# Questions tagged [focal-length]

Focal length is the distance (measured in mm) between the surface of a lens and the focal point, when the lens is focussed at infinity. Smaller focal lengths indicate wide-angle lenses, larger focal lengths indicate a telephoto lens.

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### Thin lens focal length vs perspective

Thin lens and perspective It's well regarded focal length of a camera lens does not affect perspective. While by using the thin lens equation $$\frac{1}{s}+\frac{1}{i}=\frac{1}{f}$$ we can compute the ...
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### Sensor size of Zed 2i stereo camera

I am trying to determine the sensor size in the Zed 2i stereo camera. Based on the datasheet, which can be found here, it looks like each pixel is 2um x 2um and the sensor is 2688 x 1520 pixels. That ...
1 vote
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### Crop an image taken in a known focal length, to get it in other focal length

Say that i have an image of 2560 x 1920, that is taken with a focal length of 2.8mm. Is it possible to crop the image so that it corresponds to another focal length of say 94mm, and if so, how do i ...
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### Measure the area of an object in an image by using fx and fy and given distance (distance between camera and object pixel-wise)

I want to measure the area of an object in an image. I have fx and fy of the camera and also since I am using depth camera I have the distance between camera and the object pixel-wise. Could you ...
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### Using the lens equation to make a camera out of an old projector lens

Given a 14" (~355mm) projector lens, does the film plane go a little more than 14" behind the center of the lens? I'm puzzled by the use of the canonical lens equation for long object ...
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### Does a mirrorless camera need a shorter focal length for the same FoV as an SLR?

Let's say we have a 50mm lens on a full-frame reflex camera. As mirrorless cameras have a smaller distance to the sensor, does it mean we need a shorter focal length for the same sensor size to have ...
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### Why do telephoto lenses have long focal length?

Considering the definition that a telephoto lens is one whose length is shorter than its focal length: Why do telephoto lenses (tend to) have long focal length? Why not a telephoto lens with a short ...
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### Why is aperture expressed as a function of focal length and not in absolute size?

Why is aperture expressed as a function of focal length (eg. f/2, f/4), and not in absolute size, say mm (eg. 40mm)?
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### 120mm with 35cm minimum focus distance for Macro? [closed]

Is 120mm focal length with 35cm minimum focus distance good/close enough for macro insect photography?
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### DOF formula vs the simplified DOF formula

I was calculating the theoretical difference in the change of the DOF (depth of field), between a frame achieved on a 25mm signature prime and an "identical frame" achieved by using a 75mm ...
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### Is this effect because of the lenses? what type?

https://www.instagram.com/p/CwsKDthOdST So in this video, the legs of the man look really small. I thought at first it must be because of the lenses but if it were for wide angle lenses, that would ...
1 vote
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### Is this diagram I've drawn in regards to Depth of field correct?

This diagram is modelled after a video I saw on depth of field formula and its derivation. I have now learnt a bit more regarding Depth of focus and how it refers to "the tolerance of placement ...
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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?

After having spent some time reading about DOF and the derivation of its formula, I tried calculating the theoretical DOF of different lenses. Now to my knowledge, a camera using a lens with a focal ...
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### What is the use of focal length in a Fisheye lens?

This is similar to: What does the focal length of a fisheye lens mean? What is the relationship between field of view and focal length for fisheye? But actually, I have a different question. I have ...
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### Image metadata contains Focal length and calibrated focal length but the values are different

I have a DJI drone image. When I use gdalinfo -mdd all myphoto.jpg at the command line I can read the EXIF and XMP tags for the image. I see there are two values ...
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### Focal length reducer on FF camera (24mm F2)

Lens: Zuiko OM 24mm F2, Camera: Sony A7S2 What would happen if a focal length reducer (0.71x) is used to adapt a 24mm F2 FF lens, into a FF camera? The resulting field of view should be ~17mm with ...
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### Convert Zoom % to Focal length

My Panasonic FZ70 zooms to 60x. The only focal length indicators on the lens are the minimum (20mm) and the maximum (1200mm). There is no indication of focal length as I zoom in or out; the ...
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### Determining the size of a pixel at a given distance

I have a camera looking at a flat plane that is a known distance away from it. I also know the cameras intrinsic parameters. The image from the camera looks like this: (it's grayscale but that shouldn'...
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### Does autofocus in iPhone changes focal length? [duplicate]

I was wondering how auto-focus works in mobile cameras. I understood that, mobile phones uses either 'contrast based' or 'phase-detection based' algorithms to find the optimum length position for ...
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### What focal length is best for photography of rugs?

What focal length is best for photography of rugs? I am choosing between a Sony 16-50mm lens and a 50-210mm lens. Rugs size small and medium sized 2x3m Notes :i have 2 only lenses size 16:50 mm and 50:...
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### How to calculate the optimal distance to photograph details for mobile phone lenses?

Most of the mobile phones I've owned have a primary, wide angle camera lens somewhere around 28mm. If I wanted to capture the most details/sharpness (the most pixels) and ignored perspective and ...
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### What is the definition of focal length for a camera lens?

I came across many articles where the focal length of a camera lens is defined as the distance between the optical centre of the lens and the sensor. One such article is here: https://photographylife....
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### I want to measure far distance focus in a short space, but give myself a greater FOV, how would I do that?

Building off of this question, say I put a 1 m lens in front of a camera so I can see a virtual image at 500 m to focus to, when I place an object within the focal length (0.998 m), but I want a ...
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### Does "Field of view" include radial lens distortion?

