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Old Jan 27, 2011, 12:41 PM   #1
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Default Depth of field and f-stop

Hi.

I have a Canon SX100 IS and a Canon SX130 IS.

The SX100 has f-stop range from F2.8-4.3
The SX130 has f-stop range from of F3.4 to 5.6

I'm a newbie.

One of the things I would like to be able to do is have a shallow depth of field for some of my photos.

If I understand correctly, the lower the f-stop setting the shallower the depth of field can be.

So does that mean, the SX100 can produce images with a shallower depth of field than the SX130?

Thanks.
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Old Jan 27, 2011, 12:46 PM   #2
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DoF depends on focal length as well as f/stop. That is REAL focal length, not 35mm equiv.

Assuming the two camera are used at the same focal length, the SX100 would produce a shallower DoF - by an amount that no one is likely to notice.
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Old Jan 27, 2011, 12:54 PM   #3
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Yes, but the image sensors in those cameras are pretty small, so you wouldn't notice a lot of difference. There are a number of factors that contribute to a shallow depth of field; the aperture is only one. Increasing the distance between the subject and the background will allow you to take advantage of whatever depth of field you've got.
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Old Jan 27, 2011, 3:50 PM   #4
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Agree with TCav, DoF differences are pretty much going to be a wash. The 100 has 1/2.5 sensor and a 6-60mm actual lens vs. the 130s 1/2.3 sensor and a 5-60mm lens with the 100 being about a half stop faster. The larger sensor on the 130 will mostly negate the half stop difference. Seperate the subject and the backgroung as much as you can.

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Old Jan 27, 2011, 10:48 PM   #5
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Thanks for the great info.

If many of the subjects and backgrounds are stationary, I will need to figure out another way, if there is, to reduce depth of field.

Does zooming into a the subject help reduce depth of field?

Do I need to move up to a DSLR in order to get more control over depth of fields?

Thank you.
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Old Jan 28, 2011, 5:37 AM   #6
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Quote:
Originally Posted by testers View Post
Thanks for the great info.

If many of the subjects and backgrounds are stationary, I will need to figure out another way, if there is, to reduce depth of field.

Does zooming into a the subject help reduce depth of field?

Do I need to move up to a DSLR in order to get more control over depth of fields?

Thank you.
It depends when you want that shallow DOF. With macro/close up it is quite easy, with anything else basically impossible.

The 3 elements that control DOF are:

Focal length (real not the 35mm equivalent)
Distance from camera to subject
Aperture

To reduce the DOF (less in focus) then you want the longest lens possible, get the camera close to the subject and the widest aperture.

As has been mentioned, to help enhance the shallow (ish) DOF, get the subject away from the background.

If you are looking for people to be photographed and still seeing a shallow DOF then you really need a dSLR.

Here are the sort of things you can do with a full frame camera (results from a crop camera will not be quite as good but I don't have samples) and quite fast lenses.
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Old Jan 28, 2011, 6:31 AM   #7
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G'day Testers

Another thing for you - that other helpful people above have not addressed is the f-numbers you quote and what they actually mean

You mention "The SX100 has f-stop range from F2.8-4.3"

Firstly - the lens aperture number is a fraction of the focal length ... ie: a camera lens called "f4" means that the lens aperture opening is 1/4 of the focal length, so for a 100mm lens, the lens opening at f4 is 25mm diameter, and also the same lens at f11 has an aperture of 9mm diameter

What the numbers on your camera (and other cameras) means is that as the lens zooms out from wide angle to 'narrow-angle' (called telephoto) because the diameter of the lens aperture opening does not change, it now causes the aperture number to change as it zooms

So your camera's lens at wide angle has a maximum aperture of f2.8 - however as the lens extends outwards during zooming, the maximum opening as a fraction of the zoom lens also changes and becomes a smaller fraction ... now shown as f4.3

Hope this makes sense - if not give a shout and I will try again

Photographically - for you to get the best results for very little DoF, I would take the zoom to maximum (or close to that) and use that to isolate the subject from the background. I do a lot of outdoor portraits and set the camera to 8x or 10x or 12x zoom to get lots of magnification of the subject. It means I have to walk away from the subject to get them into the photo, but it also means that the background does go nice & blurry

Hope this helps a bit
Regards, Phil
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Old Jan 28, 2011, 8:38 AM   #8
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Testers, i don't know what you want to photograph, but try getting 2 feet from your subject. make sure the camera focuses on that subject. see if the background is out-of-focus enough. if there's enough distance between your subject and the background, it might be ok. here's a shot i just did with my casio s12 which has a sensor size 1/2.33 so may be comparable. lens was 4 inches from the coffee can.
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Old Jan 28, 2011, 9:15 AM   #9
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Here is a little tool that shows how it works...You can change f stop and iso and see how it affects the photo.

http://dryreading.com/camera/index.html
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Old Jan 28, 2011, 5:52 PM   #10
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There are some great tips here. Thanks.

I think part of my problem is that the camera that I use has too wide of an in-focus 'box'. So even if I've zoomed into a tree, for example, the in-focus box ALSO includes the houses that are 200'+ behind the tree.

Regarding the Canon SX130, it was given to me to replace the SX100. I haven't opened it yet and since it doesn't seen to have a DOF advantage over the SX100, I will probably sell the SX130 and get something different/better.
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