Aperture and sensor size?

[Sarah J Connor]

Here is the link
http://forums.dpreview.com/forums/read.asp?forum=1038&message=27080036

Obviously no compact digital cameras with such lenses exist, which is
why the DP1 has better low light performance than any compact camera
with a small sensor, despite having a slower lens than most of them.

So you are saying that, under a given lightning condition, a 2/3"
sensor at F4 and 28mm equiv. lens needs longer exposure time than the
X3 sensor at F4 and a 28mm equiv.lens?

I somehow doubt this.

and see if you have more question.

O.
--
http://www.flickr.com/photos/ollivr/
http://www.flickriver.com/photos/ollivr/popular-interesting/

--
Sarah (no relation to terminator's Sarah)

--
Sarah (no relation to terminator's Sarah)

 

[GenGSP]

Tiffles:

So you are saying that, under a given lightning condition, a 2/3"
sensor at F4 and 28mm equiv. lens needs longer exposure time than the
X3 sensor at F4 and a 28mm equiv.lens?

At the same ISO, no. Lighting, ISO, aperture and exposure time combine to make an over-constrained system. Any three are enough to determine the fourth, regardless of the camera.

However, if (as Sarah seems to imply) you want the two cameras to produce the same (or similar) resultant IQ, then the 2/3" sensor has to reduce ISO or have a longer exposure time.

All of this pointless bickering really makes me sick sometimes. Yet I still join in. Sigh...

P.S. Sarah, in the future you can just 'Edit' your posts, rather than make another post, when you forget a link like that. Kinda like how I just edited this post to add this P.S.

 

[light_reaction]

OK so do I understand it right, Sarah?

The sensor makes for better light sensitivity and less overall noise, the slower lens makes for less light being passed through. Thus exposure is the same for both cameras at a given F-stop and exposure is half as long for a lens with an F/2.8 aperture. Thus the question is whether the sensor can overcompensate the stop less brightness being passed on from the lens by its inherent superiority. Which us old Foveon users certainly will confirm for many lightning conditions.

Is that correct?

Cheers,

O.
--
http://www.flickr.com/photos/ollivr/
http://www.flickriver.com/photos/ollivr/popular-interesting/

 

[light_reaction]

All of this pointless bickering really makes me sick sometimes. Yet
I still join in. Sigh...

But it is no pointless bickering, Sarah and me are learning stuff here from Philidor's Shadow!

O.

--
http://www.flickr.com/photos/ollivr/
http://www.flickriver.com/photos/ollivr/popular-interesting/

 

[Sarah J Connor]

O,

I must be honest that I am not sure what you are asking exactly? I don't think we are talking about superior but in term of the size of the sensor alone that effects the aperture.

OK so do I understand it right, Sarah?

The sensor makes for better light sensitivity and less overall noise,
the slower lens makes for less light being passed through. Thus
exposure is the same for both cameras at a given F-stop and exposure
is half as long for a lens with an F/2.8 aperture. Thus the question
is whether the sensor can overcompensate the stop less brightness
being passed on from the lens by its inherent superiority. Which us
old Foveon users certainly will confirm for many lightning conditions.

Is that correct?

Cheers,

O.
--
http://www.flickr.com/photos/ollivr/
http://www.flickriver.com/photos/ollivr/popular-interesting/

--
Sarah (no relation to terminator's Sarah)

 

[GenGSP]

Carry on..

 

[Sarah J Connor]

Gary,
Thank you, it is precisely the question I asked.

The maths is simple.

The f number is the focal length of the lens divided by the diameter
of the aperture.

So for the 16.6 mm lens on the DP-1, the aperture is 4.15 mm wide
fully open.

On typical P&S camera with a 6 mm lens, f/4 is an aperture 1.5 mm
wide. Similarly an aperture the same size as that of the DP-1 would
be equivalent to a f/1.4

As the focal length gets smaller as the sensor gets smaller (for a
given angle of view) and the physical size of any particular f stop
gets smaller in proportion.

More light has to enter the lens for the bigger sensor, but that
light is spread over a greater area, so the exposure remains the same
for any given f stop.

--
Thanks,
Gary.

--
Sarah (no relation to terminator's Sarah)

 

[light_reaction]

Philidors Shadow said the low light performance would only be equal with lenses in the F1 to F2 ballpark.

