in the viewfinder - a smaller field of view. The factor by which this is reduced (the crop factor) is found by dividing the diameter/diagonal of the lens image by the diagonal of the camera sensor/film image. Using these figures you can work out the various crop factors for whatever combination of lens/camera you want.

Using a lens for 6x4.5 format on a 35mm film camera gives a crop factor of 7.5/4.33 = 1.73, which for the 75mm Bronica lens is the equivalent field of view as a 130mm lens designed for 35mm cameras.

Using a lens designed for a 35mm camera on a Canon 440D gives a crop factor of 4.33/2.67 = 1.62, so the field of view of the 75mm Bronica on the Canon would be equivalent to a 130mm 35mm camera lens x 1.62 = 210mm, but this figure is meaningless while we all still relate to 35mm lenses.

In simple terms, fitting the Bronica 75 on the Canon will give a very similar image to using a Jupiter 37A.


PostPosted: Fri Jan 25, 2008 9:47 pm    Post subject: Reply with quote
The image circle will determine the vignetting (coverage area), but not the enlargement of the cast objects!
That is dependent only from the focal lenght, and under that respect, given the identical final recording surface, a 135mm lens with an image circle of one meter will offer EXACTLY the same absolute proportions of cast objects as a 135mm lens with an image circle of 10 cms.
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