What's the difference between a f1.5 lens and a f3.2 lens on any given SLR camera?

Question

Thank you all for helping me out on this.

Answer 1

an f-1.5 lens opens to a wider aperature than an f-3.5 hence it lets in more light. However, the lower the f-stop, the smaller the depth of field.

Answer 2

Unfortunately, Dr. Sam is mistaken with his shutter speeds. The difference from f3.2 to 1.5 is 2 stops.

Since I don't have the actual lens in question, I'll use the closest numbers I have available.

f1.4 @ 1/500
f2 @ 1/250
f2.8 @ 1/125

Allowing that f1.4 is close enough to f1.5 and that f2.8 is only .4 from f3.2, the shutter speed at f3.2 might be closer to 1/100 rather than 1/125.

EDIT EDIT EDIT

Thanks for the update, Doc. I seldom consider 1/3 stop as signifigant even though most of my cameras do have a "stepless" electronic shutter...

Answer 3

Those numbers are what they call the speed of the lens. The lower the number the higher the speed and the price. The number refers to how fast the lens will gather light, thus increasing the performance. Faster lenses allow the shutter in the camera to work faster, thus minimizing the risk of blurry photos. If you are taking photos outdoors in the daylight or inside with a flash, it's usually not that big of a deal. But, if you are in low light conditions or taking shots of moving objects (sports, pets, children playing), it can make a big difference.

Answer 4

In addition to a more shallow depth of field (less stuff is in focus), the hypothetical f/1.5 lens opening would let in four times as much light as an f/3.2 lens, so you could use a shutter speed four times faster with the f/1.5 lens. This could be important for some situations, such as a sports event where 1/500th of a second would freeze the action, but 1/125 could give you a somewhat blurry image in an action sport.

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EDIT
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Electrosmack says, "The more bokeh, the better," but this is not always the case. It's true that most people want to know how to control the bokeh and often his statement is the goal, but there are times when you want as much in focus as possible. Case in point... Pretty much everything from 5 feet to over a mile away are in acceptable focus: http://www.flickr.com/photos/samfeinstein/1250011561/

I'm not trying to argue or be critical, but I just don't think this should be put out there as a blanket statement to a beginner in photography.

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MORE EDIT - EDWIN
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How can I be mistaken with an EV that I arbitrarily made up just to use as an example? I also nominally adjusted to the conventional shutter speeds found on cameras rather than going to fractions of a shutter speed to match 1/3 stop aperture variations.

The thing is, I've never seen an f/1.5 lens, so I don't know how to calculate for that. It's less than 1/3 stop from f/1.4. f/1.5 would be darn close to f/1.4. f/3.2 is 1/3 stop over f/2.8, so I just compared used f/1.6 in my head for convenience. It is not really exactly two stops different, so I will allow that it could be more proper to assign a shutter speed of 1/100 in my example. I'd have to figure out how to do the math, though, to be any more accurate than that. Due to the simplistic nature of the question, I thought my answer was "close enough." Sorry, but thanks for keeping me sharp.

For sake of accuracy, I will have to agree with Edwin on the fine points. My example should work out close to this:

f/1.4 @ 1/500
f/2.0 @ 1/250
f/2.8 @ 1/125
f/3.2 @ 1/100 - approximation (it's probably closer to 1/105...)
f/4.0 @ 1/62.5, but we all know that we use 1/60 at this point...

1/3 stop increments are:
1.4 - 1.6 - 1,8
2.0 - 2.2 - 2.5
2.8 - 3.2 - 3.5
4.0 - 4.5 - 5.0
5.6 - 6.3 - 7.1
8.0 - 9.0 - 10.0
11 - 13 - 14
16 - 18 - 20
22 - 25 - 29
32
45
64

Even these are approximations, rounded by ALL for convenience, but there you go.

"f2.8 is only .4 from f3.2" is a mathematically correct statement, but since f-stops are ratios, it is not a linear relationship. f/3.2 is 1/3 stop (not "0.4 stop") smaller than f/2.8.

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EDWIN
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Yeah, really... What the heck is 10/43 for a shutter speed? Or 10/601? 3/5? 5/198? These are all values I have on my Flickr site. I dare anyone to put THOSE values in order! (haha)

Answer 5

Trinity got it right. The aperture, measured in f-stops, shows how wide the aperture is, which tells how much light will enter lens is. An aperture of f1.5 is much wider than f3.2. As you can see, the measuring system goes, smaller number=larger aperture=more light. Along with this, a wide aperture means a shallow depth of field. This produces a smooth background, desirable in portrait photography. This smooth out of focus area is called bokeh. The more bokeh, the better.