GOOD NEWS: Another glowing review for the Tamron SP 150-600mm.

Started Mar 7, 2014 | Discussions
Limburger
Limburger Veteran Member • Posts: 7,841
Re: Be careful not all may be as peachy...

Jerry-astro wrote:

Limburger wrote:

riknash wrote:

bugbait wrote:

Mark Goldstein at Photography Blog has published his final review of the  Tamron SP 150-600mm F/5-6.3 Di VC USD.

You can not transfer the findings 1:1 to a crop DSLR - all 1.6x crop DSLR (including the 7D and 70D) have focusing systems that are physically limited to a lens aperture of f/5.6 or better, the Tamron lens is only f/6.3 over a large proportion of it's zoom range - which means that any findings regarding focusing precision and speed that are measured on a 5DIII (whose focusing system works up to f/8) are invalid.

Can you explain why the 7D is limited to 5.6?

Correct me if I am wrong but I think what Karl means is that the AF points are sensitive up to f5.6.

When using AF this happens when aperture is open, so if a lens is 6.3 at a given fl open this may cause some trouble.

The numbers are given to the AF points to warrant a certain AF accuracy.

Mike, I believe f/6.3 is the upper limit for AF on the 7D. I'm sure the real experts here will correct me if I'm wrong.

Hi Jerry.

No expert either. A little while ago, looking at those cheap mirror lenses, I stopped looking since they had these weird apertures (up to f8), so that was a no no. I remembered loosing AF at getting one of those.

Same with 70-200 f4 with 2x TC if I remember correctly, the 1.4 should work making it a 5.6.

6.3 could be right but I remember 5.6. Another thing to look after would cross type like + as dual cross like + and x together. Not sure what to make out of it yet with 7D.

But we may be on to something

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Cheers Mike

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rdspear Veteran Member • Posts: 4,603
Re: Be careful not all may be as peachy...
1

riknash wrote:

Can you explain why the 7D is limited to 5.6?

Because Canon's firmware code say "if reported max aperture of this lens is >5.6, don't enable autofocus".

The official explanation is that a lens that allows in that little light would make the AF ineffective, with lots of hunting, lack of focus lock, etc.  Rather than customers being upset with a poor AF system, Canon just puts this limit in place.

Try attaching a 1.4TC on a 5.6 lens, and you'll see that AF is not functioning.

What you can do is tape over the three pins that report aperture to the body, and the AF will work.  I've taped over the pins on my 400 and can confirm that it will autofocus, but it is also quite iffy under anything but good light.

Many/most non-Canon lenses don't report the aperture, so this lens will probably "work", in that the AF will function.  But it might be very inaccurate or non-functional at the longest focal lengths.

I would agree that you shouldn't read a lot in an evaluation on a 5DIII when deciding on purchase for a 7D or, even more importantly, a lesser AF system.  It may be OK.  But it might not.

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Jerry-astro
Jerry-astro Forum Pro • Posts: 15,279
Re: Be careful not all may be as peachy...

Limburger wrote:

Hi Jerry.

No expert either. A little while ago, looking at those cheap mirror lenses, I stopped looking since they had these weird apertures (up to f8), so that was a no no. I remembered loosing AF at getting one of those.

Same with 70-200 f4 with 2x TC if I remember correctly, the 1.4 should work making it a 5.6.

6.3 could be right but I remember 5.6. Another thing to look after would cross type like + as dual cross like + and x together. Not sure what to make out of it yet with 7D.

But we may be on to something

Yup, my bad... I think you were right.  Thx for setting me straight.

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Karl Gnter Wnsch Forum Pro • Posts: 11,408
Re: Be careful not all may be as peachy...

riknash wrote:

Can you explain why the 7D is limited to 5.6?

Phase detection AF sensors compare two different light paths which are isolated and separated by beam splitter lenses (you can see those lenses in the bottom of the mirror box of your DSLR). These splitter lenses are taking beams from as far off the center as they are designed to - for f/2.8 sensitivity they are taking beams which are further from the center of the beam, f/5.6 is closer to the center and f/8 would be even closer to the center - the light path that f/2.8 sensors would pick up doesn't even exist for f/5.6 or worse lenses. Since the phase difference gets smaller the precision suffers - so f/5.6 is the chosen design limit for the 7D. Any worse lens aperture (wide open) and you have to contend with partially blocked light paths.

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regards
Karl Günter Wünsch

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Karl Gnter Wnsch Forum Pro • Posts: 11,408
Re: Be careful not all may be as peachy...

Jerry-astro wrote:

Mike, I believe f/6.3 is the upper limit for AF on the 7D.

Unfortunately you are wrong, it's f/5.6 - f/6.3 is close so you only get influences from diffraction and a partially blocked light path, which means that first of all precision suffers, followed by problems with challenging contrast situations (including flare and haze making their appearance when light sources or bright structures are in the frame - which can render the AF inoperative at times).

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regards
Karl Günter Wünsch

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Karl Gnter Wnsch Forum Pro • Posts: 11,408
Re: Be careful not all may be as peachy...
1

rdspear wrote:

The official explanation is that a lens that allows in that little light would make the AF ineffective, with lots of hunting, lack of focus lock, etc.

It's not the amount of light, it's the light paths through the lens which are taken to be compared - and f/5.6 is the design limit of the beam splitters used.

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regards
Karl Günter Wünsch

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tonyjr
tonyjr Veteran Member • Posts: 5,283
Thanks

No text.

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