Most recent DSLRs can now deliver good quality images at 1600 ISO and usable images at up to  25600 ISO. This is a huge leap forward from the early days of digital photography, when shooting at anything above 100 ISO produced a noisy image. Prime lenses were often the only way of getting enough light in anything but brightly sunlit conditions. Now, with the higher ISO capabilities of modern cameras, a zoom with a maximum aperture of f/4 - f/5.6 will be perfectly good enough in terms of light-gathering.

The question then, is it worth paying the premium for a fast prime lens ?  The advantages I can see for  a prime, now that its light gathering capabilies are less important, are:

 1) the wide aperture of a prime will give the ability to set a shallow depth of field

2) a prime is often of higher optical quality than a zoom 

3) a prime will give  a longer effective flash range

Other than these three points, a modern zoom lens will give much more flexibility than a prime. Serious portrait photographers will still prefer to use a prime, for the shallow depth of field. For the walk-about photographer who wants flexibility, zoom lenses have improved so much in quality that a prime now has little extra to offer.

Thanks for replying, Andreas, it is refreshing to get a response to a photographic thread, and I am pleased to see that PoE is no longer dormant (as it has been for so long) when it comes to an interest in technical matters.

I am aware that making any technical comment will risk enraging some people and I never contribute to the 'expert' forums such as DPReview for this very reason. Perhaps I should have made it clear that I am an elderly photographer who grew up in the film era, where the standard ISO for film was generally no higher than 100 for negative film and was as low as 25 for Kodachrome reversal film. Very occasionally I would use 'high speed' films such as Tri-X with an ISO of 400, but this was about as good as it got.  Obviously with these sort of ISO figures, it was necessary to use fast lenses, particularly if doing studio work with tungsten lighting. The slower apertures of a zoom would be of no use even if good zooms were available then.

The main drift of my argument is that with the wonderfully high ISO capabilities of the modern DSLR, there is no longer the necessity of using a prime lens to gather as many photons as possible. I think we are in agreement here. Which brings me back to the point that there are only a few aspects where a prime does something better than a zoom, and the main difference is that a prime lens used wide-open can achieve a much shallower depth of field than a zoom. Sharpness is a different question, and I agree with you that a professional quality zoom will usually equal or exceed the sharpness of a prime at the same aperture and focal length  setting. Having said that, there are some low end zooms where performance is truly awful and inferior to primes. I think there is still a place in the photographers kit bag for a 50 mm f/1.8 'standard' prime, which will give sharpness at that fixed focal length as good as that of a professional zoom costing ten times more. , 

Thanks again for replying, I hope we will see more photographic discussions on PoE in the future. 

P.S. Regarding the comment about flash, having done work in large venues such as  theatres, it can be diificult to get enough light to illuminate the stage even with a powerful Speedlite at maximum power. The use of a prime at f/1.8 will enable pictures to be taken which are not possible at all with a zoom (even a professional zoom will typically have a maximum aperture of f/2.8). In my opinion, it is better to get a slightly soft image with the prime wide open than not get any picture at all.

No need to say thanks, Edward, it's my pleasure. I really enjoy a civil conversation, be it technical or not.

There are very few real experts on DPR and the like, and I have stopped taking part in fruitless debates. I will gladly have a constructive discussion about things, and just as gladly admit when I'm wrong about something. What makes or breaks it for me are expertise and tone.

It is of no importance to whether or not you are 'elderly', either as a photographer or otherwise, but I did get that you have had long experience starting in the film days. Certainly, things were different then, especially when it comes to the properties of zoom lenses.
I will accept that I didn't answer your post exactly to the point.

I think the main point is the maximum aperture of zoom lenses. Back then, when there weren't any really good and fast zooms your only chance was a fast prime. Today, that has changed to a degree. So yes, we are generally in agreement, but for different reasons.

Your comment about flash and apertures really sums up my point. I'll just turn it around; if you have a zoom with the same maximum aperture as a prime, everything is jolly good. But, you're right, that zoom will have its price.

I think you will find that there are no zooms available at any price which can match the maximum aperture of even a 'standard' prime (50mm f/1.8). All the major manufacturers sell these at a knock-down price. There are even faster primes available if you want to spend more money (e.g. the Leica Noctilux 50mm f/0.95). Dream on !

