Advantages and Disadvantages of Cropping Images to Gain Extra Reach
One of the advantages of a high resolution camera, is that you can substantially crop an image and still obtain a reasonably large good quality print from a picture which is only a relatively small part of the original image.
Therefore, when deciding, for example, whether to purchase a 300mm or a 400mm telephoto lens, it is relevant to consider whether you really need a focal length of 400mm, particularly if you are satisfied with the image quality, and the print size that you can get from an image that has been taken by a camera with a 300mm lens, and subsequently cropped to the same field of view that is provided by a 400mm lens.
This page sets out some of the advantages and disadvantages of cropping images to gain extra reach, as an alternative to buying a telephoto lens that provides a longer focal length.
Note: There is a discussion here about the controversy surrounding the definition of the term reach and how the "reach advantage" should be calculated.
If an image from a full frame camera has dimensions of 6000 pixels (width) x 4000 pixels (height) and is printed at, say, 200 pixels per inch (ppi), it will provide a print width of 30 inches (6000 pixels / 200 ppi = 30 inches). So, if you have purchased a 300mm lens for your camera, you can obtain a high quality 30-inch wide print from a full-sized (24 megapixels) image captured with this lens. (Note: There is a discussion here about the controversy surrounding the number of pixels per inch needed to obtain high quality prints. Often, I have been very satisfied with the quality of images printed at 150 ppi or 175 ppi).
If you want an image which has a field of view that would be obtained if you were to use a 400mm lens, then a full-sized image that was captured with your 300mm lens, can subsequently be cropped to 75% of its original width and height, and the cropped image will have dimensions of 4500 pixels (width) x 3000 pixels (height), or 13.5 megapixels. The cropped image, if printed at 200 ppi, will provide a print width of 22.5 inches (4500 pixels / 200 ppi = 22.50 inches).
If you want an image which has a field of view that would be obtained if you were to use a 500mm lens, then a full-sized image that was captured with your 300mm lens, can subsequently be cropped to 60% of its original width and height, and the cropped image will have dimensions of 3600 pixels (width) x 2400 pixels (height), or 8.64 megapixels. The cropped image, if printed at 200 ppi, will provide a print width of 18 inches (3600 pixels / 200 ppi = 18 inches).
Note: For the purposes of the calculations shown below, the long side of the camera's image (6000 pixels) is referred to as the width of the image, because the image of the tiger was captured when the camera was in a horizontal position. The images below, of a Sumatran tiger, show to scale, the relative sizes and fields of view of the 400mm and 500mm images that have been cropped from the 300mm image.
300mm: 6000 x 4000 pixels 400mm: 4500 x 3000 pixels 500mm: 3600 x 2400 pixels
Advantages of cropping images to gain extra reach
* A 300mm lens, for example, may be considerably lighter and less expensive, than say, a 400mm or 500mm lens.
* The photographer can obtain some of the benefits of, say, a 400mm or 500mm lens, by cropping images taken with a 300mm lens. As shown in the above examples, when a 24 megapixel full frame camera is used, it is possible to obtain reasonably large good quality prints from substantially cropped images.
* It may be possible to hand-hold the camera when taking images with a 300mm lens, but this may be more difficult with a heavier 400mm or 500mm lens. However, as explained here and here , a heavy lens has more inertia than a light lens, and this may help to steady shutter / mirror vibrations.
* If you take an image with the express intention of cropping in mind, you are likely to obtain a better image by forgetting some of the 'rules' of composition. On a full frame camera, just about every lens will lose definition at the edges, so if you put the main subject as close to the centre as possible, you will get the benefit of using the sharpest part of the image. When you crop, you can then put the perfectly focussed subject where you like in the final image.
(Thanks to
“Hobgoblin” for making this point on this
Dyxum forum discussion.
Disadvantages of cropping images to gain extra reach
* Cropping an image reduces its size, so the photographer is not able to take full advantage of the resolution that can be provided by the camera. For example, if an image that was taken with a 300mm lens, is cropped to the field of view that would be provided by a 500mm lens, the width and height of the original image are reduced by 40%. In addition, with the above example, the area of the original image is reduced from 24 megapixels to 8.64 megapixels.
