Cropping, Megapixels and Focal Length

Started Jan 12, 2010 | Discussions
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Cropping, Megapixels and Focal Length

I accidentally placed this in the challenges forum.

Hello,

Is there a table or formula or general rule related to MPs lost when you crop an image and the corresponding focal length equivalent increase? As an example, I cropped an image from my 50D from 15 MPs down to 5 MP. What would be the corresponding focal length increase equivalent with a smaller area viewed on the image? I'm try to understand the MP loss related to how much of an equivalent focal length it would gain in the image area? Is it a direct relationship, where cropping by 2/3 will give me the same increase in focal length?

If I'm using a 400 mm lens, how many megapixels do I need to crop to get a field of 500 mm?

Thanks.

Paul

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Re: Cropping, Megapixels and Focal Length

apple64 wrote:

I accidentally placed this in the challenges forum.

Hello,

Is there a table or formula or general rule related to MPs lost when you crop an image and the corresponding focal length equivalent increase? As an example, I cropped an image from my 50D from 15 MPs down to 5 MP. What would be the corresponding focal length increase equivalent with a smaller area viewed on the image? I'm try to understand the MP loss related to how much of an equivalent focal length it would gain in the image area? Is it a direct relationship, where cropping by 2/3 will give me the same increase in focal length?

If I'm using a 400 mm lens, how many megapixels do I need to crop to get a field of 500 mm?

I can tell you that if you want "double" (2X) the focal length ... (400 to 800), you would need 4X the pixels.

So 2X focal length cropping requires 4X pixels for equal resolution.

To better understand this principle .... remember you will have the equivalent of 1/2 the "vertical" height .... AND 1/2 the "horizontal" width of your original photo.

So you have only 1/4 the "area".

(It is referred to as the "area-squared" rule -- albeit it goes by several other names as well.)

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Re: Cropping, Megapixels and Focal Length

Thanks JoePhoto.

I've got a 400 5.6 and I'm conpemplating getting a 500 f4. I have a 50D and was wondering about a 7D. Decisions...!

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Re: Cropping, Megapixels and Focal Length

apple64 wrote:

I accidentally placed this in the challenges forum.

Hello,

Is there a table or formula or general rule related to MPs lost when you crop an image and the corresponding focal length equivalent increase? As an example, I cropped an image from my 50D from 15 MPs down to 5 MP. What would be the corresponding focal length increase equivalent with a smaller area viewed on the image?

The image MP has reduced from 15 to 5

However, the number of pixels are related to the area of the image, the height and width must each be reduced by the square root of this. i.e sqrt(15/5) = sqrt(3) = 1.73

Or in terms of equivalent focal length, if a 100mm lens for the original 15MP image, the cropped image will be similar to that from a 100 x 1.73 or roughly 170mm lens.

I'm try to understand the MP loss related to how much of an equivalent focal length it would gain in the image area? Is it a direct relationship, where cropping by 2/3 will give me the same increase in focal length?

If you are speaking in terms of megapixels, you need to take the square root of the change in megapixels. You have cropped to retain 1/3 of the pixels, as described above.

If I'm using a 400 mm lens, how many megapixels do I need to crop to get a field of 500 mm?

Thanks.

Paul

For a distant subject, the size of the image is proportional to the focal length.

That means a simple linear crop of the sensor will simulate the effect of changing focal length. By linear, I mean crop the height and width by the same amount.

Starting from an image taken with a 400mm lens, crop by 400/500 to give the effect of a 500mm lens.

400/500 = 0.8

The effect on the megapixel count is 0.8 x 0.8 = 0.64, since both height and width are cropped by this amount.

Starting with 15MP, 15 x 0.64 = 9.6 MP

In other words over one third of the pixels must be cropped away.

Hope this helps, or at least gives a start. (Also hope I didn't make too many mistakes here).

Regards,
Peter

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Re: Cropping, Megapixels and Focal Length

apple64 wrote:

Thanks JoePhoto.

I've got a 400 5.6 and I'm conpemplating getting a 500 f4. I have a 50D and was wondering about a 7D. Decisions...!

I'm not sure if you're considering a 500 F4 for the 50D vs a 400 5.6 with a 7D, or a 500 F4 and a 7D. If it's the former consider this: A 7d will give you more pixels in your image, but it won't give you more resolving power - that is a function of the lens. A 500 F4with the 50D will be better than the 400 5.6 on the 7D. Not to mention, that as great as the 400 5.6 is, the 500 F4 is better (and has IS). Plus the 500 F4 can take a 1.4 converter and you have a 700F 5.6 that will auto focus with your camera (whether it be a 50D or 7D).

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Re: Cropping, Megapixels and Focal Length

apple64 wrote:

I accidentally placed this in the challenges forum.

