Sorry TCampbell, your explanation was very good, and a nice way of seeing lens ability etc. My reply after yours was tongue in cheek, and I know you were using examples but any examples could be used for any lens. It just a case of humour not transferring to text (and I forgot to put a smiley face)
soufiej - I understand what you are saying, a zoom does not give an accurate example of what a prime can do due its difference in sharpness and the fact the primes generally have larger aperture and less distortion etc.
However, I still think that using a zoom to give you an indicator of what FOCAL LENGTH may suit a purpose better before buying a prime is a good idea.
You suggest buying from somewhere with a good returns policy. Where I come from that means if its defective it will be replaced, and if unopened you can get a full refund within 30 days, it doesn't mean buy a lens, use for a week and return it if unsuitable. Consumer laws may be different where you live, and to be honest I'd like to try before I buy, or at least have that type of return option, but I don't, so the zoom option is at least a viable option
First, I can switch between my 50mm prime and my zoom set to approximate 50mm and there is a slight difference between the field of view each lens provides. Not much but it's there.
Maybe I'm simply not being exact enough with the zoom but, if we have someone saying their 17mm isn't wide enough and they'd prefer 16mm, then some folks simply want more than approximations can provide.
Second, deal with a reputable company. I've spent over three decades in sales and quite often equipment was loaned out for an audition. In such cases, unless it was a very familiar client who had a shared trust with the store, we'd simply record a credit card number and a DL number. The card wouldn't be run until you called to give the OK.
And, at times, equipment was sold but returned for some reason within our 30 day return policy. This is how most businesses operate. Certainly those with a subjective opinion involved in decisions tend to even encourage some clients to try before they buy. But I'm a bit uneasy giving my money to someone with an "All sales are final" sign at the register. Yes, I've seen people abuse the good will of the store and the store get's screwed. That is a part of doing business. Not pleasant but it's a part of doing business.
This may open up a can of worms for some people but this is how some business must be done. If I loan you my demo, then you have a "demo" item in that I'm never going to rebox that item and sell it as new. Your credit card says you paid full price so, if you want a demo item at full price, be my guest as to how to deal with loaners.
But, the most important part of customer service is the client has had the opportunity to handle and hold and see or hear what they are considering as a purchase.
Since a camera and a lens make up an operating system, you cannot predict what you will or will not prefer until you actually use it. Let's say you were considering the 50mm f1.8 and the 50mm f1.4. If someone says I should buy what they want me to buy without any opportunity for me to audition either lens, then I'd do without the services of that vendor.
Literally, I can buy the item on line, from a big box or most likely from another vendor in the area, use it for two or three weeks and then return it. If it is not damaged and the packaging and warranty cards are there, no legit dealer should refuse a trial period.
I want to know what I'm buying. If you don't want to assist me in that task, I can find someone else who will.
That's from someone who never wanted to see a client pull up first thing Monday morning asking if I could help him get the box out of his trunk. Some items are simply best decided on when you have them to work with and pair with your other equipment.
Honestly, to use this as a reason to push a zoom lens rather than a sharper, faster and cheaper prime is, IMO, not at all logical.