Math Help

[JTPAIN]

Argggghh Confusion!

I'm sure my stupidity is only momentary (or I hope so anyway).

What is the real focal length gap between the 24-105 and the 10-22 (e.g. 22-24)?

I know it not that hard a sum; and 'Yes' to some bright spark, I know the answer isn't 2

I think it's more complicated than that?....... .........Or is it?

Thanks for your help here guys

-The Numerically Dyslexic Dyspraxic-                   

[Tijn]

The gap is 22-24mm. Yes, it is as simple as that. Just keep in mind that a 2mm difference at the wide end (and 22-24mm is wide) is a much bigger relative framing difference than at the tele end. For example, the  difference between 10mm and 12mm is a large field of view difference, while the difference between 280 and 282 is practically unnoticable. So yes, the focal length difference really is just 2mm "wide". Bit the impact of those 2mm to framing is bigger at the wide end than at the tele end. How big? I believe just doing a fractional comparison of the mm difference works here. 10mm is 2mm wider than 12mm, which is (2/12)*100 = 17% wider. But 280mm is only (2/282)*100 = 0,7% wider than 282mm.

Not entirely sure if that's an entirely valid calculation, but it's a fact that the relative framing difference is a bit bigger at wide ends. 22mm would be 8% wider than 24mm.

Also, sometimes the lens' "actual" focal length might differ just a tiny bit from the official spec. A reported 17mm is sometimes closer to 16mm (Tamron 17-50), for example. In other words, the reported focal length isn't always bang-on. But that's not a maths thing, just something to do with how they're physically built.

[Marsu42]

Also, sometimes the lens' "actual" focal length might differ just a tiny bit from the official spec.

I've found that the reported focal length sometimes differs significantly from the actual one - esp. the cheaper zoom I've got or tried in a shop tend to exaggerate their reach in comparison to primes.

[JTPAIN]

Thanks for your help

Basically, I have found that my 24-105 is not wide enough on the 7d - what i really need is a wide angle zoom to complement my current setup - do you have any suggestions?

[Marsu42]

Basically, I have found that my 24-105 is not wide enough on the 7d - what i really need is a wide angle zoom to complement my current setup - do you have any suggestions?

Easy and affordable if you don't want full frame compatibility: Canon 10-22, Tokina 11-16 (to be replaced in August buy new version), Sigma 8-16 - use the search or read the usual review sites for advantages and drawbacks.

[JTPAIN]

personal preference?

[Marsu42]

personal preference?

... I'll get the Tokina in August - Sigma cannot use filters, Canon hasn't f2.8 aperture.

[dlleno]

just a couple of additional thoughts.  The designations "24-105" or "10-22"  are labels -- just names for the lens.  Yes these names often come very close sometimes to the actual focal length (at infinity) but just remember they not a published specification they are a  published label.  A zoom may come close to its labeled focal length when focused at infinity, but when focused close we may perceive it to be a ways off -- this is because we communicate in terms of focal length when in reality what we are thinking of is angle of view or even magnification.  Case in point:  at focus distances of approximately 1 meter, the 28-135mm zoom does not behave like a 135mm lens at all. 

yes, in simple, practical and "good enough" terms, the gap between 10-22 and the 24-105 is that the area between 22 and 24mm which is not accounted for.   Of course, when practical and affordable its a bonus to have zooms with overlapping focal lengths, instead of "no gap between them" , because sometimes the shot is gone before you can switch lenses.  On my 1.6x 40D, for example, I find it most useful to have both the 10-22 and the 17-55 in my bag, the later of which gets the vast majority of playing time.  I find that the focal length overlap (17-22) is quite satisfactory.