Sony FS100 Prime lens vs Zoom lens

I already know that prime lenses can be sharper than zoom lenses but I wanted to see for myself just how much of a difference a prime lens would make, specifically on the Sony NEX-FS100 Super35mm video camera. My hypothesis going into this, my third round of lens testing on the FS100, was that because the Super35mm sensor that Sony developed is similar in size to an APSC photo DSLR sensor but only has 2464×1394 active photo receptor pixels (3.43M pixels or 2.5K), it doesn’t require lenses with as high a resolving power in order to produce sharp video images. Sony claims that the individual photo receptors on the Exmor Super35 CMOS sensor are four times larger than the receptors on a DSLR.

The FS100 sensor has a low resolution, relative to its size, which results in photo site receptors that are four times larger than on a typical DSLR

In my previous tests I compared parfocal zoom lenses to parfocal zoom lenses in three ranges: wide zoom, normal zoom, and telephoto zoom. I didn’t notice much difference in sharpness between modern professional Nikon and Canon lenses, older Minolta autofocus A-mount, and much older manual MD mount lenses. So for this round of lens testing, I decided to do away with the apples-to-apples test of zoom lenses versus zoom lenses and do a more apples-to-oranges type of test and compare a zoom lens to a prime lens.

The test video that I shot and discussion of the test is at the end of the article in case you want to skip ahead – but first, some background information that will fill you in on my video camera and lens background and requirements.

Boring, Exciting, and Game-changing Video Sensors

I own two Sony NEX-FS100 video cameras. I love the noise free and true HD resolution that the Super35mm sensor allows. Traditional professional SD video cameras have a trio of square pixel sensors but the move to HD required more resolution, although the sensor sizes didn’t increase. As a result, the photo receptors got smaller and new methods of signal processing were required as there was just not enough room on those small sensors to fit all the photo receptors required for HD resolution. My previous video camera, a 3×1/3″ CMOS sensor Sony HVR-Z7U was capable of outputting a 1920×1080 30P video signal but the sensors weren’t really HD and in order to output an HD signal, a lot of interpolation was required. Sony famously turned the pixels on their side (ala Shreddies cereal when they advertised the “New-Exciting!” diamond shape) and used a lot of fancy algorithms (and probably lots of guess work too) in order to resolve a square pixel 1080P video signal. Ironically, the video was then converted once again when it was recorded using the lossy anamorphic 1440×1080 HDV codec.

Sony video camera Exmor CMOS sensor with 45 degree pixel shift layout

This pixel trickery, combined with a puny-by-DSLR-standards sensor, whose tiny photosites resolved a low signal to noise ratio, and the lossy HDV codec, resulted in a very noisy image and a horrible video recording compared to what you saw on the LCD and what new large sensor video cameras are able to produce.

1920x1080 HD video recorded to 1440x1080 HDV has a pixel aspect ratio of 1.33

DSLRs changed the paradigm of what was possible for a proper professional video camera when you switched from three puny sensors to one massive one that had enough resolution to resolve HD video. In this regard, the FS100 and the slightly smaller sensor-ed Panasonic AF100 are game changers because they combine the professional features a video producer requires (professional audio connections, long record time, clean HDMI output, false colour peaking, etc) with a large sensor that the DSLR pioneered but got wrong (the photo site receptors were too small and there were too many of them so only a fraction were sampled – 1/3 on the Canon 5D MKII), resulting in moiré and aliasing problems.

FS100 ownership

The problem with owning two interchangeable lens video cameras is that you need to purchase two sets of lenses, which can get expensive very quickly.

I decided that I would start by outfitting my FS100s with parfocal zoom lenses to begin with and then afterwards look into prime lenses. Parfocal lenses are lenses that hold their focus when you zoom out from telephoto to wide, which is more important for manual focus video cameras than it is for autofocus still cameras. Some video producers don’t need to change focal length at all (they move the camera to frame their subject) or cringe at the thought of changing focal length during a shot (again, they would rather move the camera with a slider or a dolly). But I don’t make movies or film actors who move to preset marks – I’m a corporate and event videographer and I need the ability to change focal length quickly between shots and often times, mid-shot. Probably the two types of productions that I film that dictate my video camera requirements are conferences and dance recitals – so sharpness is important because it makes manually focusing easier and looks better, but being video, I don’t require the ability to pixel peep and enlarge images in the same manner a still photographer would. For these reasons, zoom lenses are more important in my workflow than are prime lenses.

Shawn’s FS100 lens reviews

As I’ve previously mentioned, I have already written two articles about parfocal zoom lenses for the Sony NEX-FS100. The first was an eleven lens FS100 shootout and the article was published in the EventDV Live e-magazine Winter Edition. The article tops 6,000 words and it includes footage from each of the tested lenses:

That initial article covered normal and telephoto zoom lenses but I wasn’t able to get enough wide zoom lens options for my shootout. I tested a few after the fact and when I found one that I loved, I wrote a second article, a blog post, and shot some video comparing the new Sony SAL1650 16-50mm f/2.8 lens to two normal zoom lenses.
Sony FS100 wide angle zoom lens review

Current FS100 Lens Line-up

Let me quickly review my current lens situation before moving onto the whole point of this blog post, which is to compare the image from a great prime lens to that of a great zoom lens – both at 50mm.

Overall I’m very happy with the Sony 16-50mm f/2.8 for my wide angle zoom lens. I find it a tad sharper than any of my older used lenses and for this reason it is my go-to lens when I don’t need a long telephoto.

I don’t have the same appreciation for my normal zoom lenses. The Nikon mount Tokina ATX-Pro 28-75mm f/2.6-2.8 that I tested in my EventDV Live shootout had a focus issue so I bought a second copy, this time an A-mount copy. Unfortunately the lens design is really prone to ghosting and over-dramatic lens flare so that lens doesn’t see much action.

