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Advantages  and  Disadvantages  of  Cropping  Images

Instead  of  Using  Lenses  with  Longer  Focal  Lengths

 

 

Definition of focal length

 

When defining the term "focal length", it is mentioned in  Wikipedia  that:

In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view; conversely, shorter focal length or higher optical power is associated with a wider angle of view.

Therefore, one of the main benefits of purchasing a lens with a longer focal length is that it provides greater  photographic reach  than lenses that have shorter focal lengths. In other words, a telephoto lens with a focal length of 500mm provides greater reach than a telephoto lens with a focal length of 300mm. It also enables more pixels to be placed on a distant subject (sometimes described as "pixels on the duck") than can be obtained from using a 300mm lens.

Before discussing the advantages and disadvantages of cropping images instead of using lenses with longer focal lengths, it is worthwhile considering how the term "photographic reach" should be defined.

 

 

Definition of photographic reach

 

It is difficult to find an "official" definition of the term "photographic reach", as discussed in  this thread  by members of Digital Photography Review (DPR). One contributor to this thread said this:

 

"Reach" is a colloquialism and really doesn't warrant studious, detailed formalism. I take "Enough Reach" to mean "the relative size of the subject displayed on the image is satisfyingly big enough to enjoy". To have "not enough reach" means the image representation of the subject is too small.

 

As mentioned above, photographic reach (or telephoto reach) is often defined in terms of the focal length of the lens being used on the camera. For example, an effective way of gaining  additional  photographic reach is to use a  "longer"  telephoto lens.

 

Another way of increasing photographic reach (and the "pixels on the duck")  is to use a camera that has a higher number of megapixels. However, as explained in  this DPR post:

 

",,,when speaking of reach, we need to account not just for the pixel count, sensor size, and the focal length of the lens, but how well the lens resolves on the sensor it is used on.

 

It is also possible to gain additional photographic reach by using a  teleconverter, which is defined in Wikipedia as follows:

 

"A teleconverter (sometimes called tele extender) is a secondary lens which is mounted between the camera and a photographic lens. Its job is to enlarge the central part of an image obtained by the objective lens."

 

In  this forum,  it was mentioned that the term "reach" is just another word for "magnification" and that "reach" is just reach, it implies nothing about quality! Others consider that "reach" is how many "pixels there are on the duck", see  this forum post  and  this one.

 

 

Does a "crop" (APS-C) camera have a telephoto (reach) advantage over a "full frame" camera?

 

This topic is discussed fully in  this article  on this site. This article demonstrates mathematically that, when a full sized image from a full frame camera is cropped so that it produces the same field of view as a full sized image from an APS-C camera, if the image size of the APS-C camera is greater than the size of the cropped image from the full frame camera, this is because the  pixel density  of the APS-C camera is greater than that of the full frame camera.

 

 

Does the cropping of images provide extra reach?

 

One of the advantages of a high megapixel camera, is that the photographer 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.

 

A question that has often been discussed on  photographic forums  is whether the term "photographic reach" should include the situation where an image has been cropped so that it provides the field of view that would have been obtained if the photographer had used a lens with a greater focal length.

 

For example, let's assume that the longest telephoto lens that a photographer has available is a 300mm lens. If the photographer uses this lens to capture a distant object, such as a duck in flight, then the duck may be only a relatively small part of the overall image. But, to fill the frame with the duck in flight, it may have been necessary to use a 500mm lens.

 

Therefore, if the photographer wanted an image of the duck in flight which had a field of view that would have been obtained if the photographer had used a 500mm lens, then a full-sized image (of dimensions of say 6000 pixels x 4000 pixels) that was captured with the photographer's 300mm lens, can subsequently be cropped to 60% of its original width and height, and the cropped image will have the considerably reduced dimensions of 3600 pixels (width) x 2400 pixels (height).

 

Photographers have often used the term "reach" to describe the changed field of view that results, for example, when the image captured using a 300mm lens is cropped so that it provides the field of view that would have been obtained if the photographer had used a 500mm lens. When the term "reach" is used by photographers in this context, it is widely understood that this does not imply that cropping images increases the number of "pixels on the duck".

 

However, many photographers consider that it is misleading to use the term "reach" to describe the "changed field of view" that can be gained by cropping an image. They point out that cropping an image does not increase the number of "pixels on the duck" because this can only be achieved by using a lens with a greater focal length and / or by using a camera that has a greater number of megapixels. Therefore, for the purposes of this article, we shall not use the term "reach" to describe the "changed field of view" that results after an image has been cropped.

 

The notes that follow set out some of the advantages and disadvantages of cropping images instead of using lenses with longer focal lengths. For example, when deciding whether to purchase a 300mm or a 500mm telephoto lens, it is relevant to consider whether you really need a focal length of 500mm, 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 500mm lens.

