cropping picture - simulating longer focal length

[razorre]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

for example, this pic

https://www.dpreview.com/forums/post/37729719

 

[CubeAce]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

for example, this pic

https://www.dpreview.com/forums/post/37729719

Try this.

http://imaging.nikon.com/lineup/lens/simulator/

 

[dsjtecserv]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

Using the same camera with a longer lens will always give better image quality than cropping. However, if vignetting is your concern, you'll need to study the vignetting characteristics of the lens you are considering. All lenses vignette to some degree, though with a good lens it can imperceptibly small.

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

Divide the smaller focal length by the larger. This will give you a crop factor. In your example, 25/35 = about 0.714.

The crop factor is the percentage of the original image, in each dimension (horizontal and vertical) that you want to remain after cropping. Alternatively, subtract the crop factor from 1 to give the percentage of each dimension that you want to crop off. Example 1 - 0.714 = 0.286; 28.6%.

Using either the physical dimension of the pictures (inches or mm) or the pixel dimensions, crop to the dimensions determined above.

So if your 25 mm image has pixel dimensions of 3000 x 2000 px, you would crop it to 0.714 x 3000 = 2142 by 0.714 x 2000 = 1428 pixels to give the same field of view as a 35 mm lens used on the same camera.

for example, this pic

https://www.dpreview.com/forums/post/37729719

Dave

 

[razorre]

ok, thanks, that was what i was looking for. 25mm picture and added rectangle (71.4% - 35mm)

it's not just about quality, because both lenses are cheap (15£?) and give 'artistic' effects

 

[Max Iso]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

for example, this pic

https://www.dpreview.com/forums/post/37729719

Just remember that a longer FL will often work better. You won't lose pixel resolution, and if the F stop is the same, you won't lose as much light and your DOF control will be better.

 

[Great Bustard]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

A longer lens will give greater resolution and, if the exposure is the same, less noise.

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

for example, this pic

https://www.dpreview.com/forums/post/37729719

Let's say the original dimension of your photo is a x b pixels. Then to get the same framing from X mm as would have gotten with Y mm from the same position, you would:

• Crop the photo from a x b pixels to (a x b) · (X/Y)² pixels.

For example, let's say you took the photo at 25mm and want to crop it to the same framing as if you had taken the photo from the same position at 35mm. Then if your original photo was 6000 x 4000 pixels, you would:

• Crop the photo to (6000 x 4000) · (25/35)² = 3061 x 2041 pixels.

Alternatively, let's say you've cropped a photo taken at X mm, and you would like to know what focal length, Y mm, you should have used to get the same framing from the same position without cropping. Then if the uncropped photo is a x b pixels and the cropped photo is a' x b' pixels, the focal length you "should have" used is:

• X mm · sqrt (a' / a) for the same horizontal framing,
• X mm · sqrt (b' / b) for the same vertical framing, or
• X mm · sqrt (d' / d) where d = sqrt(a² + b²) and d' = sqrt(a'² + b'²) for the same diagonal framing.

For example, let's say you took a photo of a scene at 25mm that was 6000 x 4000 pixels and you cropped it to 4000 x 3000 pixels to get the framing you wanted. Then if you had taken the photo from the same position at:

• 25mm · sqrt (6000 / 4000) = 31mm, you'd have the same horizontal framing.
• 25mm · sqrt (4000 / 3000) = 29mm, you'd have the same vertical framing.
• 25mm · sqrt (7211 / 5000) = 30mm, you'd have the same diagonal framing.

Hope that helps!

 

[beatboxa]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

http://imaging.nikon.com/lineup/lens/simulator/

DOF simulator - Camera depth of field calculator with visual background blur and bokeh simulation.

Calculates camera depth of field and background blur and simulates it on a photo for any lens, camera and distance combination with different types of lens blur (bokeh).

dofsimulator.net

 

[razorre]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

A longer lens will give greater resolution and, if the exposure is the same, less noise.

