Aperture difference between compacts and SLR


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Apr 15, 2005
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#1
It is right to say that a f/2.8 on a compact and a f/2.8 on a SLR is of different wideness physically? Meaning the aperture hole on the SLR is larger.
 

Jul 17, 2005
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#2
of course it's differnet. i think the f/1.8 on the 85/1.8 lens is bigger than the entire compact cam lens itself.
 

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#4
Isaiahfortythirtyone said:
of course it's differnet. i think the f/1.8 on the 85/1.8 lens is bigger than the entire compact cam lens itself.
I suppose that means the actual physical aperture hole size of a particular f-stop is relative to the sensor size? The larger the sensor, the larger the hole, even if at the same f-stop?

adamadam said:
depends on the focal length :)
Oh...what about at the same focal length?
 

raptor84

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#5
The f number is realtive measurement of the diameter of the aperature with respect to the focal length. So a 50mm lens at f/2 will have an aperature diameter of 25mm.
 

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#6
raptor84 said:
The f number is realtive measurement of the diameter of the aperature with respect to the focal length. So a 50mm lens at f/2 will have an aperature diameter of 25mm.
Ahh! I think I understand now, thanks! So it has nothing to do with the sensor.

Which means because most compacts, including prosumers, has very short actual focal length (not in the 35mm equiv. form I believe), therefore the aperture diameter is very small. Which leads to huge DOF.

And compacts can only use small focal lengths because of their small sensor size?

Am I right? :D
 

singscott

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#7
Err. The main differents are the focus length and COC on these two different type of camera. Focus length on the compact are usually shorter like from 3mm to 50mm fitted lens and SLR is from 8mm to 1000mm depending you lens you use. Shorter lens the more DOF you have. But the bigger cause for concent is the COC. Circle Of Confusing. Basicly in a lay man term, lens are design differently according to their film or sensor size even they are of the same focus length. This different will cause changes in DOF. So the smaller the sensor like the ones use in copmpact will even more DOF. The bigger your film or sensor size the less DOF you have. I think the camera manufacturer design more DOF on compact on purpose, as these compact cameras are normally use by people who don't brother about shallow DOF but easier to get a clearer picture. :)
 

MDZ2

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#8
kniveswood said:
Ahh! I think I understand now, thanks! So it has nothing to do with the sensor.

Which means because most compacts, including prosumers, has very short actual focal length (not in the 35mm equiv. form I believe), therefore the aperture diameter is very small. Which leads to huge DOF.

And compacts can only use small focal lengths because of their small sensor size?

Am I right? :D
You are right. Because, of the crop ratio, a compact camera needs shorter focal length to fill the same frame as a 35mm camera.
 

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#9
I just upgraded to a DSLR and did a brief test. Seems like although the physical aperture size of the DSLR lens is larger/wider at the same f/stop (compared to compacts) and gives shallower DOF, it gives no advantage in terms of amount of light.

In other words, I shot the same subject at the same ISO, same focal length (35mm equiv.) and same aperture in aperture-priority mode with both my Nikon D50 and Canon S2 IS. And both the matrix/evaluative meterings on the cameras gave me the same shutter speed reading.

And from the pictures, both camera seem to hv taken similarly exposed pictures.

Am I doing something wrong? Or the larger/wider aperture size of the DSLR at the same f/stop really gives no light advantage? Could it be that the larger the sensor, the amount of light necessary is relatively larger?
 

singscott

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#10
kniveswood said:
I just upgraded to a DSLR and did a brief test. Seems like although the physical aperture size of the DSLR lens is larger/wider at the same f/stop (compared to compacts) and gives shallower DOF, it gives no advantage in terms of amount of light.

In other words, I shot the same subject at the same ISO, same focal length (35mm equiv.) and same aperture in aperture-priority mode with both my Nikon D50 and Canon S2 IS. And both the matrix/evaluative meterings on the cameras gave me the same shutter speed reading.

And from the pictures, both camera seem to hv taken similarly exposed pictures.

Am I doing something wrong? Or the larger/wider aperture size of the DSLR at the same f/stop really gives no light advantage? Could it be that the larger the sensor, the amount of light necessary is relatively larger?
Exposure wise they are the same regardless F2.8 on compact and F2.8 on a DSLR lenes, as it base on a common math calculation related to amount of exposure value avaliable in the reflected light itself.

But DOF they are different because of their focal lengths, focusing distance and sensor sizes. Focal length here are not the same. When the camera manufacturer say (35mm equiv), they mean this different focal length will give you the same viewing angle as 35mm equiv. on that sensor. But you are still using a focal length that shorter that the 35mm equiv. So more DOF due to the shorter focal length. The different sizes of the sensor doesn't have a effect on the exposure, but it does affect DOF. The bigger your sensors or film size, the lenes design for it will have less DOF due to COC.

DOF here means Depth Of Field and COC here means Circle Of Confusion.
 

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#11
singscott said:
Exposure wise they are the same regardless F2.8 on compact and F2.8 on a DSLR lenes, as it base on a common math calculation related to amount of exposure value avaliable in the reflected light itself.

