Focal length and depth of field

[Goldcoin79]

Hi all

I am currently reading a photography book to learn more and expand my knowledge on the subject and last knight I read a chapter on lenses, the photographer talks about wide angle lenses (lower focal lengh) having a larger depth of field and longer lenses (longer focal lengths) having a shallower depth of field.

I thought depth of field was all to do with the aperture you set and partly to do with the position you are standing and how close the subject is. Also when I set my camera to its lowest f number for the aperture then zoom in the f number increases as I zoom in which contradicts what I have read above, surly if the f number increases as I zoom in the aperture is getting smaller therefore giving a larger depth of field as the focal length gets longer.

Any one able to explain to me so I can understand, as I am now a little confused.

James

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[480sparky]

Some zooms will change the f/number when you zoom when you're using the maximum or minimum apertures. For instance, a lens may be a 25-100 f/4-5.6, meaning at 25mm, the max. aperture is f/4, but at 100mm it's dropped down to f/5.6.

At the other end, you may have f/22 at 25mm but that changes to f/32 when the lens is at 100mm.

However, in between, there shouldn't be any change. f/8 or f/11 or f/16 should be maintained, regardless of the focal length.

[Tuffythepug]

Goldcoin, you are mostly correct. smaller apertures (bigger numbers) create more depth of field and larger apertures create a shallower depth of field. distance from camera to subject and subject to background is also a factor. But it's also a fact that wider angle lenses create more DOF and telephoto lenses create shallower depth of field. You can run experiments with all these factors and see exactly what the resulting DOF is. 408Sparky is spot on with his description of the changing minimum and maximum apertures as it pertains to certain zoom lenses.

[kundalini]

<<snip>>I thought depth of field was all to do with the aperture you set and partly to do with the position you are standing and how close the subject is. Also when I set my camera to its lowest f number for the aperture then zoom in the f number increases as I zoom in which contradicts what I have read above, surly if the f number increases as I zoom in the aperture is getting smaller therefore giving a larger depth of field as the focal length gets longer.

Any one able to explain to me so I can understand, as I am now a little confused.
<<snip>>

This is a characteristic of consumer grade zoom lenses, also known as variable aperture lenses. This is easily discerned by the aperture stamped on your lens, it will have more than one value. Once you get to the higher tier, you are in the realm of constant aperture lenses. With these lenses, once you set your aperture setting, it will remain constant throughout the focal length. However, there is a caveat.
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What's in yer wallet?

[Goldcoin79]

Just been reading the below article:-

Depth of Field Myth

It is basically saying focal length does not change depth of field but when you zoom in or use a loger focal length it softens the back round and gives the impression that it has a shallower depth of field, even though the depth of field is the same but it's just the fact that because the back round is magnified it softens it giving an impression of reduced depth of field.

[KmH]

Most consumer grade zoom lenses are variable aperture zoom lenses. To keep their cost low they cannot maintain the same maximum aperture f-stop at longer focal lengths, and automatically change the lens aperture when zoomed.

However, if we consider a common kit lens like an 18-55 mm f/3.5-5.6 lens, the lens opening at 18 mm and f/3.5 is actually smaller than the lens opening at 55 mm and f/5.6.

The small f in f/3.5 or f/5.6 is whatever the lens focal length is set to. So when the lens is zoomed to 18 mm, it is 18mm divided by 3.5 and the lens opening has a diameter of 5.14 mm.
At 55 mm and f/5.6, it's 55 mm divided by 5.6 and the lens opening has a diameter of 9.82 mm, or nearly twice the diameter of the lens when it's set to 18 mm and f/3.5.

The more expensive, prosumer and pro grade constant zoom lenses only have one max aperture number, like f/2.8.

So, lets consider a constant aperture f/2.8 zoom lens that can be zoomed from 27 mm to 55 mm.
At 17 mm and f/2.8 the lens opening is 6.07 mm wide, and at 55 mm and f/2.8 the lens opening is 19.6 mm. Though called a constant aperture lens, the aperture size actually increases.
What doesn't change is the focal length to lens opening RATIO, which stays at f/2.8.

f/2.8, or f/3.5, or f/5.6 is a fraction, and fractions are a way to express ratios.