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

for example, this pic

https://www.dpreview.com/forums/post/37729719

Let's say the original dimension of your photo is a x b pixels. Then to get the same framing from X mm as would have gotten with Y mm from the same position, you would:

• Crop the photo from a x b pixels to (a x b) · (X/Y)² pixels.

For example, let's say you took the photo at 25mm and want to crop it to the same framing as if you had taken the photo from the same position at 35mm. Then if your original photo was 6000 x 4000 pixels, you would:

• Crop the photo to (6000 x 4000) · (25/35)² = 3061 x 2041 pixels.

Alternatively, let's say you've cropped a photo taken at X mm, and you would like to know what focal length, Y mm, you should have used to get the same framing from the same position without cropping. Then if the uncropped photo is a x b pixels and the cropped photo is a' x b' pixels, the focal length you "should have" used is:

• X mm · sqrt (a' / a) for the same horizontal framing,
• X mm · sqrt (b' / b) for the same vertical framing, or
• X mm · sqrt (d' / d) where d = sqrt(a² + b²) and d' = sqrt(a'² + b'²) for the same diagonal framing.

For example, let's say you took a photo of a scene at 25mm that was 6000 x 4000 pixels and you cropped it to 4000 x 3000 pixels to get the framing you wanted. Then if you had taken the photo from the same position at:

• 25mm · sqrt (6000 / 4000) = 31mm, you'd have the same horizontal framing.
• 25mm · sqrt (4000 / 3000) = 29mm, you'd have the same vertical framing.
• 25mm · sqrt (7211 / 5000) = 30mm, you'd have the same diagonal framing.

Hope that helps!

ok, thanks, it looks like jump from 25mm to 35mm gives about 2 times 'more magnified' picture (on every dimension).

• Crop the photo to (6000 x 4000) · (25/35)² = 3061 x 2041 pixels.

different formula (and results) from proposed above

• Divide the smaller focal length by the larger. This will give you a crop factor. In your example, 25/35 = about 0.714.

 

[dsjtecserv]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

A longer lens will give greater resolution and, if the exposure is the same, less noise.

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

for example, this pic

https://www.dpreview.com/forums/post/37729719

Let's say the original dimension of your photo is a x b pixels. Then to get the same framing from X mm as would have gotten with Y mm from the same position, you would:

• Crop the photo from a x b pixels to (a x b) · (X/Y)² pixels.

For example, let's say you took the photo at 25mm and want to crop it to the same framing as if you had taken the photo from the same position at 35mm. Then if your original photo was 6000 x 4000 pixels, you would:

• Crop the photo to (6000 x 4000) · (25/35)² = 3061 x 2041 pixels.

This isn't correct. You are applying the change in area, to the linear dimensions of the sensor. The change in area is indeed (25/35)² = 0.5102. But by applying that to the linear dimensions and coming up with cropped pixel dimensions of 3061 x 2041, you have an area that is 0.2603 of the original, which is the same as (25/35)^4.

The correct approach is to apply the crop factor of 25/35 = 0.714 to each liner dimensions, giving cropped dimensions of 4284 x 2856. Multiplying those give an area that is 0.5102 of the original.

Dave

 

[razorre]

yes, this square looked strange (i thought that maybe it's about area), but i wasn't sure if relationship is linear...

Thanks for help !

 

[J A C S]

Divide the FL by the crop factor. For example, you shoot at 50mm and crop to 80% (in each dimension); then the equivalent FL is 62.5mm.

 

[D Cox]

Somebody should write a book called "Elementary Mathematics for Photographers". ;-)

 

[razorre]

Somebody should write a book called "Elementary Mathematics for Photographers". ;-)

haha, yeah, looks like it's easy. i wasn't sure if relationship is linear or not...

 

[gollywop]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

A longer lens will give greater resolution and, if the exposure is the same, less noise.

