# Tag Info

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### How do I calculate the “effective focal length” of a cropped photo?

It's pretty much strictly linear unless you're talking about very close focusing distances or macro distances. For everything else, what little you may be off is probably less than the rounding error ...
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### Why doesn't the focus distance at which my lens has magnification 1 match the formula?

The equation assumes a simple single element lens that is bilaterally symmetrical. The camera lens, to mitigate the 7 major aberrations (shortcomings that degrade) is constructed using several ...

### How to estimate pixels per degree for an image from a camera phone using EXIF data?

I assume this is a still frame from a video, because 16:9 is a common video ratio. In order to maintain constant frame size, "digital zooming" needs to be reinterpolated. Thus, yes, you need to ...
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### Is it exactly true that doubling the focal length makes everything look twice as big?

Your intuition is right. To validate it, we can dig into basic high-school geometry. Although a camera lens is actually a complex lens made from many elements, conceptually and mathematically for ...

### Which SS/F-stop table is the correct one?

The quote from the book does not state that the numbers are intended for any specific lighting situation, so it appears to be indeed only concerned with showing how aperture and shutter speed can vary ...

### What is the power output from two speedlites combined?

Doubling the number of lights at the same power doubles the output. Assuming both speedlights are the same to begin with, and if they're both set to ¹⁄₆₄th power, that'll be like setting just one of ...

### Why doesn't the focus distance at which my lens has magnification 1 match the formula?

first of all, kudos on your effort to break a photography problem down to first principles. The discrepancy you've observed stems from a common oversimplification. Your 100mm Lens is actually what ...

### Shutter Speed from the Exif ShutterSpeedValue

Exif:ShutterSpeedValue is stored in rational form as an APEX value. The APEX system is a way to calculate exposure and works using base-2 logarithms. The use of base-2 means a rise of one in the ...
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### Why I am getting different values for depth of field from calculators vs in-camera DoF preview?

First, a word about what depth-of-field is and is not: In a way, depth-of-field is an illusion. There is only one plane of focus. Everything in front of or behind the point of focus is out of ...
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### Can I increase 35mm-equivalent focal length with both the crop factor and an extender?

Well, the main thing is that crop factor doesn't really affect focal length. It just affects the field of view by making it narrower. So, what you really have is a 400x1.4x => 560mm lens combination ...

### How to calculate the optimal pinhole size?

The earliest investigation of the right constant c that I am aware of is Petzval's 1859 On the camera obscura. The paper is predominantly about a new lens design he has created but he begins with an ...
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### Which SS/F-stop table is the correct one?

Like Michael says, the top chart is just hypothetical, and it only uses some random exposure, and is NOT about any specific exposure condition. Instead, it tries to show two concepts. It is an ...

### Why I am getting different values for depth of field from calculators vs in-camera DoF preview?

What am I missing? You're missing that Depth of Field is subjective. In actuality, there's only one plane that's in focus — everything else can't be. Then, there's an area around that plane that is ...

### How do I calculate the “effective focal length” of a cropped photo?

Short answer: if you halve the width, you have (effectively) doubled the focal length.
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### How do I calculate the coordinates for secondary colors on a CIE chromacity diagram?

Performing the operation directly using the chromaticity coordinates (ie: taking the midpoint between the two chromaticity coordinates) will yield incorrect results as the chromaticity diagram is ...
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### How do I correctly convert from aperture diameter into f-stops?

Sensor dimensions don't matter. F-stop is shorthand for "fractional" and what it's a fraction of is the lens focal length and the diameter of the iris and the real calculation is simply the lens ...

### Is it exactly true that doubling the focal length makes everything look twice as big?

It's not exactly true, but it's a first approximation and easily good enough for many practical purposes. Let's consider an ideal thin lens. We're already introducing one level of errors, because ...
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### How to calculate stops of light lost from a teleconverter?

Although very little (single digit percentages) light is lost to a teleconverter, the ratio of entrance pupil to focal length grows proportional to the magnification of the telecon. For example, if a ...

### What is 'sensor height?'

Sensor height is indeed the physical measurement of the vertical dimension of the sensor's active pixel area. From Wikipedia's Image sensor format article, the phone's sensor format of 1/3" (actually ...
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### Why isn't my computation of the actual size of an object in a photograph matching the expected value?

I think that you have, in fact, successfully answered this inverse question: How to test actual focal length?. It's not just infinity focus, but that lens focal lengths are generally rounded to ...

### Can I use my camera to measure lens light transmission?

In a nutshell: no, you cannot use your camera to accurately measure light transmission. The problem is with the way your ISO control works on a digital camera. ISO is not a generic "amplify signal by ...

### How do I correctly convert from aperture diameter into f-stops?

F-number equals the focal length of the lens divided by the diameter of the entrance pupil. Since both measurements are linear dimensions when the same units of measurement are used for both the focal ...
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### How to calculate the new focal length when usng a lens mount adapter

Lens mount adapters increase the pool of available lenses we are able to mount on any particular camera body. Adapters are basically mechanical coupling devices; however, some also couple electrical ...

### Is the hyperfocal distance calculation outdated?

Hyperfocal distance is a specific application of the concept of depth of field. There's only one distance that is in sharpest focus. Everything in front of or behind that distance is blurry. The ...

### How can I calculate focal length from a photograph?

I appreciate this is an old post, but I'd like to share a tool which does all of this for you and more, in case someone else comes across this question like I did. fSpy is a free tool allowing you to ...

### Is there a general formula for image size vs. print size?

The big problem you have here is that your aspect ratio is wrong. A0 (or any other A size paper) has an aspect ratio of 1:sqrt(2) or 1:1.414 or so. Your image has an aspect ratio of 2048/1152 = 1.777. ...

### Is there a formula for calculating exposure when enlarging prints?

The amount of light required to achieve the same exposure is proportional to the print area. That is the same as saying it goes with the square of the linear dimension. This is a approximation that ...

### White balance math for white, black, and grey color cards

Why are you trying to match 3 values with a linear relation? Consider only the red channel. You have 3 reference values (taken from your original photo): Rw for the white color, Rg for the gray and ...