Note: If the owner of a 24 megapixel full frame camera wishes to obtain a full-sized image (6000 pixels x 4000 pixels) with the same field of view that is provided by a 500mm lens, then the photographer must actually use a 500mm lens.
* The image quality of a cropped image may not be as good as that of a comparable uncropped image. For example, if an image taken with a 100mm lens, is subsequently cropped to the same field of view that is provided by an equivalent quality 300mm lens, it is likely that the uncropped image taken with the 300mm lens, will have better image quality than the cropped image taken with the 100mm lens.
However, if an image taken with a 200mm lens, is subsequently cropped to the same field of view provided by an equivalent quality 300mm lens, the image quality is likely to better than that provided by a comparable cropped image taken with a 100mm lens, but not quite as good as the image quality of a comparable uncropped image taken with a 300mm lens.
Note: The above conclusions are based on tests made with my Sony 70-300G lens, and these results may not be representative of similar tests made with your lenses. Therefore, it is important that you carry out your own tests with equivalent quality lenses to determine whether you are satisfied with the quality of the cropped images from these lenses.
Thanks to "Analytical" for the mathematical information provided when discussing this point on this Dyxum forum discussion.
* Because cropping an image reduces its size, if the cropped image is printed at the same pixels per inch as the original image, then in comparison with the original image, the print size of the cropped image will be correspondingly reduced.
For example, if the full sized (300mm) image of 6000 pixels width is printed at 200 pixels per inch, the print width is 30 inches. If the cropped image (field of view of 500mm) of 3600 pixels is also printed at 200 pixels per inch, the print width is reduced to 18 inches.
However, if the cropped image of 3600 pixels is printed to a width of 30 inches, in comparison with the 30 inch print from the 300mm image, any noise and other imperfections will be magnified and the print quality will be reduced, because it would be printed at only 120 pixels per inch.
* When photographing birds, wildlife and some sports events, for example, photographers may wish to use the longest reach telephoto lenses that are practical in the circumstances. Images taken with these lenses can then be cropped so that even further reach can be obtained.
Mathematical formulas
Note that the formulas given below are based on cameras that have a 3:2 aspect ratio.
Formula for calculating focal length of lens needed to capture image with same field of view as cropped image
The following formula may be used to confirm the focal length of the lens that is needed to capture an image with the same field of view as the above cropped images.
focal length of lens needed to capture an
image with the same field of view as the cropped
image =
number of pixels across the long side of the uncropped image divided by
number of pixels across the long side of the cropped image multiplied by
focal
length of lens used to capture uncropped image
The focal length of the lens needed to capture an image with the same field of view as the cropped image width of 4500 pixels =
6000 pixels / 4500 pixels x 300mm = 400mm
The focal length of the lens needed to capture an image with the same field of view as the cropped image width of 3600 pixels =
6000 pixels / 3600 pixels x 300mm = 500mm
Formula for calculating the number of pixels across the long side of the cropped image
The following formula may be used to calculate the number of pixels across the long side of the cropped image:
number of pixels across the long side of the cropped image =
focal length of lens used to capture the uncropped image divided by
focal length of lens needed to capture an image with the same field of view as the cropped image multiplied by
number of pixels across the long side of the uncropped image
The number of pixels across the long side of the cropped image for the above 400mm example =
300mm / 400mm x 6000 pixels = 4500 pixels
The number of pixels across the long side of the cropped image for the above 500mm example =
300mm / 500mm x 6000 pixels = 3600 pixels
Formula for calculating the area of the cropped image (in megapixels)
The following formula may be used to calculate the area of the cropped image (in megapixels):
area of cropped image in megapixels =
(focal length of lens used to capture the uncropped image divided by focal length of lens needed to capture an image with the same field of view as the cropped image)2
multiplied by area of uncropped image (in megapixels)
The area of the cropped image (in megapixels) for the above 400mm example =
(300mm / 400mm)2 x 24 megapixels = 13.5 megapixels
The area of the cropped image (in megapixels) for the above 500mm example =
(300mm / 500mm)2 x 24 megapixels = 8.64 megapixels
Note: The above formula was applied in this Digital Photography Review forum discussion, titled: "Cropping, Megapixels and Focal Length".