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

for example, this pic

https://www.dpreview.com/forums/post/37729719

Let's say the original dimension of your photo is a x b pixels. Then to get the same framing from X mm as would have gotten with Y mm from the same position, you would:

• Crop the photo from a x b pixels to (a x b) · (X/Y)² pixels.

For example, let's say you took the photo at 25mm and want to crop it to the same framing as if you had taken the photo from the same position at 35mm. Then if your original photo was 6000 x 4000 pixels, you would:

• Crop the photo to (6000 x 4000) · (25/35)² = 3061 x 2041 pixels.

This isn't correct. You are applying the change in area, to the linear dimensions of the sensor. The change in area is indeed (25/35)² = 0.5102. But by applying that to the linear dimensions and coming up with cropped pixel dimensions of 3061 x 2041, you have an area that is 0.2603 of the original, which is the same as (25/35)^4.

The correct approach is to apply the crop factor of 25/35 = 0.714 to each liner dimensions, giving cropped dimensions of 4284 x 2856. Multiplying those give an area that is 0.5102 of the original.

Yeah, you're right. But so is GB's original expression: (6000 x 4000) · (25/35)²

He then let the visuals of the associativity get the better of him.

It should have continued:

(6000 x 4000) · (25/35)² = (6000•25/35) x (4000•25/35) = 4286 x 2857

It often happens that those who are strong in math and big concepts are careless in arithmetic and everyday bother.

--
gollywop
I am not a moderator or an official of dpr. My views do not represent, or necessarily reflect, those of dpr.
http://g4.img-dpreview.com/D8A95C7DB3724EC094214B212FB1F2AF.jpg

 

[dsjtecserv]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

A longer lens will give greater resolution and, if the exposure is the same, less noise.

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

for example, this pic

https://www.dpreview.com/forums/post/37729719

Let's say the original dimension of your photo is a x b pixels. Then to get the same framing from X mm as would have gotten with Y mm from the same position, you would:

• Crop the photo from a x b pixels to (a x b) · (X/Y)² pixels.

For example, let's say you took the photo at 25mm and want to crop it to the same framing as if you had taken the photo from the same position at 35mm. Then if your original photo was 6000 x 4000 pixels, you would:

• Crop the photo to (6000 x 4000) · (25/35)² = 3061 x 2041 pixels.

This isn't correct. You are applying the change in area, to the linear dimensions of the sensor. The change in area is indeed (25/35)² = 0.5102. But by applying that to the linear dimensions and coming up with cropped pixel dimensions of 3061 x 2041, you have an area that is 0.2603 of the original, which is the same as (25/35)^4.

The correct approach is to apply the crop factor of 25/35 = 0.714 to each liner dimensions, giving cropped dimensions of 4284 x 2856. Multiplying those give an area that is 0.5102 of the original.

Yeah, you're right. But so is GB's original expression: (6000 x 4000) · (25/35)²

He then let the visuals of the associativity get the better of him.

It should have continued:

(6000 x 4000) · (25/35)² = (6000•25/35) x (4000•25/35) = 4286 x 2857

Yup. As I did in the second response to the OP, way back up top! :-D

It often happens that those who are strong in math and big concepts are careless in arithmetic and everyday bother.

Exactly. He is probably too much in the habit of explaining sensor area to Alfred E. Neuman.

Dave

 

[gollywop]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

A longer lens will give greater resolution and, if the exposure is the same, less noise.

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

for example, this pic

https://www.dpreview.com/forums/post/37729719

Let's say the original dimension of your photo is a x b pixels. Then to get the same framing from X mm as would have gotten with Y mm from the same position, you would:

• Crop the photo from a x b pixels to (a x b) · (X/Y)² pixels.

For example, let's say you took the photo at 25mm and want to crop it to the same framing as if you had taken the photo from the same position at 35mm. Then if your original photo was 6000 x 4000 pixels, you would:

• Crop the photo to (6000 x 4000) · (25/35)² = 3061 x 2041 pixels.

