billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

Sony, Canon, and Nikon design their lenses to work specifically with their models, Sigma and Tamron design their lenses to fit on ALL camera's, they just change the mountings, Nikon buys, grinds, and coats their OWN lenses, Sigma and Tamron buy their glass from outside sources, as Nikon changes their camera bodies, Nikon designs future lenses to go with them, Sigma and Tamron REVERSE engineer their lenses based on Sony, Canon and Nikon designs. These are minor issues but speaks a lot to the lens manufacture process. As far as I know, Canon, Nikon, Sony and others are on the cutting edge of lens production. Nikon as a matter of fact is a scientific company, camera's account for less than 5% of sales for the company, the cutting edge microscopes used professionally come from Nikon, the R&R engineers all work in one plant, they cross train, so advances in one area of glass production carry over to another making the whole process competitive and the consumer ends up winning.
Canon, Nikon, and Sony glass costs more, why? Well, most folks can figure that one out.
A few facts, Nikon, Sony, and Canon lenses will be worth about twice Sigma and Tamron five years down the road. Five years down the road most Sigma and Tamron lenses are not working as well within tolerances as Nikon, Sony, and Canon lenses. There is a reason, it is called production quality. With lenses it's what on the inside that counts, and for me anyway, I will continue to ride with the brand.

rehess Loc: South Bend, Indiana, USA

That entirely depends on what words you are using, and how you are using them.

Case 1: I mount my 300mm K-mount lens on my 4.65-crop 12MP Pentax Q-7 and take a picture.

Case 2: I mount my 300mm K-mount lens on my 1.5-crop 16MP Pentax K-30, take a picture enclosing the original picture, and crop away 2/3 of the width and 2/3 of the height of the image {because 4.65/1.5=3.1} to get essentially the same result as I got in Case 1.

Whatever language you use, three things are true

A. Comparing the original Case 1 image to the original Case 2 image is equivalent to comparing view through weak binoculars to view looking unaided at a subject. We normally say that binoculars have "magnified" the view, so using the word "magnified" here is simply being consistent in our usage.

B. Case 1 puts 12MP on the ultimate subject, while Case 2 put just 16/9 = 1.78MP on the same ultimate subject.

C. To duplicate the original picture from Case 1 using only my K-30 {no PP work} would require using a 900mm lens.

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

All I know is this, when I use my Nikon 200-500 mm f5.6 lens on my D500, I put more EFFECTIVE MEGAPIXELS on the bird itself vs my D810. And frankly, that's all I can about.

papa Loc: Rio Dell, CA

How many Sigma, Tamron, and/or Tokina lenses do you have this experience with? I'm thinking all the better build longer lasting baloney is backed up too by Nikon's or Canon's 3 to 6 year warranty. In the early 80s I was a Nikon snob with loyalty to it's superior IQ. Back then and even now the lion's share of the pro market went to Canon for their wider array of lenses; which still holds true. Try DxOmark, imaging resource, photozone, etc. for very straightforward and comprehensive scientific info on most any noteworthy lens/body combination on every level to choose for yourself. I did over several years and have rid my case of inferior Canon lenses to use only these four on the 5D Mk III and 7D: Tokina 16-28, Tammies 24-70, 70-200, and 150-600 and I'll put their IQ up against lenses costing twice the price regardless of the brand. Oh yeah, and the warranty? You guess who gives 6 years and 3 years for all nay sayers ,
OPE!

papa Loc: Rio Dell, CA

You do math, huh? So, I hear you shoot a very small part of the frame to need more of the smaller megapixels so that the extreme crop to print doesn't pixelate visually. Now, seeing how that works for you and you've got that D810 just sitting kinda piques some interest in asking what your bottom dollar is to letergo.
My recent dreams of a Sigma 135 f/1.8 paired with the D810 would serve as my go to for most of my work. Of course I could shoot the Sigma fine on my 5DMk III. By the way, have you a comparable lens to it in the Nikkor???I've been looking for this Sigma for quite awhile by any name, but as it turns out it's not Nikon,
Canon, Sony, etc., etc., etc.

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

For portraits I use a Zeiss T* Macro 100mm 2 manual focus. They also make a 135 f2, I believe both will fit the Canon.