Note that a constant aperture zoom lens has to use lens elements that have a much larger outside diameter than variable aperture zoom lenses do, which is a major factor in why constant aperture zoom lenses cost so much more than variable aperture zoom lenses.

Longer focal lengths magnify the background more, making them seem more out-of-focus. If the in focus subject scale is constant in the image frame, the blurred background a longer focal length delivers is magnified and looks more blurred than the the less magnified, blurred background a shorter focal length delivers.

Understanding photography at this level is a large part of what being a professional (expert) photographer is all about.

[Tuffythepug]

Just been reading the below article:-

Depth of Field Myth

It is basically saying focal length does not change depth of field but when you zoom in or use a loger focal length it softens the back round and gives the impression that it has a shallower depth of field, even though the depth of field is the same but it's just the fact that because the back round is magnified it softens it giving an impression of reduced depth of field.

the article may be technically correct but the result is the same: blurry background with subject in focus achieved through the use of longer focal length lens

you an easily see how depth of field changes by looking at the guides on your lens barrel. whether it's wide angle or telephoto lens you are using you can see the exact range of acceptable focus with the guide numbers at various apertures

[snowbear]

Just been reading the below article:-

Depth of Field Myth

It is basically saying focal length does not change depth of field but when you zoom in or use a loger focal length it softens the back round and gives the impression that it has a shallower depth of field, even though the depth of field is the same but it's just the fact that because the back round is magnified it softens it giving an impression of reduced depth of field.

I haven't read the entire article, but the author states he physically moved the camera back from the subject, then increased the focal length so the subject's size matched the previous photo. He hasn't changed the magnification at all, so he wouldn't get a shallower DOF.

[Derrel]

SNIP>

Understanding photography at this level is a large part of what being a professional (expert) photographer is all about.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^THAT^^^^^^^^^^^^^^^^^^^

[deeky]

f/2.8, or f/3.5, or f/5.6 is a fraction, and fractions are a way to express ratios.

KMH,

Thank you for that explanation. I've been through all of that before, but it has never clicked like it did here. Factions, duh. I've never had it explained that way before.

As far as focal length and a zoom lens - so focal length doesn't really shrink dof, but rather just essentially 'flattens' the image and makes it appear to have a shallower dof?

[Dao]

Just been reading the below article:-

Depth of Field Myth

It is basically saying focal length does not change depth of field but when you zoom in or use a loger focal length it softens the back round and gives the impression that it has a shallower depth of field, even though the depth of field is the same but it's just the fact that because the back round is magnified it softens it giving an impression of reduced depth of field.

This is correct.


DoF is related to Circle of Confusion (CoC)

When we say about DoF, we actually means such as taking a photo, let's say with 24mm lens, within that DoF range, the objects will appear to be sharp when we develop it on a 8x11 paper and view it at arm length distance.

What happened if you take a the same photo with a 200mm lens at the same location and same camera? Your Angle of View (AoV) is smaller, so the same object your saw in the photo taken with 24mm is now bigger. So it is magnified.

When you print the new photo with a 8x11 paper and view it at the same arm length distance, some objects may appear to be out of focus now so the DoF appeared to be different.


DoF is related to magnification. If you print the photo on a 24" x 36" paper and view it at arm length distance, it will also affect the DoF.

[Goldcoin79]

I didnt know that the apeture was calculated from the focal lengh, I keep learning from this forum which is why I joined as I am reasonably new to photography and am wanting to learn lots about it.

So if the actual apeture is focal lengh/f number = apeture diameter would it be correct to say that a longer focal lengh will give a wider apeture and therefore shallower depth of field but also because of the magnification of a longer focal lengh this also gives an effect of a shallower depth of field.

Can any one confirm if I have this right in my head now.

James