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Effective focal length vs full-frame 35-mm equivalent focal length

Cem_Usakligil

Cem_Usakligil

Well-known member
(EDIT by CU: This technical discussion has been separated from this thread in the Photography as Art forum.)

....The picture was taken with a 200mm lens and a 1.6x crop factor sensor so the effective focal length is 320mm. As such, I did not have to correct much of the geometry, only a slight rectification as you've aptly put. .....
 
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Asher Kelman

Asher Kelman

OPF Owner/Editor-in-Chief
Wolfgang Plattner said:
Hi,
Sorry, but it is not an effective focal length, it is a crop similar to a focal length of 320mm ...
But despite of this, the pic is interesting ...
Click to expand...

Wolfgang,

I'm fascinated by this! why would 320mm not be an effective focal length? After all the with the distance used, likely the DOF is sufficient. Or do you mean it should be much greater, say 600 mm in order to provide the bird cage with more resolution by framing a smaller portion of the face of the building.

Asher
 
Doug Kerr

Doug Kerr

Well-known member
Hi, Woldgang,

Wolfgang Plattner said:
Hi,
Sorry, but it is not an effective focal length, it is a crop similar to a focal length of 320mm ...
But despite of this, the pic is interesting ...
Click to expand...
Indeed, "effective focal length" is not desirable. It suggests a basic optical parameter that is not in effect.

Of course, our intent is to describe field of view of a leng in terms of the focal length that would provide the same field of view on a camera using some other familiar format size.

A useful term in this regard for the common practice in this genre of cameras is " full-frame 35-mm equivalent focal length".

The long phrase "full-frame 35-mm" of course deals with the fact that this notation is only meaningful for a particular "reference" format size. "35-mm" does not do it, since there are cameras using 35-mm film with a format size of 18 mm x 24 mm. "Full-frame" doesn't do it, since there are "full frame" versions of every film type. (For "8x10" film, the full frame format size is nearly 8" x 10"!)
 
Cem_Usakligil

Cem_Usakligil

Well-known member
Doug_Kerr said:
Hi, Woldgang,


Indeed, "effective focal length" is not desirable. It suggests a basic optical parameter that is not in effect.

Of course, our intent is to describe field of view of a leng in terms of the focal length that would provide the same field of view on a camera using some other familiar format size.

A useful term in this regard for the common practice in this genre of cameras is " full-frame 35-mm equivalent focal length".

The long phrase "full-frame 35-mm" of course deals with the fact that this notation is only meaningful for a particular "reference" format size. "35-mm" does not do it, since there are cameras using 35-mm film with a format size of 18 mm x 24 mm. "Full-frame" doesn't do it, since there are "full frame" versions of every film type. (For "8x10" film, the full frame format size is nearly 8" x 10"!)
Click to expand...
Hi Doug,

You know me so I owe you an apology. I was explicitly trying to avoid this terminology mistake as I know how you and some others feel about it but have somehow gotten distracted while typing. I guess my mind was more occupied with the picture itself and what it meant to me than the technical details of it.

Cheers,
 
Cem_Usakligil

Cem_Usakligil

Well-known member
Asher Kelman said:
Wolfgang,

I'm fascinated by this! why would 320mm not be an effective focal length? After all the with the distance used, likely the DOF is sufficient. Or do you mean it should be much greater, say 600 mm in order to provide the bird cage with more resolution by framing a smaller portion of the face of the building.

Asher
Click to expand...
Hi Asher,

Wolfgang and Doug are right. "Effective focal length" is not the same as the "full-frame 35-mm equivalent focal length. The focal length of the lens doesn't change when used with a camera housing a different sensor size. The word I meant to type was "equivalent", not "effective".
 
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Asher Kelman

Asher Kelman

OPF Owner/Editor-in-Chief
Doug,

What's significant is that the perspective is identical whether it's a cropped sensor or a prime 600 mm lens as that, as we all know depends on Cem's position in reference to the building. The other factor is depth of field and that, in practice, at this distance, will have no likely influence on the lenses ability to describe the birds cage. The advantage of a crop factor is the ability to put more pixels on the small area of interest. If that is in play, then there may be an advantage.

What we call the devotion of a 1/1.6 area to the cameras pixel array, is not so important as the quality of the lens And choices of aperture, speed, time of day , position and so forth. so I don't mind what we call it. I just want the picture or the camera!

Asher
 
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