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.
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?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.
Oh...what about at the same focal length?adamadam said:depends on the focal length
Ahh! I think I understand now, thanks! So it has nothing to do with the sensor.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.
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?
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?
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.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 for your input as well, zcf! Good point about more light for focusing.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.
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 would say the physical aperture opening is also a factor.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.
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!).
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.