When manufacturers state the "field of view" of a camera, is that including lens distortion or not? I work a lot with CCTV and dashcam footage which always has barrel distortion. For a given ...
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### Why is the commonly utilized aperture for moon photography much higher than that provided by DoF formula?

in this Moon Photography Guide, a F-Number equal to f/11.0 is suggested. In many answers in this forum, similar values are suggested (ranging from f/8.0 to f/11.0). Even if someone suggests f/5.6, it ...
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### Clarifications about the Reproduction Ratio Formula

The reproduction ratio formula is: ...
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### What does pixel width mean in the context of calculating camera placement distance?

I am setting up cameras for an experiment filming animals. In a experimental protocol paper that I'm following, it states that: Based on a pinhole camera model, the bound x_min on the pixel span of ...
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### Equivalent lens to periscope camera?

I am trying to figure out an equivalent telephoto lens to the newer periscope cameras that report focal lengths in 35 mm equivalents. There are some similar threads such as How do I convert lens focal ...
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### Are most (if not all) lenses' focal length described as the 35mm equivalent? [duplicate]

2 thoughts made me ask this question: Different lens looks different on the different-sized sensor and has a different 35mm equivalent Many normal and wide-angle lenses' optical center is not ...
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### What does the focal length of a fisheye lens mean?

Say an ordinary lens, Sigma 30mm f1.4 for example, is designed for APS-C and thus produces images that have a 35mm-equivalence of 48mm. But what about the fisheye ...
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### How to calculate extension tube length when the focal length is unknown

A 16mm objective has a minimum focus distance of 1m. When an extension tube is used to decrease the minimum distance to 0.1m, how long is this extension tube? If b_1...
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### Capturing License Plates From 50-100 ft (And Night)?

I'm attempting to build something with a Raspberry Pi that can hopefully capture moving vehicle's license plates 50-100 ft away (and at night). I understand very little about photography, but through ...
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### What are the equivalent focal lengths for iPhone digital zoom?

I would like to know what the equivalent focal lengths are when in digital zoom on an iPhone 13 Max Pro – greater than 3X to 15X.
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### Focal length, zoom-x, magnification, and their relationship to image size projected on sensor

I have been doing remedial research on zoom and magnification. Here are some of my conclusions ("sensor" here assumes a digital camera): The zoom-x number of a zoom lens is the maximum ...
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### Is it possible to calculate field of view and aperture given focal length, k1 and k2?

I have some data coming from an API that looks like this: "camera_parameters": [0.785028, 0.032519, -0.053622] and the data structure that I would like to ...
1 vote
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### Does the focal length on cropped lenses include a crop factor? [duplicate]

I'd like to be certain about the focal length written on the cropped lenses, e.g. I have a Sony NEX-5N with the original OSS 18-55mm kit lens. Is it actually an 18 to 55mm focal length with no "...
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### Does focal length correlate to the required ND filter in sun photography?

I got my hands on an M42 500mm lens and want to try sun photography. I also have a 2x and a 3x converter, thus I'm able to get a focal length range of 500-3000mm. Back in the day I bought a Baader ...
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### What is the relation between aperture number and exposure, keeping all other parameters fixed?

My language is German, so forgive my lack in English words, especially with regard to photography. I tried to understand at a very basic level the relation between aperture number and exposure and ...
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### Motion tracking/VFX: How to get needed info for iPhone camera?

Basically, I want to do VFX with my iPhone SE 2nd Generation front-facing camera. The software being used is Blender up-to-date. I'm not sure what to set Blender for in the track camera settings field,...
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### Distance marking on a lens - flange distance or film plane?

Some lenses have distance/ hyperfocal markings - is that with reference to the flange (outer mount point of the lens where it meets the camera body) or the film/sensor plane?
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### Can I use a 127.5mm focal length on a crop camera to see what the view would be like using an 85mm full frame lens?

I have a Nikon D7500 which is a DX format camera. I am thinking of getting a Nikon 85mm lens but I only see one that is in FX format. I have a 70-300 lens in DX format and was wondering if I set the ...
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### APS-C + speedbooster vs full frame

I own a Nikon in DX-format (Nikon name for APS-C sensor, crop factor ~1.5) and I'm considering buying a 50mm prime lens soon for it. (To be complete, my camera is a D5200, although I don't think the ...
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### Is maximum focal length of P1000 similar to 300mm kit lens provided with APS-C cameras?

Is maximum focal length of P1000 similar to 300mm kit lens provided with APS-C cameras? Either in terms of 35mm or 1/2.3(sensor size found in P1000) format.
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### How can I get more magnification when shooting wading birds?

I have a Canon EF 70-300mm 1:4-5.6 IS USM lens. Will all 300mm zoom/telephoto lenses give the same magnification of the subject as mine does or, to get more magnification do I need to get a zoom or ...
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### How to determine via a math formula when a lens is wide angle or macro

Lets say I have a device with 3 separate lenses. One will be the main reference lens, and then the two others may be a wide angle lens or a macro lens, or also even a replica of the main reference ...
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### Is x50 zoom camera able to zoom 500 meter target area from a 6 km distance?

I have a x50 zoom camera and here is some specs of the camera and lens: Focal length: 6 mm ~ 300 mm Field of view: 58.4° ~ 1.4° Resolution: 2 MP I drew a scheme to explain my question properly, you ...
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### matching field of view between apsc and full frame

I'm sure this is a dumb question. I'm upgrading from a sony a6 aps-c to a sony a7Rii full frame and trying to determine which focal length of FE primes I want. I'm using my aps-c a6 as a reference to ...
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### Focal length for a specific distance?

I have the following information: s: Image sensors dimensions: 6.00mm, 4.80mm, 3.60mm (diagonal, horizontal, and vertical) f: ...
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