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Old Aug 1, 2006, 5:16 PM   #1
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Depth of field doesn't seem to be very restricted even at f4.0 on full telephoto. I understand that with the smaller ditial sensors (I have a Canon A620), although you can restrict the depth of field somewhat,creating a very short depth of field is almost impossible (as compared to my film SLR which is fabolous at f4.0 full tele). QUESTION: Does anyone know if using a teleconverter lens (say 2X) on the A620 would overcome this problem? Or would it just balance out because in most cases you would have to move further away from your subject to get the same composition ... and this greater distance might just counteract any benefit the teleconverter provided ... even if the teleconverter did provide a benefit technically??? Sorry if this seems confusing, but you guys out there who know what you're doing will underastand what I'm getting at. Thanks folks!
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Old Aug 2, 2006, 7:24 AM   #2
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I answer this question periodically. So, I'm just copying an answer from another recent thread to here:

Get closer, frame tighter.

As a result of a very tiny sensor compared to 35mm film, the lens on most non-dslr digital cameras can have a much shorter actual focal length, to get any given 35mmequivalentfocal length (i.e, the same angle of view as you'd have with the lens on a 35mm camera)..

Look at the front of your lens and you'll probably see the actual focal length printed (along with it's aperture ratings for the wide angle and full telephoto zoom positions).

So, your subject occupies a much larger percentage of the frame at any given actual focal length, compared to a 35mm camera at the same distance to subject with most digicams, and you'll have much more Depth of Field compared to a camera with a larger sensor (or film).. This isbecause Depth of Field is computed by the actual versus 35mm equivalent focal length,focus distance,and aperture.

Your ability to blur the background for any given aperture depends on your subject size, the percentage of the frame you need it to occupy (which you can use focal length or the distance to your subject to change), and the distance to the background that you want your subject to stand out from. Of course, using the largest available aperture (represented by the smallest f/stop number) helps - but this is usually not enough to achieve the desired results for larger subjects with most non-DSLR models (i.e., your people photos).

Your best bet is to frame as tightly as possible (fill the frame by getting in closer or using more zoom). In other words, go for a tight head and shoulders, versus a full length shot. You'll want to use the camera's largest available aperture (smallest f/stop number), and put as much distance as possible between the subject and background.

You could also try focusing in front of the subject (so that your subject is barely in the area of acceptable sharpness).

Load this Depth of Field Calculator and selectyour camera model. Then, plug in the *actual* focal length of the lens, focus distance and aperture to calculate Depth of Field. Keep in mind that "acceptable sharpness" is not the same thing as "acceptable blur" though.


Of course, keep in mind that when you use more optical zoom, you'll need to be further away from your subject for it to occupy the same percentage of the frame (hence, cancelling out thebenefits of longer focal lengths in some shooting conditions where you'd want less Depth of Field, especially since the largest available aperture requires the wide angle lens position with most compact digital cameras).

Although the perspective changes (more compressed background from shooting further away) can give the illusion of a shallower depth of field, since blur in out of focus areas will be more obvious (even if the real depth of field isn't changing, since you need to take the photo from further away if you use more zoom for the same framing).

So, for many scenarios, unless you can budget for a DSLR model (which have much larger sensors compared to non-DSLR digital cameras), your best bet is to try and use software to simulate a shallow depth of field for larger subjects. You may want to check in theEditors forum to get some tips. Here is a thread with a couple of different methods mentioned:


Sbooting small, subjects from close distances is one thing. Trying to blur the background with larger subjects is something else entirely (since you need to be further away to get them fitted in the frame, increasing depth of field).

That's one of the appeals of a DSLR (the ability to control Depth of Field for helping your subjects stand out from distracting backgrounds). The sensors are just too small for that in non-DSLR models (unless your shooting smaller subjects).

A non-DSLR model with much greater depth of field like your camera can be a good thing, too.

You may want more depth of field versus less, and with a DSLR model, you'd need to stop down the aperture (smaller aperture represented by higher f/stop numbers) to get it (often requiring much higher ISO speeds or slower shutter speeds to achieve what you can get with a non-DSLR model shooting at wide open apertures.

There are pros and cons to both types of systems.
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Old Aug 2, 2006, 2:44 PM   #3
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Thanks Jim. Great answer, exactly what I needed to know.
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