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Focal Length of Rokinon 135mm with asi1600MM

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#1 powasam5000

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Posted 26 March 2020 - 12:48 PM

Hello all. I was wondering what the focal length of the rokinon 135mm lens ( canon)  would be with the asi 1600mm. I read that the focal length is 202.5mm with an aps-c sensor. But the 1600mm is a 4/3 sensor correct? Would that alter the focal length number and if so what is the calculation? ( I want to make sure I plug the right numbers in asiair)


Edited by powasam5000, 26 March 2020 - 12:56 PM.


#2 sg6

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Posted 26 March 2020 - 12:58 PM

Cameras do not change the focal length. It stays as 135mm.

Stop reading stuff on the web.

Actually don't read the Rokinon page below (description), what they say is basically wrong and some marketting idiot is trying to convince you that you are getting something for nothing.

 

R135

 

The sensor size will change the field of view, not the focal length.


Edited by sg6, 26 March 2020 - 01:01 PM.

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#3 S.Boerner

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Posted 26 March 2020 - 01:12 PM

On your ASIAir just put in zero(0).  Take a picture, plate solve, and the AIR will figure it out for you.



#4 markb

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Posted 26 March 2020 - 02:21 PM

The relative focal length calculation allows people to visualize 'regular' photograph image scale relative to, usually, thr 35 mm film camera 'everybody' was used to using. The same relative calculation was done way back when people would compare 35 mm film cameras to six by six or six by seven film cameras like the classic hasselblad or big Pentax ('normal' lens were 90mm instead of 50mm IIRC).

 

So if you use the identical lens on a full-frame camera and on an aps-c camera (as you can do in many digital camera systems), and then enlarge the resulting photograph to 8x10 the image scales will appear different despite the fact that the lens focal length did not change. The smaller sensor captures a smaller part of the image circle projected by the lens. The basic lens design has to be varied to give the appropriate image circle to cover the film plane for larger sensors or films, so 90mm Hasselblad lenses looked much different and are much larger than the same focal length lens specifically made for a 35 millimeter camera or, nowadays, specifically made for a small sensor digital camera.

 

Regular photography and astrophotography have totally different ways of looking at things but the physics don't change.


Edited by markb, 26 March 2020 - 02:25 PM.


#5 powasam5000

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Posted 26 March 2020 - 03:33 PM

Thank you everyone for the replies. I did initially go to the rokinon site so It confused me. Ive only ever used a telescope so a camera lens is uncharted waters for me. Thanks again!



#6 MikiSJ

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Posted 26 March 2020 - 03:54 PM

A camera lens is a telescope lens meant for a camera. For marketing reasons the focal length, always a larger number is given for for the camera lense.

 

For example: a 135mm lens with a stated wide-open aperture (diaphragm) of 2.0 means it has an objective group sizing of 67.5mm. What would you rather buy - a 135mm lens or a 67.5mm lens?

 

(Note: This is an oversimplied explanation. Hard balls not welcome but soft comments are.)



#7 WadeH237

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Posted 26 March 2020 - 05:23 PM

A camera lens is a telescope lens meant for a camera. For marketing reasons the focal length, always a larger number is given for for the camera lense.

 

For example: a 135mm lens with a stated wide-open aperture (diaphragm) of 2.0 means it has an objective group sizing of 67.5mm. What would you rather buy - a 135mm lens or a 67.5mm lens?

I suspect that the reason that camera lenses are characterized by focal length is that virtually every camera lens has an adjustable aperture, whereas lots of camera lenses (primes) have fixed focal length.

 

The leads to primes being named for their one-and-only focal length, and zooms being named for the focal lengths at the short and long end.  The names also include the focal ratio of the lens when it's wide open.  They could just as well state the wide open aperture, but the focal ratio is a much more useful number for photographers in terms of determining proper exposure and depth of field.  I don't see this as marketing fluff, as much as being a useful way to describe the lenses.

 

I agree with the gist of some of the comments that the camera and lens manufacturers have made things overly confusing with terms like "crop factor" and "focal length equivalence".  It would make more sense to me if they just described the physical characteristics.



#8 SonnyE

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Posted 26 March 2020 - 05:47 PM

"Cameras do not change the focal length. It stays as 135mm.

Stop reading stuff on the web."

 

confused1.gif  How's he going to get an answer then? confused1.gif

 

lol.gif


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