[normhead]

All you need to know about f-stops, you learn in the first day of photography class. Each ƒ-stop doubles the amount of light or halves it. A photographer doesn't need to know how its determined.

I felt the same way about my lens design class. A photographer doesn't have to know how to design lenses, he better know how to use them though.

But carry on if you wish.

[pres589]

But setting the lens to a larger or smaller f-stop number does more than just "make stuff brighter". It affects sharpness, rendering, depth of field, et cetera. Knowing those things is still well within the realm of being a photographer before leaping to lens designer. I think the question of "if you're going to keep shooting at f11, why pay for f2.8?" is a good one. Knowing how to use your equipment is a good thing and not overreach.

[Rondec]

Sharpness isn't everything. Most lenses will be "sharpest" a stop or two stops down from their max aperture. But not every image is about having everything super sharp. Opening up the lens makes your depth of field more shallow which lets you blur the background more.

This is the DA *55 at f1.8



at f2.8



f14



I will say that for portraits I typically shoot at f2 to f2.8 and for landscapes f8 on APS-C and f11-14 on my K-1. The reason for stopping down for landscapes really has nothing to do with sharpness, but more because I typically like everything to be in focus and at f4 it may not be.

[photoptimist]

The rules of thumb about the "best aperture" depend on which type of sharpness you want:

1) The sharpest part of the image is as sharp as possible (use about 2 to 3 stops down from fully open); or

2) Most of the image is as sharp as possible (use f/8 or f/11 to sharpen the corners and bring more of the foreground and background into focus).


Of course, maximum sharpness is not the only possible goal. Other sharpness-related goals might include:

3) Isolate the subject by blurring the foreground and background distractions (use the lens wide open or nearly so);

4) Have focus from some distance all the way to infinity (use the hyperfocal equation to compute a specific aperture);

5) Create a very specific range of focus distances (use depth-of-field equations to compute a specific aperture); or

6) Minimize the percentage of the image that is totally out-of-focus (close the lens down to f/16, f/22, or even f/32 as in the case of many macro images).


If your photographic goals are #2, #4 or #6, then you are right that you don't need a fast lens.

If your photographic goal is #1 or #3, then high-quality fast lenses are better.

If you seek goal #5, a fast lens can sometimes help if you want a specific narrow depth-of-field and blur everywhere else.

(Of course, a major advantage of a fast lens is that it also lets you use lower ISO or faster shutter speeds in darker conditions but that's outside of sharpeness or focus goals.)

[normhead]

The rules of thumb about the "best aperture" depend on which type of sharpness you want:

1) The sharpest part of the image is as sharp as possible (use about 2 to 3 stops down from fully open); or

2) Most of the image is as sharp as possible (use f/8 or f/11 to sharpen the corners and bring more of the foreground and background into focus).


Of course, maximum sharpness is not the only possible goal. Other sharpness-related goals might include:

3) Isolate the subject by blurring the foreground and background distractions (use the lens wide open or nearly so);

4) Have focus from some distance all the way to infinity (use the hyperfocal equation to compute a specific aperture);

5) Create a very specific range of focus distances (use depth-of-field equations to compute a specific aperture); or

6) Minimize the percentage of the image that is totally out-of-focus (close the lens down to f/16, f/22, or even f/32 as in the case of many macro images).


If your photographic goals are #2, #4 or #6, then you are right that you don't need a fast lens.

If your photographic goal is #1 or #3, then high-quality fast lenses are better.

If you seek goal #5, a fast lens can sometimes help if you want a specific narrow depth-of-field and blur everywhere else.

(Of course, a major advantage of a fast lens is that it also lets you use lower ISO or faster shutter speeds in darker conditions but that's outside of sharpeness or focus goals.)

A major advantage of a variable aperture lens is that it's likely to be on your camera instead of sitting home on shelf or buried deep in your camera bag.

[photoptimist]

A major advantage of a variable aperture lens is that it's likely to be on your camera instead of sitting home on shelf or buried deep in your camera bag.

Quite true.

Big bright lenses might gather more light but they also gather more dust!

[clackers]

I believed that it ends with ratio of light intake between aperture and lens diameter, also this f stop varies and light intake varies also.

Well, that's just wrong, Penview, and the sooner you dispose of that notion, the better!

There are many photography classes where you can learn about aperture and its implications.

Some of the other posts here show great examples.

[surfar]

the first day of photography class. Each ƒ-stop doubles the amount of light or halves it

Thanks for the free lesson, i'll look forwrd to tomorrow.