But DOF they are different because of their focal lengths, focusing distance and sensor sizes. Focal length here are not the same. When the camera manufacturer say (35mm equiv), they mean this different focal length will give you the same viewing angle as 35mm equiv. on that sensor. But you are still using a focal length that shorter that the 35mm equiv. So more DOF due to the shorter focal length. The different sizes of the sensor doesn't have a effect on the exposure, but it does affect DOF. The bigger your sensors or film size, the lenes design for it will have less DOF due to COC.

DOF here means Depth Of Field and COC here means Circle Of Confusion.
Thanks singscott! That's so helpful, now I understand. Previously, I thought the larger aperture would certainly result in more light. Now i know otherwise.

Hmmm...then I guess the IS on a compact effectively cancels out a DSLR's ISO advantage in low light. Now I see why constant/wide aperture telephoto lenses and IS lenses are so highly priced (a few exceptions of coz. eg. 50mm f/1.8). :D
 

zcf

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#12
Though the exposure/time are the same. There are more available light for the SLR camera to focus under low light condition, so SLR can generally focus better under low light than compact camera. And one of the main advantage of DSLR is you can up the ISO with acceptable noise increase, if compare to compact camera.
 

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#13
zcf said:
Though the exposure/time are the same. There are more available light for the SLR camera to focus under low light condition, so SLR can generally focus better under low light than compact camera. And one of the main advantage of DSLR is you can up the ISO with acceptable noise increase, if compare to compact camera.
Thanks for your input as well, zcf! Good point about more light for focusing.

But as for ISO, I believe IS on a compact effectively cancels out the advantage? Or maybe leaving only 1 stop advantage. That's assuming that no IS/VR lens is used (not exactly affordable to most!).
 

michhy

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#14
zcf said:
Though the exposure/time are the same. There are more available light for the SLR camera to focus under low light condition, so SLR can generally focus better under low light than compact camera.
I dont get this.
 

Nerd

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#15
Given the same f-stop and same focal length, the aperture on a DSLR lets in more light than the compact camera since the hole is larger, thus autofocus can work function better.
 

#16
sorry to interrupt, i've been trying to understand the reason why compact cameras couldn't archieve shallow DOF as compare to DSLR.

in short, can i say that the small sensor size and the short focal length of compacts are the 2 factors that cause more DOF to appear in the picture?

reasons are small sensors require shorter focal length to actually archieve the wideness of bigger sensors dued to the crop factor, and shorter focal length theorically increase the DOF?

I'm trying to write in my own words for better understanding, pls correct me if i'm wrong.
 

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#17
jack_low said:
sorry to interrupt, i've been trying to understand the reason why compact cameras couldn't archieve shallow DOF as compare to DSLR.

in short, can i say that the small sensor size and the short focal length of compacts are the 2 factors that cause more DOF to appear in the picture?

reasons are small sensors require shorter focal length to actually archieve the wideness of bigger sensors dued to the crop factor, and shorter focal length theorically increase the DOF?

I'm trying to write in my own words for better understanding, pls correct me if i'm wrong.
I would say the physical aperture opening is also a factor.
 

Apr 12, 2005
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#18
kniveswood said:
Thanks for your input as well, zcf! Good point about more light for focusing.

But as for ISO, I believe IS on a compact effectively cancels out the advantage? Or maybe leaving only 1 stop advantage. That's assuming that no IS/VR lens is used (not exactly affordable to most!).
IS/VR is no perfect substitute for better sensor with lower noise at higher ISO.

If proper exposure is :
F/2.8 @ 1/15, ISO 100, focal length 50mm (actual, not 35mm equivalent).

If you use a prosumer with IS/VR, then you may be able to prevent handshake blur because of IS/VR. However any fast action will appear as motion blur on the picture because of a slow shutter speed 1/15.

In contrast, if you use ISO 800 on a DSLR, your shutter speed can go up to 1/125 to have the same exposure as ISO 100. Not only handshake blur can be prevented, any reasonably fast action will be freezed and most motion blur will be prevented.
 

#19
24 f2.8 ===> @f/2.8 len is 8.57 mm diameter
35 f2 ===> @f/2 len is 17.5 mm diameter
50 f1.8 ===> @f/1.8 len is 27.7 mm diameter.
80-200 f2.8 ===> @f/2.8 len is 28-71mm diameter


most of them are already quite big. some are even bigger than the whole size of compact, width-wise.
 

singscott

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#20
jack_low said:
sorry to interrupt, i've been trying to understand the reason why compact cameras couldn't archieve shallow DOF as compare to DSLR.

in short, can i say that the small sensor size and the short focal length of compacts are the 2 factors that cause more DOF to appear in the picture?

reasons are small sensors require shorter focal length to actually archieve the wideness of bigger sensors dued to the crop factor, and shorter focal length theorically increase the DOF?

I'm trying to write in my own words for better understanding, pls correct me if i'm wrong.

You got it about right. But smaller sensor also made the camera manufacturer to make lens with smaller image circle. So this will course smaller COC facter, smaller the COC you will have more DOF. So it is both the shorter focal length lens and smaller sensor size that give compact more DOF;)
 

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