If I could place the RX100 sensor in A500 for bird shooting, .........

The 1" inch sensor seems good enough for me. And my 100-300 APO becomes 270-810 f5.6. Cool.

Better still, the viewfinder still provide 150-450mm (x1.5) which is much easier to locate the bird far away.

Will Sony make such kind of camera? Is anyone interested in it?

It's called a Nikon 1 V2, only 16 megapixels not 20, but 1" sensor and you can just get an adaptor for any DSLR lens (simplest with Nikon as you get full coupling). You can go even further and get a Pentax Q body with a truly tiny sensor.

The A500 does not give you what the V2 does, a full electronic finder view and focusing from your 'super tele' lens at eye level.

David

No. The crop factor also applies to aperture.

So if you have a crop factor of 2.7 and use a 100-300 f/5.6 lens, it will behave as a 270-810 f/15 lens would on full frame.

That is:

Depth of field will be the same.

Light gathering ability will be the same.

Noise in the full picture will be the same if both sensors are of equal technology.

But:

The maximum amount of light which can be gathered will be heavily reduced, so you will not have the same dynamic range. With the small sensor it will be easier to blow out highlights.

You can mount RX100 on spotting scope such as the high end KOWA 88mm version. It's manual focus of course but you get much big magnification with very goodefeat her details.

Goo defeat her details?

Good feather details?

Heh. I had to thing hard and long before I understood that one.

That, sir, is wrong! The DOF will change but the relative aperture (f-number) will stay the same.

Yes, I am also considering Nikon V2 but I don't have any nikon lens at the moment.

On the contrary, I have Minolta 100-300APO, Minolta 200f2.8HS, Sony 500f8 Reflex .........

But you'll have a very high pixel density sensor resolving very precisely the output of a lens that was never created to the required high precision. You might be very deceived by the output quality, and not gain much compared to a shot from your A500 cropped to the same size.

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I am afraid your understanding is not 100% correct.

The f-value of the lens will remains the same for different sensor size.

Think of using full frame lens in APS-C sensor. f-value won't change.

No, not wrong at all.

The relative aperture to the REAL focal length will stay the same. So his 100-300 f/5.6 will still be a 100-300 f/5.6.

But the OP converted the focal length to 35 mm equivalent focal length. And you can't convert the focal length and think that you get the same relative aperture to the equivalent focal length. You don't. You will have to use the crop factor on both the aperture number and the focal length if you want to find the real equivalent numbers.

I repeat: The DOF and the light gathering ability of a 100-300 f/5.6 on a crop 2.7 sensor will be the same as a 270/810 f/15 on full frame.

See my reply to Luebke.

And if you don't believe me, start doing the math yourself. Or use Google. This is well known and not something I am inventing.

Perhaps the easiest way to convince you is looking at the front element of your lens.

A real 810 mm f/5.6 will require a front element of at least 810/5.6 mm = 145 mm. The front element of your lens is much smaller. It will never be able to gather the same total amount of light from the scene at the same distance and view angle.

I'm not buying it. My 50mm f1.7 FF lens on my A65 is still an f1.7 50mm lens despite the fact the Crop factor makes it a 75mm equiv.

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Tom
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Yes, you are buying it. You just haven't realized it yet, but what you write above is actually in agreement with what I wrote:

It is still a 50 mm f/1.7. It is not a 75 mm f/1.7.

The light gathering ability of any lens (its speed) is determined  by the physical characteristics of the lens only.  That is the ratio between the entrance pupil size and the lens focal length.  It is focal length / diameter of the entrance pupil size (which is the effective aperture).   Nothing else.

For example a 200mm lens set to F4 will have a pupil size of 50mm.  This pupil size does not change dependant on what camera you mount it on.  Therefore what this means is it will always produce the same level illuminance at the focal plane for a subject of a given luminance regardless of what is situated at the focal plane.  It doesn't matter whether the sensor at the focal plane if full frame, aps-c or a 1 inch sensor the light gathering ability of the lens is identical in all cases.