I asked him whether the exposure time would be different for same apertures/FLs and alternative sensor sizes because I doubted this. This was the only question I had.

You then referenced the thread from the Leica forum which had another, much wider scope (image quality) than what I asked for (exposure time). Due to the wider scope you brought into the play, I myself tried to adjust to it and put things into perspective and asked if what I wrote was correct.

O.

OK so do I understand it right, Sarah?

The sensor makes for better light sensitivity and less overall noise,
the slower lens makes for less light being passed through. Thus
exposure is the same for both cameras at a given F-stop and exposure
is half as long for a lens with an F/2.8 aperture. Thus the question
is whether the sensor can overcompensate the stop less brightness
being passed on from the lens by its inherent superiority. Which us
old Foveon users certainly will confirm for many lightning conditions.

Is that correct?

Cheers,

O.
--
http://www.flickr.com/photos/ollivr/
http://www.flickriver.com/photos/ollivr/popular-interesting/

--
Sarah (no relation to terminator's Sarah)

--
http://www.flickr.com/photos/ollivr/
http://www.flickriver.com/photos/ollivr/popular-interesting/

 

[Sarah J Connor]

Philidors Shadow said the low light performance would only be equal
with lenses in the F1 to F2 ballpark.

I asked him whether the exposure time would be different for same
apertures/FLs and alternative sensor sizes because I doubted this.
This was the only question I had.

You then referenced the thread from the Leica forum which had
another, much wider scope (image quality) than what I asked for
(exposure time). Due to the wider scope you brought into the play, I
myself tried to adjust to it and put things into perspective and
asked if what I wrote was correct.

Ok, I did not mean to open up another variance. For now I just want the equivalent of aperture between the size of the sensors. Thanks for explaining, I rather get one thing right before expanding, less mudding at this time.

O.

OK so do I understand it right, Sarah?

The sensor makes for better light sensitivity and less overall noise,
the slower lens makes for less light being passed through. Thus
exposure is the same for both cameras at a given F-stop and exposure
is half as long for a lens with an F/2.8 aperture. Thus the question
is whether the sensor can overcompensate the stop less brightness
being passed on from the lens by its inherent superiority. Which us
old Foveon users certainly will confirm for many lightning conditions.

Is that correct?

Cheers,

O.
--
http://www.flickr.com/photos/ollivr/
http://www.flickriver.com/photos/ollivr/popular-interesting/

--
Sarah (no relation to terminator's Sarah)

--
http://www.flickr.com/photos/ollivr/
http://www.flickriver.com/photos/ollivr/popular-interesting/

--
Sarah (no relation to terminator's Sarah)

 

[SandyF]

...today at the Norfolk zoo
http://www.pbase.com/sandyfleischman/image/94116818/large
and basking in the sunshine
http://www.pbase.com/sandyfleischman/image/94116826
Best regards, Sandy
[email protected]
http://www.pbase.com/sandyfleischman
http://www.flickr.com/photos/sandyfleischmann

added: one more http://www.pbase.com/sandyfleischman/image/94117257

 

[Sarah J Connor]

Without goats' sure footing makes one a bit unsure. Nice shots and cute subject to add.

...today at the Norfolk zoo
http://www.pbase.com/sandyfleischman/image/94116818/large
and basking in the sunshine
http://www.pbase.com/sandyfleischman/image/94116826
Best regards, Sandy
[email protected]
http://www.pbase.com/sandyfleischman
http://www.flickr.com/photos/sandyfleischmann

added: one more http://www.pbase.com/sandyfleischman/image/94117257

--
Sarah (no relation to terminator's Sarah)

 

[Philidors shadow]

So you are saying that, under a given lightning condition, a 2/3"
sensor at F4 and 28mm equiv. lens needs longer exposure time than the
X3 sensor at F4 and a 28mm equiv.lens?

I somehow doubt this.

And you're absolutely right to doubt this, if the ISO is the same. Then the exposure time must be the same, because the intensity of the light on the sensor is the same. But the fact that many more photons in total fall on the larger sensor per unit time, at the same f-stop, i.e. the larger lens transmits more light at the same f-stop, means that the larger sensor can deliver a higher signal-to-noise ratio. This provides better quality. Or the ISO can be increased for the same image quality, offering more flexibility with shutter speed and depth of field.

