Rifle and pistol lens are very different as is the sighting principle.

With a pistol the aim is more an area aim and the lens will be what ever is neccessary for you to get sharp focus on the foresight, the top shooting specific Opticians work on a focal length of 1.3 mts, when I shot pistol seriously I always took my pistol to the optician and they got it exactly spot on.
As its an area aim the target blur is irrelavent, the aiming mark is just to give a relation to the centre, you do not aim at it, and you do not want to be tempted to try and focus on it, it is the best way to get flyers! If you can see the target clear then you are not focusing on the sight picture and you will get a bigger error from misallingned sight picture than any wobble or placement of the sight picture against the target. Try shooting with the target reversed (no aiming mark), If your technique is good you should get the same group and very similar scores, possibles, (50's) with the card backwards are very achievable for a good shot.
An iris merely complicates the focus, as it draws the focal length back out towards the target, the few shooter who shoot well with an iris have their focal lens set up with the iris, but really its a complication most good shooters have tried and rejected, its best avoided.

Now rifle shooting is a different kettle of aquatics, you are looking through an aperture (which changes your focus), you wish to get a sharp focus on the foresight, and a good enough view (carefully chosen word, not focus) of the aiming mark, which is at infinity, as we are talking of a very precise aim to achieve the top score (10.9), so we have two focal lengths that we need to gain focus on at the same time in a perfect world, and that is impossible with the human eye so we work on the best comprimise.

The accepted technique is to have the foresight in sharp focus, its the bit we move and control to achieve the good shot, the aiming mark is fixed, and we judge the white gap in the foresight around the aiming mark, as long as we have a good enough "view" of that aiming mark and we see it as
round we can achieve a perfect aim. If your focus goes to the aiming mark you are doomed, as you will loose the precision of the equal ring of white around the fore sight element, the shot can go any where, its the biggest reason for flyers with the shooter convinced it was a good shot!

The compromise focal length as Tim has correctly stated is 2 mts and that relates to a +0.5 diopter correction on long vision to achieve that, so your long prescription is +1.5 so your shooting prescription is +2.00. Beware to do the correct maths if you have a minus prescription i.e. -1.5 your shooting prescription is -1.0.

I read into the OP's question being around what do you do if you have perfect vision? With perfect vision you may happily see perfectly well both rifle and pistol, then the lens works more as an assist to discipline to focus on the sights with pistol, I always used a pistol lens when I shot pistol at top level and with perfect eyesight, and I knew many others who did the same.

My concentration is now with rifle coaching, and with rifle a +0.5 can help those with perfect 20/20 or infinity vision, it can reduce eyestrain, but its a personal choice, I know several top level rifle shooters who have perfect vision, but use a +0.5 lens. I know one who at a cataract operation had the right eye lens coutesy of Bausch and Lomb set at the perfect 0.5 rifle variance, now that's dedication, and she shoots 100's! We currently coach a young rifle talent with infinity right eyesight, who has constantly complained of blurred picture and eyestrain from aiming, we have tried a +0.5, problem gone. Her long vision was so good it was a struggle to focus at 2 mts.

Although there are some simple possible answers, its a complex subject, our eyesight is valuble and critical to us, and it is always best to talk to a qualified optician, and idealy one who is experienced in the foibles of shooting vision.

Good Shooting
Robin