question about DOF

[shingfan]

say i use a 50mm at f/2.0 to take a shot

would i be able to achieve the same DOF using a 200mm at f/4.0?

edit: how do i calculate DOF (focus range) using focal lenght, aperature, and focus point

 

[fmw]

Perhaps you can test it and tell us. I wouldn't know without trying it myself.

 

[shingfan]

i dont have the equipment or setting to do such test...but i'll give it a try with what i have to see what i can come up with...would be interesting to know

 

[Digital Matt]

Depth of field depends on distance to subject, as well as focal length. I would guess that at the same distance to subject, the 200mm at f/4 would have less depth of field. The composition would obviously be much different however, so if you matched the composition, your distance to subject would be drastically different.

 

[shingfan]

yes...i'll be further away to compose a similar frame with different focal length........but i like to know a bit more about finding the DOF so that at times i can achieve desired DOF without having to change the lense

 

[markc]

The apertures are also different. 50mm/2 = 25mm aperture. 200mm/4 = 50mm. But there is something else funky going on with the physics. My guess would have been that 50mm@f/1.0 would be a better comparison to 200mm@f/4 at the same distance. Or 200mm@f/8 to 50mm@f/2. But it doesn't pan out. Here's a calculator that you can use to check for yourself. At the same distance, 50mm@f/2 = 200mm@f/32. Nuts, huh?

--edit--
Actually, that would have been a poor guess, as the distance from aperture to film is going to impact relative aperture size. 200 = 50 x 4. 32 = 8 x 4. So that pans out. Makes sense to me now.

 

[Digital Matt]

Here is an interesting read.

http://www.luminous-landscape.com/tutorials/dof2.shtml

 

[markc]

I've seen that before. It throws a wrench into the concept of "acceptable sharpness" though doesn't it? Doesn't that depend on viewing distance to the print? If you enlarge the source, that changes the DOF. What was acceptable sharp now becomes more blurry because it is enlarged. If you change the size or viewing distance on the calculator, it changes the DOF.

At the size those images are shown on the web, the tower is not even close to acceptably sharp in the 400mm image. Once it gets down to 17mm, it's starting to look ok. Heck, the size and JPG artifacts are causing more problem with seeing it than any blur. Once you enlarge it, you have to enlarge the other images and compare details at that level. The Gremlin looks blurry to me in the 28mm shot, so I don't find this that great of a comparison anyway. And the JPG artifacts really screw things up.

I understand what he's getting at, but I think he's missing an aspect of DOF that's important.

 

[ksmattfish]

Make your own DOF calculator out of paper.

http://www.dofmaster.com/custom.html

 

[ksmattfish]

I understand what he's getting at...

I don't. He's making a big deal about the fact that if you use a longer focal length, and then increase the distance to subject, the DOF is about the same. Amazing how if you change a factor that decreases DOF, and then adjust another DOF factor to increase it, that it evens out. What a profound concept. If focal length is the only factor changing, then shorter focal length equals more DOF than a longer focal length.

 

[markc]

I didn't think he was increasing the distance to subject. I didn't mean I agreed with him, just what he was trying for because of faulty logic. His definition of DOF is lacking an important component.

 

[shingfan]

Here is an interesting read.

http://www.luminous-landscape.com/tutorials/dof2.shtml

this guy's statement is totally incorrect...he is talking about blurrness...not DOF.....DOF is how much you can view within the focus point......for sure he is thinking in a wrong direction and not realize it....or to be precise....DOF is the focus range

and from his site...i cant beleive so many ppl agree to his explanation (faulty logic).....or probably written by himself...lol

 

[Digital Matt]

Thanks Shingfan, I know what dof is The title of that article is, "Do Wide Angle Lenses Really Have Greater Depth of Field Than Telephotos?", which is a common belief, and ties directly to what my original point was, which was your distance to subject difference between the two lenses.

 

[ksmattfish]

No, I think he's absolutely right. He just presents it as if it's something new.

To get more DOF do any, or combine several, of the following:
decrease focal length
increase focusing distance
decrease aperture size

To get less DOF do any, or combine several, of the following:
increase focal length
decrease focusing distance
increase aperture size

If I increase focal length, and increase focusing distance, the one DOF factor is equalizing the other.

 

[shingfan]

Thanks Shingfan, I know what dof is The title of that article is, "Do Wide Angle Lenses Really Have Greater Depth of Field Than Telephotos?", which is a common belief, and ties directly to what my original point was, which was your distance to subject difference between the two lenses.

o Matt....pls dont get me wrong....i totally know that you understand what DOF is....and i love your work (you never teach me how you get such natural skin tone on your other post?).......and i know you put up this link only for a laugh....