If shooting at f/22, is a fast 1.8 lens still considered fast?

Started Apr 17, 2011 | Discussions thread
noirdesir Forum Pro • Posts: 13,519
Re: And Wikipedia is wrong in this regard

edwardaneal wrote:

so in one sentence you say "it is physically impossible" for the extra light of an f/2.8 lens to have an effect

It is physically impossible for all PD-AF cameras except a number of of Canon bodies. Happy now? This thread obviously was about Nikon AF unless stated otherwise.

and then later you acknowledge that Canon puts special sensors in some of their cameras that take advantage of that extra light

So tell me why would they have special sensors if it was physically impossible for the extra light to get to those sensors?

It is a design decision by them to have both f/5.6 and f/2.8 sensors, other camera makers made a different design decision. And you still have not acknowledged that Canon explicitly states that some sensors profit from faster lenses whereas other sensors do not, which pretty much states that these f/5.6 sensors exist and that those sensors do not profit from faster lenses.

Also if you looked at my second post about the canon af system you will see that Canons af system knows when faster lenses are mounted and makes finer - more precise - af adjustments. this alone would give a focus accuracy advantage to faster lenses.

Yes, in principle that would give Canon an advantage but in practice a lot of other factors come into play and the advantage of Canon is not exactly that big.

its funny I have googled this and I find source after source that says phase detect AF works better with faster glass - and many of these sources are well known and reliable - so why should I think they are all wrong and you are right?

Crowd sourcing has its limits, particularly with questions that seem to surpass the appetite for technical detail of the average poster.

Especially considering the fact that I have been able to reliably test it with my primes and f/2.8 zooms vs every variable aperture lens I have ever owned.

I don't know what more I could do to explain this to you when you are apparently so stubborn to not even be willing to consider the meaning of 'correlation is not causation' and seem far from going as far as making an honest effort in understanding the technical explanations I have linked to including the very practical ones by Marianne Oelund.

noirdesir wrote:

edwardaneal wrote:

forget focus speed - if you are correct please tell me why my f/5.6 lens hunts for focus and sometimes cant even lock focus in low light yet my primes dont hunt and easily lock focus in exactly the same light

Since it is physically impossible for the extra light an f/2.8 lens provides over a f/5.6 lens (in reality the cut-off will be slightly higher than this), there must be another explanation, eg, the faster lens being sharper in the relevant f/5.6 ring than the slower lens or the slower lens having slower and less precise AF mechanics and electronics.

I have tested this many times and with every variable aperture lens I have owned and I have always been able to duplicate it

Again correlation is not causation, in particular if the proposed explanation is physically impossible.

No matter what the sensor type, however, it will usually be more accurate with a wider aperture lens . Remember, during autofocus the camera automatically opens the lens to its widest aperture, only closing it down to the aperture you’ve chosen just before the shutter curtain opens. Phase detection autofocus is more accurate when the light beams are entering from a wider angle. In the schematic below beams from an f/2.8 lens (blue) would enter at a wider angle than those of an f/4 lens (red), which are still wider than an f/5.6 lens (yellow).

edwardaneal wrote:


The speed of the AF system is highly dependent on the maximum aperture offered by the lens . F-stops of around f/2 to f/2.8 are generally considered optimal in terms of focusing speed and accuracy. Faster lenses than this (e.g.: f/1.4 or f/1.8) typically have very low depth of field, meaning that it takes longer to achieve correct focus, despite the increased amount of light.

And that only applies to certain Canon bodies which have distinct f/2.8 AF sensors. Think about it, why would Canon explicitly declare that a limited set of their AF sensors produces higher accuracy with f/2.8 lenses but other sensors do not? It is simply that some their sensors use the f/2.8 ring and others the f/5.6 ring. No other camera makes a similar claim.

Have a look at the link and images I posted in another post her today, tell me what you don't understand about it. And have a look at Marianne's posts on the subject, for those not believing all the technical descriptions of how PD-AF works she has done some tests that show that when you block the f/5.6 ring, no light reaches the AF sensor.

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My kit - D200, 10.5mm f/2.8D, 35mm f/1.8G, 50mm f/1.4G & 70-300VR
SB800, SB600 and other misc lighting equipment

Lenses worth mentioning owned and sold– 12-24 f/4, 17-55 f/2.8, 35-70 f/2.8, 80-200 f/2.8, 20mm f/2.8, 35mm f/2, 50mm f/1.8, 50mm f/1.4D, 60mm f/2.8D, 85mm f/1.8, 105mm f/2D-DC, 180mm f/2.8, 300mm f/4D-ED

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