Cropping and fov

[Kiwimac]

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This is not specific to Leica although in my case it is.

If I shoot a 60MP file with a 50mm lens and crop the centre 50%, is there an easy way to find out what focal length that equates to in fov terms?

I am sure it is not as easy as adding 50% to the original 50mm - or is it?!

[williamj]

4 hours ago, Kiwimac said:

This is not specific to Leica although in my case it is.

If I shoot a 60MP file with a 50mm lens and crop the centre 50%, is there an easy way to find out what focal length that equates to in fov terms?

I am sure it is not as easy as adding 50% to the original 50mm - or is it?!

Because it is a two dimensional measure you have to use squares and square roots, but you can get a reasonable approximation as follows.

Your lens is 50 mm so divide 50 by square root of 0.5 because you are looking at half the 'area', square root 0.5 is approximately 0.7 and so the result is approximately 71 mm or a 71 mm lens. Going the other way, twice the area would mean dividing 50 mm by inverse of square root of 0.5 or the square root of 2, which is approximately 1.4, and dividing by that gives a result of 35 mm. A 35 mm lens covers about twice the area of a 50 mm lens.

Obviously, this crop may not look exactly like the result you get with a 71 mm lens due to the differences in compression.

These are approximates :). Cheers.

Edited January 22, 2024 by williamj

[dpitt]

41 minutes ago, williamj said:

Because it is a two dimensional measure you have to use squares and square roots, but you can get a reasonable approximation as follows.

Your lens is 50 mm so divide 50 by square root of 0.5 because you are looking at half the 'area', square root 0.5 is approximately 0.7 and so the result is approximately 71 mm or a 71 mm lens. Going the other way, twice the area would mean dividing 50 mm by inverse of square root of 0.5 or the square root of 2, which is approximately 1.4, and dividing by that gives a result of 35 mm. A 35 mm lens covers about twice the area of a 50 mm lens.

Obviously, this crop may not look exactly like the result you get with a 71 mm lens due to the differences in compression.

These are approximates :). Cheers.

Correct if you want see what MP count you get.

It is much simpeler if you take one dimension only. The width and height of your picture at the same distance is a linear relation to the crop factor.
 

So a picture taken with a 50 mm lens will yield the same FOV as a 100mm lens when you crop so that the width and height is half of the original. This results in 60/2/2 = 15 MP in total.

It is easy to use this rule if you want to guess what the ideal FL would be to take a shot. If a 50mm lens only gets half the building in the picture then you need a 28mm or 25 mm to get the job done... Of course stitching several shots can do the job sometimes.

Edited January 22, 2024 by dpitt

[Greenhilltony]

4 hours ago, williamj said:

Because it is a two dimensional measure you have to use squares and square roots, but you can get a reasonable approximation as follows.

Your lens is 50 mm so divide 50 by square root of 0.5 because you are looking at half the 'area', square root 0.5 is approximately 0.7 and so the result is approximately 71 mm or a 71 mm lens. Going the other way, twice the area would mean dividing 50 mm by inverse of square root of 0.5 or the square root of 2, which is approximately 1.4, and dividing by that gives a result of 35 mm. A 35 mm lens covers about twice the area of a 50 mm lens.

Obviously, this crop may not look exactly like the result you get with a 71 mm lens due to the differences in compression.

These are approximates :). Cheers.

The so-called compression is often a wrong mixing of perspective and DOF(bokeh). When the distance to subject is fixed, the perspective of any lens is the same–referring to the relative size comparison for the front ground and background. The closer you are to the subject, the larger the size difference between it and the subjects in distance, and vice versa. The frequently mentioned "compression" from tele-lenses, is due to you have to use them in a much longer distance to the subject, say, 10-20m, which reduces the size comparison between the foreground subjects and background subjects.

The composition, or say, the relative size of the subject in the final image, with fixed distance, is dependent on the FOV. FOV is the result of considering both the size of the sensor/image area and the focal length of the lens, and that's why we usually use the "135 format equivalent focal length" to discuss the focal lengths and usages of lenses in APS-C, full frame, middle formats, large formats, etc. The lens with longer focal length physically has smaller DOF and hence the background blurring is stronger, for example, when you compare the shots between 50mm + 50% cropped and 71mm lens taken in the same subject distance and on the same size of sensor/film.

[williamj]

50 minutes ago, Greenhilltony said:

The so-called compression is often a wrong mixing of perspective and DOF(bokeh). When the distance to subject is fixed, the perspective of any lens is the same–referring to the relative size comparison for the front ground and background. The closer you are to the subject, the larger the size difference between it and the subjects in distance, and vice versa. The frequently mentioned "compression" from tele-lenses, is due to you have to use them in a much longer distance to the subject, say, 10-20m, which reduces the size comparison between the foreground subjects and background subjects.

The composition, or say, the relative size of the subject in the final image, with fixed distance, is dependent on the FOV. FOV is the result of considering both the size of the sensor/image area and the focal length of the lens, and that's why we usually use the "135 format equivalent focal length" to discuss the focal lengths and usages of lenses in APS-C, full frame, middle formats, large formats, etc. The lens with longer focal length physically has smaller DOF and hence the background blurring is stronger, for example, when you compare the shots between 50mm + 50% cropped and 71mm lens taken in the same subject distance and on the same size of sensor/film.

Well put. Cheers. 

[Kiwimac]

That’s all very helpful. Thank you!

 

I want to get a 75 for my M11 to pair with my 35 Apo. I’m just trying to envisage the effect in terms of how it would look in use. 
 

50 is the longest I have just now so I’m using that and cropping to save $7500 until I have it handy!

[jaapv]

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On 1/22/2024 at 8:12 AM, williamj said:

Because it is a two dimensional measure you have to use squares and square roots, but you can get a reasonable approximation as follows.

Your lens is 50 mm so divide 50 by square root of 0.5 because you are looking at half the 'area', square root 0.5 is approximately 0.7 and so the result is approximately 71 mm or a 71 mm lens. Going the other way, twice the area would mean dividing 50 mm by inverse of square root of 0.5 or the square root of 2, which is approximately 1.4, and dividing by that gives a result of 35 mm. A 35 mm lens covers about twice the area of a 50 mm lens.

Obviously, this crop may not look exactly like the result you get with a 71 mm lens due to the differences in compression.

These are approximates :). Cheers.

 

[jaapv]

If you shoot from the same position there will be zero difference in compression the only difference between a native focal length and a crop is the DOF and, of course, pixel count. 

[williamj]

@jaapv, I agree compression was the wrong word to use as several have pointed out already, and @Greenhilltony has summarised it very well. I was trying to express an opinion that 1) different lenses render a scene differently, 2) while Leica makes a 75 mm lens it does not make a 71 mm lens, and 3) there will be slight differences in the images even though the field of view will be the same. I was trying to head off one sort of argument and ended up in a different one due to a bad choice of words . Cheers.

[jaapv]

No problem. It is just that the forum is read by outsiders as well. As this is a common misconception, linking perspective to focal length, we don’t want the forum to propagate it. 

[perb]

Why don't you simply use the frame line lever on your M11?