Neptune Photo Newsletter
April 2017 Volume 63
by Steve Zimic
Understanding Crop Factor When Choosing a Camera
Having worked in the photo retail business for many years, I noticed when discussing the crop factor for various cameras, much of the time I saw those glazed over eyes staring back at me. I'm hoping that the following will help to clear up your vision in that regard. Back in the days of film the most popular choice was 35mm film which recorded an image that was 24mm x 36mm. In today's modern digital age, among the myriad of different image sensor sizes (see chart below), that size is known as a full frame digital sensor. The only reason that size became the standard in digital photography by which other sensor sizes are compared, is because there were millions of film lenses owned that were designed to cover 35mm film and they could be used on digital cameras as well, plus most folks were familiar with the focal lengths required for various tasks. TBT, there were other film sizes, many of which were considerably larger than 35mm such as the 6cm x 9cm (120 roll film) or the 4"x5" and 8"x10" sheet films that were used by great photographers like Ansel Adams. It was true then and today, that the larger the film size, the better the image quality and the bigger the camera. The same quality difference and camera size holds more or less true with regard to the size of digital image sensors. Advancing technology though has been able to deliver much higher quality images in a smaller form factor compared to film. For example in my opinion, and I'm sure some will disagree, the best full frame (FF) camera delivers quality similar to 4"x5" sheet film, while the latest micro 4/3 (MFT) cameras are close to 120 roll film. The point here is that when choosing a camera, the FF camera or even an APS-C camera may be more than you need. Of course there are other things to consider besides image quality when buying a camera, but your sales associate can help you there. So how do you choose lenses based on the sensor size of your camera using the crop factor in the table below? All photographic lens are specified by their focal length and maximum aperture. Unfortunately there's no obvious correlation between focal length and the more desirable specification - angle of view - unless you do the math, which we're not going to do. Instead we'll consider the 50mm lens when used on a FF camera, which delivers a horizontal field of view of 40 degrees and closely matches the perspective and field of view of the human eye. If we put that same 50mm lens on a camera with a smaller sensor, say a micro 4/3 (MFT) for example, the field of view will be much narrower, resulting in an image that looks like a 100mm lens on a FF camera (50mm times a crop factor of 2). Going the other way, to get that same 40 degree angle of view on a MFT camera you'd need a 25mm lens (50mm divided by the crop factor of 2). Simple, right?
That crop factor is also useful when selecting aperture values between cameras with different sensor sizes. Most camera lenses have variable openings, AKA apertures or f-stops, controlled by a diaphragm. Once again the numbers used that represent different apertures are not at all intuitive and actually seem backwards from what you'd expect. For example an aperture of 2.8 lets in more light than say 5.6, and in this case 4 times more light. Yes, there's math involved once again, but we're staying away from that. it's enough to know that a lens set with a wide opening (F2.8) will render less things in focus, (known as a shallow depth of field), than if that same lens is set to a smaller opening (F8.0). So now we'll use the same example as above and apply the crop factor to the aperture. Here we'll choose a relatively large depth of field by choosing f8.0 on a 50mm FF camera which on a MFT camera will need a 25mm lens at f4.0 to produce the same depth of field ( f 8.0 divided by 2). If we take a typical phone sensor (1/2.3") and find the same 50mm FF equivalent lens and aperture, we use the 5.8 crop factor from the below chart, giving the result of an 8.6mm lens shot at f 1.4. I haven't done that much research on phone apertures, which by the way are not variable, but I'm pretty sure none of them have such a wide aperture. This is the reason why pictures from your phone have so much depth of focus and since the aperture isn't variable, there's no way to effectively isolate your subject from the background the way a camera with a larger image sensor can do. If you're planning on purchasing a camera, I hope this helps to clear up some of the misconceptions regarding crop factor and will allow you to focus on other aspects in your choice. Just remember, the best camera is the one you have with you.
Pentax KP, 24MP APS-C DSLR
Other than the original Spotmatic, Pentax has always been a small player compared to Nikon and Canon. You would think that with seriously dwindling DSLR sales in general that they would have closed up shop by now. However with some clever innovations, Pentax has managed to hold their DSLR sales steady and has gained a bit by some reports. I would imagine that their in-body 5 axis, 5 stop image stabilization (SRII system) has played a significant part in that regard. Another unique feature carried over from previous models is Pentax's Pixel Shift Resolution mode. The Pixel Shift mode takes 4 images in rapid succession, shifting the sensor by one pixel for each image, in order to capture full color information for each output pixel. The resolution remains at 24MP, but there's a huge increase in detail and dynamic range using this mode because there's no anti-aliasing filter as found in most digital cameras. Although the time is extremely short between the 4 captured images, subject motion can be a problem. Pentax nicely addresses this problem with an in camera motion correction feature that will take care of subtle changes in subject position which it applies to JPEG images and Raw image. Unfortunately, Raw images will have the motion correction applied only if you use the included Silky Pix software. To be clear, the motion correction is not supported by Adobe's ACR. Astrophotographers love the Astro Tracking feature which automatically shifts the sensor to track stars - best used with the optional GPS unit. The LCD even has a night mode which shifts the control panel colors to red in order to preserve your night vision. This new model is weather sealed and borrows much of its styling (i.e. more physical controls) from the full frame K1, thus targeting the enthusiast market. Street price is $1099 for the body only, although various lens kits are offered. More info on the Pentax website.
Sigma Compact 100-400 F5-6.3
Long telephotos lenses are traditionally rather expensive, plus when you add features like optical image stabilization and weather sealing they're often priced beyond the reach of most folks. Sigma breaks that stigma with this newly announced FF lens which is available for preorder at $800. In addition to the weather sealing and stabilization, you get 4 super low dispersion elements which should yield ultra sharp results even wide open. A handy feature is that the lens can be zoomed by either a push/pull motion or by rotating the zoom ring. The lens weighs in at around 2.5 pounds and is only a bit over 7 inches in length when collapsed. Lens mounts available will be Nikon, Canon and Sigma SA with the Nikon version being the first to hit the market. If you read the article on crop factor in this newsletter, you'll know that on a Nikon APS-C camera, the lens will yield the equivalent of 150mm-600mm - sweet. Any firmware updates to the lens can be done by the end user via the optional Sigma USB dock. More info on the Sigma website.
Fuji X-T2 - version 2.0 - This is a huge firmware update with 24 improvements and no bug fixes listed - commendable. There are improvements to focusing, bracketing range, video features and much more. Click on the above link for the full list, new owner's manual and link to the download.
Nikon Cameras - D810, D810A, D500, D750, D7200 - Various bug fixes plus support for the WT-7 wireless transmitter. Click on the above individual camera links for the firmware download page.