Formula for calculating the crop factor
Note: Click here to see a Wikipedia article about the crop factor. This article explains that:
"The most commonly used definition of crop factor is the ratio of a 35 mm frame's diagonal (43.3mm) to the diagonal of the image sensor in question; that is, CF = diag 35mm / diag sensor. Given the same 3:2 aspect ratio as 35mm's 36mm x 24mm area, this is equivalent to the ratio of heights or ratio of widths; the ratio of sensor areas is the square of the crop factor."
Therefore, there are several ways of calculating the crop factor, as illustrated here. For the purposes of this article, we shall calculate the crop factor using the following formula:
crop factor = width of sensor of full frame camera divided by width of sensor of second camera
To illustrate an application of the crop factor, we shall compare two "theoretical" cameras which both have the same 3:2 aspect ratio and also the same pixel density:
Camera "FF" (a full frame camera) has a sensor size of 36mm x 24mm, image dimensions of 6000 pixels x 4000 pixels, and a pixel density of 1666.67 pixels per linear centimetre (6000 / 3.6).
Camera "APS" (an APS-C camera) has a sensor size of 24mm x 16mm, image dimensions of 4000 pixels x 2667 pixels, and a pixel density of 1666.67 pixels per linear centimetre (4000 / 2.4).
From this information, the crop factor may be calculated as follows:
crop factor = width of sensor of FF divided by width of sensor of APS
Therefore, with the above example, the crop factor is 1.5x (36mm / 24mm).
The following example provides an illustration of how the crop factor may be used when calculating the field of view provided by a 300mm lens on APS.
When a 300mm lens is used on APS, the image is cropped by the camera so that a field of view of 450mm is obtained [ 300mm x sensor width of FF (36mm) / sensor width of APS (24mm) ], which in this example, where both cameras have a 3:2 aspect ratio, represents 300mm x the crop factor of 1.5.
Note that a 300mm lens on FF will provide a field of view of 300mm. In addition, remember that the actual focal length of a photographic lens is fixed by its optical construction, and does not change when it is attached to an APS-C camera.
With this example, because both cameras have the same pixel density, the above answer of 450mm can be checked using the formula given previously:
focal length of lens needed to capture an
image with the same field of view as the cropped image =
number of pixels across the long side of the uncropped image divided by
number of pixels across the long side of the cropped image multiplied by
focal length of lens used to capture uncropped image
Therefore, the focal length of the lens needed to capture an image with the same field of view as the cropped image width of 4000 pixels = 6000 pixels / 4000 pixels x 300mm = 450mm
Note: Click here to read a full article about the crop factor and associated issues. The article is titled: "Full frame" cameras vs "APS-C" cameras: Analysis of the "telephoto advantage" of an APS-C camera".
Digital Photography Review: "Open Talk" Forum: "Official definition of
reach", January 2011
Dyxum Forum Discussion: Cropping images
to gain extra reach: pros and cons
Rob's Photography: Analysis of the "pixel
density advantage" of two "theoretical cameras": Template for calculating
the approximate mathematical relationships between pixel density, pixel
size, and image size (for both linear and area relationships)
Examples of the
practical application of the "pixel density advantage" template:
Sony SLT-A77 / A65 compared with the Sony A900 / A850
Sony SLT-A55 / A580 compared with the Sony A900 / A850
Sony A700 compared with the Sony A900 / Sony A850
Sony NEX-3 / NEX-5 compared with the Sony A900 / A850
Nikon D300S compared with the Nikon D3S
Canon EOS 7D compared with the Canon EOS 5D Mark II
Sony A900 compared with the Nikon D3S
Sony R1 compared with the Sony A900 / Sony A850
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