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DPChallenge Forums >> Tips, Tricks, and Q&A >> Depth of field
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05/07/2007 03:36:10 PM · #26
What's been said above and plenty of practice.

How far along the Ridgeway have you been?
05/07/2007 03:57:29 PM · #27
Originally posted by tosk:

If all lenses are soft after f11 or f14, why do they offer so many apertures after that point, if no one (who knows what they're doing) would use them?


The question is not this! The "film"/lens couple is the limiting factor. A lens that delivers a f32 minimum aperture really can or not deliver a clear crisp image. It depends of the MTFs and it´s another history. But many lenses in the market can really deliver great images at 2 or 3 stops before minimum.
There are too a "de facto" standard that at the half of aperture a lens will give you the best quality. This is factual but not really technical. Some lens can go farter and farter.

The question here is Camera sensor, with tiny pixels, with lots of space between them. The sensor size is now the most constraint factor in DoF and Sharpness. A Rebel XT has a shallow DoF than 5D due to sensor size. A 35mm film SLR Camera has a deepest DoF than any Digital SLR due to film properties and so on.

This question goes farter in technical concepts but let me guide your thougts to other area now. Lets think in art. Some time you doesn´t want that spetacular sharpness, but you have so much light. What do you do? I like night shots. You have noticed that due to large exposure times is very hard to get a crisp clear night shot? Well if crisp is so hard, why not goes farter? Unlimited by sharpness of you aperture you can get gret light effects working freely with you "f" number.

Much more than crisp pictures, some one can have a pallete of options to picture out your world! Know your limits is usefull. But you are an artist or an engenier?

Why the cars are made to develop a final velocity times over the limit speed of the law?

---Edited to spell

Message edited by author 2007-05-07 16:03:03.
05/07/2007 03:58:08 PM · #28
Originally posted by tosk:

If all lenses are soft after f11 or f14, why do they offer so many apertures after that point, if no one (who knows what they're doing) would use them?

They have their uses, aperture isnt all just about getting your DoF, its for controlling how much light enters the camera, so controlling your exposre, on super bright days and you need to doa longer exposure, youll need to compensate for that by using a larger f-stop, even if it means compromising some quality loss
05/07/2007 06:54:41 PM · #29
GoodEnd - I guess you mean that the Rebel XT has a DEEPER DoF than the larger-sensor 5D?

* To get the same image area you need to use a shorter lens (smaller focal length f) on the XT
* A shorter lens with the same relative aperture size, "f-number", has a smaller absolute aperture size since "f / number" is smaller when f is smaller. I.e. a 50 mm objective at f/5 has a 10 mm aperture, but a 40 mm objective only has an 8 mm aperture.
* Absolute aperture size together with focus distance dictates DoF. Think of the cone of light that the lens takes in from the object.

I know that sensor/film/printer/viewer resolution also matters when defining the DoF. Let's just say it's the same for both cameras, or that both images are scaled down to 640 px size for DPChallenge.
05/07/2007 08:11:40 PM · #30
Don't be fooled by the comparison of sensor size and depth of field.

Depth of field describes a distance over which objects are acceptably sharp. It is a function of lens focal length, f/stop, distance to the object from the lens and the distance to the focal point where an image is focused.

In terms of pure physics it has nothing to do with the sensor size. None whatsoever. It does not matter if you have a medium format camera, 35mm "full frame" or a 1.6 crop sensor. DOF is the same for all of them.

Confusion comes into the argument when people describe DOF in terms of field of view which is a side issue brought on by camera manufacturers making small sensor sized dSLR cameras. You will be told that for an equivalent field of view that a 1.6 sensored camera will have 1.6 more DOF than a full framed camera. Makes it sound like you are getting more for your money. You aren't.

The reason you get 1.6 more DOF with a small sensor for the same field of view is simple. An object has to be 1.6 times FURTHER away to fit the same field of view as a 35mm camera onto the tiny 1.6 cropped sensor. 1.6 times further away, approximately 1.6 times more DOF. Of course, what they don't tell you is that you have a lot less light to work with and the objects are recorded with a lot less data because of the distance and smaller sensor.

Physics hasn't change, just advertising. LOL!!!!

Message edited by author 2007-05-07 20:14:16.
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