To return to the original discussion, you will find that if you are struggling to light a large space with flash, and you are at the limits of flash power and acceptable limits of ISO, a prime will be the only option of getting that shot.  

Prime lenses still are the best for quality results. My two favourites are the Nikon 50mm f1.4 and 85mm f1.8. The images are really of very good quality, and make ideal portrait lenses.

However, my "workhorse lens" is the Sigma F4 24-105 DG Art. It's quite big and heavy, but what a performer! It is so easy to put this onto my Nikon D750 or D300 when I go out walking looking for urban shots and street images / candids. It's a very convenient and easy option. one I find hard to resist. I recenty did a wedding for two friends, and the old church was delightful, but dark! I used the 85mm and 24-105mm leneses never taking anything under ISO 1250 as flash was strictly out of the question under pain of being ejected, by the Bishop no less! I used the combination of the D300 with my Sigma 10-20mm  for exterior shots of the church. My 85mm lens was soon back in the bag, as I just needed the felexibility of a zoom lens for groups and snatched candids. Over 300 guests made speed a must. Legs knackered! 

Conclusion. If you have lots of time.........go for prime. 

Prime lenses deliver, but zooms are so very tempting.  At the end of the day too, you only get what you pay for. Quality tells! Second hand lenses always an option too. Some people swap their lenses like socks, well not quite, but I hope you get the drift.

I avoid using flash more and more as the D750 delivers at very high ISOs. My aim is for as natural looking images as possible. 

You see, this is exactly what I meant in my first posts of this thread.
It is often true, yes, but it is not generally true anymore. You need to compare two (or more) lenses on the same body under controlled conditions.

It may very well be true for the Sigma 24-105 and the Nikon 50 and 85 on your D750. I have never shot any of those or delved into reviews, so I simply don't know.

On the 60D it is definitely not universally true for the Canon 24/1.4 II, 24/2.8, 28/1.8, 28/2.8, 35/1.4, 35/2. In all these comparisons the Sigma is at least as sharp and often sharper at all apertures. At all the primes' maximum apertures the Sigma is sharper.
You will find similar circumstances with the Sigma 50-100 and 24-35, depending on the body used for comparison.
If you are into this kind of thing, do spend some time on TDP:
http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx

With the above in mind you can get more for less. This is the case for some of the zoom-prime comparisons I mentioned (especially when you sum up the cost for several primes), and also when you compare certain zooms.
As an example, a friend of mine shoots a Canon 16-35/4 on his 70D. I compared the 16-35 with my Sigma 18-35 on both the 60D and 70D. Not only is the Sigma sharper at all corresponding focal lengths and apertures, it is also way cheaper. Fairness, though, commands mentioning the fact that the 16-35 is designed for full-frame bodies.

Now, I understand that sharpness is only one factor among many that make up the overall quality of a lens. But for me it is the single most important factor, and in that I am definitely not alone.

Let me sum up my point. Everybody may buy and like whatever he or she wants (or needs or can afford).
What we should leave behind are statements that claim universal, all-encompassing truths without being supported (or even despite being contradicted) by evidence from controlled testing.

Please note that I am not trying to be a beeeep, I merely champion what is true.

When comparing lens characteristics, it is clear that the majority of zooms cannot match the large maximum aperture of a prime. Therefore the sharpness comparison argument becomes irrelevant when you reach the maximum aperture of the zoom. Even a cheap prime has several stops in hand compared with the zoom before it reaches maximum aperture. I agree that doing a comparison of a zoom and a prime at smaller apertures e.g. f/5.6 may show that the zoom is sharper, but this ignores the other capabilites of the prime.

In those situations where an extremely shallow depth of field is required, the prime is the winner.

However, for general purpose use, the convenience of a zoom coupled with the high ISO capabilities of a modern DSLR  is the way to go. Rod's example of his wedding shoot is a good example where the  convenience of a zoom outweighs the possible artistic advantages of a prime. This is exactly what I said in the introductory posting.

1) True.

2) Not in general. Usability of primes at large apertures is a major factor. I consider the Canon 50/1.4 unusable until stopped down to at least f/2.2.

3) Depends on exactly which lenses you compare. Even before you factor in usability you have reached the point where the general statement is not valid anymore.

4) The comparison I made between the Sigma 18-35 and the primes was about the zoom's maximum aperture of f1.8.

5) Again, depends on exactly which lenses you compare and how usable they are.