[lotech]

While day dreaming an idea pops up on my mind, to make DSLR thinner.

As far as I understand, on traditional SLR design, the metering cell was put on the pentaprism, except models with inter-changable prism, what if we put the image sensor on one surface of the prism, and meter with another on anther surface ? I don't know if that is technically possible due to the optical path, and we will need a super clear prism also a very durable and clear mirror, or better to seal the section to protect from scratch and dirt, similar to Sigma's SD Quattro with removable IR filter.


If my 'design' can be done then :

1. We will get true "what we see is what we get", since the image sensor is on the same optical path we preview the image.

2. We only need a fixed mirror no vibration.
3. We will have full time electronic shutter and no black out.

4. We will have a thinner body (to move the image sensor and shake reduction unit to the prism)


But problems are :

1. We need a very large prism to make a full frame body.

2. May not be easy to put the shake reduction unit on the prism.
3. Very hard to put a physical shutter.
4. If no shutter, the sensor may suffer and degrade from full time expose to strong light (need mirrorless users to share their experience)

The above is just a wild thought, even can be done may not worth while due to the cost and complex engineering !

[UncleVanya]

I’m not sure I get this.

If the sensor is on any surface of the prism - light stops there. If it reflects off that’s lost light for our exposure. If it doesn’t reflect off what do we look at?

Metering directly on the sensor is what mirrorless cameras do. Why do I need a separate sensor to meter with in this design? Sensors generate heat - where will it go?

[lotech]

I’m not sure I get this.

If the sensor is on any surface of the prism - light stops there. If it reflects off that’s lost light for our exposure. If it doesn’t reflect off what do we look at?

Metering directly on the sensor is what mirrorless cameras do. Why do I need a separate sensor to meter with in this design? Sensors generate heat - where will it go?

As far as I understand, traditional SLR design put the metering cell on the prism and does not cause blackout to the view.

[Fogel70]

I do not see the point in making the camera thinner by making the prism housing much much bigger.

Every light used for capturing the image can not also be used for the optical viewfinder. You will need a way to direct the light to the sensor or the viewfinder. Or divide the light so like halv of it goes to either one. Making the viewfinder darker and leading to worse sensor performance.

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Last edited by Fogel70; 06-10-2022 at 11:14 PM.

[lotech]

I do not see the point in making the camera thinner by making the prism housing much much bigger.

Every light used for capturing the image can not also be used for the optical viewfinder. You will need a way to direct the light to the sensor or the viewfinder. Or divide the light so like halv of it goes to either one.

Then how would you explain traditional SLR metering sensor on the prism does not 'eat up' the light ? I simply replace the meter cell with image sensor.

[shyrsio]

I think there already is such a camera, the SLT models of sony alpha line.

[Fogel70]

Then how would you explain traditional SLR metering sensor on the prism does not 'eat up' the light ? I simply replace the meter cell with image sensor.

They do eat up light, but they just use a small amouny of the light entering the prism. A sensor configuration like that would loose multipla stops of light.

[lotech]

I think there already is such a camera, the SLT models of sony alpha line.

Thanks for the tips, as far as I know there were at least 2 SLR used fixed 'half mirror', both are from Canon and I played with one an EOS.

---------- Post added 06-11-22 at 03:15 PM ----------

They do eat up light, but they just use a small amouny of the light entering the prism. A sensor configuration like that would loose multipla stops of light.

It depends on how dim the final view is due to the sensor, so the design will work but may not worth to do it.

[Fogel70]

It could maybe work with a moving mirror in front of the sensor that let most light back to the viewfinder before capture, the moving away letting all light to the sensor during capture, blacing out the viewfinder.

[biz-engineer]

I'd rather have pancake lenses from 20mm all the way up to 600mm.

[BigMackCam]

It could maybe work with a moving mirror in front of the sensor that let most light back to the viewfinder before capture, the moving away letting all light to the sensor during capture, blacing out the viewfinder.

Let's see - a prism big enough that an APS-C or full-frame sensor would fit completely on one surface, then a moving mirror in front of it... I'd love to see the prism housing on this thing

[StiffLegged]

Inside an SLR or DSLR are two places an image may be focussed. One is the film plane (SLR) or image sensor (DSLR) and the other is the focussing screen directly under the prism. The focussing screen and what is (or isn’t) focussed there is what you’re looking at through the viewfinder until you press the tit and the mirror flips out of the way so the image is now focussed (hopefully…) on the film or sensor to take the shot. The exposure sensor looks at the focussing screen too, wherever in the prism housing it may be fitted.

That’s why there isn’t room to shrink the DSLR below the existing mirror box/prism housing constraints. A fixed semi-silvered (pellicle) mirror such as Canon played with years ago reduces both viewfinder brightness and light actually reaching the film or the sensor. That’s why they didn’t catch on.

[lotech]

... I'd love to see the prism housing on this thing

It would be about the size of the head of the lead engineer working on the project

[Fogel70]

Let's see - a prism big enough that an APS-C or full-frame sensor would fit completely on one surface, then a moving mirror in front of it... I'd love to see the prism housing on this thing

Don't forget adding IBIS to the sensor and support for IBIS on the prism.

[biz-engineer]

Don't forget adding IBIS to the sensor and support for IBIS on the prism.

You can add a stabilizing rig outside the thinner DSLR camera.