What would give me a sharper picture?

How low light are you thinking about? Something like the subway station above, or outside at night with just a streetlight 50 feet away? Are you going to be manually focusing, or using AF? If AF, will your body AF at f/4 in as low of light as you're expecting?

Lexar wrote in post #16072722
Yes for anything that might have motion the prime would be better!
But I am talking static architecture.

Real example is a picture of buildings, very low light, handheld with:
prime Sigma30 at f1.8 with 1/13 sec.
Versus a picture with zoom 15-85 at f4 with 4 sec. (4 stop IS)

So what be clearer and sharper?

Well, for a start, one of those will be badly exposed as those settings are very different exposures.

f/4 is only two and a third stops slower than f/1.8, yet you have given a shutter speed that is almost 6 stops different, giving over 3 stops more exposure. The shutter speed to give equivalent exposure with f/4 would be around 1/2 a second (actually slightly shorter, but I can't be bothered to look up the third of a stop figure).

It is hard to say which would be sharper, so much depends on your handholding technique. Theoretically, as you are well into potential camera shake territory here (ideal being 1/FL x crop factor, so 30mm on a crop "needs" 1/50th second) the prime is going to suffer from some camera shake as you are two stops below that ideal shutter speed. That shake may be very slight to positively disastrous, depending on how good you are at hand holding.

The IS, on the other hand, is good for 4 stops, so should allow you to shoot just 2 stops below "ideal" shutter speed without problems, however the slower aperture puts you some 4.5 to 5 stops below ideal speed. That should still give the edge to the zoom (as once you knock off the 4 stops of IS, you are only one stop too slow, compared to two with the prime). But, at longer such shutter speeds, it is possible to move the camera quite significantly if you aren't careful, and that can go beyond the IS capability to counteract.

With either lens though, you are pushing your hand holding ability. I would try and rest the camera on something solid as a brace, or lean against a wall to give you some support from the side. With good bracing and shooting technique, you should be able to get a decent shot, but you will need to concentrate on keeping the camera as steady as possible, and gently squeezing the shutter button, so you don't move the camera when you press it.

With that in mind, I would think the zoom with the IS should be slightly better for avoiding camera shake, but a lot depends on how accurate the claim for "4 stop IS" actually is. If it is only 3 stops in practice, the verdict could go either way.

Note, this answer applies only to that specific scenario (although at slow shutter speeds, 4 stops of IS will always be better for avoiding camera shake than 2 1/3 stops of aperture, assuming the subject isn't moving) and doesn't take into account the possible difference in optical sharpness of the actual lenses, as that is of secondary importance to camera shake. It doesn't matter which lens is slightly sharper optically, if you get blur from camera movement using it.

sandpiper wrote in post #16072909
Well, for a start, one of those will be badly exposed as those settings are very different exposures.

f/4 is only two and a third stops slower than f/1.8, yet you have given a shutter speed that is almost 6 stops different, giving over 3 stops more exposure. The shutter speed to give equivalent exposure with f/4 would be around 1/2 a second (actually slightly shorter, but I can't be bothered to look up the third of a stop figure).

It is hard to say which would be sharper, so much depends on your handholding technique. Theoretically, as you are well into potential camera shake territory here (ideal being 1/FL x crop factor, so 30mm on a crop "needs" 1/50th second) the prime is going to suffer from some camera shake as you are two stops below that ideal shutter speed. That shake may be very slight to positively disastrous, depending on how good you are at hand holding.

The IS, on the other hand, is good for 4 stops, so should allow you to shoot just 2 stops below "ideal" shutter speed without problems, however the slower aperture puts you some 4.5 to 5 stops below ideal speed. That should still give the edge to the zoom (as once you knock off the 4 stops of IS, you are only one stop too slow, compared to two with the prime). But, at longer such shutter speeds, it is possible to move the camera quite significantly if you aren't careful, and that can go beyond the IS capability to counteract.

With either lens though, you are pushing your hand holding ability. I would try and rest the camera on something solid as a brace, or lean against a wall to give you some support from the side. With good bracing and shooting technique, you should be able to get a decent shot, but you will need to concentrate on keeping the camera as steady as possible, and gently squeezing the shutter button, so you don't move the camera when you press it.

With that in mind, I would think the zoom with the IS should be slightly better for avoiding camera shake, but a lot depends on how accurate the claim for "4 stop IS" actually is. If it is only 3 stops in practice, the verdict could go either way.

Note, this answer applies only to that specific scenario (although at slow shutter speeds, 4 stops of IS will always be better for avoiding camera shake than 2 1/3 stops of aperture, assuming the subject isn't moving) and doesn't take into account the possible difference in optical sharpness of the actual lenses, as that is of secondary importance to camera shake. It doesn't matter which lens is slightly sharper optically, if you get blur from camera movement using it.

Thanks!! That is a very good answer and quite detailed...

I made a typo on my example... It was zoom with f4 and .4 sec.

So the conclusion is that at those light levels its all about the holding technique... If I am good then the zoom with IS is likely better then prime.

kin2son wrote in post #16072627
Wrong...

http://www.the-digital-picture.com …omp=0&FLIComp=0​&APIComp=2

Zoom @ 35mm f4.5 (no 30mm on the site), prime @ f2.

The zoom demolishes the prime in terms of pure sharpness.

Actually the prime is doing better there than I had expected. The zoom is better, but I don't know if I would classify that as demolition.