Beginners Technical Question Regarding DoF & Prime lenses

[bbtom10]

Hi there,

I am an amateur and looking to see what my next purchase will be - aside from a couple of filters I think that the 'nifty fifty' prime seems a great option for everyday photography which is what i am focussed on.

I do have a question though, I recently attended a photography course and was told that you can only get proper DoF when the lens is in telephoto mode rather than wide (i.e. zoomed in rather than zoomed out) - in a prime where you don't zoom in or out, what affects the DoF (aside from the Aperture which I understand)?

If anyone also wants to say how great their 50mm prime is and how I should definitely get one, then please feel free! I am limited in my choices by my Nikon D40 and need to have an AF-S (or equivalent) auto-focussing lens.

Thanks
Tom

[dcclark]

Depth of Field is a description of how much of an image is in focus. EVERY image has DOF. "Proper" DOF is a meaningless term, because choosing the amount of DOF in your image is an artistic choice. Perhaps your instructor meant "narrow" DOF, but in many cases (such as landscapes), you don't want a narrow DOF.

Ultrawide lenses tend to have very big depth of field (a lot is in focus) by the nature of their focal length. My Nikon 10-24 has enormous depth of field at 10mm. On the other hand, telephoto lenses can have narrow DOF for the opposite reason.

However, the major controlling factor is aperture. A "nifty 50" can usually open up all the way to f/1.8 or f/1.4, which will give you very narrow DOF. A kit lens like the 18-55, on the other hand, will only open up to f/3.5 -- it's not bad, but you won't get the sliver-thin focus like you can at f/1.4.

However, for your D40, I do NOT recommend a 50mm lens. The focal length is just too long for general use -- it's more of a short telephoto. But luckily, Nikon has a 35mm f/1.8 AF-S which retails for about $200 -- which is a steal, considering the excellent quality of the lens. I'd definitely recommend getting it. Using a prime can teach you a lot about composition (and not relying on zooms), and this is a cheap way to try it out.

[bbtom10]

Fantastic explanation, and thanks for the recommendation too - I think DoF is the most difficult thing to understand conceptually, because big numbers = small holes so it just doesn't quite add up.

One thing the instructor did explain which really helped was the paint splat process.

Drop paint through a big hole quickly, and it will splatter everywhere - just like a small Aperture (Big Aperture Hole) - the light will splatter and most of it will therefore be out of focus.

Drop paint through a small hole and it drips slowly onto a defined point - just like a large Aperture (Small Aperture Hole) - the light will focus the entire picture really well.

I didn't really understand what he meant by having to be in tele- mode so I guess I don't really need to.

Thanks again.

Tom

[jdepould]

You can get shallow DoF on a wide angle, it just takes a little more work. If you use something like a 24/1.4 wide open, you'll still get background blur.

Camera-to-subject and subject-to-background distance also matter a whole lot.

Putting your subject right up against a background (say, a brick wall) will put them closer to the same focal plane, so the brick will be sharper than it would if they were far away from it.

The focal depth also narrows as you bring the focal point closer to the camera. Putting your subject close to the camera and far from the background maximizes the effect.

[Elmo]

Tele-mode. Not actually true but the longer the focal length of the lens the smaller the APPARENT DOF. However given the same image, the longer the lens the further away you need to be to capture the same image.

I am a big user of controlled DOF. I generally shoot wide open, except for landscapes and micros, I then adjust the DOF with the focal length where possible.

Also the same DOF usage applies to both prime and zoom lenses.

[RecurrentNerve]

Drop paint through a big hole quickly, and it will splatter everywhere - just like a small Aperture (Big Aperture Hole) - the light will splatter and most of it will therefore be out of focus.

Drop paint through a small hole and it drips slowly onto a defined point - just like a large Aperture (Small Aperture Hole) - the light will focus the entire picture really well.

You'll confuse yourself less if you think of a large f-number instead of large aperture. So:

Drop paint through a big hole quickly, and it will splatter everywhere - just like a small f-number (Big Aperture Hole) - the light will splatter and most of it will therefore be out of focus.

Drop paint through a small hole and it drips slowly onto a defined point - just like a large f-number (Small Aperture Hole) - the light will focus the entire picture really well.

[inkista]

Originally Posted by bbtom10

I think DoF is the most difficult thing to understand conceptually, because big numbers = small holes so it just doesn't quite add up.

Actually, it does, if you write them out as f/16 or f/8, rather than as f16 or f8, or (as it's sometimes written on lenses) 1:16 or 1:8.

The f-number is a ratio. It's how much bigger the focal length of the lens is compared to the diameter of the aperture opening.

f-number = focal_length/aperture diameter (e.g., a 50mm lens at f/2 has an aperture that's 25mm across; at f/8, it's 6.25mm across).

Therefore, the aperture = focal_length/f-number.

Which is why we write an aperture as f/8 or f/16. the f = focal length. That way, it's the same number on all lenses, regardless of focal length.

And just as 1/2 is larger than 1/16, f/2 is larger than f/16.

[bbtom10]

Originally Posted by RecurrentNerve

You'll confuse yourself less if you think of a large f-number instead of large aperture. So:

Drop paint through a big hole quickly, and it will splatter everywhere - just like a small f-number (Big Aperture Hole) - the light will splatter and most of it will therefore be out of focus.

Drop paint through a small hole and it drips slowly onto a defined point - just like a large f-number (Small Aperture Hole) - the light will focus the entire picture really well.

Initially that was even more confusing, but I get you, and yes, it is actually a better way to think about it - thanks!

Tom

[bbtom10]

Originally Posted by inkista

Actually, it does, if you write them out as f/16 or f/8, rather than as f16 or f8, or (as it's sometimes written on lenses) 1:16 or 1:8.

The f-number is a ratio. It's how much bigger the focal length of the lens is compared to the diameter of the aperture opening.

f-number = focal_length/aperture diameter (e.g., a 50mm lens at f/2 has an aperture that's 25mm across; at f/8, it's 6.25mm across).

Therefore, the aperture = focal_length/f-number.

Which is why we write an aperture as f/8 or f/16. the f = focal length. That way, it's the same number on all lenses, regardless of focal length.

And just as 1/2 is larger than 1/16, f/2 is larger than f/16.

Genius - thank you - I am very comfortable with thinking about mathematics and physics and this makes a lot more sense - why didn't I realise it was fractions before!

Thanks again - you guys ROCK!

Tom

[inkista]

Originally Posted by bbtom10

Genius - thank you - I am very comfortable with thinking about mathematics and physics and this makes a lot more sense - why didn't I realise it was fractions before!...

I know. The lightbulb just went "bing" over my head when someone wrote out the f-number=focal_length/aperture diameter thing for me, too.

And now as a reward (or punishment) you get the square-root-of-2 lecture! [Usually nobody makes it this far.]

The other funky thing is the numbers that are used for the full-stop scale. They're not linear. And they seem kind of randomly odd, but they're actually a square-root-of-2 scale. Because the amount of light isn't proportional to the diameter of the aperture--it's proportional to the area of the aperture opening. And y'know, πr²=area and all that.

So, while stops mean a doubling of the light, and with iso and shutter speed, simple doubling/halving will get you the stops, with aperture, it's the square of the f-number that's doubling (with much approximation and rounding):

square root of 1 = 1
square root of 2 = 1.4
square root of 4 = 2
square root of 8 = 2.8
square root of 16 = 4
square root of 32 = 5.6
square root of 64 = 8
square root of 128 = 11
square root of 256 = 16