Your post implies a 300mm F5.6 lens loses almost 4 stops of light gathering ability because you mounted it on a small sensor camera.  That is clearly not the case.  Whether that is what you intended to say I don't know but that is how it reads.

I think you are looking at this the wrong way around.   If you want an 810mm F5.6 lens you need to build one that allows you to set a pupil size of approximately 145mm.

The fact when you set your 810mm lens to F15 you end up with a pupil size of 54mm which is close to the same size of pupil you get if you set a 300mm lens to F5.6 does not mean an 810mm F15 lens on a full frame camera is as fast as a 300mm F5.6 lens mounted on a small sensor camera.

They key is the focal length.  If you change the focal you affect the size of pupil size required to illuminate the focal plane with a given amount of light on the subject.  So an 810mm lens requires a pupil size of 145mm to be as fast as F5.6.  A 300mm lens only requires one of 53.5mm to be of the same speed.  What camera you stick them on does not alter this.

That, sir, is wrong!

The f-number is a physical property of the lens and is fixed. What the OP did is correct. The focal length of a lens does not changed just because the sensor behind has a different viewing angle.

The DOF does change, the f-number does not.

The amount of light that is reaching the sensor does not matter when it comes to the f-number. Aperture is a lens property.

This will give you the same amount of light per sensor area.

It will not give you the same total amount of light if the sensors are of different area.

The total amount of light which hits the sensor will be reduced with a factor of 2.7^2 = 7.3x.

If you want to compare that in stops, it is ln(7.3) / ln(2) = 2.9 stops. I don't know where you got "almost 4".

Which is exactly what I wrote in one of my other replies.

It will gather exactly the same total amount of light.

Focal length is a lens property.

Aperture is a lens property.

As long as you respect that for both properties, you will not do anything wrong.

But if you start converting one of the properties to the 35 mm equivalent value, you will also need to convert the other. Otherwise you are making a mistake which leads to a large overestimation of the total amount of light gathered with this lens.

Even dpreview understands this, sometimes. If you look in their tests of pocket cameras, you will see that they often not only state the 35 mm equivalent focal length but also the 35 mm equivalent aperture.

I see your point but I still think you are mixing stuff up.

The equivalent aperture is a possibility to compare the DOF and the total amount of light captured by the sensor. But it does not have any influence on the shutter speed.

Therefore the equivalent aperture is far less important than you think and the real aperture is much more important.

A 50mm F.14 FF lens mounted to an APS-C sensor gives you the same shutter speed as a 75mm F1.4 APS-C lens mounted to an APS-C sensor. The difference between those lenses are the DOF and the amount of glass needed.

The total amount of light gathered is far less important than the resultign shutter speeds. It actually does not matter at all. All that matters is the resulting picture and the noise. The total amount of light has a huge impact on the noise but it's not the only factor that matters.

Using the "real" aperture to determine shutter speeds, re-calculating the correct DOF and testing for noise is much easier to handle and gives you better tools to compare different sensors or lenses than using the equivalent aperture.

Maybe the bigger problem is that sometimes people do not understand that the crop factor does not change the focal length.

with Allan a few weeks ago.  He is exactly right and I did the imaging experiments to prove it with my A77 and A99.

Yes it is true that the lens parameters do not change vs. "crop factor".  But Alan's point about the effective f/# is true when you add the statement that the *same scene* must be imaged onto the full extent of each detector.  That is, you can't just take the scene on the FF camera and crop it to your detector size and see the effect that Allan is talking about.  However, you will see it when you change the position of the camera to image the exact same scene onto your smaller detector.  The apparent speed of the lens on the FF sensor increases greatly.

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That isn't what you said unless I completely miss understood your post. I thought you said it affected the f stop.

"So if you have a crop factor of 2.7 and use a 100-300 f/5.6 lens, it will behave as a 270-810 f/15 lens would on full frame.

That is:

Depth of field will be the same.

Light gathering ability will be the same."

The exposure which is effected by f stop would still be f5.6 not f/15 as you stated. Everything else you said is true.

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Tom
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