Full Frame Mirrorless: Sideline or Replacement of dSLR?

Mar 2, 2012
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Since the difference between f-number and T-stop with those lenses often varies widely from one focal length to another, the difference between f-number and T-stop can not be fully explained by transmission loss. It seems that the actual entrance pupil size does not grow at the same rate with the 28-300mm from, for example, 70mm to 100mm as it does from 35mm to 70mm.

View attachment 179591

As with all variable aperture zoom lenses, the size of the entrance pupil, as magnified by the optical elements between the aperture diaphragm and the front of the lens, is not keeping up with the focal length as it is zoomed to longer focal lengths. Yet at certain points in the zoom range of these lenses they seem to "make up" some of the loss. If it was all transmission loss one would expect the difference between f-number and T-stop to be constant and the above graph would be flat lines for each of the lenses. Link to lens comparison at DxO Mark. As always, ignore the "number scores" and go to the actual measured data by clicking on 'Measurements' and then 'Transmission'.

That’s all very interesting, but my inquiry was to the application of lens speed (introduced by the term “slower”) as it pertains to specified compatibility with the line sensors and their spacing.

Better stated, when you say canon has broken the rule and various zoom lenses are “slower” than their specified maximum, do you mean that the exit diameter as viewed by the PDAF unit is smaller than f/5.6 would imply because it doesn’t benefit from the magnification of the zoomed geometry? Or did you really mean “slower” to mean that although the light is falling in the right location on the sensors, there isn’t enough of it?
 
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Michael Clark

Now we see through a glass, darkly...
Apr 5, 2016
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That’s all very interesting, but my inquiry was to the application of lens speed (introduced by the term “slower”) as it pertains to specified compatibility with the line sensors and their spacing.

Better stated, when you say canon has broken the rule and various zoom lenses are “slower” than their specified maximum, do you mean that the exit diameter as viewed by the PDAF unit is smaller than f/5.6 would imply because it doesn’t benefit from the magnification of the zoomed geometry? Or did you really mean “slower” to mean that although the light is falling in the right location on the sensors, there isn’t enough of it?

I think it is a bit of both. The entrance pupils appear to vary more than can be accounted for by the change in magnification as the lenses are zoomed. So at some focal lengths the actual entrance pupil seems to be slower than f/5.6 by up to 1/2 to 2/3 stops. Which seems to support what I suggested earlier, based on a lot of user's experience, that the "f/5.6" limit may actually be the "a little less than f/8 limit." In other words, the f/5.6 limited AF points may not actually be cut off, based on their geometry, until a lens is somewhere between f/6.3 or f/7.2 and f/8.

Just for kicks, I can put my EF 24-105mm f/4 L IS + 2X TC (non-Canon) on a 5D Mark II and it will AF when pointed at reasonably bright scenes with targets that have decent contrast. It's a bit slow, but no slower than, for example, the Tamron 70-200mm f/2.8 Di Macro (about 2 generations back from their current 70-200/2.8) is on the same body.

If I put the same lens + TC on a 7D it won't AF on anything except a bare light bulb on a dark background.

Addendum: The EF 24-105mm f/4 L IS a T-5.0 or slower lens across its entire zoom range. Either the lens' elements absorb/reflect 2/3 stop or some of that loss is due to the entrance pupil not being a true f/4 at any focal length.
 
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Jul 21, 2010
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Which seems to support what I suggested earlier, based on a lot of user's experience, that the "f/5.6" limit may actually be the "a little less than f/8 limit." In other words, the f/5.6 limited AF points may not actually be cut off, based on their geometry, until a lens is somewhere between f/6.3 or f/7.2 and f/8.
Certainly. From the piece I wrote for TDP several years ago:
Note that these thresholds are not absolute - a lens with a narrower aperture than the threshold might still work, but at reduced effectiveness, accuracy, and speed. Thus, Canon limits the functionality to the rated aperture for a given AF sensor.
Clearly, the absolute limitation is based in the firmware. This becomes very obvious if you look at the 1D X – at launch, it was limited to f/5.6 for all AF points. Soon there after, Canon released a firmware update allowing AF at f/8 with the center point. I presume that in response to use the requests (previous 1-series cameras had f/8 capability), Canon empirically tested the performance and found it to be acceptable.
 
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Michael Clark

Now we see through a glass, darkly...
Apr 5, 2016
4,722
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Certainly. From the piece I wrote for TDP several years ago:

Clearly, the absolute limitation is based in the firmware. This becomes very obvious if you look at the 1D X – at launch, it was limited to f/5.6 for all AF points. Soon there after, Canon released a firmware update allowing AF at f/8 with the center point. I presume that in response to use the requests (previous 1-series cameras had f/8 capability), Canon empirically tested the performance and found it to be acceptable.

The limitation is based in the firmware for many *newer* EOS models. Many older EOS models will try, and sometimes succeed, to AF with f/8 lens + TC combos.

These older models that still try to AF with f/8 lens + TC combinations don't all seem to be trying to fool the camera using the "this is an f/5.6 lens set to f/8 with a manual aperture ring" trick. When the EOS system was launched way back in 1987 it seems there were allowances made for as-yet-to-be-designed tilt-shift lenses to have mechanical aperture rings on the lens. Thus there are two apertures reported by the lens: maximum aperture and current aperture setting.
 
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