One thing digital cameras and post processing software have brought to photography is the ability to crop either in post processing or when capturing the image.  Cropping an image allows you give the image a sense of being closer or using a longer focal length lens.  this can be very useful when long focal length lens are expensive and getting closer is not possible.  

I use both a Nikon D7000 and a D7100, the difference between these is a 16MP sensor and a 24MP sensor, with the D7100 having more available pixels for greater cropping.  The D7100 has an additional 1.3 crop method to allow cropping in camera when capturing an image.  So what are the differences between cropping in camera (1.3 crop mode), cropping in post processing and using a teleconverter to increase focal length? 

The 1.3 Crop Mode- How useful is it?

There was much hype on internet forums about this 1.3 crop mode and how it would extend the effective field of view (FoV) when used. The 1.3 crop mode effectively alters the D7100's 24MP 1.5x crop sensor to a 15.4MP 1.95x crop sensor. The overall used area of the sensor would be reduced from a 24x16mm sensor in the standard mode to just 18x12mm in the 1.3 crop mode, meaning many more pixels would be squeezed into a smaller sensor area compared to the D7000. The forums were full of people thinking that their 300mm f4 lens would be a near equivalent of the expensive 600mm f4 lens. In reality how useful was it?

The main benefits I can see with using the 1.3 crop mode would be an increased number of frames that could be stored in the buffer before it reached its capacity, and smaller sized image files. For a 12 bit lossless compressed NEF (RAW) file there is an increase in the buffer capacity from 7 frames to 12 frames, this allows more continuous frames to be captured before the buffer capacity is reached. This can be useful when trying to capture fast moving wildlife. The second benefit of smaller file sizes, a 12 bit lossless compressed NEF (RAW) file reduces from 22.7MB to 15.1MB, meaning an increase from 191 to 295 images can be stored on a 8GB memory card (an estimated value from Nikon, in reality more images can be stored).  Another benefit that has been talked about with the 1.3 crop mode would be able to use a lower aperture on a variable aperture lens. If a variable aperture lens was f4.8 at 200mm and f5.6 at 300mm, the D7100 would give an extra half stop of light in the crop mode at 200mm and a similar MP count compared to the D7000 at 300mm, both giving a roughly similar field of view. I can of course get similar results by doing the same by cropping in post, and having the choice of the extra wider shot if I want it.

Although I thought I would use the 1.3 crop mode, I've found I rarely use it. Image file size is not too much of an issue for me as I carry a 4 8GB memory cards (2 slots used at once with the second slot set to backup). The 1.3 crop mode basically throws away the outside perimeter of the frame and only captures the inside 18x12mm area of the sensor. I find I would rather shoot and capture the whole image and later crop to the composition I would like. This allows me to use a similar sized post processing crop as the 1.3 crop mode would give but allow me to move the composition to the either top, middle or bottom of the frame, giving better options on the final framing and composition of the image.

Crop in post or use a teleconverter?

As the 1.3 crop mode was not that useful to me what I really wanted to know was how good the 24MP sensor was.  As a previous owner of the D7000 the D7100 had an extra 8MP, those extra MP's should give more room for cropping but how much?  Would cropping be possible instead of using a teleconverter?  By not using a teleconverter I could benefit by not losing the stops of light yet still get the extra reach I wanted.  During a recent shoot I had taken similar images of a Water Vole with the camera mounted on a tripod, this gave me an opportunity to test this theory.

The two images below were  both taken with the Niokn AF-S 300mm F4 lens, but one was taken using a 1.4 teleconverter and one without the teleconverter but cropped to similar field of view in post processing.  Can you guess which is which?

The two images were taken using the following camera settings

300mm AF-S F4 lens and post processing crop:

           f4, 1/250, ISO2000 at 300mm cropped to a 630mm effective focal length.

300mm AF-S F4 lens using a 1.4 teleconverter:

           f5.6, 1/200, ISO3600 at 420mm, effectively a 630mm focal length.

From the settings above it’s clear to see the benefit of not using a teleconverter, by being able to use a one stop lower aperture ISO can be reduced by a stop too.  This is the benefit of not having to use a teleconverter to give a similar field of view. 

Looking closely at the two images there is a slight difference between the two images.  The right image seems to be a little sharper but this is only marginal.  Considering one image was taken with a teleconverter and there is one stop of ISO difference with the other image the difference in image quality is only small and hardly noticeable.

One of the biggest differences is the size of the images, the one taken with the 1.4 teleconverter is very close to a full 24MP (3971x5956) image with only a slight crop to straighten the image, whilst the other image taken without the teleconverter and cropped to similar field of view is a 12MP (2826x4240) image.  A 12MP image is plenty for web images or printing to reasonable size.   

So far there does not seem to be much difference between images taken with or without a teleconverter.  Of course there was no extra cropping of the image taken with the teleconverter.  The two images below are further extra crops from the two images above.   

The image file size has been reduced further due to the extra cropping.  The image taken with the teleconverter was now 8MP (2271x3407) whilst the image taken without a teleconverter was now 4MP (1623x2435).  An 8MP image would be a reasonable sized image of web use and for printing but the 4MP image would limit the possible size of a final print but would still be a reasonable size for web use.  The quality of the images is quite reasonable especially considering one image is quite a large crop of its original size image. 

Conclusion

These image comparisons have made me think again about the use of teleconverters with the D7100.  Although the D7100, Nikon AF-S 300mm F4 and Nikon 1.4TC are a very good combination I can now see that cropping can give similar results in certain situations where I would like a similar field of view of what the 1.4 teleconverter would give.   This means that in certain situations where I previously used a teleconverter I now don’t need to as I now know I have that little extra cropping available.  If I need a little extra focal length beyond this then a 1.4 teleconverter will be quite useful to provide a reasonably sized image for web use and printing.  The decision on the amount of cropping or the use of teleconverters really depends on the image quality and image size you require.  Personally I don't print many images large and most of my images are viewed on the internet so the MP size difference from 24MP to 12MP is not too much of a concern for me.  For others this is could be a problem and this is where you have to make a personal choice on what is best for you.

Finally I've remembered I’ve yet to answer my earlier question regarding which image is which, the image on the left was taken with a 1.4 teleconverter.   I would be interested to know if any of you worked out which image was which and why you came to that conclusion.