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Aperture on compact cameras vs. SLR lenses.
I am planning on buying the new 450d when it hits the stores. I have now a Canon G3 with aperture f2.0-f3.0 and the kit lens on 450D has f3.5-f5.6 which is a big difference. To get a zoom lens with the same aperture I have to pay around 5-10 times more than the whole G3 camera cost. So is the SLR lens worse than the G3 lens if you compare the aperture?.

Is the aperture a relative measure?..

Comments (8)

Bias wrote:.

Is the aperture a relative measure?.

Yes. It is relative to the focal length of the lens. The notation f/4, for instance, means the aperture is 1/4 the focal length of the lens. For a 100mm lens, that would be an opening of 25mm.http://www.flickr.com/photos/p3jason/..

Comment #1

Smaller sensor cameras (compacts) need a shorter focal length to get the same field of view. So a 'standard' lens on a DSLR might have a focal length of 30mm (giving a field of view about the same as human eye) but on a compact it may be, say, 8mm..

The f-number is (more or less) the focal length of the lens divided by the size of the hole that the light goes through. So for f/2, a DSLR lens needs to have a hole 15mm across, but the lens on a compact only needs to have a hole 4mm across. Conversely, if you make a very small lens with a maximum hole size of 2mm, that equates to f/4 on the compact camera (not bad) but only f/15 on the DSLR (rubbish)..

The consequence is that cameras with larger sensors, and longer focal length lenses, tend to have smaller maximum f-numbers to keep the size and weight down. You can get f/2.8 for a standard DSLR zoom (18-55mm or thereabouts) but they are much more heavy and expensive than the smaller-aperture f/3.5 - f/5.6 kit lenses which are small, light and relatively cheap. It doesn't mean that they have poor optical quality: some of the 18-55 kit lenses are very good..

If you can afford it, you can get the DSLR body only and add the lens of your choice, and something like an 18-55 f/2.8 would be a popular choice as it offers an obvious advantagfe in light-gathering power compared to an f/3.5-f/5.6 kit lens..

Best wishesMike..

Comment #2

Bias wrote:.

I am planning on buying the new 450d when it hits the stores. I havenow a Canon G3 with aperture f2.0-f3.0 and the kit lens on 450D hasf3.5-f5.6 which is a big difference. To get a zoom lens with the sameaperture I have to pay around 5-10 times more than the whole G3camera cost. So is the SLR lens worse than the G3 lens if youcompare the aperture?.

If f-stop is your only measurement, than yes the DSLR kit lens is "worse" or slower than the G3 lens..

Is the aperture a relative measure?.

No, but f-stop is .

Technically speaking, aperture is size of the entrance pupil, while f-stop is the focal length/aperture. In terms of exposure, it is the f-stop that counts. Note that the three things that are important for exposure are f-stop, shutterspeed, and ISO..

Now, you may be asking yourself, why get a DSLR if the lens is worse? You are giving up 1.67 stops on the wide end and almost 2 stops on the long end. Well, the trade-off is that you get a cleaner high ISO shot with a DSLR. You get to shoot at ISO1600 on the 450D, while the G3 is limited to ISO400 (2 stops). You also gain more control over DoF and other creative controls. You also get a faster response time (almost no shutter lag and almost instant on). And if you are willing to get a "faster" lens, you loose the only drawback on the DSLR...

Comment #3

The G3 has a DOF that will be difficult at best to nearly impossible at worse to equal on a DSLR..

However with the DSLR you gain:.

1. operating speed2. more creative control.

Technologist @ Large- Mark0..

Comment #4

Mbloof wrote:.

The G3 has a DOF that will be difficult at best to nearly impossibleat worse to equal on a DSLR..

Really ? Let's see some numbers.....

Comment #5

Sigezar wrote:.

Mbloof wrote:.

The G3 has a DOF that will be difficult at best to nearly impossibleat worse to equal on a DSLR..

Really ? Let's see some numbers....

You can go here and plug the numbers in yourself..

Http://www.dofmaster.com/dofjs.html.

Remember to use the ACTUAL numbers from the front of the lense. (7.2-28.8mm).

Point and shoot cameras have VERY short focal lenghts that allow VERY large DOF's. A real PLUS if thats what you want, however a real drawback if you want to have a much smaller DOF..

Technologist @ Large- Mark0..

Comment #6

Given the OP I naturally assumed you meant a shallow depth of field....

Mbloof wrote:.

Sigezar wrote:.

Mbloof wrote:.

The G3 has a DOF that will be difficult at best to nearly impossibleat worse to equal on a DSLR..

Really ? Let's see some numbers....

You can go here and plug the numbers in yourself..

Http://www.dofmaster.com/dofjs.html.

Remember to use the ACTUAL numbers from the front of the lense.(7.2-28.8mm).

Point and shoot cameras have VERY short focal lenghts that allow VERYlarge DOF's. A real PLUS if thats what you want, however a realdrawback if you want to have a much smaller DOF..

Technologist @ Large- Mark0..

Comment #7

Ok I think I get it somewhat I have to do some reading about DoF and f-stops in more detail. Thanks for all the answers!..

Comment #8

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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