April 19, 2014, 08:27:12 AM

Author Topic: Effective focal length of some telephotos  (Read 3550 times)

AlanF

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Re: Effective focal length of some telephotos
« Reply #30 on: November 15, 2013, 06:21:15 AM »
OK, I can't come up with anything more quantitative, which I don't think anyone can. I think we can agree on the following points. For high class telephoto lenses like the series II big whites, the resolution is pretty close to being limited by the sensor, filters etc. For that reason, the measured MTFs on cameras with different sensors are close to to 1 (as I wrote previously, 0.978 is good enough for me).

When you get MTFs that become significantly less than 1, the lens itself is the weak link in the whole resolution system and its MTF largely determines the overall measured MTF. For a lens like the Sigma 150-500, which has a measured MTF of 0.52 at f/6.3 and 500mm, the resolution of the system is pretty close to that of the lens itself.

So, I would say to a first, very rough, approximation, you would have to get twice as close to the subject to make it 2x wider and 2x higher in order to get an image to reveal as much detail as a very good 250mm lens at the same distance. So, I would equate that to the Sigma having an effective focal length of ~250mm for telephoto purposes.

The 400mm f/5.6 L lens is described by many of its devotees as being very sharp. It is not tack sharp, but if you get closer to the subject than you would have to get close with a tack sharp lens, like the 400mm f/2.8, you will get an apparently very sharp image because the image has a larger size and covers more of the sensor. I estimate in practice that it performs as about as well as a 330mm tack sharp lens. But, of course, there are other factors that come in as well.

I'll leave it at this point and thank everyone who joined in the discussion.
5D III, 70D, Powershot SX50, 300/2.8 II, 1.4xTC III, 2xTC III, 70-200/4 IS, 24-105, 15-85, Sigma 10-20, Tamron 150-600.

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Re: Effective focal length of some telephotos
« Reply #30 on: November 15, 2013, 06:21:15 AM »

Pi

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Re: Effective focal length of some telephotos
« Reply #31 on: November 16, 2013, 12:00:43 AM »
You still need to define what xx% means. What is the 100% base? You'd better stick with a fixed sensor (and a fixed MTF value). You can take the base to be the Nyquist frequency. Depending on the way resolution is measured, some lenses can surpass Nyquist but so what. Then you will get 110% or so. Another way is to use a reference lens as a base. In all cases, the numbers you get can be compared to each other (you can say Lens A is 20% sharper than lens B, or has, say, 30% more reach) but their values would be just relative, like using meters vs. feet, etc.

The question you raised is very practical, indeed. A sharper (and faster) shorter lens can easily beat an older an longer one but you would still get what you pay for, I guess. Oh, here is an idea: add another column: mm per $.  :)

takesome1

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Re: Effective focal length of some telephotos
« Reply #32 on: November 16, 2013, 01:45:53 AM »

The question you raised is very practical, indeed. A sharper (and faster) shorter lens can easily beat an older an longer one but you would still get what you pay for, I guess. Oh, here is an idea: add another column: mm per $.  :)

I think the proper comparison would be the additional cost of additional Effective Focal Length per MM.

For instance in the OP the 300mm F/4 at F/4 is listed at 254, the 300mm f/2.8 II at F/4 is listed as 293.

The difference is 39mm. Since 254 can be delivered for about $1400 and the cost of the 300mm f/2.8 is about $6700 the additional cost for 39mm would be $135.90 per mm.

AlanF

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Re: Effective focal length of some telephotos
« Reply #33 on: November 16, 2013, 11:09:48 AM »
The SX50 at a total cost of $0.3/mm of effective focal length, including camera, wins hands down.
5D III, 70D, Powershot SX50, 300/2.8 II, 1.4xTC III, 2xTC III, 70-200/4 IS, 24-105, 15-85, Sigma 10-20, Tamron 150-600.

Pi

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Re: Effective focal length of some telephotos
« Reply #34 on: November 16, 2013, 11:45:38 AM »
The difference is 39mm. Since 254 can be delivered for about $1400 and the cost of the 300mm f/2.8 is about $6700 the additional cost for 39mm would be $135.90 per mm.

You should move the decimal point, to get something like less than $20 per mm for the $6k lens, etc.

Which makes my 15 fisheye a really luxury item and low value: $40 per mm!

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Re: Effective focal length of some telephotos
« Reply #34 on: November 16, 2013, 11:45:38 AM »