A question

[Deleted User]

Not sure where to post but as the forum title says if you have to ask it go here!!

Can somebody explain to me the Aperture "F" number in plain english and in less than 65 words so that a real numpty like me can understand it :scratch

My understanding is that the lower the "F" number the sharper\quicker speed the picture is taken at but the depth of field is shorter e.g. the item you focussed on is clear and sharp (in a perfect world) but the background is blurred, the higher the "F" number the less sharp the focussed area but the greater the depth of field..........................or am I barking up the wrong tree.

I look forward to the your comments

Tim

ShepsMom

Think of it as your eye. When it's dark, your eye opens up wide to get more light. Same with apperture, the darker it is, the larger apperture you need. Large apperture means small f number and vs. versa.


Did it help any?

bkatz

And if you can't open your eye wide enough to let more light in you will go to a faster film speed (Higher ISO - needs less light) or a slower shutter speed (to let light in longer). That is why you want a zoom with a constant low aperture at night so you can still use a fast shutter speed at a decent ISO - at least for me when I shoot sports.....:D

Cheers

[Deleted User]

So (to repeat what you guys said) the smaller the "F" number e.g. f1.8 then the larger the aperture to allow more light throught the lense in a dark situation. Once the largest aperture setting is set then you have to change to a higher ISO rating or slower shutter speed.

The clever bit comes in combining these settings to meet your surrondings.

If its not to differcult to ask, where does the DoF come into it??

Tim

jdryan3

draggin, you may want to do a search on Photography books, but one I like to recommend is Bryan Peterson's Understanding Exposure. It does a great job of defining the interaction between aperature (f-stops), shutter speed, and ISO.

He then ties out the impact of changing each of those: a small
f-stop number lets in more light (bigger 'hole') but also reduces the DoF, meaning the amount of the image that is in focus (or blurred) in front of and behind the actual focused upon subject.

A shallow DoF may be great for a sports action shot where you are trying to isolate the athlete from all the distracting elements around them, but usually not what you want to do if shooting the Grand Canyon. Or maybe even a group portrait.

z_28

OK - simple as in redneck's school :

F1.0 is as you should see, no lost of light inside lens, DOF is minimal.
Any higher number, smaller hole, less light, more DOF.

Actual DOF may be vary in different lenses due to design, quality etc.

Books don't bite (usually)

[Deleted User]

OK I think I have an understanding of the basics. The problem I have is that when you read an article on the subject there are very few that start at a level I can understand, but now having you guys point out the basics I have a much better idea of how it works.

Thanks

Tim

Tommyboy

The f-number is a actually a fraction. Let's use whole numbers to make this easier.

f/4.0 is actually 1/4.
f/8.0 is actually 1/8.

In the above examples, the diameters of the apertures are 1/4 and 1/8 the focal length of the lens. The aperture f/16 has a diameter that is 1/16th the length of the lens.

To state it another way, an aperture of 1.0 (which is very unusual) would have a diameter equal to the length of the lens.

This is why the larger the number (a smaller fraction), the smaller the aperture.

This is what finally made me understand the significance of the f-number when I learned it umpteen years ago. . . .

Not to confuse things, but. . . .

Depth of field is controlled by three factors:

1. The aperture (larger apeture, i.e. f/1.8 shallow DoF; smaller aperture, i.e. f/16, greater DoF)
2. Distance from film plane to subject (the closer you are, the shallower the depth of field)
3. Focal length of lens (the longer the lens, the shallower the DoF)

If you're paying attention, numbers 2 and 3 are related as they both refer to magnification.

Chew on this for a while and note it in your photos. It will all "click" before you know it.