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One approach to saving weight while increasing telephoto reach is to use a smaller sensor. e.g. that Nikon 300mm f/2 lens can be mounted to a modern Micro Four Thirds camera (e.g. Olympus or Panasonic) for 600mm of equivalent reach while retaining the f/2 light gathering ability and unmodified optics. Adapters are very inexpensive (e.g. Voigtländer) compared to the cost of lenses like this or good quality bodies.

The other path to doubling reach would be a 2x teleconverter which would reduce light gathering to f/4 and introduce glass elements that won't do image quality any favours.

Small sensor downside: high ISO ability. Probably two stops lost, but with modern MFT sensors you can still shoot up to ISO 6400 with great results.

Small sensor downside or upside, depending on your needs: deeper depth of field (twice the depth of field, in this case, which is still pretty amazing for 600mm!)

2 comments

At the risk of sounding pedantic - the f-stop* roughly equates to "light per area" or brightness, so if you're using crop factor, you need to apply it to the f-stop as well as the focal length.

So for a given full-frame lens at a given f-stop, if you drop down to an APS-C sensor, you're reducing the amount of light gathered by about one stop (half). On an m43, you're obviously losing 75% of the light gathering capacity (two stops).

Putting a 300mm f/2 lens on a micro 4/3rds body should result in images similar to a full-frame camera with a 600mm f/4 lens.

*Obviously t-stop would be better, since it's an actual measurement of brightness.

Except you're not. This is a popular talking point in 35mm circles, but the amount of light on a given area on the sensor remains unchanged, which is what actually matters.

The smaller sensors are challenged to keep their noise down, given they typically have a smaller pixel pitch to keep up the megapickel count.

Noise is the key, as far as I can tell a FF sensor usually has at least 1 stop more usable ISO compared to a crop of the same generation.

So to get a comparable shutter speed, assuming 1 stop less ISO, you still need that 400mm f/2.8 on crop to match the 600mm f/4 on FF.

Humorously, they have the same front element size.

> the amount of light on a given area on the sensor remains unchanged

This isn't the only thing that matters. It also matters that you're amplifying the noise more by enlarging it 4x as much as you would have on the larger sensor. It's analogous to film in that a larger format will exhibit less obtrusive grain than a smaller one despite using the exact same film speed. You're using the same exposure over a larger area, and thus gathering more light to drown out the noise.

You can either think of it in terms of total light gathered compared with the underlying noise of the sensor (signal to noise ratio) or in terms of enlargement of the image. The result is the same, smaller sensors have "more noise" simply because they're gathering less total light and amplifying everything more.

And don't be fooled by the old "higher megapixel = more noise" fallacy. All else being equal, it's the same amount of noise. However each pixel is gathering less light (signal) to offset the noise, so the signal to noise ratio is worse when comparing pixel for pixel. However, if you were to downscale the output of the higher res sensor to match the lower resolution sensor, you'd find roughly the same amount of noise in both images.

John Hess has a good explanation of these trade offs. https://youtu.be/MytCfECfqWc

> One approach to saving weight while increasing telephoto reach is to use a smaller sensor

I make a 'soft' change to the the sensor size on my Nikon D850 by switching between FX (full frame) and DX (crop) mode. DX gives me a 1.5 effective multiplication of the focal length, but a still very good resolution of 5408x3600 pixels (uncropped FX is 8256x5504).

In some shooting modes (e.g. slow motion video at 1920x1080;30p x4) the camera automatically switches into DX mode, presumably to maintain a high framerate for a sustained period. I've now learned to select a DX crop in advance if there is a chance I might need slow-mo so that I don't get a jarring change in effective focal length mid-way through a shoot.