Interesting focal length shown in Capture One (8.3.3)

[Bill Caulfeild-Browne]

I've been playing with my new SK 120 LS Macro ("blue ring") lens. I'm very impressed but also a little puzzled.

When shooting at distances great than a a metre or a few feet, C1 shows 120 mm and the f-stop, ISO and shutter speed under the file. Quite right!

But when shooting moderately close - a couple of feet - it shows a focal length of 110 mm. When shooting really close, it shows 90 mm!

The Metadata tab always shows the correct lens and 120 mm focal length. It's only the information shown under the picture when in Viewer mode. Some sort of software glitch I suppose - not a serious problem by any means but a a bit puzzling to be told you have a zoom lens when you don't! (I could understand if the focal length showed longer when close-up because the lens is indeed longer and the effective FL is greater.)

Anyone else noticed this?

 

[trond]

Dear Bill,

I see the same thing with my XF/SK120LS.

Long distance shows 120mm, 1.5 feet shows 115mm, close distance shows 90mm.

Best regards

Trond

 

[jerome_m]

(I could understand if the focal length showed longer when close-up because the lens is indeed longer and the effective FL is greater.)

Actually, most modern macro lenses see their focal length reduced when focussed close. If their focal lengths were not reduced, they would be even longer (at least twice as long as when focussed far away).

Long distance shows 120mm, 1.5 feet shows 115mm, close distance shows 90mm.

That is probably the real focal length, as defined by the optical engineer. Knowing the real focal length is necessary for certain tools, because it is linked to distance and magnification.

 

[Kolor-Pikker]

Actually, most modern macro lenses see their focal length reduced when focussed close. If their focal lengths were not reduced, they would be even longer (at least twice as long as when focussed far away).

Not just macro lenses, nearly all lenses reduce in focal length as they are focused close, and this is called "focus breathing". The Nikon 70-200 2.8 II is infamous for this, reducing to about 180mm at minimum focus when set at 200mm.

Only cinema lenses are corrected for this, since it's assumed that this "defect" is irrelevant to stills photographers, but in a film it would be pretty obvious if the magnification changed as focus is pulled from one point to another.

 

[jerome_m]

Not just macro lenses, nearly all lenses reduce in focal length as they are focused close,

Only lenses with internal focussing or floating lenses can change their focal length when they are focussed. Indeed, most of the time, the focal length is reduced.

and this is called "focus breathing". The Nikon 70-200 2.8 II is infamous for this, reducing to about 180mm at minimum focus when set at 200mm.

Only cinema lenses are corrected for this, since it's assumed that this "defect" is irrelevant to stills photographers, but in a film it would be pretty obvious if the magnification changed as focus is pulled from one point to another.

At small distances, the fact that the entrance pupil moves when the lens is focussed will also produced a noticeable breathing effect, even for lenses without internal focussing.

 

[jlm]

by definition, focal length is the image distance to lens center when focused at infinity: 1/f=1/OD +1/ID.
so the focal length changing becomes semantic, i would think.

 

[jerome_m]

by definition, focal length is the image distance to lens center when focussed at infinity: 1/f=1/OD +1/ID.
so the focal length changing becomes semantic, i would think.

1/f=1/OD+1/ID works at all distances, so f can be defined at all distances.

 

[jlm]

the formula is useful because if you know the "focal length", f, of your lens and either I of O, you can calculate the other. but the specific case where O is inf defines "f" as the focal length. if you were to work backward, and empirically measure a group of O and I distances, and from the lensmakers formula calculate f, you would theoretically arrive at the same f for all the calculations.

to the OP then, if you did the above and got different values for f, then the formula would be misleading; it is for a theoretically "perfect" lens

 

[jerome_m]

to the OP then, if you did the above and got different values for f, then the formula would be misleading; it is for a theoretically "perfect" lens

You particular formula is for a simple lens, for multiple lens systems you have principal planes, entrance and exit pupils, etc... and the focal length of the complete system can vary.