Are low f-stop lenses better or sharper?

Hi,

Lower aperture lenses are usually better quality lenses than "slower" lenses of comparable designs but they often sacrifice some sharpness at the edges of the image to get the increased brightness. As VanGork says wider apertures will have shallower depth of field but he repeats a common misconception regarding "bokeh". (Many people believe that bokeh is a term to describe the areas that are out of focus in a picture when there is very shallow depth of field or when there are shaped apertures such as hearts, stars, etc. used to give defocused areas a particular appearance. Bokeh actually refers to the QUALITIES or PROPERTIES of out of focus portions of an image and is a subjective evaluation of how pleasing out of focus areas in an image appear to be).

One major advantage of using wider aperture lenses on reflex viewing systems is that they give a brighter viewfinder image. This is beneficial in several ways - 1) It makes it easier to compose an image in the viewfinder 2) It usually increases the response speed of auto focus mechanisms and 3) It makes it easier to focus manually.

As a general rule an f/2.0 lens will give better sharpness at f/4.0 or f/5.6 than a lens with a maximum aperture of f/3.5 and a lens with f/1.4 will perform better at f/2.0. That is primarily because the more steeply curved edges of the glass elements on a lens are occluded by the diaphragm mechanism as it is closed down which helps reduce aberrations. However, as lens manufacturing processes have improved and lens elements are molded rather than ground the difference in performance between faster and slower is being reduced.

Most lens designs produce their optimum results at the middle range of f stops losing some image quality at the higher apertures such as f 11 or 16 because of diffraction (the lens begins to act like a prism spreading the colors of the spectrum at very small lens openings).

The properties of faster lenses can be used to advantage by understanding these characteristics. Using a lens wide open generally means that the edges will appear softer, flattering for portraits and pleasing when doing close up/macro photography to emphasize the central image. These lenses will usually give exceptional results for landscape photography when used at their middle range of f stops.

Hope that helps. Email me if you would like more information.

1

The 70-200 f/4L IS is a good lens, and half the price of the 2.8L, which is one stop faster. Lens speed pertains to its maximum effective aperture [diameter], the smaller the f/number, the larger the opening - the larger the opening [aperture], the more light the lens can gather and expose to the image sensor. The three exposure settings are: aperture, shutter speed, ISO. The aperture determines how much light is exposed to the image sensor. The shutter speed determines how long that light is exposed to the image sensor. And the ISO determines how sensitive the image sensor is to that light, higher ISO = more sensitive. Larger maximum effective aperture lenses will allow you to make a shot in an area with less light, at a lower ISO (less sensitive). This makes for an image with less noise. Less noise means more sharpness, better detail. Your 40D has good sensitivity. And is how you decide on the importance of speed when buying a lens. It's always a trade off, and in this case your trade off is 1 stop of sensitivity vs. cost. And in the case of canon's 70-200 telezooms, cost can be steep as I am sure you are aware. But canon does something that nikon [or others] do not. They offer this lens in 4 different flavors, each either f/2.8 or f/4, with or without IS - all based on cost. Because either way, canon's pro telezoom is about the best lens ever (and I'm a nikon/olympus guy, so you know they must be good). So, if you've got $1900, get the 2.8 IS. You couldn't run out of situations that its use would be highly beneficial to you. But, f/2.8 does not mean "better". Because there is no better, compared to the f/4 version, only appropriate. With image stabilization, you can use f/4 as you would an f/2.8 without it, at up to 4 stops (so canon claims, anyway). The f/4 IS is cheaper and lighter, and still as sharp, just not as fast - which is not a big deal. Were it me, I would get the f/4 IS and save money.

Although f-stop and sharpness are not directly linked, you can almost always be sure that a lens with a lower minimum f-stop is going to be sharper than one without it. For instance, the 50mm f/1.4 is sharper than the 50mm f/1.8. Even at a higher f-stop like f/11 or f/16.

Virtually any lens is sharper than a kit 18-55mm lens though. Those lenses are pretty terrible if you're concerned with maximum sharpness.

The fact that the lens has those low f-stops in and of itself does not make it a better lens, but it IS a general indication that you are using a higher quality lens. If the maker went to the trouble to make a wide aperture lens, you can usually count on the lens being a better lens than a $100-125 kit zoom lens. There are other factors that go into lens quality beside the size of the aperture and you should expect better performance throughout the entire range. (less distortion, less color abberations, sharper image, whatever...)

Check here a good photography course online:

http://photography-course.info/

You can be like the masses of humanity and buy a camera you can afford that has auto this and that for worry free picture taking. And learn through trial and error how to use something like aperture or shutter priority auto modes or even attempt to use the thing on full manual mode.

You seem to know already there is some thinking to using a camera and to take pictures. A good place to start is by reading the owners manual that comes with a camera. Read the information and look at the illustrations with part names and look at the real camera. Handle the camera and take pictures. Let me rephrase that. Take pictures to learn how to use the camera and maybe even to keep some. Don't start with important stuff you cannot photograph over again such as birthdays, a toddlers first steps. That puts picture taking out of the learning phase and puts the pressure and emphasis to taking pictures for real and to keep.

These first pictures are for you to learn how to use your camera. And you have learned how to use your camera when you can take pictures with it and can teach others how to use it. Honest. You can also go to a college in your area and take a beginning photography course. There you will be taught the basics even a pro must learn and do in their work. Camera handling and use, taking exposures with film and/or digital cameras, and maybe even some photo assignments to get some real time learning. In this learning do not take serious pictures you must keep as that detracts from the learning aspect of the class. Do so only if you have mastered the use of your camera before class is over.

It's like growing up in a way. And I am happy you know there is a way to learn how to use a camera and take pictures. It's like learning how to drive I suppose. Someone can teach you or you can get taught at a driving school. Both will get you a drivers liscense. One though will really teach you the fundamentals you can use for the rest of your driving career.

They are simply "faster" lens.

This means that the opening glass at the end of the lens by the mount, are a lot larger than average kit lens, thus allowing more light to hit the sensor, meaning you won't have to use extremely high ISO's to get a decent hand-held image in low-light.

You also get a more shallow depth of field and better quality bokeh with "fast lens"

DSLR photography doesn't need to be over-complicated. This online photography course has been developed for beginners - intermediate levels and will teach you how to make the best use of your DSLR camera. https://tr.im/LtY2W

Learning how to confidently use your DSLR will help you get full value out of this awesome camera you have already paid for!

This course has been developed after seeing many potential photographers give up far too soon, wasting good money they have spent on the purchase of their DSLR camera.

2