My Konica Minolta 28-75 f/2.8 lens returns a slightly yellow image, which can be colour-corrected in post or with a manual white balance, but it bothers me enough that it too doesn’t get used much. That leaves me with a Minolta MD 35-70mm f/3.5 lens that works beautifully but it is only a 2x lens and slightly slower than the other options.

I’ll probably end-up getting the $800 Sony SAL2875 28-75mm f/2.8 SAM lens for when I need more than 50mm and less than 70mm. I previously tested a used copy and it is parfocal but its build quality is below that of the SAL1650 and the zoom is not as smooth either.

I’m very happy with my Minolta 70-210 f/4.0 beercan lens as a telephoto lens. So much so that I’m going to get a 2nd copy for when I need both my cameras to have that range but I might look into the newer and much more expensive 70-200mm f/2.8 lenses again at a later date. I know f/4.0 is a whole stop slower than f/2.8 but my used beercan cost me 1/10 the price of a new or used Sony 70-200 f/2.8 lens (and the second beercan seller is only asking $50) and the FS100 handles gain very well its Super35mm sensor, with its large photo receptors, has a high signal to noise ratio.

So at this point you’re mostly caught-up on my lenses. I have several more but they aren’t noteworthy and are going back on Craigslist.

Let me get you caught-up on the adapters I’m using as well. I have several fotodiox adapters and the Sony LA-EA1 adapter but am waiting for new FS100 firmware (03/12 target release) as it will allow me to use the newer Sony LA-EA2 adapter, which has autofocus and smooth iris changes. This adapter, with its ability to autofocus when needed, is one of the main reasons I’m going with A-mount lenses, rather than Nikon lenses. Cost is the other.

Zoom Lens versus Prime Lens

OK – on to the test. Professional photographers want the sharpest lenses they can afford. The main reason is that you can really see the difference with full frame photography at resolutions exceeding 20 megapixels and larger than life prints.

Cinematographers have similar needs when it comes to shooting with the sharpest lenses because when viewed on a movie screen, you can see the difference.

But what about when you are “only” shooting video at 1920×1080 and displaying infrequently on 60″ HDTVs or projecting on 100″ projection screens and more frequently on computer monitors? Can you really see the difference, especially seeing that the Sony FS100 has massive photosites that are 4x larger than those on a typical DSLR sensor and as a result, doesn’t require as high a lens resolving power? Or will it be like putting high octane gas into an economy car where if there are any returns, they are hardly discernible and unnecessarily expensive – not that the FS100 is an economy video camera as it has a premium engine with its Super35mm sensor.

One of the big reasons I’m taking the bold move of comparing a zoom lens to a prime lens is that in my initial shootout I had a very hard time discerning a difference between the video quality and sharpness between $1,800 modern professional lenses and a used 33 year old one that I paid $20 for at a thrift store. Part of the reason was that I was using live models as my subjects and people don’t have hard edges that make lens sharpness differences obvious.

For my Prime lens versus Zoom lens test I filmed a simple fruit bowl and set my focus manually to the word “banana” on the sticker. I didn’t want to pass-up on the opportunity to pay tribute to my favourite hockey player, Teemu Selanne, and his December 17th return to Winnipeg for an NHL hockey game for the first time in 15 years – his Winnipeg Jets figurine also made it into the shot. The long absence is because Winnipeg lost its franchise to Phoenix (now the Coyotes) in 1996, Selanne now plays for the Anaheim Ducks, and Winnipeg bought the Atlanta Thrashers franchise. The Jets beat Anaheim by a score of 5-3 and Selanne picked-up two assists.

My test lenses are the Minolta MD Rokkor-X 50mm f/1.4 prime lens and the Sony SAL1650 DT 16-50mm f/2.8 wide angle zoom lens.

Make sure to watch the video in full screen – it is 1920×1080. I’m using Wistia to host this video, which will auto-adjust to the highest resolution your bandwidth will allow, so you don’t need to look for an HD button. Also check-out the Wistia “video stats” – they’re pretty cool.

What did you see?

I saw a very slight advantage for the prime lens at f/2.8 in both sharpness and contrast but not much of a difference afterwards. The prime was also a bit more consistent when it came to light transmission across the different exposures. I was also interested to see the depth of field differences at the various f-stops.

Obviously an 50mm f/1.4 lens enjoys a two-stop low light advantage and shooting at f/1.4 creates an incredibly shallow depth of field but it isn’t always practical or desired to shoot with a large iris.

So unless I’m trying to achieve an f/1.4 look, or absolutely need the extra two stops for low light situations, I’m not going to bother switching to the prime for interviews because the extra sharpness would likely be lost on a moving subject and I lose the ability to change focal length without physically moving the video camera. I will, however, pull it out for establishing, decor, and architectural shots when I am able to move around and don’t need the adjustable focal range of a zoom lens. By the way, I paid $45 for my Minolta MD Rokkor-X 50mm f/1.4 lens on Kijiji, so it was definitely worth the money, but I wouldn’t bother if I had to pay a few hundred or even a thousand dollars for an equivalent lens.

The next prime lenses on my wish-list are these old manual Minolta primes that have larger-than-f/2.8 irises:

Minolta MC or MD Rokkor-X 85mm f/1.7 or MD 85mm f/2.0
Minolta MD Rokkor-X 35mm f/1.8.

Ok – that’s it for this third lens review for the Sony FS100. Please leave your questions and comments below & feel free to Like, Tweet, or G+ this article and follow me on Twitter @shawnlamvideo. And let me know what other tests you would like to see me conduct for the Sony NEX-FS100.