Speed booster confusion in DOF

Started Jun 6, 2013 | Discussions thread
EinsteinsGhost Forum Pro • Posts: 11,977
Re: Speed booster confusion in DOF

viking79 wrote:

EinsteinsGhost wrote:

viking79 wrote:

To get exact same images between APS-C and 135 format full frame:

APS-C: 35mm f/1.8 1/100 ISO 100

Full Frame: 50mm f/2.8 1/100 ISO 250

This will give you the same noise performance (given similar sensor performance, like Sony 16 MP vs Sony 24 or 36 MP full frame) same field of view same shutter speed and same depth of field.  These are "equivalent" settings.

To figure these out, use the crop factor to get effective aperture and focal length and for ISO is it crop factor^2?  So if your crop factor is 1.52 you would multiply ISO by 2.31, rounding up to 250 since that is closest 1/3rd stop.  I haven't thought about the ISO one much, but instead just use an ISO chart and go up or down 1 1/3 stops [1] (actual is like 1.23 stops, but 1 1/3 is rounded).

The speed booster basically converts your lens focal length and aperture to a full frame equivalent (or nearly).  This is actually changing the physical focal length and f/stop, but the net result is it is a full frame converter for the camera.


You're also increasing the amount of light reaching the sensor by using a lens as a concentrator, which would otherwise be spread over a larger area. Consequently, increasing exposure.

Correct, that is the how, by compressing the image circle you enlarge the f/stop of the lens, remembering f/stop is focal length divided by aperture.  If you decrease the focal length of the lens with a rear converter (not with a front converter) your aperture remains the same apparent size from the front.  This means your f/stop increases and more light is hitting the sensor.

A teleconverter is the opposite.  A 1.5x TC converts your APS-C lens into a lens usable on full frame.  I.e. you could mount any APS-C lens and get a full frame image circle on a 1.5x TC and use it on your full frame camera.  This direction is usually less desirable though


Actually, it is about more light hitting over a smaller area than before, that increases the exposure, the lens in the adapter serving as a light collector. Each pixel is now getting more photons than would be available without the lens element in the adapter, or on FF.

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