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Fidgeting with Focal Length

Posted in Lenses, Photography, tagged focal reducer, Lens Turbo, Metabones, Speedbooster, tele-converter on July 10, 2018| 4 Comments »

Last time I mentioned focal reducers and I thought I would go over them in detail this time round. Focal reducers are in essence the opposite of a tele-converter. Rather than increasing the effective focal length of a lens they decrease the effective focal length of the lens. How do they work and are they any good? Let’s find out.

First I want to start with tele-converters as they have been around in photography much longer. These are often referred to as “doublers” because the most common ones effectively doubled the focal length. I call it ‘effective’ because the lens focal length does not physically change. With a tele-converter the way the focal length is “doubled” is by having a lens assembly that crops away 75% of the image and then projects the remaining 25% in the center across the entire sensor or film. Because 75% is cropped away, the remaining amount of light is only 1/4 as great and so two stops of additional exposure is required. With the less common 1.4x units only half the image area is cropped away and thus only 1 stop of additional exposure is required. Remember that the image area is two-dimensional so when cropping 1/2 of each dimension you actually remove 3/4 of the image area. As an example, the image area seen by a 100mm lens is only 1/4 as much area as a 50mm lens. Why? Because both dimensions tighten up by 1/2. An 8 foot by 12 foot sign that fills the image with a 50mm lens taken from the exact same spot with a 100mm lens will only show 4 foot by 6 foot. The area seen by the 50mm lens is 96 square feet the area seen by the 100mm lens is 24 square feet. The tele-converter optics take that 25% or 50% image area (2x vs 1.4x) and spread it out over the entire film/sensor area to create the effective doubling or 1.4x focal length. The lens is producing enough light to fill a full frame but the tele-converter is focusing on a small cropped portion and then optically spreading it out to cover the full frame. This is why there is an effective loss of two stops with a doubler and 1 stop with a 1.4x. It’s all math.

Canon M5 with Leica R 90/2.8 and 0.72x FR ISO 400 1/1250 second @ f/2.8 (effective 104mm f/2.0)

Focal reducers are much newer to photography, but have been around the astronomy scene for decades. These devices are designed to take light that is “wasted” and squeeze it onto a smaller area. When telescopes started getting really long on focal length, astronomers found that viewing large more faint objects became nearly impossible. The focal reducers were designed to effectively make their scope a wide field unit with brighter images. These reducers became a coveted advantage in astro-photography by cutting exposure times dramatically.

In photography we now have these modern mirrorless cameras that can accept a wide variety of lenses due to their extremely shallow flange to sensor distance. This means lenses designed to cover larger formats can be used but the sensor is cropping away much of the image. In the case of a micro four thirds camera with a full frame lens mounted, it is the same 75% that is unused. A 50mm f/2.0 lens on a 4/3 body will only see the image area in the center making it shoot like a 100mm f/2.0. A focal reducer aims to capture some or all of the wasted light that would have filled the 35mm full frame sensor. The optics in the focal reducer gather that light and focus it onto the smaller sensor. If we were to take a 0.5x focal reducer and use a FF lens on a MFT camera we would gain 2 stops of effective speed as we can utilize all the lens light gathering ability and focus it onto the smaller sensor. The focal reducer at 0.5x makes the lens an effective 25mm f/1.0. A 25mm lens on MFT shoots like a 50mm on full frame. Metabones does make a 0.5x unit for 35mm/FF to MFT!

The focal reducer is essentially the opposite of the tele-converter. The tele-converter takes a small portion of the light coming through the lens and spreads it out over a larger area thus “thinning” the light but granting a tele-photo effect. The focal reducer removes all or part of the cropping effect of smaller sensor cameras by taking the larger amount of light and forcing it onto the smaller sensor. One key difference is that a tele-converter does not require a lens for a larger format camera. A focal reducer does. If you were to attach a focal reducer to a full frame camera and use a full frame lens with it you would get severe vignetting.

The main players in focal reducers are Metabones, a Canadian company that uses American optics to produce amazing results, and Zhongyi Optical from China offering a solid performance at a more reasonable price. Metabones markets their focal reducers as “Speed Booster” while Zhongyi calls theirs, “Lens Turbo.” Neither of these two companies offer a unit that will mount to a Canon M body so I bought one from a lesser known Chinese company. I got a Canon EF to EF-M focal reducer in 0.72x for less than $100 and have had surprisingly good results. In fact if either Zhongyi or Metabones will make an EF-EF-M unit, I’ll buy it.

Canon M5 50mm Zeiss Pancolar f/2 with 0.72x FR (effective 36mm FF or 58mm on APS/c) focus @2 feet. ISO 400, 1/80 second @ f/2 (effective 1.4)

Metabones units have electronics that pass lens information through to the camera body. For Canon EF to Sony E they have a software chip to convert Canon language into Sony. This allows for full autofocus in some cases. The Lens Turbos do not have electronic pass through. So until Metabones gives me an option to buy their product, I am relegated to this cheap unit. You here me Metabones? Come on the flange distance is identical to Sony and you don’t need the cpu converter! If you shoot Sony, Metabones makes all kinds of these adapters. If I had a Sony I would get the EF-Sony E mount model as it allows full pass through of Canon EXIF data and autofocus although the focus is sluggish due to the electronic conversion chip. Canon EF mount is readily adaptable to a wide variety of other lenses like Nikon, Pentx K, M42, Yashica/Contax and more. If you have a FF Sony A7 camera you need to shoot in APS/C crop mode when using the Metabones or other focal reducers. Remember the focal reducer takes the light from full frame and squeezes it onto a smaller sensor.

