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13 votes
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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 ...
Michael C's user avatar
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13 votes
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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 ...
Alan Marcus's user avatar
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12 votes

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 ...
scottbb's user avatar
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11 votes
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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 ...
mattdm's user avatar
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10 votes

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 ...
mattdm's user avatar
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9 votes

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 ...
PhotoScientist's user avatar
9 votes

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 ...
xiota's user avatar
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8 votes
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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 ...
inkista's user avatar
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8 votes

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 ...
brett stevens's user avatar
7 votes

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.
osullic's user avatar
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6 votes
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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 ...
Patrick Hughes's user avatar
6 votes

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 ...
Peter Taylor's user avatar
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6 votes
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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 ...
PhotoScientist's user avatar
6 votes

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 ...
scottbb's user avatar
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5 votes

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 ...
Michael C's user avatar
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5 votes
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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 ...
Alan Marcus's user avatar
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5 votes

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 ...
Michael C's user avatar
  • 175k
4 votes

calculating aperture and shutter speed inversion

The best way to work with f/numbers is to memorize the f-number set. The number set for the f/stops: 1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22 – 32 Each number going right is its neighbor on the ...
Alan Marcus's user avatar
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4 votes

More megapixels to compensate for small zoom (Or: How good is Sony's Smart Zoom?)

However, it costs much more than I was planning to pay. As the size of the sensor increases the cost of the lens needed to provide an equivalent picture (same field of view, or depth of field, or ...
Michael C's user avatar
  • 175k
4 votes

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 ...
oculometric's user avatar
4 votes

How do I get the right size image for a specific small photo frame?

It's easy: you take the desired DPI (dots per inch) and multiply it with the frame size. Say, your photolab (or printer) prints with 300 dpi. Then your picture must be at least 2.5*300 x 3.5*300 = ...
Zenit's user avatar
  • 1,791
4 votes
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More megapixels to compensate for small zoom (Or: How good is Sony's Smart Zoom?)

Cropping and zooming are basically equivalent, so this is easily calculated. Zooming in 2× — like, focal length 36mm to 72mm — is like cutting the frame in half. Of course, that's each dimension, so ...
mattdm's user avatar
  • 143k
4 votes

What is 'sensor height?'

I find that the diagonal measure of this sensor image is 5.867mm. From specification I can't find, the height and length. However, based on the height to length ratio 1920 pixel length by 1080 high I ...
Alan Marcus's user avatar
  • 39.3k
4 votes

How can I use a non-TTL lightmeter beyond its built-in ISO range?

Yes, you can definitely use set the light meter at 400 and figure out what the exposure has to be at 800. It's a one stop difference so if it's saying f/5.6, set your aperture to f/8. Most old lenses ...
carloandaya's user avatar
4 votes

Does field of view change for different lens model for the same focal length?

There are at least two issues involved here: Focal lengths, particularly with zoom lenses, are approximated to the nearest "standard" number. You'll almost never see a lens marked as a 242mm or 256mm ...
Michael C's user avatar
  • 175k
4 votes

What is the formula to calculate aperture ("diaphragme" in French)?

The equation is: $$ {N^2\over t} = \frac{E \times S}{C} $$ for \$N=\$ f-number; \$t=\$exposure time (in seconds); \$E=\$ illuminance; \$S=\$ ISO value; and \$C=\$ incident-light meter calibration ...
mattdm's user avatar
  • 143k
4 votes
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Shutter Speed from the Exif ShutterSpeedValue

What camera is that with this Exif format? I recognize the math, but not the values. The regular math is this way: If you simply number the f/stops starting at f/1 = stop number 0, f/1.4 = stop ...
WayneF's user avatar
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4 votes
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Is the hyperfocal distance calculation outdated?

In short: There are two possible approaches to the sharpness in digital photography: getting the best possible sharpness given the sensor adequate sharpness when the image is viewed The first ...
hpchavaz's user avatar
  • 367
4 votes
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Is x50 zoom camera able to zoom 500 meter target area from a 6 km distance?

By calculation angle (which is to the camera) of this triangle is around 4.7 degree. So on one of the length of zoom you will be able to fill the frame with this object. But the quality on such ...
Romeo Ninov's user avatar
  • 12.2k
4 votes

How to determine via a math formula when a lens is wide angle or macro

For valid reason, which I will explain later, the “normal” focal length for any size camera is a focal length that is approximately the same length as the diagonal measure of its format size. ...
Alan Marcus's user avatar
  • 39.3k

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