Speed booster confusion in DOF

Started Jun 6, 2013 | Discussions thread
kkx
kkx
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Re: Speed booster confusion in DOF
In reply to EinsteinsGhost, Jun 7, 2013

EinsteinsGhost wrote:

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.

Eric

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

Eric

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.

That was my understanding too.

But Eric's explanation is also very popular and seems logical so I am still a bit confused now.

Adding the speech booster to a 100mm F2.8 lens will give an effective 100/1.5 mm F2 lens for my nex-6 ?

using DOF calculator at : http://www.dofmaster.com/dofjs.html

on nex-7/nex-6:

no speed booster: 100mm F2.8 at 10f DOF = 0.33f

with speed booster: 66.7mm F2 at 10f DOF = 0.54f

FOV will be 1.5 crop vs FF, at ISO 100, shutter speed for second case (with speed booster) will be faster (2x) due to effective F2 on the lens.

IS this correct?

I was thinking :

no speed booster: 100mm F2.8 at 10f DOF = 0.33f

shutter speed = 1/1200, ISO 100

with speed booster: 100mm F2.8 at 10f DOF = 0.33f

shutter speed = 1/2400, ISO 100

give the same exposure, FOV of 1.5 crop vs FF (because I lock the focus distance at 10f for both). Same DOF.

Shutter speed is faster because light is concentrated so per-mm-square on the sensor will get more light.

So I am now not sure which is correct. Maybe these are just two view of the same thing, but I am still missing something, I think.

Because, for view 1, DOF change from 0.33 to 0.54, but view two DOF is constant. Help me to clarify this please. If I have the speed booster, I could run some test (shooting a ruller). But I have yet to buy one.

-kk

edit: spacing was messed up, fixed

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