Shockingly, the low-priced product performs very well. The only issue I have found, which was disclosed by the manufacturer, is an occasional purple/blue ghost flare under extreme lighting conditions. The contrast, color, and sharpness all appear to be excellent. I can’t wait for Zhongyi or Metabones to pull their heads out of their asses and build an EF to EF-M version. For the record I bought my cheap FR on ebay for $79 from China, took a couple of weeks to arrive. Mine is a 0.72x focal reducer that reduces exposure time by one stop. So the 50mm f/2 from the example above becomes an effective 36mm f/1.4. A 36mm lens on my Canon M5 shoots like a 58mm would on FF.

Canon 5D Mk III Zeiss 58mm f/2 Biotar focused at @2 feet ISO 400, 1/30 second @ f/2

It is important to understand that the lens you shoot with either a tele-converter or focal reducer is still the same focal length and aperture. Tele-converters and focal reducers change the angle of view, not the actual focal length and aperture. For example the 50mm f/2.0 lens remains a 50mm f/2.0 lens when using either the FR or TC. The depth of field doesn’t change nor does the characteristics of the original lens. The optics inserted behind the lens simply modify what the sensor sees. The use of light is what changes. With a TC we are using less of the available light gathering of the primary lens to achieve a telephoto effect and with the FR we are using more of the light gathering ability of the lens to get a wider angle lens and more light.

So for the math people here is how it works. F-stop is a simple ratio equation. The focal length divided by the aperture in like terms. A 50mm f/2.0 has an aperture of 25mm. When a 2x TC is used we end up with an effective 100mm f/4 reverse the math 100/4 = 25. Remember the TC or FR doesn’t change the physical-mechanical primary lens it still has 25mm of aperture. With the FR using the above example of a 0.5x FR on the same 50mm f/2.0 we end up with an effective 25mm f/1.0 and that is easy math 25/1 = 25, again, the physical max aperture doesn’t change, it is always gonna be 25mm on that 50/2.0. With the doubler the optics are spreading a small bit of light out over the whole sensor. With the FR the optics are taking a full frame light source and concentrating it onto a smaller sensor which increases the intensity of the light thus improving exposure time and effective f-stop. The math pans out.

For the record we are talking about F-stop not T-stop so F-stop is just pure basic math where as T-stop values are actual measured light transmission taking all the variables like air-glass surfaces, multi-coating, et al. You need to have a post-grad degree in physics to calculate the pure math on all those T-stop variables. T-stop is another post, today just F-stops 🙂

Generally speaking, it is harder to maintain image quality with a tele-converter than it is with a focal reducer. This is especially true with modern digital sensors that prefer MORE light than less light. High end manufacturers TC units are very expensive and this is because they have to overcome the tendency of TCs to impair image quality. The moral here is NEVER buy a cheap tele-converter! The more pixel dense your sensor, the less it will like a TC.

Canon M5 with Carl Zeiss 180/2.8 and 0.72x FR ISO 400 1/800 second @ f/2.8 (effective 207mm f/2.0)

Since the focal reducer is concentrating more light on the sensor, they tend to maintain and in some cases improve overall resolution by giving the sensor what it really wants… more light! This may be why I have gotten respectable results from a cheap FR. Smaller sensor cameras are generally more pixel dense. My Canon 5d Mk III has 22.6 MP on a 24x36mm sensor. That works out to 160 pixels per millimeter. My EOS M5 has a 24 MP sensor on 22.2 x 14.8mm sensor. That works out to 270 pixels per millimeter. The smaller sensor is far more pixel dense. It likes more light. (this phenomenon is why phone cameras look good outside but suck in low light) I wrote about sensors here.

Below are 5 sample images. I have a set of Tamron Adaptall tele-converters, both the 2x and 1.4x. I only have 3 compatible lenses for the TC units a Tamron SP 17mm f3.5, Tamron 28mm f/2.5 and Tamron SP 500mm f/8. The 500 was the logical choice, but it is a reflex lens fixed at f/8. It is a good reflex lens but in general these cat lenses are not super sharp. But I was able to take all the variable shots from the same vantage point on both the 5D Mk III and M5 with the lens. I made the same blanket adjustments on all the images in Lightroom except the 5D shots have a slight color temperature shift to compensate for the different white balance on that body. The captions under each photo explain the lens-camera-adapter combo and the effective values include the TC or FR and crop sensor value. The shot taken with the 500mm lens and the 1.4x TC on the M5 is an effective 1120mm lens in 35mm/FF terms. here’s the math. 500mm x 1.4x TC = 700mm. 700mm x 1.6 crop factor = 1120mm. My effective values are converted into 35mm/FF equivalents.

Look at the depth of field; it really doesn’t change at all in any of the photos. The lens has a fixed aperture and the focus distance is identical. In theory the depth of field should be much more shallow on the longer focal length shots. It isn’t, because the lens is what it is, a 500mm f/8. That is why the focal lengths with the tele-converter or focal reducer are “effective” focal lengths. The only thing that changes using these devices is the angle of view.

In short, I like focal reducers very much. I will buy a Metabones unit for Canon EF lenses on the EF-M bodies as soon as they make one! Those would have full AF and electronic connection and I could still use my manual lenses with the various EF adapters I have. I recommend the Metabones and/or Lens Turbo focal reducers.