Alternatively, to equal the signal-to-noise ratio of the large-sensor camera, the small-sensor camera could employ a much faster lens (around f/1.2 to f/1.5 to compare to the Sigma's f/4 lens, as discussed in previous posts). A lens of this speed on a small-sensor camera would additionally provide more or less the same depth of field control as the DP1 offers. But making such a lens that offers comparable quality at f/1.2 to the DP1's lens at f/4, is no trivial matter. In fact, it would likely cost much more than the entire DP1.

A lens roughly comparable to the DP1's lens, but for the much smaller 2/3" format, does actually exist. It's called the Carl Zeiss DigiPrime, and it has a focal length of 7 mm (slightly wider than the DP1's lens) and a speed of f/1.5 (slightly higher photon throughput than the DP1's lens at f/4). It has an MTF value of over 90% at 56 cycles/mm, which is only slightly sharper than the DP1's lens at f/4, when taking into account the format size difference. Despite only marginally beating the DP1's lens in these parameters, it requires 18 elements in 14 groups to achieve this performance. The lens therefore weighs about 6x more than the entire DP1 camera, and is 164 mm long and 95 mm in diameter. It costs tens of thousands of dollars and is used for digital cinematography.

You can read about the Zeiss DigiPrimes here: http://www.digiprimes.com/

 

[Roland Karlsson]

Better check you physics. At f4 on any lens you will get the same
light energy per area. On a sensor with the same ISO you will get the
same exposure. Has nothing to do with resolution. Has noting to do
with ISO.

I am very knowledgeable in physics. What you say is correct - except the part that I have to check my physics.

It is you that have added "per area" though - not me.

The total light energy that hits the entire sensor is the amount of light (potentially) collected.

And if the sensor is bigger - it collects more light at the same F-stop.

A photograph is about the light per unit area captured - not the
total light energy captures.

Not at all. It is about the amount of light that you capture from the object you want to take a photo of. Larger sensors means larger cameras means larger apertures means more light collected per time unit.

Sure you capture more total light energy
on an 8x10 sheet of film than you do on an ASP sensor by the ratio of
the sensor area. However, for energy per square mm - they are the
same.

Yes - and therefore you either have a potential for more resolution or a potential for using a more sensitive film. Both are gains caused by the larger size of the thing.

As I said - I am very knowledgeable in physics - and it is my experience that way more than 50% of those that discuss the topic of collected light (which happens now and then) do not understand physics. You do understand physics - but you have not seen the light yet regarding how it best is used to make images

--
Roland

 

[Roland Karlsson]

As Roland said, you're quite right in your thinking here, Sarah,
though few photographers including practising pros understand this
well. Roughly speaking, to get a similar depth of field from two
cameras with different sensor sizes, with lenses of equivalent angle
of view, the apertures (or entrance pupils, to be more accurate) must
be of equal diameter. This would also deliver the same amount of
light to the sensors, resulting in equal noise performance if all
other factors are identical (which they never are, with smaller
sensors often performing better than expected).

I am amazed! Someone that have seen the light!

The DP1 has a lens of 16.6 mm focal length and minimum f-stop 4. The
maximum aperture is therefore 16.6 / 4 = 4.15 mm.

The DP1's sensor size is 20.7 x 13.8 mm, while a 1/1.7" sensor (as
used in the Canon G9, for example), is 7.60 x 5.70 mm, or about 2.6
times shorter in the diagonal. So a 28 mm-equivalent lens on such a
sensor would require a focal length of 6.3 mm. And for a 6.3 mm lens
to have a maximum aperture of 4.15 mm would require a minimum f-stop
of f/1.5.

If the sensor were 1/2.5", another common size, the lens would have
to open up to fully f/1.2 to get comparable depth of field and photon
throughput as the DP1's f/4 lens.

Obviously no compact digital cameras with such lenses exist, which is
why the DP1 has better low light performance than any compact camera
with a small sensor, despite having a slower lens than most of them.

My compliments! Well done!

--
Roland

 

[Ed_S]

But - if you mean "amount" of light per time unit and area unit, i,e, light
intensity - then you are of course right.

For better or worse you restored my sanity.

Kind regards,
--
Ed_S
http://www.pbase.com/ecsquires

 

[rick decker]

Ship that goat over to Silver Oaks Ranch...my girl goaties have been asking for him!!

--
It's a tough job living in Hawaii but somebody's got to do it.

See Sigma Lens Tests At:
http://www.lightreflection.com/sigmalenstests/matrix.htm



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