DoF questions

AaronLLockhart · Jun 18, 2012

Alright, I'm not new to photography by any means. However, DoF is becoming very confusing here the past month or so. I remember when I was using a 70-300 telescopic lens on my d60, to increase DoF, you could simply use a higher f-stop and a slower shutter at the long range photo. With this 50mm f/1.8, I notice that the lowest f-stop and the fastest shutter create the most bokeh for me.

Can someone please explain why this is? And if I have it screwed up, how DoF works?

SCraig · Jun 18, 2012

Unless I misunderstand your description it is behaving correctly. As you said, to increase depeth of field use a higher number f-stop (smaller diameter). To decrease depth of field use a smaller f-stop (larger diameter) which will then increase the bokeh.

AaronLLockhart · Jun 18, 2012

SCraig said:
Unless I misunderstand your description it is behaving correctly. As you said, to increase depeth of field use a higher number f-stop (smaller diameter). To decrease depth of field use a smaller f-stop (larger diameter) which will then increase the bokeh.

Ah, you answered my question. As I recall you are right. Looking through my old photos, I just had the terminology confused. Thanks!!!

Robin Usagani · Jun 18, 2012

I think you confused background blur and DOF mixed up.

tirediron · Jun 18, 2012

Schwettylens said:
I think you confused background blur and DOF mixed up.

Ehh? Howzat?

KmH · Jun 18, 2012

A smaller aperture is actually a smaller f-stop.

F/16 is a much smaller number than f/2 is, because they are fractions.

The f-stop describes the size of the lens aperture as a function of the lens focal length. In other words, the letter f is a variable that is the lens focal length.

A 100 mm lens set to f/4 has a lens aperture that has a 25 mm diameter - 100/4 = 25. A 100 mm lens set to f/8 has a lens aperture that has only 12.5 mm diameter - 100/8 = 12.5
A 200 mm lens also set to f/4 has a lens aperture that has a 50 mm diameter - 200/4 = 50. A 200 mm lens set to f/8 has a lens aperture that has a 25 mm diameter - 200/8 = 25

So you can see that a 200 mm lens set to f/8 has the same lens aperture diameter as a 100 mm lens that is set to f/4 - 25 mm.

Basically, a lens aperture of 25 mm lets in the same amount of light regardless the lens focal length.

You may want to note however - A 'stop' of lens aperture doubles, or halves, the amount of light entering through the lens aperture. The amount of light a lens aperture can let in is dependent on the aperture's area, not it's diameter.
Your camera is likely set to the factory default of adjusting lens aperture in 1/3 stop steps.
The full stop steps are f/1 - f/1.4 - f/2 - f/2.8 - f/4 - f/5.6 - f/8 - f/11 - f/16 - f/22 - f/32 - etc.
In other words, if lens focal length is not changed f/5.6 lets in twice as much light as f/8 does, or f/8 lets in 1/2 as much light as f/5.6 does.

AaronLLockhart · Jun 18, 2012

KmH said:
A smaller aperture is actually a smaller f-number.

F/16 is a much smaller number than f/2 is, because they are fractions.

The f-number describes the size of the lens aperture as a function of the lens focal length. In other words, the letter f is a variable that is the lens focal length.

A 100 mm lens set to f/4 has a lens aperture has a 25 mm diameter - 100/4 = 25. A 100 mm lens set to f/8 has a lens aperture that has only 12.5 mm diameter - 100/8 = 12.5
A 200 mm lens also set to f/4 has a lens aperture that has a 50 mm diameter - 200/4 = 50. A 200 mm lens set to f/8 has a lens aperture that has a 25 mm diameter - 200/8 = 25

So you can see that a 200 mm lens set to f/8 has the same lens aperture diameter as a 100 mm lens that is set to f/4 - 25 mm.

Basically, a lens aperture of 25 mm lets in the same amount of light regardless the lens focal length.

You may want to note however - A 'stop' of lens aperture doubles, or halves, the amount of light entering through the lens aperture. The amount of light a lens aperture can let in is dependent on the aperture's area, not it's diameter.
Your camera is likely set to the factory default of adjusting lens aperture in 1/3 stop steps.
The full stop steps are f/1 - f/1.4 - f/2 - f/2.8 - f/4 - f/5.6 - f/8 - f/11 - f/16 - f/22 - f/32 - etc.
In other words, if lens focal length is not changed f/5.6 lets in twice as much light as f/8 does, or f/8 lets in 1/2 as much light as f/5.6 does.

That's actually VERY good information. Not only have I not read it that way, I've never heard it explained that way either. I wish this explanation would have been around when I first started using SLR cameras!!!

Solarflare · Jun 18, 2012

> A smaller aperture is actually a smaller f-number.

Um, no.

A large aperture = a large opening of the lens = a small f-number.

For example my 35mm F/1.8 can open up to an f-number of 1.8 - this is however a large opening and a large aperture.

> F/16 is a much smaller number than f/2 is, because they are fractions.

Thats correct, but the f-number of F/16 is 16 ! Thus, a larger f-number is a smaller aperture.

> [more stuff]

Agreed.

AaronLLockhart · Jun 18, 2012

Solarflare said:
> A smaller aperture is actually a smaller f-number.

Um, no.

A large aperture = a large opening of the lens = a small f-number.

For example my 35mm F/1.8 can open up to an f-number of 1.8 - this is however a large opening and a large aperture.

> F/16 is a much smaller number than f/2 is, because they are fractions.

Thats correct, but the f-number of F/16 is 16 ! Thus, a larger f-number is a smaller aperture.

> [more stuff]

Agreed.

I think he was implying the "fraction size" of the aperture is smaller. Because with an f/16, your aperture closes double the distance as f/8 on the same focal length in lens.

yerlem · Jun 18, 2012

Solarflare that doesn't make any sense. You either view the numbers as fractions or you don't, whichever way is easier for you to remember. If you view it as fractions a larger number means a larger aperture (1/2 being larger than 1/16). If you view it as whole numbers* a larger number means smaller aperture (16 being larger than 2).

*I'm a native Spanish speaker, I don't know how you call numbers that are not fractions

Helen B · Jun 18, 2012

Solarflare is correct. For an aperture of f/16 the f-number is 16. It's that simple. The f-number isn't 1/16 or f/16, it is 16. Large apertures correspond to small f-numbers and vice versa.

DiskoJoe · Jun 18, 2012

smaller number = larger hole and shallower depth of field. and vice versa.

tcphotography · Jun 18, 2012

yerlem said:
Solarflare that doesn't make any sense. You either view the numbers as fractions or you don't, whichever way is easier for you to remember. If you view it as fractions a larger number means a larger aperture (1/2 being larger than 1/16). If you view it as whole numbers* a larger number means smaller aperture (16 being larger than 2).

*I'm a native Spanish speaker, I don't know how you call numbers that are not fractions

You did fine, it's a whole number!

And in all honesty, technical meaning of camera works is confusing unless you look at a lense. The numbers and dof change equally if you pay attention to the bottom number. I.e. 1/16 (emphasize the 16) has a deeper dof compared to 1/4 (emphasize the 4), which is very shallow.

Now if you want to talk about the opening of the lens, 1/4 is larger number overall and thus the opening is larger compared to 1/16.

Just memorize the meanings and it will come naturally!

fjrabon · Jun 18, 2012

JFC, we're not going to get into another argument about whether or not f-stop numbers are fractions or not, are we?

At the very least, we can all agree that the f-stop numbers at least come from fractions? Then it's just a matter of whether or not you want to think of them as fractions, or think of them as whole numbers and then 'think backwards'.