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6 votes
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If focal length does not affect DOF, why doesn't a 25mm lens at an f-stop of 4 give me the same DOF as a 100mm on an f-stop of 16?

Because magnification (focal length and subject distance) affects the Depth of Field more than aperture does (approx twice as much generally). Start with 50mm @ f/8 focused at 10 ft. If you halve the ...
Steven Kersting's user avatar
5 votes

Convert Zoom % to Focal length

20mm is the wide view. A 2x zoom lens would zoom to 40mm. A 3x zoom lens would zoom to 60mm. A 10x zoom lens would zoom to 200mm. And a 60x zoom lens would zoom to 1200mm. That's basic multiplication ...
osullic's user avatar
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5 votes

If focal length does not affect DOF, why doesn't a 25mm lens at an f-stop of 4 give me the same DOF as a 100mm on an f-stop of 16?

Focal length totally affects DOF. That's one of the reasons that older video camera footage and current-day cell phone video doesn't look "filmic". The sensor is tiny, so a short focal ...
Robert M.'s user avatar
  • 151
4 votes

If focal length does not affect DOF, why doesn't a 25mm lens at an f-stop of 4 give me the same DOF as a 100mm on an f-stop of 16?

In short: there are two reasons your expectations don't hold true: In order for crop 'equivalence' to hold true, everything must scale with crop factor, including subject distance and subject size. ...
scottbb's user avatar
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4 votes

If focal length does not affect DOF, why doesn't a 25mm lens at an f-stop of 4 give me the same DOF as a 100mm on an f-stop of 16?

Circle of confusion is an absolute value. Circle of confusion can be drawn on the image itself. If you were to scale the image from 25mm lenses to match the image from 100mm you will find out that ...
Euri Pinhollow's user avatar
4 votes

Why is aperture expressed as a function of focal length and not in absolute size?

The brightness of the image is proportional to the amount of light admitted by the lens. We are interested in quantifying image brightness because we will use this value to determine what settings are ...
Alan Marcus's user avatar
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3 votes
Accepted

Convert Zoom % to Focal length

The Panasonic FZ70 has a focal length of 3.58-215mm (20 - 1200mm in 35mm equiv.) This is right from the Panasonic website for your camera. LUMIX® Digital Camera DMC-FZ7 The actual “500 Rule” formula ...
Mike Sowsun's user avatar
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3 votes

Does a mirrorless camera need a shorter focal length for the same FoV as an SLR?

No, we don't need a different focal length – a lens designed for a mirrorless camera is just intrinsically different to a lens designed for a traditional (D)SLR camera. The image that a mirrorless ...
osullic's user avatar
  • 12.4k
3 votes

Crop an image taken in a known focal length, to get it in other focal length

Yes, since the spacial relationship of objects in the scene is dictated only by the perspective (distance), you can crop a wider recorded FOV for the same resulting image composition as if a longer FL ...
Steven Kersting's user avatar
2 votes
Accepted

DOF formula vs the simplified DOF formula

The simplified formula makes two assumptions; that subject distance is greater than the focal length, and that the subject distance is less than the hyperfocal distance. And that is true when the DOF ...
Steven Kersting's user avatar
2 votes
Accepted

Is this diagram I've drawn in regards to Depth of field correct?

So I am thinking, if this graph I have made is incorrect, since the distances between \$i-i_2\$ and \$i_1-i\$ should be the same, as depth of focus is symmetrical. (Emphasis mine) Your assumption ...
scottbb's user avatar
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2 votes

Is this diagram I've drawn in regards to Depth of field correct?

i1-i and i2-i should not be the same. What makes them symmetrical w/in the depth of focus is that their resulting blur radiuses are the same size at the image plane... i.e. o1 and o2 record equally ...
Steven Kersting's user avatar
2 votes

Why is aperture expressed as a function of focal length and not in absolute size?

What is important for determining exposure is the amount of light per unit area on the sensor. For a given object and aperture absolute size, a larger focal length lens will produce a larger image ...
Not_Einstein's user avatar
2 votes

Why do telephoto lenses have long focal length?

Let's put it the other way round: If you were to construct a lens for a desired focal length, under which circumstances would you use a telephoto lens construction? To make it concrete, let's talk ...
Ralf Kleberhoff's user avatar
1 vote
Accepted

Using the lens equation to make a camera out of an old projector lens

The film plane will be located, for objects at infinity, 14 inches from the rear nodal of the lens. If a simple lens is involved, we measure from its center. A complex lens has a forward and a rear ...
Alan Marcus's user avatar
  • 39.2k
1 vote

Does a mirrorless camera need a shorter focal length for the same FoV as an SLR?