 

Note: As part of the research for writing this article, I initiated a very useful discussion  here  about the issues associated with cropping images instead of using lenses with longer focal lengths. I am grateful to all the forum members who contributed to this informative Dyxum forum discussion. This topic was also discussed  here.

 

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.

 

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).

 

The mathematical formula for making the above calculation is shown below. The cropped image, if printed at 200 ppi, will provide a print width of  22.5 inches (4500 pixels  / 200 ppi = 22.50 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 instead of using lenses with longer focal lengths

 

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.)

 

An image taken with a 300mm lens, that has been cropped to the field of view that would be provided, for example, by a 500mm lens, may have greater depth of field than could be obtained with an equivalent image captured by a 500mm lens.

 

For further information on this complex topic, please read the relevant posts on this Dyxum forum discussion. Thanks to "cputeq" for raising this interesting point, and to "pegelli" for the additional information provided.

 

 

Disadvantages of cropping images instead of using lenses with longer focal lengths

 

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, some photographers may wish to use the longest focal length telephoto lenses that are practical in the circumstances, even if these are rather heavy and expensive!

 

 

Mathematical formulas

 

Note that the formulas given below are based on cameras that have a 3:2 aspect ratio.

 

 

Formula for calculating cropped image dimensions needed to achieve the required "effective" focal length

 

The following formula may be used to calculate how much an image needs to be cropped to achieve a specified focal length:

 

cropped number of pixels across the long side of the image needed to achieve the required "effective" focal length  = 

 

focal length of lens used to capture the the original image      divided by

 

required "effective" focal length  after  cropping the original image     multiplied by

 

number of pixels across the long side of the original image

 

For example, if you captured the original image (dimensions 6000 pixels width x 4000 pixels height) with a 300mm lens, but you would like a cropped image which has a field of view that would be obtained if you were to use a 400mm lens, the cropped image width needed to produce an "effective" focal length of 400mm is 4500 pixels, which is calculated as follows:

 

cropped image width = 300mm / 400mm x 6000 pixels = 4500 pixels

 

cropped image height = 4500 pixels / 1.5 = 3000 pixels

 

area of the cropped image = 4500 pixels x 3000 pixels = 13.5 megapixels

 

 

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 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  hereFor 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".

 

 

Note that Appendix 2 of the above article includes the following sections:

Calculation of pixel pitch

Calculation of the area of one pixel

 

Why is the pixel size and sensor size of a digital camera important? - Does increasing pixel count increase noise? Big pixels vs small pixels. (Click  here  to read the full article)

 

Pixel level vs image level in digital photography (also refers to the new  Studio Test Scene  published by Digital Photography Review)

 

Case study: High ISO low light images of the Sony A77 compared with those of the Sony A55

 

Downscaling and upscaling images for comparative purposes  (can apples be compared with oranges?)

 

Index of practical examples of the application of the "pixel density advantage" template

 

The following supplementary notes are designed to provide further information about how to compare the cameras listed in the above index:

 

 Relationships between crop factor, field of view, photographic reach, image size, pixel density, and pixel size

 

Sony A900 full frame 24 megapixel camera -- examples of the amazing crisp details that have been captured in selected Sony A900 images. To give you an appreciation of how these images will appear when greatly enlarged, the full-sized images are displayed, together with crops of just small areas of the images. Some valuable links are included following the picture thumbnails.

 

Other technical articles on this site are indexed  here  and include articles about  determining print size, and  upgrading your digital camera.



Links




Rob's Photography: Full frame cameras vs APS-C cameras: Analysis of the crop factor and "telephoto advantage of an APS-C camera

 

Rob's Photography: Index of camera comparisons showing the mathematical relationships between image size, pixel size, pixel density, reach etc.

 

Digital Photography Review: "Sony Alpha Talk" Forum: "How do you calculate the reach advantage? Sony A900 vs Nikon D3S, April 2010

 

Digital Photography Review: "Open Talk" Forum: "Official definition of reach", January 2011

 

Digital Photography Review: "Photographic Science and Technology Forum", "What is reach", September 2013

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)

 

Rob's Photography: Examples of the outstanding resolution of crops made from images captured by the full frame Sony A99 and Sony A900

Rob's Photography: Determining print sizes for prints from digital cameras

Digital Photography Review "Open Talk" forum discussion: "Cropping, Megapixels and Focal Length", January 2010

Dyxum Forum Discussion: Sony A900: Do you really need 24 megapixels?


BobAtkins.com: Crop Sensor (APS-C) Cameras and Lens Confusion

 

 

 



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