This isn't correct. You are applying the change in area, to the linear dimensions of the sensor. The change in area is indeed (25/35)² = 0.5102. But by applying that to the linear dimensions and coming up with cropped pixel dimensions of 3061 x 2041, you have an area that is 0.2603 of the original, which is the same as (25/35)^4.

The correct approach is to apply the crop factor of 25/35 = 0.714 to each liner dimensions, giving cropped dimensions of 4284 x 2856. Multiplying those give an area that is 0.5102 of the original.

Yeah, you're right. But so is GB's original expression: (6000 x 4000) · (25/35)²

He then let the visuals of the associativity get the better of him.

It should have continued:

(6000 x 4000) · (25/35)² = (6000•25/35) x (4000•25/35) = 4286 x 2857

Yup. As I did in the second response to the OP, way back up top! :-D

Yep.

It often happens that those who are strong in math and big concepts are careless in arithmetic and everyday bother.

Exactly. He is probably too much in the habit of explaining sensor area to Alfred E. Neuman.

If there were anyone who could, it would be he. But it appears there are a number on these fora who would do well to do as well as Alfred. Alfred is lucky to have the likes of GB around.

--
gollywop
I am not a moderator or an official of dpr. My views do not represent, or necessarily reflect, those of dpr.
http://g4.img-dpreview.com/D8A95C7DB3724EC094214B212FB1F2AF.jpg

 

[Great Bustard]

Hi

Do you know formula to calculate (roughtly) longer focal length from cropped picture ?

I have some pictures and i want to crop them (because of vigneting), but maybe instead of cropping, i could buy longer lens (which works better)

A longer lens will give greater resolution and, if the exposure is the same, less noise.

If i have a picture from 25mm lens, how much picture should i crop (in percentage?) to get 35mm lens equivalent ? (i want to experiment and check the results).

for example, this pic

https://www.dpreview.com/forums/post/37729719

Let's say the original dimension of your photo is a x b pixels. Then to get the same framing from X mm as would have gotten with Y mm from the same position, you would:

• Crop the photo from a x b pixels to (a x b) · (X/Y)² pixels.

For example, let's say you took the photo at 25mm and want to crop it to the same framing as if you had taken the photo from the same position at 35mm. Then if your original photo was 6000 x 4000 pixels, you would:

• Crop the photo to (6000 x 4000) · (25/35)² = 3061 x 2041 pixels.

This isn't correct. You are applying the change in area, to the linear dimensions of the sensor. The change in area is indeed (25/35)² = 0.5102. But by applying that to the linear dimensions and coming up with cropped pixel dimensions of 3061 x 2041, you have an area that is 0.2603 of the original, which is the same as (25/35)^4.

The correct approach is to apply the crop factor of 25/35 = 0.714 to each liner dimensions, giving cropped dimensions of 4284 x 2856. Multiplying those give an area that is 0.5102 of the original.

Right you are! That was more than a little embarrassing! But thanks for pointing it out!

 

[Gerry Winterbourne]

Do you know formula to calculate (roughly) longer focal length from cropped picture ?

I have some pictures and I want to crop them (because of vignetting), but maybe instead of cropping, I could buy longer lens (which works better)

If i have a picture from 25mm lens, how much picture should I crop (in percentage?) to get 35mm lens equivalent ? (I want to experiment and check the results).

As you say in a later post "haha, yeah, looks like it's easy. I wasn't sure if relationship is linear or not..."

One of the reasons we use focal lengths to describe lens, rather than angle of view, is exactly that: the width (and/or height) of the picture is directly, linearly proportional to focal length. Angle of view is a trigonometrical function (tangent) and is nonlinear.

For your example the width/height ratio id 25/35 - you need to crop to 25 35ths of whatever dimensions you are working in (pixels, inches, cm).

You'll lose quality as you crop, but that's unlikely to matter unless you view at large sizes. Very roughly, it's usually safe to crop by a factor of about 1/2 (that's 25mm to 50mm in your example) for an ordinary kit type of lens, or by about 1/3 (25mm to 75mm) for a good quality lens.