[Kameraten]

While pixel-peeping my lenses I've stumbled across a surprising phenomena: The field of view of certain lenses do not correspond to their stated focal lengths.
Either I have not understood the laws of physics (optics) correctly, or the focal lengths claimed do not hold.
Most recently I've noticed this comparing my DA 60-250 and my DA 200, both on the K-1. It turns out the magnification of the DA 200 is greater than of the DA 60-250 – and not only with the zoom at 200 either, but even with the zoom at 250! Judging from this, the real maximum focal length of the zoom is about 180 mm.
I also compared the two lenses to the Tamron 70-200 – and here the 200 really was identical to that of the DA 200.
I've earlier noticed the same kind of thing with the FA 28-105/3,2-4.5, which really should be a 28-88/4-5,6, if the comparisons with other lenses are anything to go by. I assumed Pentax having cut corners a bit with that lens – old, affordable – but not with the DA 60-250.
For me, my first takeaway on this is that the DA200 gives me more of a reach than the DA 60-250, somewhat surprisingly.

Any thoughts?

[victormeldrew]

Firstly, yes focal lengths are always rounded to the nearest 'whole' name. A 21-88 might be called a 20-90. for ease. Sometimes it is only reasonable rounding (89.8 to 90 maybe) other times it is quite adventurous rounding...

Secondly, in many designs the focal length is affected by focus distance (focus breathing) and so close focusing at 200mm might leave you with 180mm as opposed to infinity focus. You can see this in the viewfinder as you focus from one extreme to the other. Zooms tend to suffer this more than single focus lenses.

[swanlefitte]

They should be very close at infinity but breathing happens at closer distances due to elements moving. Some lenes breath more than others. Google focus breathing.

[Kameraten]

Yeah, I did some more testing on the long zooms and found that focusing on infinity evens out the field. The "focus breathing" phenomena seems to reach up to some 30 feet with these lenses.
Still, it means that if you are into for example flowers or butterflies you should reach for the 200-mm DA 200 and not for the longer zoom, which will give you a smaller magnification at it's closest distance (tested that, too). And the Tamron also appears not to be affected by the focusing distance.

[victormeldrew]

And the Tamron also appears not to be affected by the focusing distance.

The Tamron is very good but there is a small amount at extremes. I think the performance is down to the internal focusing design, but I don't know enough to say it with any certainty.

[Rondec]

Most internal focus lenses have, as you say, focus breathing. This means that they only achieve their stated focal length at infinity and at closer distances they have a shorter focal length. It's a pretty common thing, but some lenses do it worse than others.

[ffking]

I think I'm right in saying that internally zooming lenses are more prone to focus breathing too - it's possible to correct it, but at a cost. It's also something people didn't really used to worry about, they just accepted that that was what they gor and photographed accordingly.

[jpipg]

The interesting corollary to this is that a lenses maximum magnification is not only a factor of its MFD and focal length, but also of its design. I wonder how many published maximum magnification figures take that into account,

[UncleVanya]

Ironically there are internal focus designs that don't focus breathe. The 50-135 for example has minimal focus breathing if any. The da 18-135 has some, and the 60-250 has a lot.

[LensBeginner]

Brain fart, scrapped.

[UncleVanya]

@Kameraten
if this concerns you, just buy a parfocal lens. Video lenses are usually built this way.

Parfocal and varifocal usually refer to the ability of the lens to keep the focus while zooming. The shifting of focal length isn't something I'm familiar with in reguard to parfocal.

[CarlJF]

By convention, the official focal length of a lens is given at infinity focus. When the focus isn't at infinity, the FL can change. You may physicaly noticed it with lenses that aren't internal focus: the barrel will extend or retract when the focus ring is turned. Some lenses are more affected by this than others, and zooms seem more to this phenomena than fixed FL.

The 60-250 is notorious for heavily showing this effect. At minimum focusing distance, it has about the same reach as the 50-135m... So, what you observed when comiparing with the DA* 200mm is normal and expected, although somewhat confusing. At infinity, both lenses should give about the same coverage, but at minimum focusing distance the 60-250 should behave like a lens with a significantly shorter FL.

[stevebrot]

Most internal focus lenses have, as you say, focus breathing. This means that they only achieve their stated focal length at infinity and at closer distances they have a shorter focal length. It's a pretty common thing, but some lenses do it worse than others.

I was mildly dismayed when I found that my Sigma 17-70/2.8-4 (C) at 5' has the FOV of a 50mm prime. That extra reach was part of the purchase decision. Internal focus is presented as a premium feature and so it is if the alternative is huge extension, but for a 4X zoom to lose 30% of its reach is disappointing. I wonder what the FOV of the Sigma 17-50/2.8 is at 5'.


Steve

[stevebrot]

By convention, the official focal length of a lens is given at infinity focus. When the focus isn't at infinity, the FL can change.

Yep, and nobody is really bothered by the fact that conventional focus using extension "breathes" the other direction with an increase in effective focal length. I am not sure, but I suspect that the advantage of internal focus is that the moving mass is much less and that allows lighter and faster in-lens focus motors.


Steve

[CarlJF]

Yep, and nobody is really bothered by the fact that conventional focus using extension "breathes" the other direction with an increase in effective focal length. I am not sure, but I suspect that the advantage of internal focus is that the moving mass is much less and that allows lighter and faster in-lens focus motors.

The non-rotating front element also makes IF more convenient when using polarizing or graduated filters.