# 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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### 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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### 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 ...
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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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### Pixel diameter of a given field of view calculation equisolid (equal-area) fisheye lens

I have a 180° equisolid (equal-area) fisheye lens with a focal length of 8 mm. I want to crop images to a circle defined by an angular field of view from the zenith angle of the lens (i.e. the middle ...
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### Calculation of the heigth of the camera

I am trying to find the height of a drone using the camera only. I can calculate the distance of the object top and bottom. Object is a square platform with width=0.2m. I used the equation provided by ...
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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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### How big is the sun in an image made with Canon APS-C and 300mm focal length?

I am trying to decide what lens should I use for eclipse and I am wondering if my 70-300 IS USM would be enough or should I get (rent) a lens with higher magnification. Since my solar filters are ...
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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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### 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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### 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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### Why does getting closer or zooming in decrease depth of field?

I know that aperture changes the depth of field (DOF), but why is it that by making the image appear larger (by getting closer to the camera or zooming in) there is less DOF?
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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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### 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 ...
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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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### 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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### Calculation for getting dimension of object in image not working

I am trying to calculate the dimensions of an object in an image. This is a sample image I took to get height of blue shirt: I am following the math from this page with sastanin's answer to How do I ...
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### How do I calculate the distance of an object in a photo?

If I took a picture of a windmill on the horizon — given that I know the sensor size and the focal length of the lens and other factors to do with the shot — could I calculate how far away an object ...
1 vote
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### Bad photos (no focus). What are these photos called?

I'm creating an open Neural Network that could identify if the photo is blurred. This way I need to find many photos of objects at close range, and the focus should be upset. The same as you can see ...
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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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### How to calculate Sensor Pixel Pitch?

I need to compute the “Sensor Pixel Pitch (in mm/px)” for some calculation. I have the following data available from the manufacturer: ...
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### Do bigger focal lengths capture more light?

I never understood what the f really stands for in the f-stop values, like f/1.8. Wikipedia explains it in various articles, but I still find it very confusing. What I understand, though, is that it ...
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### APS-C crop factor 1.5x or 1.53x

I'm having issues wrapping my head around how to get crop factor based on 35mm. I understand what crop factor is but my math is confusing me. For example, everyone knows that aps-c is a 1.5x crop ...
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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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### How can I explain focal length to someone who isn't into photography?

Context So my friends and family are curious about how far my lens can reach. I usually explain with focal length and show them a few pictures. Question Is there any easier way to explain what ...
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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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### 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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### 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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### 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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### Clarifications about the Reproduction Ratio Formula

The reproduction ratio formula is: ...
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### How to capture the scene exactly as my eyes can see?

What settings of my DSLR camera will emulate the scene exactly as I can see through my naked eyes? I think it's not possible to get the contrast and colors exactly as my eyes can see, and it may vary ...