Hello,

Is there a table or formula or general rule related to MPs lost when you crop an image and the corresponding focal length equivalent increase? As an example, I cropped an image from my 50D from 15 MPs down to 5 MP. What would be the corresponding focal length increase equivalent with a smaller area viewed on the image? I'm try to understand the MP loss related to how much of an equivalent focal length it would gain in the image area? Is it a direct relationship, where cropping by 2/3 will give me the same increase in focal length?

If I'm using a 400 mm lens, how many megapixels do I need to crop to get a field of 500 mm?

(400/500)^2 * 15MP = 9.6MP.

But don't forget that you also are now using a sensor that collects 64% the area, so you get more noise. A good TC is a better way to get more reach, and you don't waste resolution.

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John

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Re: Cropping, Megapixels and Focal Length

iMac, therefore iAm wrote:

I'm not sure if you're considering a 500 F4 for the 50D vs a 400 5.6 with a 7D, or a 500 F4 and a 7D. If it's the former consider this: A 7d will give you more pixels in your image, but it won't give you more resolving power - that is a function of the lens.

No; both can lower the MTF curves, and in this case, the 500 is a very sharp lens, and there is NO way that if it is stable and in focus, that it is going to be the main limiter of resolution with any current pixel densities, or any soon to come.

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John

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Re: Cropping, Megapixels and Focal Length

I was just wondering what the cropping capabilities of the 7D and 500mm combo would give me and then one can also factor in the 1.4X convertor. I usually do BIF. My 400 5.6 IS a great lens, in that it is lightweight, portable, and very quick to position. Is it worth an additional \$8K for the 500mm, 7D and tripod...your information will help me greatly decide that.

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Re: Cropping, Megapixels and Focal Length

If you're serious about birding the 500 and 1.4 will be a HUGE benefit. Get those, then if you need more cropping get the 7D at a later date, although I seriously doubt the 3mp increase will be very noticeable.

apple64 wrote:

I was just wondering what the cropping capabilities of the 7D and 500mm combo would give me and then one can also factor in the 1.4X convertor. I usually do BIF. My 400 5.6 IS a great lens, in that it is lightweight, portable, and very quick to position. Is it worth an additional \$8K for the 500mm, 7D and tripod...your information will help me greatly decide that.

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Re: Cropping, Megapixels and Focal Length

Thanks IMac!!

With a bare 500, the difference between it and the 400 5.6, on a 50D, is 160 mm. Is that worth \$5K? Is 1120 mm with the 1.4X on it too long, especially for BIF? I think if the majority of your time was stationary, small birds, then the 500 would be a winner...for flight I'm not so sure! IMHO

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Re: Cropping, Megapixels and Focal Length

sherwoodpete wrote:

apple64 wrote:

I accidentally placed this in the challenges forum.

Hello,

Is there a table or formula or general rule related to MPs lost when you crop an image and the corresponding focal length equivalent increase? As an example, I cropped an image from my 50D from 15 MPs down to 5 MP. What would be the corresponding focal length increase equivalent with a smaller area viewed on the image?

The image MP has reduced from 15 to 5

However, the number of pixels are related to the area of the image, the height and width must each be reduced by the square root of this. i.e sqrt(15/5) = sqrt(3) = 1.73

Or in terms of equivalent focal length, if a 100mm lens for the original 15MP image, the cropped image will be similar to that from a 100 x 1.73 or roughly 170mm lens.

I'm try to understand the MP loss related to how much of an equivalent focal length it would gain in the image area? Is it a direct relationship, where cropping by 2/3 will give me the same increase in focal length?

If you are speaking in terms of megapixels, you need to take the square root of the change in megapixels. You have cropped to retain 1/3 of the pixels, as described above.

If I'm using a 400 mm lens, how many megapixels do I need to crop to get a field of 500 mm?

Thanks.

Paul

For a distant subject, the size of the image is proportional to the focal length.

That means a simple linear crop of the sensor will simulate the effect of changing focal length. By linear, I mean crop the height and width by the same amount.

Starting from an image taken with a 400mm lens, crop by 400/500 to give the effect of a 500mm lens.

400/500 = 0.8

The effect on the megapixel count is 0.8 x 0.8 = 0.64, since both height and width are cropped by this amount.

Starting with 15MP, 15 x 0.64 = 9.6 MP

In other words over one third of the pixels must be cropped away.

Hope this helps, or at least gives a start. (Also hope I didn't make too many mistakes here).

Regards,
Peter

Hello Peter, I have just come across this thread, but I agree with the approach you have taken. I have a web page which deals with this topic. It also sets out the advantages and disadvantages of cropping images to gain extra reach:

http://www.robsphotography.co.nz/focal-length.html

In the example given in this thread, 15 megapixels would provide image dimensions of approx. 4743 pixels x 3162 pixels. If you crop this 400mm image to the same field of view that you would get with a 500mm lens, the image dimensions (original x 0.8) would be approx. 3795 pixels x 2530 pixels, or 9.6 megapixels.

The above web page is currently being discussed here:

http://www.dyxum.com/dforum/cropping-images-to-gain-extra-reach-pros-and-cons_topic57942.html