Bug ... here's a suggestion for you ... go to your local camera shop with both a FX and a DX of your liking ...

Ask to see a Tamron SP 70-300 VC USD ....

Attach it to your DX camera and take a picture ... then, attach it to your FX camera and take a picture ... return it to them ...

Then come on home and transfer both pics to your computer ... then take a look for yourself ... see if you can find any obvious differences ....

Well, in some cases it can make reasonable sense. Let me preface my comment by saying I don’t think it is a necessity to have both formats. One always has the option of cropping down an image taken with a FF to get the same field of view one would get with a crop sensor camera. To me, need is a case of not being able to get the job done without having both, and I can’t think of cases where not having both formats would be preventative.

Anyway, why do I have both and use both? First off, I often take extended photo trips to places far remove from home. After a near disaster on one of these trips, I decided it was in my best interest to carry two bodies as insurance against finding myself without a working camera while 1500 miles from home half way into a ten day trip. So, I invested in a second body. My primary body has been a FF. When I bought the second body I took advantage of a good deal and picked up a decent crop body. I did this with the idea that when in the field I could set up body/lens combinations that would allow me to take advantage of the crop body’s field of view [“reach”] and compose shots without having to plan for cropping later. It was a convenient way of aiding my ability to compose shots. It also allows me to have both wide angle and telephoto option available without having to swap lens in the field, a nice convenience. Again, not a necessity, but helpful.

All sound reasons, Jim ... but the soundest thing you've written here - is that there must be a backup camera for your in-use system. This presumes - if your primary is a DX, then, the backup should also be a DX. By the same reasoning - if your primary is an FX, then your back-up should also be.

The contention one is better off with one dig cam FF and the other dig cam - crop sensor, is like adding milk to your coffee cup to the brim, and then topping off with a teaspoon of coffee. Not sure whether that makes much sense to you, but it does, to me. If you drink coffee, in other words - make it so, as Jean-Luc Picard would say. And if you drink tea - make sure it's Earl Grey!!! ... I hope you get my drift, Jim ....

Well, I guess I can’t agree with the use of “should” here. One might presume that a person might, more than likely, duplicate their body’s format when adding a second body, but I don’t see that as a given. Should would imply right vs. wrong, and I see nothing wrong with following a different alternative if it offers possibilities that work for the individual. In my case, I like the ability of taking advantage of the crop body’s “extended reach” in some situations, where it allows me to compose shots without having to consider cropping later.

A good example of where I did this was when I hiked the Virgin River Narrows in Zion NP. I set up my wide angle on the FF, and my longer 24-105 on the crop body. This effectively let me compose shots covering a field of view of 17-168mm without the need to carrying a 70-200 lens or switch lens while knee deep in the middle of the river. And, I could compose the longer shots without having to think about cropping down later. Arguably small advantages, but worked well for me, an was more effective than it would have been had I had two FF bodies only.

Is this is in Utah, Jim ... or somewhere in northern Cal?

Not that familiar with western territory ... although I did drive down the west coast from Vancouver to LA, once ....

Now, the 24-105 is a lens designed for FF bodies, right? ... But you used it on a DX ....

Look, who am I to downplay your choice to go with a body of each format? If it works for you ... go with it, Jim ....

I tend to think the extra wide thing, is just as easily done on a DX as it is on an FX. I mean you can use 10mm, even 8mm ....

Can you imagine doing THAT on an FX? ... The widest you can go is - what - 15mm ... 14mm ... 12 mm (or is that already a fisheye?)