You need a different flange distance, not a different focal length. Flange distance is sometimes referred to as "flange focal length" or "flange focal distance"... but that is best ...
Steven Kersting's user avatar
1 vote

Does a mirrorless camera need a shorter focal length for the same FoV as an SLR?

No, it means the lens' rear nodal point must be that much further in front of the flange so it can be the same distance in front of the sensor. Let's assume we have a Canon EF lens and a Canon EF-M ...
Michael C's user avatar
  • 175k
1 vote

Does a mirrorless camera need a shorter focal length for the same FoV as an SLR?

No. The actual length of the lens does not correlate with its focal length, at least not directly. The rear element of a typical 50 mm lens on a full-frame mirrorless camera is a bit inside the lens ...
scottbb's user avatar
  • 32.8k
1 vote

Why do telephoto lenses have long focal length?

Ironically enough, a 10 mm or 20 mm lens (on a full frame or crop sensor DSLR body) sort of is a telephoto lens, in the sense that it has be a retrofocus lens (in order to not intrude in the mirror ...
scottbb's user avatar
  • 32.8k
1 vote

Why do telephoto lenses have long focal length?

Put simply, every time you bend light it reduces the potential IQ; every lens element, aperture restriction, etc... there are definite negatives to making a lens more complicated than necessary; ...
Steven Kersting's user avatar
1 vote

Why do telephoto lenses have long focal length?

Law of physics In the image above, assume that the "object" is sufficiently far way from the camera so that the distance between the sensor plane where the "Image" is projected ...
xenoid's user avatar
  • 21.4k
1 vote

Why do telephoto lenses have long focal length?

Let us say we want a focal length of 10mm. In order to be "shorter than its focal length", our lens design needs to be shorter than 10mm and still be large enough to illuminate our sensor. ...
user24582's user avatar
  • 781
1 vote

Why is aperture expressed as a function of focal length and not in absolute size?

Because that way you don't have to keep doing math for every lens you use. A given aperture diameter lets in different amounts of light for different focal lengths. f/2 (where f is focal_length) lets ...
inkista's user avatar
  • 52.2k
1 vote

Why is aperture expressed as a function of focal length and not in absolute size?

My explanation in simpler terms. Because it is actually a relationship, not an absolute size. Any division is a relationship between the top number and the bottom one. When defining the aperture we ...
Rafael's user avatar
  • 24.9k
1 vote

120mm with 35cm minimum focus distance for Macro?

Magnification (how does size of image compare with real life size) M = lens to image distance divided by lens to object distance. All values in millimeters M = 120 / 350 = 0.3. This tells us that the ...
Alan Marcus's user avatar
  • 39.2k
1 vote
Accepted

Determining the size of a pixel at a given distance

If you know the focal length, find out the field of view from https://www.nikonians.org/reviews/fov-tables then it's just trigonometry: pixel size = 2 * tan ( horiz.fov /2 ) * distance / image width ...
FarO's user avatar
  • 1,329
1 vote

Determining the size of a pixel at a given distance

How accurate do you expect/need to be? IMHO between the inaccuracy of the focal length (especially since you are not focused to infinity) and your lens correction, the GIGO principle says that you ...
xenoid's user avatar
  • 21.4k
1 vote

What is the use of focal length in a Fisheye lens?

I know this is a late answer but I have been looking at the TTArtisan 11mm fisheye for Nikon-F FX. The DSLR version of this lense is actually a different optical formula than the mirrorless version. ...
Ethan Queen's user avatar
1 vote

What is the use of focal length in a Fisheye lens?

A few years ago, I had a similar curiosity and this is my takeaway…which may or may not be entirely accurate. “Fisheye” is a term of art. It does not have a concrete technical meaning. “Fisheye” ...
Bob Macaroni McStevens's user avatar
1 vote

Image metadata contains Focal length and calibrated focal length but the values are different

The focal length is 8.8mm. Often this value is expressed in terms of the equivalent focal length lens that delivers the same angle of view when mounted on a 35mm full frame camera format 24mm height ...
Alan Marcus's user avatar
  • 39.2k

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