A photographer doesn't have to know how to design lenses, he better know how to use them though.

Yes, and a lens designer needs to knowhow to do that.Ricohs one does,he designs great lenses.Hes a slow worker though![COLOR="Silver"]

---------- Post added 09-20-17 at 12:38 PM ----------

A major advantage of a variable aperture lens is that it's likely to be on your camera instead of sitting home on shelf or buried deep in your camera bag

Yes, very much agree.The DFA stack of primes 28-105mm is great...beats all the DFA WR primes under 100mm.

---------- Post added 09-20-17 at 12:40 PM ----------

There are many photography classes where you can learn about aperture

We learnt that first day!

[mikesbike]

Thanks in advance for your help. Sorry if this is a basic question. I keep reading and hearing that you should be taking pictures with a f stop around f 11 . This is where the lens is sharpest. Why buy a fast lens with a f 2.8 if you are going to shoot at an aperture at f 11? Thanks again .

Welcome to Pentax and to the forum. No need to apologize for any question you may have. There are a lot of experienced people responding here with very good information. You are probably getting a better education in this thread than you would in a week's worth of photography classes! I never attended one. When I started there was just film and there was no auto anything. You either had a camera with a fixed focus, or a more advanced model where you could select an aperture, manually focus the lens, and even select a shutter speed. With my first SLR I could do all of these things, and take light metering using the camera's built-in meter as a guide for exposure. On advice from a friend who had taken photography courses, I shot only slides at first. That was cheaper so I could waste more film, and as he said, results were more accurate for exposure- what you got is what you shot. This is still true today. This still has the least extraneous post-shot manipulation. The rest was learn by doing and by reading over the years.

You have not stated which equipment you are using. There have been many good posts here. I especially like the thoroughness and practicality shown in normhead's. Whatever you read about taking pictures mainly at around f/11 or at any other f/stop is nonsense, as I believe by now you've come to realize. A number of Pentax camera models have had a selectable program line called MTF, where the camera would recognize a particular Pentax lens mounted on it and automatically choose the apertures where the lens has been tested to be at its sharpest performance, as long as available light permits at that ISO setting. I don't know if the current models, some of which I own, still have this feature, because I have rarely used it.

If photographers have a fast f/2.8 zoom lens, they have it because sometimes, or often, they need f/2.8 for the advantages it affords. As to sharpness, when it comes to a very good lens, even when not at its sharpest aperture it is still pretty sharp. You can see by normhead's mushroom shot examples, that where the first shot was taken at the lens's better aperture and should exhibit much greater overall sharpness, instead, due to reduced depth-of-field, it does not. According to test-lab lens evaluations, the f/22 shot should look much worse. Instead, it looks sharper overall than the f/5.6 shot. In fact, the mushroom itself looks comparatively sharper in its entirety because it is entirely in focus. On the other hand, the f/5.6 shot in such a closeup defocusses the surroundings so the mushroom stands out more, if that is the photographer's intention. There are good reasons for choosing a particular aperture.

Going back to sharpness when using a zoom lens capable of f/2.8, there are times when without this capability you will be unable to attain good sharpness, even by using the lens's sharpest aperture. Shooting fast action in less than very bright lighting is a common example. You might need 1/500 sec shutter speed to freeze action or will otherwise wind up with soft images or even blurred ones. To get that shutter speed, you might need f/2.8 or otherwise go to an extremely high ISO, which would compromise quality in terms of sharpness, noise, etc.

But your shooting needs might very seldom include the particular properties a fast zoom lens can offer, so why spend the money for one and lug around the size and weight? There are very good zoom lenses available having just average aperture capabilities. That said, it is also a good idea to include a fast prime lens, some of which can be had at a reasonable cost, so you can explore the particular aspects that a large aperture can offer. That is the classic basic lens kit- a versatile zoom lens of good quality, plus a good, fast prime lens of mid focal length.

[noblepa]

To the OP: If you're still confused about aperture sizes (f/stops), I suggest you get a copy of Bryan Peterson's "Understanding Exposure". It explains how apertures are calculated, and how and when to use the various options.

As others have said, aperture size, among other thijngs, affects depth of field. Peterson explains the almost infinite combinations of "correct" exposure values for any situation, and then discusses which one to choose for the photographic effect you are looking for.

I've been an amateur photographer since 1967 and I still learn something from the book every time I look at it.